Light management in luminescent solar concentrators : aligned organic dyes and organic

Light management in luminescent solar concentrators :aligned organic dyes and organic wavelength selectivereflectorsCitation for published version (APA):Verbunt, P. P. C. (2012). Light management in luminescent solar concentrators : aligned organic dyes andorganic wavelength selective reflectors. Eindhoven: Technische Universiteit Eindhoven.https://doi.org/10.6100/IR740226

DOI:10.6100/IR740226

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https://doi.org/10.6100/IR740226

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Light management in luminescent solar concentrators

Aligned organic dyes and organic wavelength selective reflectors

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de

Technische Universiteit Eindhoven, op gezag van de

rector magnificus, prof.dr.ir. C.J. van Duijn, voor een

commissie aangewezen door het College voor

Promoties in het openbaar te verdedigen

op 1 november 2012 om 16.00 uur

door

Paul Pieter Catherina Verbunt

geboren te Roermond

Dit proefschrift is goedgekeurd door de promotor:

prof.dr. D.J. Broer

Co-promotoren:

dr. M.G. Debije

en

dr.ing. C.W.M. Bastiaansen

A catalogue record is available from the Eindhoven University of Technology Library

ISBN: 978-94-6191-451-4

Copyright © 2012 by Paul Pieter Catherina Verbunt

The research described in this thesis has been financially supported by the Stichting voor

Technische Wetenschappen (STW) VIDI Grant 7940

“Wish you were here”

Pink Floyd

Veur mam en pap

Table of contents Summary ...................................................................................................................................................... ix

1 Introduction............................................................................................................................................ 1

1.1 Energy in the built environment .............................................................................................. 2

1.2 Solar Energy ................................................................................................................................. 2

1.2.1 Photovoltaic cells .............................................................................................................. 3

1.2.2 Types of photovoltaic cells .............................................................................................. 4

1.2.3 Geometrical solar concentrators ..................................................................................... 8

1.3 Scope and outline of the thesis ................................................................................................ 11

2 Luminescent solar concentrators ...................................................................................................... 13

2.1 Introduction ................................................................................................................................ 14

2.2 Working principle of luminescent solar concentrators ........................................................ 15

2.3 Losses of luminescent solar concentrators and their proposed solutions ........................ 18

2.3.1 Re-absorption of emitted photons by other luminophore molecules .................... 19

2.3.2 Luminophore losses: Limited absorption, limited stability and luminescence efficiency ....................................................................................................................... 22

2.3.3 Photovoltaic losses .......................................................................................................... 34

2.3.4 Waveguide losses ............................................................................................................. 35

2.4 Conclusions ................................................................................................................................ 38

3 Spatial distribution of emitted photons from dichroic dye ensembles ....................................... 41

3.1 Introduction ............................................................................................................................... 42

3.2 Spatial distribution of emitted photons from isotropically distributed dichroic dye molecules .................................................................................................................................... 42

3.3 Spatial distribution of emitted photons from dichroic dye molecules in a planarly aligned liquid crystal host ........................................................................................................ 46

3.3.1 Introduction to liquid crystals ....................................................................................... 46

3.3.2 From molecular dichroism to macroscopic dichroism ............................................. 49

3.3.3 Directional emission from planarly aligned luminophores ...................................... 51

3.4 Spatial distribution of emitted photons from dichroic dye molecules in a homeotropically aligned liquid crystal host .......................................................................... 55

3.5 Spatial distribution of emitted photons from dichroic dye molecules in a tilted aligned liquid crystal host ...................................................................................................................... 56

3.6 Conclusions ................................................................................................................................ 60

vi

4 Emission from planarly aligned dichroic dyes ................................................................................ 61

4.1 Introduction ................................................................................................................................ 62

4.2 Dichroism in fluoresencence ................................................................................................... 62

4.3 Spatial distribution of emitted photons .................................................................................. 63

4.3.1 Theoretical approach ...................................................................................................... 63

4.3.2 Methods ............................................................................................................................ 64

4.3.3 Experimental results and discussion ............................................................................ 67

4.4 Application of silver mirrors or a scattering background to planarly aligned dichroic dyes in LSCs .............................................................................................................................. 70

4.5 Conclusion .................................................................................................................................. 72

5 Surface loss in luminescent solar concentrators ............................................................................. 73

5.1 Introduction ................................................................................................................................ 74

5.2 Theoretical approximation ....................................................................................................... 74

5.3 Methods ....................................................................................................................................... 77

5.4 Results and discussion ............................................................................................................... 78

5.5 Conclusions ................................................................................................................................ 84

6 Reduction in surface loss by dye alignment .................................................................................... 87

6.1 Introduction ................................................................................................................................ 88

6.2 Surface loss from planar and homeotropically aligned dye ensembles ............................. 88

6.2.1 Methods ............................................................................................................................ 88

6.2.2 Results and discussion .................................................................................................... 89

6.4 Tilted dye ensembles ................................................................................................................. 97

6.5 Conclusions ..............................................................................................................................103

7 Organic wavelength selective reflectors .........................................................................................105

7.1 Introduction ..............................................................................................................................106

7.2 Methods .....................................................................................................................................109

7.3 Narrowband reflectors ............................................................................................................113

7.4 Broadband reflectors ...............................................................................................................119

7.4.1 Theoretical approach ....................................................................................................119

7.4.2 Experimental results .....................................................................................................129

7.4.3 Patterned waveguides ...................................................................................................131

7.5 Other luminophores ................................................................................................................133

7.6 Conclusions ..............................................................................................................................135

Table of contents

vii

8 Wavelength selective reflectors and indirect sunlight ..................................................................137

8.1 Influence of the angle of incident light on the performance of cholesteric reflectors .138

8.1.1 Influence of the angle of the incident light on the light that passes through the reflector .......................................................................................................................138

8.1.2 Dependence of maximum possible LSC efficiency with change of the angle of the incident light ........................................................................................................139

8.1.3 Indirect sunlight .........................................................................................................141

8.2 Special dispersion cholesteric reflectors ...............................................................................143

8.2.1 Introduction ...................................................................................................................143

8.2.2 Transmission of sunlight ..............................................................................................145

8.2.3 Efficiency towards surface emitted light ...................................................................146

8.2.4 Angular dependent increase in LSC efficiency .........................................................147

8.3 Conclusions ..............................................................................................................................150

9 Technology assessment and future possibilities ...........................................................................151

9.1 Luminescent solar concentrator: the future ........................................................................152

9.1.1 Energy generating applications ...................................................................................152

9.1.2 Day lighting applications ..............................................................................................154

9.2 Aligned luminophores .............................................................................................................154

9.2.1 Static alignment of luminophores in LSCs: what can be gained? ..........................154

9.2.2 Dynamic alignment of luminophores in LSCs: window applications ..................156

9.3 Wavelength selective reflectors in luminescent solar concentrators ...............................156

Appendix A: Perylene perinone dye .....................................................................................................159

A.1: Introduction ............................................................................................................................159

A.2 Methods ....................................................................................................................................160

A.3 Results and Discussion ..........................................................................................................161

A.4 Conclusions..............................................................................................................................165

References .................................................................................................................................................167

Samenvatting ............................................................................................................................................181

Acknowledgements .................................................................................................................................185

Curriculum Vitae .....................................................................................................................................189

List of symbols .........................................................................................................................................191

List of definitions ....................................................................................................................................195

List of publications ..................................................................................................................................197

Summary

Dwindling oil reserves have turned world governments towards the use of renewable energy

sources. In particular, buildings, which account for 40% of all energy usage in well-developed

countries, have attracted special attention for the possibility of implementing energy generating

devices from renewable energy sources. For buildings, the devices must be adaptable to the

demands of the architect and should not detract from the appearance of the structure, while

maintaining a low costs for the generated energy (~€0.06/kWh). An energy source readily

available for each building is the sun. The implementation of standard photovoltaic cells in

these constructions has been difficult, since the cost of the energy generated is still too high and

the adaptability to the demands of the architect is limited. An alternative to standard

photovoltaic cells is a luminescent solar concentrator (LSC) which holds promise for low cost

and of better meeting the demands of the architects.

LSCs are simple devices that consist of an optically transparent plastic or glass plate acting

as waveguide. A luminophore is embedded in or as a thin film on top of the waveguide and

absorbs the incoming sunlight, re-emitting this light as photons with a longer wavelength. Part

of this emitted light is waveguided in the plate due to total internal reflection and transported to

the edges of the plate where small photovoltaic cells are placed which convert the photons into

electricity. The efficiency of LSCs has been limited due to several loss mechanisms. One of the

most important losses is the photons lost through the surfaces of the plate, a result of a limited

trapping efficiency of emitted photons combined with sequential re-absorption and re-emission

of photons in the waveguide mode (called ‘photon recycling’).

In this work, I present a simple model that predicts the spatial distribution of emitted

photons from dichroic dye ensembles in an isotropic or liquid crystalline host. The model

predicts the emission from dichroic dyes in an isotropic host is non-isotropic when illuminated

with a collimated light source. The model furthermore predicts that by changing the alignment

of the dichroic dye molecules, the spatial distribution of the emitted photons may be altered.

The model is tested by experiments in which the molecular order of dichroic dyes in a

planar aligned liquid crystal is changed. LSC waveguide with the dye molecules aligned parallel

to the top and bottom surfaces and parallel to two edges (planar) were made, and I measure the

light energy emitted from the edge parallel to the alignment direction can be 60% higher than

from the edge perpendicular to the alignment direction, supporting the calculated result.

The amount of surface loss from LSCs with dichroic fluorescent dye molecules randomly

distributed in the LSC is calculated and determined experimentally. Calculations show that the

trapping efficiency is 74.3%, meaning 25.7% of the photons are lost through the surfaces of the

x

LSC waveguide. Experiments show that the photons lost though the surface of LSCs made

from polycarbonate is actually ~ 50% of all emitted photons for LSCs containing BASF

Lumogen F Red 305, the standard dye used in LSCs. This large surface loss is caused by photon

recycling events of waveguided photons and the consequent change in propagation path.

Aligning dichroic dye molecules can be used to reduce surface loss of LSCs. Calculations

and experiments both show aligning dye molecules in a homeotropic fashion result in a

reduction of surface loss to less than 10%, but with concurrent reduction in the incident light

absorbed. Tilting the director of the dye ensembles at an intermediate angle can combine the

advantages of planar (increased absorption) and homeotropic (decreased surface loss) dye

ensembles. Prediction of the optimal tilt angle and order of the dye ensemble with the model

presented in this thesis was not possible. I suggest this model needs implementation in

simulation software so the influence of photon recycling is better represented.

A second method to reduce the surface loss in LSCs is the application of organic

wavelength selective reflectors transmitting the incoming sunlight and reflecting the light

emitted by the luminophore. Calculations show approximately 90% of all surface emitted

photons from luminophore BASF Lumogen Red 305 may be reflected back into the LSC

waveguide, leading to an increase in LSC efficiency of 66% if a 400 nm bandwidth reflector is

used, or by 45% with a 175 nm reflector. Experimentally, the increase in energy leaving the

edge of an LSC containing Red 305 with a peak absorbance of 1.0 is only about 5%. Reducing

the number of photon recycling events in an experimental system with a 175 nm bandwidth

reflector led to enhancement in LSC efficiency of around 20%, showing it is photon recycling

that limits the effectiveness of the wavelength selective reflectors.

The organic wavelength selective reflectors show angular dependency: the reflection band

shifts to shorter wavelengths with increasing angle of incidence, possibly leading to the

reflection of absorbable incident sunlight at these larger angles. Calculations show that for non-

direct sunlight or direct sunlight at oblique incidence angles there is a decrease in effectiveness

of the reflectors and potential decrease in device efficiency. Simulations show that special

dispersion cholesteric reflectors (SDC) have a less pronounced angular dependency. Using SDC

reflectors will enhance the LSC efficiency in any solar condition and only incident light at angles

larger than 70° are reflected away by the reflector.

Both luminophore alignment and organic selective reflectors presented in this thesis

reduce the surface loss of the LSC, and increase the device efficiency. Experimentally, the

increase is not as large as expected from the calculations. To increase the LSC efficiency the

methods presented in this thesis need to be employed using a luminophore with a larger Stokes

shift, so photon recycling is limited. In addition, the amount of sunlight that can be absorbed

by the luminophore molecules (e.g. the spectral coverage) needs to be increased for the LSC to

become viable in the marketplace, as this is still the single largest loss in LSCs.

1 Introduction

Chapter 1

2

1.1 Energy in the built environment

With the realities of dwindling oil reserves now affecting much of everyday life, world

governments have increasingly turned towards renewable energy technologies. In particular,

there is an increasing interest in bringing solar energy systems to the built environment.

Buildings account for about 40% of total energy use including 70% of total electricity use, and

40% of emissions in the more developed countries [1], and about 25% of all energy use globally

[2]. Much of this use is related to our inability to control sunlight: tremendous energy resources

are used to both heat and cool buildings and provide artificial lighting systems. [1] The scene of

an office with shuttered windows and burning lights in the daytime is familiar to everyone.

The European Committee has decreed that all new-to-be-built buildings should be near-

zero energy by 2020. [2] This demands that architects and the building industry must integrate

energy saving and energy generation into the design of new buildings. This puts considerable

pressure on the architect wishing to meet these requirements: how to create buildings both

pleasing to the eye, yet energetically neutral? To give the maximum freedom, the devices

incorporated to save and/or generate energy have to be easily implemented into the design and

adaptable to the situation. Adaptability could also make retrofitting of existing structures during

renovations easier and cheaper. One energy source that is readily available in a built

environment is the sun, which is a clean, safe, inexhaustible and reliable energy source.

1.2 Solar Energy

The sun emits energy in the form of electromagnetic waves with a total of 174 PW reaching the

Earth’s atmosphere. Before this energy reaches the Earth 30% is lost via reflection from the

atmosphere and clouds and from absorptions in the atmosphere. World energy consumption is

in the order of 15-20 TW, so if the energy from the sun could be converted into usable energy

like electricity, the sun alone could produce enough energy to provide the world needs many

times over.

The photons emitted by the sun range from almost 0 eV to nearly 4 eV, where the energy

( E ) of these photons is related to the wavelength ( ) or frequency ( ) by

hc

E h (1.1)

where h is Planck’s constant (6.626x10-34 Js) and c is the speed of light in a vacuum (~3x108

ms-1). As described above, the solar radiation loses approximately 30% of its energy before

reaching the earth. Scattering and absorption of photons by molecules and particles present in

the upper atmosphere are partially wavelength dependent, so the solar spectrum reaching the

Earth’s surface will change. Solar irradiance is defined according to the optical air mass (AM),

which is related to the position of the sun relative to the zenith position. The solar spectrum

Introduction

3

just above Earth’s atmosphere is called Air Mass 0 (AM0) and the standard terrestrial solar

spectrum AM1.5. The AM0 and AM1.5 spectra are depicted in figure 1.1.

Figure 1.1 Solar spectrum of AM 1.0 (black) and AM1.5 (gray).

Solar energy can be converted into usable energy using photo-thermal, photo-chemistry or

photovoltaic systems. Photo-thermal uses the sunlight to heat up a material such as water.

Photochemistry uses the energy in the photons to start chemical reactions, while photovoltaics

convert the suns photons into electricity. Photovoltaics is a clean method to produce energy,

which can be used directly. In the rest of this chapter the photovoltaic method for solar energy

conversion is discussed in greater detail.

1.2.1 Photovoltaic cells

In a photovoltaic cell, absorption of a photon by the semiconducting material promotes an

electron from the valence band to the conduction band, creating an electron-hole pair. In

silicon, a well-known semiconductor, the minimum energy needed to promote an electron into

the conduction band is ~1eV, called the bandgap. Photons which do not have enough energy

to overcome the bandgap will not be absorbed by the semiconducting material. The electron-

hole pair generated has a very limited lifetime, and re-combines. To keep the electron and the

hole separated from each other, asymmetry is built in to the photovoltaic device, which pulls

the electrons away from the holes, creating a potential difference. To increase this asymmetry,

the silicon is doped with other atoms to create p-n junctions. P-type doping adds a different

atom with fewer electrons in the crystal lattice of the silicon, while n-type doping adds an atom

with more electrons. In a p-type semiconductor material the dopant needs an electron to match

the lattice of the semiconductor, which is taken from the valence band of the semiconductor

material, leaving a hole in the valence band and a negatively charged ion in the lattice. In an n-

type semiconductor material the dopant has one electron left after addition to the

Chapter 1

4

semiconductor lattice. This electron is added to the valence band creating a positively charged

ion in the lattice. Adding a p-type and n-type semiconductor together will lead to a depletion

region, where the excess holes and electrons in the valence band recombine, leading to a

negatively charged p-type semiconductor and a positively charged n-type semiconductor,

creating a difference in potential energy between them. This difference in potential energy will

separate the created charges and act as a diode. (figure 1.2)

Figure 1.2 A schematic depiction of a photovoltaic cell, where the open circles depict the holes and the closed

circles depict the electrons in the valence band.

1.2.2 Types of photovoltaic cells

In the previous section silicon is mentioned as a semiconducting material that can be used in

photovoltaic (PV) cells, but there are more types of semiconductor material used in PV-cells

like type III-V cells, thin film cells, and organic cells.

1.2.2.1 Silicon photovoltaic cells

Silicon was used in the first solar cells made in the Bell Laboratories in the 1950s. The bandgap

of silicon is 1.11 eV, meaning that silicon can absorb photons with wavelengths up to

approximately 1100 nm. Silicon is used in several different forms in PV-cells. The first is a

multi-crystalline state in which the PV-cell consists of several grains of crystalline Si, where the

crystal-lattice is differently organized in each grain. The second uses one single crystal of Si,

called ‘single crystalline’ or “monocrystalline’ Si PV-cells. Both types of crystalline Si have an

indirect bandgap, [3] which leads to low absorption. An indirect bandgap is a result of a

difference in momentum between the lowest energy in the conduction band and the maximum

energy in the valence band and consequently optical transitions between free electrons and

holes are forbidden. The absorption coefficient of crystalline silicon is in the order of 2x103

cm-1 in the green part of the spectrum. Grain boundaries in multicrystalline Si cause a loss of

photo generated charges at the boundaries. Thus, monocrystalline Si PV-cells have a higher

efficiency. Large single crystals of silicon are rare in nature and so it is more expensive to

produce single crystalline Si solar cells.

Introduction

5

To improve the absorption of Si cells, amorphous Si (a-Si) is used. In contrast to

crystalline Si, a-Si has a direct bandgap and therefore the absorption coefficient is one order of

magnitude greater. This makes it possible to decrease the cells’ thickness and makes it possible

to construct more flexible PV-cells. The disadvantage of a-Si PV cells is that they lack good

stability. In table 1.1 the current record efficiencies of Si PV cells are listed.

Table 1.1 Silicon PV cell efficiencies in 2011

Photovoltaic cell Record Research

cell efficiency [%]

Single crystalline Si 25.0 [4]

Multicrystalline Si 20.4 [5]

Amorphous Si 10.1 [6]

A disadvantage of silicon PV-cells is still the cost of these cells. The combination of costs

with the efficiency of the Si PV-cells leads to a high price per Watt of the generated electricity.

1.2.2.2 Thin film photovoltaic cells

Thin film photovoltaics use only thin layers of the semiconductor material and could therefore

be made less expensive. Decreasing the thickness of the semiconductor layer decreases the

amount of light that can be absorbed, so the semiconductor material needs to have a high

absorption coefficient. Two examples of thin film photovoltaics which have been extensively

investigated are Cadmium-Telluride (CdTe) and Copper-(Indium)-Gallium-Selenide (CIGS).

Cadmium-Telluride is commonly used in combination with Cadmium-Sulphide (CdS),

where the CdTe forms the p-type semiconductor and CdS the n-type. CdTe is the absorbing

material and has a bandgap of approximately 1.5 eV (~830 nm), which is slightly larger than the

bandgap of silicon. Cells made from CdTe have demonstrated efficiencies above 15%, but

progress towards increased efficiencies has been very slow over the past 10 years, with a record

efficiency of 16.7%, [7] which is still higher than of thin film a-Si. The availability of Tellurium

is somewhat restricted, [8] which makes it difficult to provide enough solar cells to meet

demand. Another concern is the safety of Cadmium, which is a toxic metal. [9] Recycling of

Cadmium is prohibited according to European regulations. [9]

CIGS cells are in principle a solid solution of Copper-Indium-Selenide (CIS) and Copper-

Gallium-Selenide. Like CdTe, CIGS are mainly used in combination with CdS. The bandgap of

CIGS cells can be tuned by altering the content of Indium and Gallium in the solution. [10]

The general structure of CIGS is CuInxGa(1-x)Se2 with variation of x from 0 to 1 to change the

bandgap linearly from 1.0 to 1.7eV. The reported efficiencies for CIGS cells have been the

highest for thin film PV-cells, and the record efficiency is 20.0%. [11] Since the most efficient

CIGS-cells are heterojunction cells of CIGS and CdS the production cost of the cells rise due

Chapter 1

6

to the multiple layers needed. Furthermore Cadmium is introduced, which gives concerns on

safety.

1.2.2.3 Type III-V photovoltaic cells (multi-junction solar cells)

Type III-V cells, like GaAs have been reported to show the highest efficiencies of all materials

used in PV-cells. Thin film GaAs has a reported efficiency of over 28%. [12] Instead of thin

film applications, these materials are mainly used in multi-junction solar cells, which results in

very high efficiencies at the expense of high production costs.

Multi-junction solar cells are a combination of different cells made from different

materials, each with their own bandgap. This combination of multiple materials uses the ability

of high photon to electron conversion of these materials for relatively small spectral

bandwidths. In other words, each layer is only used to convert the light for a limited portion of

the incident spectrum, that part where it demonstrates the highest efficiency. Multi-junction

cells mainly consist from three or four junctions. III-V materials used in these triple multi-

junction cells are, for example InGaP which has a bandgap of 1.8-1.9 eV, InGaAs with a

bandgap of 1.4 eV and Germanium having a bandgap of 0.67 eV, GaAs is also used with a

bandgap of 1.42 eV. Other combinations of these materials have also been employed.

These multi-junction cells have reported efficiencies over 40%, [12] but the materials used

and the production of these cells is very expensive and are currently too expensive for use in

large area energy conversion. Instead, these cells are mainly used in space applications, [13]

where the energy generated per kilogram of materials is much more important than the costs, or

terrestrially in combination with solar concentrators, which will be discussed in section 1.2.3.

1.2.2.4 Organic photovoltaic cells

All the types of cells described so far have been made from inorganic semiconductor materials.

The discovery of organic semiconductor materials induced extensive research into organic

materials than can be used to produce PV-cells.[14] In organic semiconductor materials the

bandgap is formed by the energy difference between the highest occupied molecular orbital

(HOMO) and the lowest unoccupied molecular orbital (LUMO). By absorption of a photon an

electron is excited from the HOMO to the LUMO level, and separating this electron from a

hole that is left in the HOMO level creates an exciton.

Early research on organic PV-cells concentrated on single layer organic materials.[15] In

these cells the active organic semiconductor was placed between an anode and a cathode. The

exciton forms a free electron and hole pair which must transfer to the collection electrodes.

This separation is driven by the electric field created by the difference in work functions of the

two electrodes. In these single layer organic PV-cells the electric field created by the electrodes

was not sufficient to separate the electron and the hole, which rather recombined, leading to

very low photovoltaic efficiencies (<1%).

Introduction

7

One way to solve this problem was to use two layers of active materials placed between

the two electrodes in so-called bilayer organic PV-cells. [16] One material is an electron

acceptor and the other an electron donor. Excitons formed by light absorption feel a strong

local electric field at the interface between these materials, which will lead to separation of the

electron and the hole, since they are attracted to the electron acceptor material and the electron

donor material, respectively. These materials have one big disadvantage; the diffusion length of

the excitons in these materials is only ~10 nm. Therefore, light collection layer thicknesses of

approximately only 20 nm can be used in these PVs, since excitons formed further away than

10 nm from the interface between the electron donor and acceptor material will be lost. To

reach sufficient absorption, these cells require a thickness of 100s of nanometres.

To achieve these thicker active layers, but maintain short migration distances for the

charges a dispersive active layer was created. [17] This creates a much larger interface between

the materials, and formed excitons have better chance of reaching this interface with the

diffusion length of ~10 nm.

There has also been a large amount of research performed to create different kinds of both

electron acceptor and electron donor materials. Some examples are the buckyball, C60 and C70

and perylenebisimides as electron acceptors and derivates of poly(phenylene-vinylene) (PPV),

poly(alkyl-thiophenes) and phtalocyanine as electron donors. (i.e. [18]) So far all this research

has led to a record efficiency of ~10%, and this efficiency is rapidly increasing. [19]

Organic solar cells could become less expensive than the inorganic solar cells discussed

earlier, and viable for production on large scale. Another advantage is that these cells can be

made flexible, which initiates many different application possibilities. The modest efficiency and

the long term stability of these materials are still disadvantages of the organic PV-cells. [20]

1.2.2.5 Other types of photovoltaic cells

There are still a number of types of cells which do not belong to these main classes of solar

cells. Two types of cells that will be discussed in this section are dye-sensitized solar cells

(DSSC) and quantum dot solar cells.

A dye-sensitized solar cell is an electrochemical cell with organic dyes that acts as an

absorber attached to an oxide electrode. Light entering a DSSC will be absorbed by the organic

dye, promoting an electron to an excited state. This electron is then transferred to a porous

TiO2 layer between the dye and the oxide electrode, and from this TiO2 layer the electron is

transferred to the electrode. The organic dye is left with a lack of an electron. The dye molecule

then replaces the electron with an electron donated by the electrolyte in the electrochemical cell.

Iodine is commonly in the electrolyte of the cell can be used as this donor and is oxidised to

triiodide. This triiodide then diffuses to the reduction electrode in the cell and is reduced back

into iodine. The dyes used in these types of materials are mostly ruthenium based complexes

with organic ligands. The efficiency of these DSSCs are modest, the record efficiency is ~11%.

[21] DSSCs are relatively inexpensive if the amounts of Ruthenium and Platinum used in the

Chapter 1

8

electrolyte is kept low. The main disadvantages of these cells is the liquid electrolyte which has

stability problems; if the temperature decreases it can freeze and upon temperature increase it

can expand, leading to rupture of the seal preventing the electrolyte to leak out of the cell. The

electrolyte often contains volatile components that can leak out of the cells, [22] so these cells

must be sealed with care.

Quantum dots can also be used as the semiconductor material. The advantage of quantum

dots is that their bandgap can be tuned by the material of which the particles are made, but also

by the size and the size distribution of the particles. More on the quantum dots can be found in

Section 2.3.2.2. This tuneable bandgap makes them viable for multijunction solar cells, made

from a single material. This technique to produce solar cells is relatively new, and the record

efficiency is still modest (~5%). [23] Expanded research on these cells could improve the

efficiency and they could become an important factor in the PV-cell industry if their theoretical

potential with efficiencies up to 65% is met. It is known that quantum dots have stability issues

when exposed to air, which makes it necessary to seal the cells. Furthermore there is some

concern on the safety of quantum dots. [24-26]

1.2.2.6 Building integrated photovoltaics

For architects it is very important that energy generating and saving devices that may be

implemented in a building do not impede with the appearance of the building. Since PV cells

are relatively thin their implementation on flat rooftops of a building will not change the look

of the building much. Implementation of solar cells on a non-flat rooftop or on the façade of a

building raises more issues. In this case the device is directly in the view. Silicon PV-panels are

only available in black and dark blue and they are not transparent. Some research has been

conducted to change the colour of the PV-panel. [27,28]

Mounting a solar cell to the side of the building sets limitations to the weight of the device.

Devices which are too heavy, like single crystal and multicrystalline silicon PV-panels require

extra care during installation. Thin film photovoltaics do not have this problem, but all

materials have their own challenges towards implementation in the building environment, such

as safety issues, stability and efficiency.

The second important factor in integrating photovoltaics in buildings is the costs of the

energy produced. The cost should be low enough to compete with grid prices and approaching

$0.06/kWh. At the moment the average costs of energy produced from silicon photovoltaic

modules is on the order of $0.30/kWh, so still too high.

1.2.3 Geometrical solar concentrators

To reduce costs of solar energy, concentrating systems were developed. Concentrating systems

replace a large area of expensive photovoltaic cells with less expensive materials. These

concentrating systems concentrate the sunlight from a large area to a small area creating a high

Introduction

9

radiance flux. Traditional solar concentrators are based on geometrical optics like reflection

(mirrors) and refraction (lenses).

The first reflection-based system was a point-focus concentrator, [29,30] a parabolic

mirror that reflects incoming sunlight to one focal point. At this focal point a small PV-cell was

placed. By focusing of all incoming light to this focal point the flux increased, creating

concentration of light. A schematic depiction of a focus point concentrator can be seen in

figure 1.3a.

A more evolved version of this point focus concentrator is a compound parabolic

concentrator, also known as CPC. [31-33] CPCs are two parabolic mirrors that intersect each

other at a point below the focal point of the parabolic mirrors. A small opening is created in

this CPC at the focal point of the parabolic mirrors and a PV-cell is placed at this focal point.

This insures that all the photons that enter the CPC will be focused on the PV-cell. The CPC

does not create an image of the sun on the PV-cell, but the principle of concentration is known

as non-imaging optics. An example of a CPC can be seen in picture 1.3b.

Refraction or focusing concentrators are concentrators based on lenses, primarily Fresnel

lenses. [34-37] Fresnel lenses are plastic or glass sheets containing grooves in a linear (2D lens)

or circular (3D lens) pattern. The grooves in Fresnel lenses form asymmetrical prism-like

structures. These structures mimic the curvature of spherical lenses. The light going through a

Fresnel lens is focused at a focal point just like a spherical lens. The disadvantage of such a

Fresnel lens over a spherical lens is that the focusing is less precise, but the decrease in

thickness makes these Fresnel lenses more likely to be used in concentrating systems. Another

disadvantage of Fresnel lenses is that the acceptance angles for the incident light are rather

limited.

Figure 1.3 Schematic depiction of (a) a point focus concentrator and (b) a compound parabolic concentrator.

Both reflecting and focusing concentrators must meet the thermodynamic conservation

law of etendue. This conservation of etendue limits the concentration of light, especially for

incoming light with a large solid angle. Light entering and leaving a concentrator have both an

Chapter 1

10

area ( inA and outA ) and a solid angle (in and out ). The law of conservation of etendue (U )

upon concentration describes that the maximum achievable concentration ( maxC ) by a

concentrator, and depends on the refractive index of the material in which the receiver (for

instance the PV-cell) is embedded and by the incoming and outgoing solid angle. (see equation

1.2) [38]

out inU U

1 2

out out out in in out out

in out in in in

E L U L U nC

E A A n (1.2)

where E (Wm-2) is the irradiance, L (Wm-2ster-1) is the radiance of the light and n is the

refractive index.. Maximum concentration is achieved when the solid angle of the outgoing light

is 90° and the solid angle of the incoming light is a small as possible.

For completely diffuse light reaching a concentrator in air the maximum concentration is

defined as:

2max,diff concC n (1.3)

Equation 1.3 demonstrates that the maximum concentration for these concentrators for

both diffuse light and direct sunlight that moves along the horizon is relatively low (if the cell is

embedded in a material with refractive index 1.5 the maximum concentration is 2.25). For

direct sunlight the maximum concentration of these systems can be very high since the solid

angle is zero, but this direct sunlight should enter the concentrator always at the same angle. To

achieve this, concentrators are normally combined with a sun tracking system. [39-42] The

disadvantage of these tracking devices is that they add costs to the concentrating system and

that they add size to a relatively small concentrator. This makes these concentrating systems

undesirable for implementation in buildings. [43,44]

Figure 1.4 Flat plate concentrator. Sunlight is coupled into a waveguide made from glass or a polymer by

diffractive elements, light trapping, special backside mirrors, or refractive-index variations.

There is a need for flat panel solar concentrators that do not need a tracking device for

achieving higher concentration. Flat plate concentrators are based on thin polymer or glass

plates which act as a waveguide (see figure 1.4). The sunlight is coupled into this waveguide by

Introduction

11

diffractive elements, [45] light trapping, [46] special backside mirrors, [47] or refractive-index

variations. [48] However, each of these designs presents their own challenges to produce

electricity efficiently enough to be economically feasible. A flat plate concentrator that has great

potential is the luminescent solar concentrator (LSC). The LSC uses (as the name already

suggests) luminescent species to concentrate light. These luminescent species are embedded in a

polymer or glass plate that is transparent. The LSC will be discussed in more detail in chapter 2.

1.3 Scope and outline of the thesis

The scope of this thesis is to determine and increase the trapping efficiency of photons in the

LSC waveguide, and therefore reducing the surface loss of emitted photons. In chapter 2, the

principle functionalities and losses of luminescent solar concentrators are described, and a

background of the research that has contributed to reducing these losses over the past thirty-

odd years is discussed. This chapter is a purely informative chapter with no original research by

the author. In chapter 3, a model is presented that predicts the spatial distribution of photons

emitted by an ensemble of dichroic dye molecules in both isotropic and liquid crystalline hosts.

In chapter 4, the model is validated and the change in spatial distribution of emitted photons by

aligning the dichroic molecules planarly with different order is shown. In chapter 5, the surface

loss from dichroic dye molecules in an isotropic host is determined and the results compared to

the trapping efficiency determined by the model presented in chapter 3. The use of aligned dyes

in LSCs to reduce surface loss is described in chapter 6. Surface loss in LSCs can also be

reduced by application of a wavelength selective reflector. The influence of the application of

organic wavelength selective reflectors on the LSC performance is presented in chapter 7.

These reflectors are angular dependent and this angular dependency is described in chapter 8,

along with a possible solution. The thesis will be concluded by chapter 9, which presents a

technology assessment.

2 Luminescent solar concentrators1

1 Also published in:

P.P.C. Verbunt and M.G. Debije, “Progress in luminescent solar concentrator research: Solar

energy for the built environment”, in proceedings of the World renewable energy congress,

2011.

M.G. Debije and P.P.C. Verbunt, “Thirty years of luminescent solar concentrator research:

Solar energy for the built environment” Advanced Energy Materials, 2, 12-35, 2012

Chapter 2

14

2.1 Introduction

The desire is to take the advantages of a reflective or focusing solar concentrator (collecting

light from a larger area and focusing it onto a smaller area) while avoiding the disadvantages

(the need for direct sunlight, tracking, and often awkward dish-like shapes and large sizes) for

integration in urban environments. An alternative to traditional semiconductor devices that

takes the advantages of concentrator systems but which could still be used in the built

environment could be the luminescent solar concentrator (LSC), which was originally

introduced more than three decades ago. The earliest reference to an LSC type device

architecture appeared as a grant proposal in 1973 by Lerner, [49] soon followed by papers in

the general literature. [50-52] There was a great flurry of activity in the field, leading to a

number of patents. The drop in oil prices in the 1980s led to a near-abandonment of the

research. Recent surges in oil prices and increased awareness of the effects of power generation

from traditional fuels on the global environment have revitalized research on the LSC, and it

has again become the interest of many groups around the world. In particular the past five years

have demonstrated a considerable reawakening of interest in the LSC. This has been due to a

number of factors, including the increasing awareness by the general public of the dwindling

fossil-fuel availability, limitations on the performance and thus capability of deployment in

urban areas for traditional PV systems and the desire from the architectural and building sectors

for more freedom in their design choices.

LSCs were initially proposed as low cost alternatives for standard silicon PV-cells. Recent

research by Farrell et al. showed that for standard first generation LSCs the costs of the LSC

plate should be less than 11% of the costs of an equal size Si-photovoltaic module. [53] The

costs of these modules have dropped lately and are in the order of $350/m2. The costs of the

LSC are determined by the cost of the waveguide material (~$4.5/kg) and the used

luminophore. The costs of an LSC were determined to be ~$35/m2,[54] which is about 10% of

the cost of a Si photovoltaic module. So the costs of the energy produced by LSC and Si

photovoltaic modules is approximately the same. To lower the cost of energy produced from

LSCs the efficiency of the LSC has to be improved, while the costs are kept low.

LSCs promises potential low costs for energy generation [55,56] and they could provide

adaptability to the needs of the architect in that they can be made in a variety of colours,

shapes, and transparencies, could be made flexible, and should weigh less than silicon PV

panels, for example, which could make LSCs more viable for mounting to the side of a

building. [57] In addition, the LSC could function well in both direct and diffuse light, [58,59]

of particular interest for countries with frequent cloud coverage, such as the Netherlands or

areas of persistent shady conditions, such as are typical in cities, for example. If the photon

in/photon out efficiency is high enough, the cost of electricity generated by the LSC could be

competitive with grid electricity. [55] While the efficiency of an LSC will be lower than an

Luminescent solar concentrators

15

equivalent area of a silicon PV due to decreased spectral usage, the reduced cost and

tremendous flexibility in design could make them a viable alternative for the urban area where it

would be too expensive, inappropriate, or impractical to use standard solutions.

2.2 Working principle of luminescent solar concentrators

The basic LSC design allows sunlight to penetrate the top surface of an inexpensive plastic or

glass waveguide. This light is absorbed by luminescent molecules (which could be organic dyes,

inorganic phosphors, or quantum dots, which will be discussed in section 2.3.2), which are

embedded in the waveguide, applied in a separate layer on the top or bottom of the waveguide,

or contained in a liquid solution between two glass plates. [51,60,61] The absorbed light is re-

emitted at a longer wavelength and a fraction of the re-emitted light is trapped in the waveguide

by total internal reflection, becoming concentrated along the edges of the plate. Small PV cells

can be attached to the edges of the waveguide to collect the light and convert it to electricity.

figure 2.1 summarizes the functionality of the device.

Figure 2.1 Working principle of a luminescent solar concentrator. Incoming sunlight (black) is absorbed by the

luminophores in the waveguide. The luminophore re-emits this light at longer wavelengths (gray). A part of this

re-emitted light is trapped inside the waveguide by total internal reflection. The light is concentrated at the edges of

the waveguide, where a photovoltaic cell converts the light into electrical current.

Other flat-plate concentrating systems that do not rely on luminescence have been

proposed. These systems have a variety of forms, for example, relying on diffractive elements

[45], light trapping [46], special backside mirrors [47], or refractive-index variations [48].

However, each of these designs presents their own challenges to produce electricity efficiently

enough to be economically feasible.

The portion of the emitted light that is trapped inside the waveguide depicted in figure 2.1

is determined by the refractive index of this waveguide. According to Snell’s law, all photons

approaching an interface between a material and air at an angle higher than the critical angle will

be totally reflected. This critical angle is defined as

Chapter 2

16

1 1sinc

n (2.1)

where n is the refractive index of the waveguide. This means that for an isotropic emitter and a

waveguide with a refractive index of 1.5–1.6, approximately 75% of all emitted photons will be

internally reflected. These photons will waveguide towards the side of the LSC, where a

significant fraction is coupled out of the waveguide by a photovoltaic cell that transforms the

photons into electrons. However, the efficiency of an LSC is not only dependent on the

trapping efficiency of the waveguide.

One description of the optical efficiency, opt , of an LSC is [51]

(1 )* * * * * * *opt abs PLQY Stokes trap host TIR selfR (2.2)

where R is the reflection of solar light from the waveguide surface, trap is the trapping efficiency

of emitted photons, abs is the fraction of solar light that is absorbed by the luminophore(s),

PLQY is the photo luminescent quantum yield of the luminophore(s), Stokes is the energy lost

due to the heat generated during the absorption and re-emission event, host is the transport

efficiency of the waveguided photons through the clear waveguide, TIR is the reflection

efficiency of the waveguide determined by the smoothness of the waveguide surface, and self is

the transport efficiency of the waveguided photons related to re-absorption of the emitted

photons by another luminophore.[51] The transport efficiency of the waveguided photons related to reabsorption of emitted

photons (self ) is determined by the number of sequential re-absorption and re-emission

(photon recycling) events that take place in the waveguide. Each time an emitted photon in

waveguide mode is reabsorbed there will be chance that it will be re-emitted (PLQY ), it will lose

some energy on heat generation (Stokes ) and only a part of these photons is trapped within the

waveguide (trap ). So self can be described as a product of different efficiencies already in

equation 2.2:

, ,

1

x

self PLQY Stokes i trap i

i

(2.3)

where x is the average number of photon recycling events. The number of photon recycling

events is determined by several factors, like the concentration of luminophore molecules, the

pathlength of a photon through the luminophore layer of the LSC, the extinction coefficient of

the luminophore at the wavelength of the emitted photon, the overlap between the absorption

and the emission of the luminophore and the angle between the polarization of the photon that

is reabsorbed and the optical axis of the luminophore when the luminophore is dichroic, but to

the knowledge of the author no dependency of these factors on the number of photon


17

recycling events has been determined.

The losses influencing all the efficiencies in equation 2.2 will be described later in this

chapter as well as the solutions proposed by several research groups over the past thirty years.

Thermodynamics limits conventional geometric concentrators, like CPCs and Fresnel

lenses, which consequently are inefficient at concentrating diffuse light. [62,63] As described in

section 1.2.3, the maximum concentration that these devices demonstrate is close to the square

of the refractive index of the material in which the concentrator is embedded. Therefore, these

concentrators are not very useful in a built environment, where the sunlight is very diffuse due

to scattering from buildings, cars, or trees. The absorption and re-emission of photons in LSCs

changes the entropy of the system; the maximum concentration of an LSC is therefore

dependent on the heat generation during the absorption and emission event, and thus from the

Stokes shift of the luminescent molecules. The maximum power concentration of an LSC can

be approximated by:

32 1 2

max 301

expe e e

CkTe

(2.4)

where 2e and 1e are the photon energies of the emitted and absorbed photons, respectively, k

is Boltzmann’s constant, and 0T is the ambient temperature. [62,63] For a luminophore with a

Stokes shift of 0.2 eV, the maximum concentration based purely on thermodynamics is

approximately 2000 times.

Considerable effort has been put forth developing a wide range of computational models

using many different approaches to describe the results of the existing devices, as well as to

predict optimal, higher-efficiency LSC designs. Random-walk theory has been used to illustrate

re-absorption, [64] and Monte Carlo simulations have been used to investigate the performance

of single and double-film stacked LSCs. [59,65] Different thermodynamic models have

examined waveguide losses [63,66] which report results in very good agreement with

experiments on test modules. Recently, Meyer et al. have compared the re-absorption probability

model reported by Weber et al. to ray-tracing results of luminophore-impregnated thin-film and

liquid-concentrator systems. [67] Other work has used ray tracing to predict the performance of

LSCs, including filled [68-71] and thin-film [72] waveguides. Each of the theoretical approaches

has their advantages and disadvantages. The thermodynamic modelling requires minimal input

and respond with swift answers but is often limited to simpler geometries and limited

luminescent species. Ray tracing allows much more freedom in device design and number of

luminophores and other details, but is quite computationally demanding. Potentially high

performance levels for the LSC have been predicted by, for example, detailed balance theory of

a single stage fluorescent collector using a high efficiency luminophore and wavelength-

selective surface reflectors. Such a device was predicted to demonstrate efficiencies of up to

90% that of a directly illuminated cell of equivalent size.[73]

Chapter 2

18

2.3 Losses of luminescent solar concentrators and their

proposed solutions

The LSC has not yet been extensively commercialized, primarily owing to their modest

efficiencies. [74,75] A diagram depicting a number of identified loss mechanisms for LSCs

taking into account the factors described in equation (2.2) is graphically depicted in figure 2.2.

Figure 2.2 Loss mechanisms in luminescent solar concentrators: 1) Light emitted outside the capture cone; 2) re-

absorption of emitted light by another luminophore molecule; 3a) incident light not absorbed by the luminophore;

3b) limited stability of the luminophore; 3c) internal quantum efficiency of the luminophore is not unity; 4) solar

cell efficiency; 5a) Fresnel reflections from the waveguide surface; 5b) waveguide absorption of the emitted photons;

5c) waveguide scattering; 5d) surface scattering.

The first loss of the LSC is light emitted by the luminophore under an angle which is refracted

out of the waveguide through an ‘escape cone’ rather than being reflected internally (figure 2.2,

1) which affects trap . The second loss is due to the re-absorption of emitted photons in the

waveguiding mode by subsequent luminophores due to an overlap of the emission and

absorption bands (limited Stokes shifts, figure 2.2, 2), which affects trap and self . The third

loss is a combination of losses related to the luminophore used: one is that the luminescent

molecules have limited spectral absorption bands which lead to incomplete incident light

absorption (figure 2.2, 3a) affectingabs . This light simply passes through the waveguide and is

lost through the bottom surface. Another loss related to the luminophore is the absorption of

high energy UV photons that leads to either direct photo degradation of the molecules (figure

2.2, 3b) such that in time a fraction of the molecules will be broken down and total emission of

the LSC will decrease, or degradation of other molecular species within the vicinity of the

luminophore that subsequently react with it and degrade the luminophore performance. Some

of the absorbed photons are not re-emitted by the luminophores due to limited emission

quantum yield, but instead lost as heat and vibrations (figure 2.2, 3c) affecting PLQY and self .


19

The PV cell at the waveguide edge has a non-uniform spectral response, with a fraction of

incident photons being lost due to the finite conversion efficiency of the PV cell leading to a

fourth loss (figure 2.2, 4). Even optically perfect waveguides can suffer from losses; a small part

of the input light is lost through reflection from the surface of the waveguide, due to Fresnel

reflections( R in equation2.2, figure 2.2, 5a). Waveguides used for LSCs can cause losses

through parasitic absorption, especially in the near infrared (figure 2.2, 5b) affecting host .

Imperfections of the waveguide surface can cause photons in the waveguide mode to leave the

surface and be lost (figure 2.2, 5d) affecting TIR , while imperfections in the waveguide bulk

also scatter waveguided photons (figure 2.2, 5c) affecting host , leading to additional surface

losses.

In the next sections of this chapter an overview of research done on LSCs towards

reducing these losses since their discovery in the 1970s is given. The surface losses are

described in the next chapters of this thesis.

2.3.1 Re-absorption of emitted photons by other luminophore

molecules

The Stokes shift of a luminophore is the wavelength separation between the peak of the most

red-shifted absorption band and the most blue-shifted emission band. Most organic

fluorophores used in LSCs have quite small Stokes shifts, leading to relatively large overlaps

between the absorption and the emission spectrum. [76] As a consequence, fluorophore-

emitted photons can be re-absorbed by subsequent fluorophores encountered during

waveguiding through the waveguide. These re-absorptions are not necessarily losses themselves,

but become a loss if the reabsorbing fluorophore does not again emit a photon (due to non-

radiative relaxation pathways) or if it does emit, but in a direction within the escape cone of the

waveguide. [64,77-80]

One manner in which the problem of re-absorption events has been approached is in the

design of new luminophores with larger Stokes shifts. By reducing the overlap in the absorption

and emission spectra of the luminophores, the re-absorption losses are minimized. A number

of different molecular species with increased Stokes shift have been employed in the LSC,

including lanthanides, [81] phosphors, [82] bipyridyls, [61] and quantum dots. [83-87] These

classes of luminophore bring additional challenges to the production of LSCs. Inorganics often

suffer from low solubility in organic matrices, and often also display a lower absorption. More

on luminophore research is described in Section 2.3.2.

Another technique to reduce the amount of re-absorption is the introduction of a polar

and highly mobile material (in this case, thionin) as a dopant while forming the luminophore

containing polymer waveguide plates. In this work it was postulated that the dopant assisted by

increasing the separation of the absorption and fluorescence bands of the molecules by altering

the electronic states around the luminescent molecules in the polymeric plate, thereby

Chapter 2

20

increasing the Stokes shift and reducing the overlap between the absorption and emission

spectra. [88]

Another option to reduce the re-absorption losses in devices using more than one

luminophore is to physically stack a number of waveguides on top of another, each containing a

luminophore that absorbs in a different part of the spectrum. [51,56,89,90] This is particularly

useful when one wishes to include a luminophore with absorption approaching the infrared,

which to date have generally demonstrated lower fluorescent quantum yields. In this way, by

stacking the more efficient luminophores on top, the light is collected, converted and

transported to the attached cell without ever encountering the reduced-efficiency luminophore.

The added benefit is that luminescence emitted through the escape cone could be captured by a

luminophore in an adjacent waveguide rather than being lost. [89]

To reduce encounters of emitted light with the luminophores, one may apply them as a

thin layer (from sub-micrometer thickness to hundreds of micrometers) to the surface of the

waveguide rather than filling the luminophore within the bulk of the waveguide, [56,71,72,91-

95] or as multiple thin layers stacked on one another. [96] In this way, emission light may be

transported predominantly in the clear host material, and only encounters the luminescent layer

with every second internal reflection. Theoretically, the thin layers should perform as well as the

filled waveguides. [97] In practice, care must be exercised as the limited solubility of the

luminophores often results in an underperformance of the thin layers due to aggregation of the

luminophores, creating non-emitting absorption centers, also known as quenching. [72]

An advantage in utilizing the thin luminophore-filled layer on top of a blank waveguide is

in production: one could envisage the large-area, inexpensive application of thin layers on either

glass or polymer hosts using a variety of techniques (including spin coating, [72] bar coating,

casting, [71] sol–gel techniques, [98] doctor blading, spraying, Langmuir–Blodgett techniques,

or printing). The thin layers could consist of luminophores in acrylates, [99] cellulose triacetate,

[71] polymerized liquid crystals (see chapters 5 and 6 of this thesis), or a host of other materials.

It is also possible to reduce the losses due to the Stokes shift by using more than one

luminophore, and employing Förster resonance energy transfer (FRET). FRET is the direct

exchange of energy from an excited molecule to another nearby molecule without the emission

of a photon. FRET is very sensitive to distance and other factors: the probability of a transfer

being related to the orientations of the interacting molecules as well as the distance:

, 6

0

1

1

PLQY FRET

r

R

(2.5)

where, ,PLQY FRET is the quantum yield of the energy transfer, 0

r

R is the ratio between the

donor and the acceptor ( r ) and the Förster distance ( 0R ).


21

By mixing a number of luminophores at high concentrations it was possible, through a

chain of virtual and real emissions and absorptions, to transfer short-wavelength input light into

long-wavelength output light at relatively high efficiency. [56,100] However, the high

luminophore concentrations necessary for the FRET effect are often not achievable unless the

molecules are brought closer together by other means. [101,102] Another material design taking

advantage of FRET uses ordered dye–nanochannel antennae (based on zeolites), which could

allow enhanced directionality of light emission as well as reduced photon recycling losses.

[103,104]

Another option to try to reduce re-absorption and thus photon recycling losses is to

physically space thin luminophore layers with stretches of empty waveguide using spatially

separated patterns of luminophores, so that the number of re-encounters that emitted light

could have with other molecules was reduced: a depiction of the functionality of such a device

is shown in figure 2.3. [105] The transport efficiency of the photons through the LSC increased

as the overall fluorescent-dye coverage decreased from 100% to 20%, and was relatively

independent of the shape of the dye pattern.

Figure 2.3 The functionality of patterned fluorescent dye layers on a clear waveguide. Top: In standard thin layer

LSCs incident light (black) is absorbed and re-emitted at a longer wavelength (gray). The emitted photons in

waveguide mode encounter the dye layer each time they reach the bottom of the waveguide. Each encounter with

the dye layer the photons have chance of being re-absorbed and possibly re-emitted (dashed). Bottom: In LSCs

with patterened dye layers the re-emitted photons (gray, solid) do not encounter the dye layer each time they

encounter the bottom of the waveguide, reducing the chance of getting re-absorbed.

However, reduction in total light absorption due to the physical gaps between absorption

regions resulted in a decrease of the total system output. [106] In order to increase the amount

of light absorbed in a patterned system, a lens array on top of the LSC is being developed that

would increase the system output. The lenses focus incoming light from a wide area onto small,

patterned regions of dye on the waveguide surface, leading to improved dye absorption.

Chapter 2

22

Current lens designs have demonstrated relatively efficient focusing of light over a range of

±30°. By using illumination through the lenses themselves to induce crosslinking of the dye-

containing acrylate layer, one would be able to tailor the dye pattern to the expected solar light

positions and so effectively minimize the area coverage requirement, as well as eliminate the

difficulty of correctly aligning a separately produced lens array with a dye pattern. While this

device demonstrates increased efficiency under direct lighting, measures still need to be taken to

improve performance in diffuse light.

A very recent approach uses resonance shifting, where spatially confined emission from

bilayer cavities interacts off-resonance with the luminophore layer upon subsequent

interactions. This is accomplished by careful variation of the luminophore thin-film thickness

across the area of the waveguide. The thickness variation is on the order of a few tens of

nanometers and is most appropriate for samples with luminophore layers on the order of a

micrometer in thickness. In this way, initial work appears to allow light propagation with

practically no re-absorption losses, and as such promises potentially significant performance

improvements if the design may be simply and reproducibly applied. [107]

2.3.2 Luminophore losses: Limited absorption, limited stability and

luminescence efficiency

Luminophores are essential for concentration in luminescent solar concentrators. Due to their

absorption and reemission, luminophores decouple the direction of the incoming sunlight from

the direction of the emitted light, overcoming the limitations of geometrical optics in trapping

light within waveguiding modes. The absorption and re-emission of the luminophores alter the

entropy of the LSC, so higher light concentrations can be reached, in accordance with

thermodynamic laws (etendue). [62,63] According to the laws governing etendue (equation 2.4)

the maximum concentration achievable by the waveguide is determined by the Stokes shift of

the luminophore used. Besides the maximum concentration of an LSC, the luminophore is the

most determining factor of light transport efficiency. In Goetzberger’s equation (equation (2.2))

for LSC efficiency, the fraction of solar light absorbed, the photoluminescent quantum yield,

the energy lost due to the heat generation during the absorption and emission events, the

transport efficiency of the waveguided photons related to re-absorption of the emitted photons,

and the spectrum of the photons reaching the PV cell are all determined by the characteristics

of the luminophore. Combining the needs for maximum concentration, maximum efficiency,

and maximum lifetime, the luminophore is the single most important component in the device.

An effective luminophore must meet all the following requirements:

• Broad spectral absorption

• High absorption efficiency over the whole absorption spectrum

• Large Stokes shift (no or low overlap in absorption and emission spectra, to reduce photon


23

recycling)

• High luminescent efficiency (quantum yield)

• Matching the emitted photons to the spectral response of the PV-cell (≈1.14 eV for silicon)

• Solubility in the host matrix material

A wide variety of luminophores have been studied in an effort to meet these requirements.

What follows is an overview of the major efforts in these areas, broken down into the following

categories: 1) organic dyes, 2) quantum dots, and 3) rare earth ions.

2.3.2.1 Organic dyes

Organic dyes are π−conjugated organic molecules, where the core of the molecule is planar

with all atoms of the conjugated chain lying in a common plane and linked by σ-bonds. The

π-electrons form a cloud above and below this plane along the conjugated chain. Absorption

bands of these organic dyes are the promotion of these π-electrons from a ground energy state

to an excited higher energy state. [108] The transition moment of these absorption events is

mostly parallel to the conjugated plane, mostly the molecular axis of the dye, but some

transitions are perpendicular to the molecular axis. These perpendicular transition moments are

observed at short wavelength absorptions. The main absorption wavelength (abs ) can be

estimated by:

28

1abs

mc L

h N (2.6)

where m is the electron mass, c is the velocity of light in a vacuum, L is the chain length of

the π−conjugated plane, h is Planck’s constant, and N is the number of π electrons. The

absorption band of organic dyes is mainly determined by the chain length and the number of π

electrons in the conjugated plane of the molecule. [108]

Since the first papers on LSCs, organic dyes have been investigated as luminophores of

choice due to their solubility, high fluorescence yields, and large absorption coefficients. The

dyes investigated as possible components of LSCs belong to the following classes of molecule:

bipyridines, [61] coumarins, [71,91,92,109-113] dicarbocyanine iodides, [91] dicyano

methylenes, [56,91,92,110,114] lactones, [111] naphtalimides, [111,113,115] oxazines, [91]

perylenes and perylenebisimides, [71,89,94,111-113,115-121] perylenebisimidazoles, [89](see

appendix A) phtalocyanines, [122] phycobilisomes, [123] porphyrins, [56,122] pyrromethenes,

[111] rhodamines, [50,52,91,92,108-111] sulforhodamines, [91,119] tertiary amine derivates of

tetra-cyano-p-quinodimethane, thioxanthenes, [124] (iso)violanthrones, [117] and some

unspecified dyes including BASF K1, [108,119,125] BASF K27, [125] and BASF Lpero. [125]

Characteristic examples of these dyes are depicted in figure 2.4. As can be seen, there have been

a wide variety of dyes explored for use in LSCs over the past few decades.

Chapter 2

24

Figure 2.4 Examples of organic dyes used in LSCs: a) a bipyridine derivate, b) coumarin 6, c) a dicarbocyanine

derivate (DODCI), d) a lactone, e) DCM, f) oxazine 720, g) a naphtalimide derivate, h) phtalocyanine, i)

hematoporphyrin, j) DEMI, k) pyrromethene 580, l) thioxanthene, m) sulfoRhodamine B


25

Figure 2.4 (continued) n) Rhodamine 6G, o)3,9-diisopropionicacid-4,10-dicyanoperylene, p) perylene-3,4,9,10

tetracarboxylic acid-bis-(2’-6’diisopropylanilide) (Perylenebisimide), q) perylene-1,7,8,12-tetrachloro-3,4,9,10

tetracarboxylic acid-bis-(2’-6’diisopropylanilide) (Perylenebisimide), r) perylene-1,7,8,12-tetraphenoxy-3,4,9,10

tetracarboxylic acid-bis-(2’-6’diisopropylanilide) (Perylenebisimide), s) a derivate of an (iso)violanthrone, and t) a

derivate of perylenebisimidazole (called perylene perinone in the rest of this thesis)[117]

Chapter 2

26

The most commonly used dye types for LSCs have been the rhodamines, coumarins, and

perylene(bisimides) derivatives. Rhodamines, like Rhodamine 6G, and coumarins, like

Coumarin 6 (also known as Coumarin 540), belong to the group of dyes mentioned in the

earliest stages of LSC research, [50,109] while the perylenes and perylenebisimides are mostly

mentioned in the more recent papers on LSCs, and were first described for use in LSCs in the

late eighties. [94,117] Rhodamines are known for their high quantum yields and high molar

extinction coefficients, but also for their small Stokes shifts. These small Stokes shifts lead to

much re-absorption and consequently to increased LSC losses, both surface and quantum

(nonradiative) losses. For Rhodamine 6G, the luminescence was reduced when incorporated in

poly(methyl methacrylate) (PMMA) in comparison to the solution luminescence, but no

suggestion has been made to explain this behaviour as this is contrary to common performance

of organic fluorophores in a polymer matrix. [110] In other types of dyes, like Coumarin 540

and dicyanomethylene (DCM), the luminescence increases when they are incorporated in a

solid PMMA matrix, probably caused by an increased molecular rigidity, limiting nonradiative

relaxation of the molecule in the excited state [110] or isolation from radicals or other

impurities. [126] Rhodamines 590, 575, 6G and B show very limited photostability [108,111] in

comparison to other types of dye molecules like perylenes and some coumarins.

Coumarins can have a larger Stokes shift compared to Rhodamines. [91] The overlap

factor, af , of the absorption and emission spectra of Coumarin 540A is 0.12, while for

Rhodamine 6G this overlap factor of the absorption and emission spectra is 0.48. [110] There

have been coumarins tested with moderate to very good quantum yields: Coumarin Red G has

a quantum yield of over 80% [112] (87% was also reported [71]), while the quantum yield of

Coumarin 540A and CRS040 have been reported as nearly unity (98%). [71,111] Even though

the photostability of coumarins have proven to be better than that of rhodamines, they still

have been reported to reduce stability in comparison to perylene-based dyes. [71]

Perylene dyes and their derivates, like perylene bisimides, perylenebisimidazoles, and

(iso)violanthrones, are known for their intense fluorescence and good photostability. [117]

Perylene itself has low photostability, due to easy electrophilic substitution. To improve the

properties of this dye, new side groups were added: 3,4,9,10-tetracyanoperylene has been

synthesized, the cyano groups are electron acceptors and when added at these positions around

the perylene core they improve the photostability. The solubility of this tetracyanoperylene is

moderate, therefore 3,9-diispropionicacid-4,10-dicyanoperylene (figure 2.4o) with a quantum

efficiency of 91% and good photostability was synthesized. [117]

Perylene bisimides are known for their intense fluorescence and good photostability, but

generally exhibit low solubility. To improve the solubility of these perylene bisimides, ortho-

alkylated aromatic bulky groups are added to the bisimides (figure 2.4p). These groups are

twisted 90° with respect to the perylene core due to steric hindrance, which increases the


27

solubility of the dyes. Addition of large side groups on the perylene core (1,7,8,12 positions;

figure 2.4q) shows a chlorinated perylene core) cause the two sides of the perylene core to twist

by 42° with respect to each other. This twist bathochromically shifts the absorption and

emission bands of the perylene dye. Furthermore, the tetraphenoxy-substituted perylene

bisimides (figure 2.4r) have good solubility in organic matrices due to the phenoxy side groups,

especially in comparison to the chlorinated perylene bisimides (figure 2.4q). Component 2.4r

has an absorption band peaking at 578 nm and an emission band peaking at 613 nm with a

quantum yield of 96% with both good solubility [127] and photostability. The violanthrones

[117] and perylene-bis-imidazoles (figure 2.4t and appendix E) [89] have more red-shifted

absorption and emission bands compared to component 10r, with reasonable quantum yields

of, respectively, 55% and 80%. If the conjugated core of the perylene is shortened to a

naphthalene core, the absorption and emission bands are blue-shifted. An example of this type

of dye is the BASF Lumogen F Violet 084, which is a naphtalimide. [115] This work

demonstrates that by changing the side groups or changing the length of the core of an organic

dye, a perylene in this case, one can alter all important properties of a dye, allowing for

engineering of more suitable and robust molecules.

As described earlier, perylene based dyes are known for their high quantum efficiency

[115] and for their large spread in spectral absorption and emission. Combining multiple

perylene dyes in one single device could thus lead to both broad spectral absorption (the whole

visible spectrum) and reasonably high quantum yields. [115]

Two different types of dicyano methylenes have been used in LSCs, DCM (4-

(dicyanomethylene)-2-methyl-6-(pdimethylaminostyryl)- 4H-pyran) [91,92,110,128] and DCJTB

(4-dicyanomethylene)2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9- enyl)-4H-pyran). [56] DCM has

a broad absorption spectrum and a large Stokes shift with a reasonable quantum yield (≈80% in

PMMA), [91,110] but the photostability is limited. DCJTB has been used in combination with a

platinum–porphyrin derivate, whereby DCJTB transfers its absorbed energy to the platinum

complex. This combination is calculated to form an LSC with a 6.8% power conversion if

paired with GaAs PV cells. [56] Other dyes have also been investigated in LSCs, as mentioned

before, but the published research on these dyes is more limited. Bipyridines [61] have a large

Stokes shift, due to excited-state intramolecular proton transfer and twisted intramolecular

charge transfer, but the quantum yield is low (up to 3.6% in butanol). Tertiary amine derivates

of tetracyano p-quinodimethane like DEMI (4-[1-cyano-3-(diethylamino)-2- propenylidene]-

2,5-cyclohexadiene-1-ylidenepropane dinitril) are red-light absorbing dyes, but their

photostability is low. [124]

Nature-based luminophores, like phtalocyanines, [122] hematoporphyrin, [122] and

phycobilisomes [123] have also been proposed for LSCs. The phtalocyanines and

hematoporphyrin have intense absorptions and good thermal stabilities, but the quantum yields

are 20% and 8% respectively. The quantum yield of the phycobilisomes is below 50%.

Chapter 2

28

Sulforhodamines, like sulforhodamine-B and sulforhodamine- 101 have small Stokes

shifts, similar to Rhodamines [91] resulting in many re-absorption events. Oxazine 720, oxazine

750, DODCI (a dicarbocyanine iodide: 3,3’-diethyloxadicarbocyanine iodide) and DOTCI (a

dicarbocyanine iodide: 3,3′-diethyloxatricarbocyanine iodide) suffer from similar small Stokes

shifts. [91] Thioxanthene dye D 315 orange, Lactone dye D 838 yellow, and Pyrromethene dye

580 have been shown to be relatively unstable towards illumination with visible light. [71,111]

IR-144 (a dicarbocyanine) is only mentioned in the paper of Batchelder; [91] it appears to have

a reasonable Stokes shift and absorption coefficient, but no further information on the

performance of this dye in LSCs is available.

One of the disadvantages of organic dyes in general is the limited breadth of their spectral

absorptions. To increase this breadth a combination of several dyes can be used. These dyes

can be used in a stack of LSCs with one dye in or on each waveguide [56,89,92,110] or all dyes

in one LSC. [71,91,92,100,109,115,118] In the first case each plate will act as an LSC itself.

Light emitted in the escape cone by one LSC can penetrate another LSC where it can be re-

absorbed (if the light is in the absorption band of the dye of the second LSC) and partly re-

emitted in the waveguide mode. This is described in more detail in the part of this chapter

focusing on the re-absorption problem of the LSC. In case of all dyes in one LSC, all dyes will

absorb energy and all this absorbed energy is transferred to the dye with the smallest bandgap,

via nonradiative or radiative transfer. For FRET the dye molecules have to be very close to

each other, typically less than 10 nm, so this only happens in LSCs with very high dye

concentrations.

Numerous studies have been performed on the photostability of organic dyes:

[71,92,94,108,111,113,117-119,124-126,129] much depends on the processing conditions and

polymeric environment of the fluorophore. Photo degradation of dyes in polymers can occur in

two ways: 1) by direct interaction of the dye molecule with the sunlight, leading to

decomposition, or 2) by the attack on a dye molecule by an active species formed due to

photodecomposition of a residual molecule in the polymer matrix or by singlet oxygen. [111]

These residual molecules are mostly materials used during polymerization or processing of the

matrix material. Two examples of reduced luminescence in LSCs caused by other molecules

present in the polymer matrix under solar illumination are 1) photo reduction, where the dye in

its excited state takes an electron from an electron donor, forming a nonradiative stable anion,

and 2) photo-oxidation where the dye in its excited state donates an electron to an acceptor,

forming a nonradiative cation. [117] Direct interaction of the dye molecule with solar light can

lead to molecular changes, caused by high-energy photons.

Photochemical decomposition of the dye molecule can lead to a reduction in absorption

and thus also a reduction in fluorescence or to a blue-shift in absorption and emission. [108] An

extensive comparison of the photostability between several types of dyes, including

rhodamines, coumarins, perylenes, a naphtalimide, a thioxanthene, a lactone, and a


29

pyrromethene, showed that the perylenes from the Lumogen Series of BASF had the best

photostability, both under illumination of UV light as well as visible light. [111]

There are several options beyond the engineering of more robust dyes towards extending

the photostability of the LSC device. Introducing UV absorbers and hindered amine light

stabilizer (HALS) molecules could reduce the photo degradation of the dyes under UV

illumination. Using copolymers of PMMA has also been shown to increase the photostability of

the dyes. [129] Decomposition of the dye molecules is much faster in a singlet oxygen

environment than in a nitrogen environment, with a small recovery of luminescence occurring

after some time in the dark. [117,130] Thus, it may be that during dark periods during the night

some of the luminescence lost due to photo degradation in the daytime could recover.

2.3.2.2 Quantum Dots

Quantum dots (QDs) are nanostructures from semiconducting materials with dimensions in the

order of 10–100 nanometers. The size of the dots is in the order of the de Broglie wavelength

of the electron. As a consequence of their restrictive size, excited electrons are confined in the

semiconductor, which exhibits optical and electrical properties similar to those of atoms. QDs

promise several advantages over organic dyes if used in luminescent solar concentrators. The

absorption threshold of the QDs may be tuned by judicious choice of the particle diameter.

Colloidal InP QDs, for example, have been shown to be capable of absorbing the whole visible

spectrum. [131] QDs may also have large Stokes shifts, which are determined by the spread in

dot size. [132,133] Their crystalline semiconductor composition should make them more stable

than organic-based dyes. [84] Quantum dots as luminophores in luminescent solar

concentrators were first suggested and modelled by the group of Barnham, [66,83,84] which

predicted a concentrator efficiency up to 20% when type III–V solar cells were used in

combination with high quantum efficiency (QE = 1) QDs.

As mentioned above, QDs promise good photostability, but outside a solid matrix they are

quite sensitive to oxygen and light, [69,134-136] which becomes a challenge for module

engineering. Several research groups claimed that the incorporation of QDs into a solid matrix

could lead to a blue-shift in both their absorption and emission, caused by surface oxidation of

the QD during the manufacturing process. [85,134,137] In addition to a blue-shift in emission

wavelength, the emission intensity in QD solar concentrators is also dependent on the nature of

the host matrix. For example, CdSe/ZnS QDs in a solid matrix lose 22.5–96% of the emission

intensity they exhibited in solution. [85] Several factors can cause this loss, including increased

scattering due to particle clustering owing to decreased solubility, and matrix absorption of the

emission light.

The spectral emission of QDs may be tuned by adjusting the QD size. With increasing

QD size, the bandgap will decrease. The cluster size distribution of the dots can be tuned in situ

during the production of the QD concentrator. By increasing the annealing temperature of CdS

quantum dots in silicon dioxide films, the photoluminescence spectra red shifted, attributed to

Chapter 2

30

an increasing CdS dot cluster size. [138] These results were supported by calculations and direct

X-ray diffraction (XRD) measurements of the QDs, determining that the cluster size grew

more at increased annealing temperature. [137] Schuler also showed that CdS-rich silicon oxide

layers show an absorption band close to the bandgap of bulk CdS, while low concentration

layers had bandgaps of higher energy, corresponding to finite well and tight-bonding

calculations. [139,140]

Naturally, by altering QD materials one may also define alternative bandgaps. PdS used as

a QD material, for example, has an absorption band up to wavelengths in the IR part of the

spectrum. [87] In addition to a larger spectral absorption, these QDs also have a larger Stokes

shift, leading to less overlap between the absorption and emission band. Where the CdSe/Zns (

abs 600 nm) QDs used by Shcherbatyuk [87] have a Stokes shift of 23 nm and an absorption

of 0.68 (normalized to the peak absorption) at the peak emission, the PbS ( abs 750 nm) QDs

had a Stokes shift of 122 nm and a normalized absorption of 0.24 at the emission peak. A

disadvantage of these PbS QDs is that the molar extinction coefficient is one order of

magnitude lower than that of the used CdSe/ZnS QD (2 × 104 L mol−1 cm−1 for PbS and 3 ×

105 L mol−1 cm−1 for CdSe/ZnS). Red-absorbing cadmium based QDs also show a decrease in

absorption coefficient, [141] but because the spectral absorption is higher for these materials

the total absorption efficiency is still higher than that of cadmium based QDs absorbing only

up to the blue or yellow part of the spectrum. Kennedy et al. [141] showed that the optical

absorption efficiency for NIR, orange, and green-emitting Cd-based QDs was, respectively,

23.1%, 21.7%, and 11.6%. The increase in the Stokes shift for the lower bandgap QDs leads to

less re-absorption of waveguiding photons, which in turn may also reduce the surface losses of

the QD solar concentrator (QDSC). NIR-emitting QD solar concentrators were modelled to

have 43 ± 1% surface loss in comparison to the green-emitting QD solar concentrators, which

had 58 ± 5% surface loss. For 60 mm × 60 mm × 3 mm QD concentrators, the reduced re-

absorption translated into a modelled optical efficiency of 13.2% for the NIR-emitting device,

in comparison to 5.0% for one that was green-emitting, assuming similar quantum yields of

unity in each device. [141] PbS QDs can also show similar results. The absorption efficiency of

the PbS QDs is 40% in comparison to 22% for the CdSe/ZnS QD used, but the luminescent

quantum yield for the PbS QDs is lower (30%) than that of the CdSe/ZnS QDs (50%). As a

result, the resultant optical efficiency for the PbS QD solar concentrator was set at 12.6%. [87]

The emission spectra of the QDs used in these experiments demonstrated that increased

pathlength of the emitted light through the waveguide resulted in a spectral shape change, with

the short wavelength peak decreasing with respect to the longer wavelength peak,

demonstrating that re-absorption still plays an important role in these QD concentrators.

[87,138]

QD-based concentrators have been compared directly with organic dye containing LSC

via both modelling and experimental studies. One such work compared CdSe/ZnS QD and


31

Lumogen Red F 300 containing concentrators. [85] The best QD concentrator described in this

paper reached 58% of the performance of the device containing Red 300 containing, mainly a

result of the lower quantum yields (QY 0.1–0.6) of the dots compared to that of the organic

dye (fluorescence quantum yield FQY ≈ 1). Another work has compared the optical efficiency

of CdSe/ZnS QD containing concentrators with several other organic dye containing

concentrators, including Rhodamine B, a red fluorine (Red F), and several other laser dyes

(LDS698 and LDS821). [86] Simulations and experiments described in this work showed the

optical efficiency of the QD concentrator was within 10% of the optical efficiency of the Red

F- and Rhodamine B-containing LSCs. The optical efficiencies of the LSCs containing the LDS

series dyes were comparable to that of the QD concentrator. These relatively low optical device

efficiencies reported were again attributed to the relatively low quantum yield of the QDs.

Increasing the quantum yield of the dots closer to that of Rhodamine B (stated to be ≈95%)

would lead to increased optical efficiencies of the device: as it was, the QD-based LSC

performed at 60% of the optical efficiency of the Rhodamine B-containing LSC. Calculations

done on PbS containing QD concentrators predict such devices could achieve power-

conversion efficiencies (PCEs) more than double the PCE of a Rhodamine-containing LSC, at

3.2% and 1.3%, respectively. [87]

It should be noted there is some concern about the use of the QDSC due to the

potentially toxic nature of many of the materials used in the QD. [24-26] There appear to

remain considerable uncertainties as to the true toxicity of these materials, and the level of

toxicity often is related to not only chemical composition, but also processing conditions,

environmental factors, and a number of other details. Nevertheless, there is continued research

aimed at reduction of the potentially harmful effects of QDs, such as the use of ‘jelly dots’ [142]

or on QDs based on more benign materials, like silicon. [143,144]

2.3.2.3 Rare earth ions

Rare earth ions (sometimes complexed with ligands) are investigated as luminophores for usage

in LSCs primarily because of their promise of high photostability and their large Stokes shift,

although the presence of organic ligands may compromise the effective lifetime of the

molecules. Levitt and Weber [52] described already in 1977 the use of Neodymium (Nd3+)-doped

glasses as materials for LSCs. Neodymium mainly absorbs around 580 nm, but also absorbs

photons at longer wavelengths. Emitted photons have wavelengths around 880 nm (4F3/2 →

4I9/2) and 1060 nm (4F3/2 →4I11/2). The photons emitted at 880 nm can be reabsorbed

(4I9/2 → 4F3/2) and the energy of the photons emitted at 1060 nm is slightly lower than the

bandgap of silicon. These characteristics of Neodymium lead to low efficiencies of Nd3+-

doped glasses as LSCs. [145]

To increase the efficiency of Neodymium-doped LSCs, codoping with Ytterbium (Yb3+)

has been discussed. [145] Energy absorbed by the Neodymium ions is transferred to the

Ytterbium ions, this accomplishes two goals: it will circumvent the limitations due to self-

Chapter 2

32

absorption, and the Neodymium emission at 1060 nm will decrease due to depopulation of the

4F3/2 state of Neodymium by the 2F5/2 state of Ytterbium. Emission from Ytterbium ions is

around 970 nm (2F5/2 → 2F7/2), which is slightly higher in energy than the bandgap of

silicon, but the response of silicon to photons with this wavelength is still high.

The energy transfer from Neodymium to Ytterbium depends on the type of glass: in

Borate Tellurite and Germanite glasses the energy transfer efficiency can reach up to 90%.

[145,146] Ytterbium itself can also be used as a luminophore in LSCs, but the solar absorption

of these ions only occurs in the NIR (2F7/2 → 2F5/2 in 4f13 configuration) or in the UV

(4f12 configuration), so large sunlight absorption is only possible if ytterbium-doped glasses are

codoped. [147] In the case of Neodymium, the Neodymium ions are codopants for the

ytterbium. A disadvantage of Neodymium and ytterbium is their low absorption efficiency.

[148]

Uranyl ions (UO22+) have been reported [148] in LSCs because of their high absorption

efficiency. Uranyl ions have five orders of magnitude higher absorption efficiency than

Neodymium ions, but they absorb only in the blue part of the spectrum (maximum absorption

around 430 nm). The fluorescence of uranyl ions is maximum at 500–530 nm, [148] and the

quantum yield is 0.67. [149] Uranyl ions have also been used as codopants in combination with

Neodymium, [150] and ytterbium together with Neodymium. [151]

Chromium(III) ions have a large spectral absorption with peaks at 450 nm (4A2 → 4T1)

and 650 nm (4A2 → 4T2) and so could be useful for LSCs, but a major drawback is the limited

quantum efficiency (up to 25%). [151] Higher quantum yields have been found in quartz-like

(FQY = 50%), pentalite-like (FQY = 75%), and gahnite (FQY = 100%) glass ceramics.

[152,153] Chromium(III) ions can be used for codoping Neodymium- and ytterbium-doped

glasses, as they increase the absorption range of the glass and then transfer their energy to the

high-quantum yield Neodymium and/or ytterbium ions. The energy-transfer efficiencies from

chromium(III) to Neodymium and ytterbium have been determined at 92% and 88%

respectively in lithium lanthanum phosphate (LPP) glasses. [151] Other rare earth ions like

Sm2+ can also be used in LSCs. [149,154]

To increase the absorption of rare earth ions, organic ligands coordinating to the ions have

been proposed. [81,151,155] In such a complex, the ligands absorb energy and the electrons go

from their S0 to their S1 state. From the single S1 state, energy is transferred to the triplet state

of the ligand (T1) via intersystem crossing. From this triplet state, energy is transferred to the

rare earth ion. Due to all the energy transfers, these complexes have a very high Stokes shift

(>200 nm). Moudam et al. [155] synthesized a Eu3+ complex (Eu(hexafluoroacetylacetonate)3-

(bis(2-(diphenylphosphino)phenyl)etheroxide). This complex absorbs light in the UV region

(<350 nm) and emits it at 613 nm with an FQY of 86% in PMMA. In 2011, Katsagounos [156]

showed four europium-based complexes with increased luminescence compared to the single

europium ion and using these dyes in LSC-type down converters increased the efficiency of


33

multicrystalline-silicon solar cells up by 17%, for the complex with a pyridine derivate as a

ligand. The spectral absorption of Eu-complexes is not very broad because low-energy photons

create a triplet state lacking sufficient energy for transfer to the europium ion, but may have a

large Stokes shift . [81] Other rare earth ions like Neodymium and ytterbium can be excited by

lower-energy triplet states, creating NIR-emitting complexes with broad absorption in the

visible region of the solar spectrum. A problem occurring with these ions is that they can be

deactivated by surrounding vibrational states of O–H, N–H, and C–H bonds. Therefore the

ligand has to been fluorinated or deuterated, because C–F and C–D absorptions occur at lower

energies.

The emission of this fluorinated ligand and Ytterbium complex peaks at 970 nm when

excited at 320 nm. In perfluoromethylcyclohexane, the luminescent QY of the Ytterbium

complex with the fluorinated ligand was determined to be 2.6%; not high enough for LSC

applications.

2.3.2.4 Enhanced fluorescence by metal nanoparticles (surface plasmons)

A newer research area for LSCs is in the field of using plasmonics to enhance the system

response. [95] It has been shown in many research publications that when a fluorescent

molecule is brought within close proximity of a small metallic nanoparticle, such as silver, there

may be a considerable enhancement of the fluorescence. [157-161] There has been previous

application of surface plasmonics in PVs to enhance device performance. [162-168] Very

recently, experiments have coupled such plasmonic photovoltaic cells of LSCs, with predicted

large enhancements of the efficiency. [169] The introduction to plasmonic structures within the

LSC waveguide could open up the use of a considerable range of dyes that had heretofore been

rejected on the basis of low quantum efficiencies or photostability, as both of these important

dye characteristics could be improved by the application of some type of plasmonic based

system.

2.3.2.5 Enhanced absorption by back scattering layer

To aid in luminophore absorption, it is standard practice to apply a rear layer to an LSC to act

as a reflector. The reflecting back layer effectively doubles the path length of incident light

through the dye layer for enhanced absorption. Some of the initial experiments used a silver

mirror, [51] but such a mirror absorbs in the visible range and can result in significant

absorptive losses for longer transmission distances. To avoid absorptive losses, most recent

work has employed a white scatterer. [74,75,100,128,170-173] The effectiveness of the scatterer

depends on both the size of the waveguide (the scatterers have been shown to be beneficial for

waveguides tens of centimetres long) and the concentration of dye within the waveguide. [173]

The scatterer also may direct that fraction of incident light that cannot be absorbed by the dye

directly at the PV cell, allowing it to generate electricity. The separation of the scatterer from

the waveguide by a low refractive index layer is quite important, as one desires to maintain

Chapter 2

34

waveguided light in the trapping modes of the waveguide: every encounter with the attached

scattering layer redistributes the light, and a significant fraction of this redirected light will be

outside the waveguide modes of the system.

It was demonstrated that the effectiveness of the scatterer is greatest near the edges of the

device, where direct scattering of non-absorbed light into the solar cell directly is prevalent.

However, over long distances the direct scattering component is less important, and the

primary advantage given by the scattering layer is the return of unabsorbed light back into the

waveguide for possible absorption on the second pass, but this effect can also be minimized in

highly dye-doped waveguides. [173] This suggests a possible transparent LSC design with only

the close edges using a scattering layer.

The functionality of the rear layer may be enhanced by adding a luminophore to the

scattering layer. This can potentially improve performance by converting light otherwise lost

due to lack of absorption into light of longer wavelengths that can be absorbed by the

luminophores within the waveguide [171,174] while still maintaining the benefits of scattering

described earlier.

2.3.2.6 Enhanced absorption and emission by confinement effects

Other designs may be contemplated that will reduce the limitations of the dye materials through

confinement effects. Recently, a slot waveguide using a nanometer-sized low-refractive-index

slot sandwiched by two high-refractive-index regions was calculated to enhance emission by the

luminophore due to the Purcell effect, to increase the effective absorption length of

luminescent centers and improve their fluorescence quantum yield. [175] The physical

requirements of this device, however, are quite extreme, with a 10 nm slot demanding use of

materials with refractive indices >2. While immediate use of such architecture is not likely, the

design does provide an interesting framework to build upon.

2.3.3 Photovoltaic losses

The standard silicon-based PV has a bandgap corresponding to a photon of around 1100 nm

(≈1.1 eV). Photons with energies above this threshold may still be processed by the solar cell,

of course, but the excess energy of the photon is wasted, and converted most often into heat,

and there is a reduction in the response of the cell for these shorter wavelengths. Energies

lower than this bandgap are not sufficient to generate any current.

However, an LSC does not emit a spectrum even remotely similar to the solar spectrum.

Rather, it emits a narrow range of wavelengths, most often centered at red and NIR

wavelengths (630–720 nm at the maximum). This gives the advantage that the LSC should

remain relatively cool under standard operating conditions, as they do not absorb much of the

infrared light. In consequence, the light incident on the edge mounted photovoltaic will contain

minimal infrared components, preserving high performance [58] as generally photovoltaics

perform less well when heated. [176]


35

In addition to reduced heat loads on the cell, by matching the spectral response of the

solar cell to the output spectra of the waveguides it may be possible to obtain enhanced

performance, although the overall effect may be limited due to the generally broad response of

many cells to light over the common wavelengths emitted by luminophores. [128] However,

this tuning could find application in amorphous silicon [177,178] and various type III–V cells.

Using larger-bandgap solar cells would deliver similar currents, but at larger open circuit

voltage.[71]

Down conversion, which is the process of converting a high energy photon into a lower

energy photon with a wavelength more readily utilized by the photovoltaic, has been studied

using luminophores attached directly to the surface of the PV cells [179-182] or within the

encapsulation layers of the PVs. [27] The goal of these layers is generally to shift the incoming

‘blue’ portion of the spectrum to longer wavelengths better suited to the PV. In other cases,

colours are applied to the photovoltaic surface for more aesthetic reasons, to add alternative

colours to the PV panels. [183] A similar application of this type, but this time using small,

sliver silicon-based photovoltaics located within the waveguide itself, promise improved

performance. [114]

To better exploit the emission spectrum in the LSC, researchers have used type III–V PV

cells based on GaAs and InGaP [74] and obtained record-setting efficiencies. If these cells

could be produced economically, [184] it could hold promise for widespread adoption of the

LSC in the future. Another option could be the use of organic-based PV cells, which often have

a ‘sweet spot’ in the spectral range where the LSC emits. [185]

Another option to enhance the response of photovoltaic cells is to use individual dye-filled

LSC waveguides, where each waveguide is separately attached to a solar cell specific for that

emission wavelength. In the so-called luminescent spectral splitter (LSS) concept, [186] the

incoming light is collected over a large area and funnelled into a smaller region where it is

absorbed by a stack of what are essentially LSCs, each attached to a separate photovoltaic cell

tuned to the specific wavelengths emitted by the LSC. In this concept, the LSCs themselves do

not need to transport the light over extended distances. Yet another option is to employ an

LSC with edge-mounted photovoltaic directly on top of a second photovoltaic: in this

configuration, the light that passes through the LSC due to its limited absorption range will be

collected by the underlying cell. [187]

2.3.4 Waveguide losses

Around 4% of incoming sunlight is reflected from each of the waveguide surface (the refractive

indices of poly(methyl methacrylate) (PMMA) and poly(carbonate) (PC) being between about

1.49 and 1.58) and never enter the waveguide, and could thus be considered a loss. While

antireflection coatings are very common in PV cells, they have not yet been applied to LSCs. As

the LSC relies on total internal reflection from two smooth surfaces, textured systems as used

Chapter 2

36

in many antireflective coatings may not be a viable option. [188] Rather, coatings utilizing

different refractive indices can reduce these reflective losses and can be applied to polymeric

materials. [189] It remains to be seen how these coatings affect LSC performance.

Naturally, it would be a great advantage to be able to produce the LSC panels on the size

of square meters in many applications, as this could allow for a more ‘seamless’ look, and

reduces the amount of wiring that needs to be done thus lowering installation costs, among

other things. However, there are all the losses described in this section that must be accounted

for which places severe limitations on the maximum size for the device. A drop-off of edge

output with distance has been reported.[66,172,173] These transport losses are generally the

result of two main loss mechanisms: re-absorption of emitted light by subsequent dye

molecules (described earlier) and parasitic absorption of the emission light by the host

waveguide material. Other features are certainly at play as well. For example, imperfections in

the surface of the LSC waveguide will also lead to losses and the presence of dust or minor

imperfections can result in the scattering of light outside of the waveguiding mode and lost to

the environment. [190] Generally, the devices should be kept as scratch free as possible,

suggesting the use of hardcoats or the like for particularly soft waveguide materials. There is

also potential for a significant degree of scatter from unwanted scattering centers located within

the waveguide. [190] These scattering and absorption events dominate for large waveguides: re-

absorption by the dyes is primarily a short-range effect, [191] although the absorption ‘tail’ of

the luminophore may have quite a large impact over long distances. [79,172]

One desires to extend the dye absorption as far to the infrared as possible. However, this

is complicated in that the polymeric waveguides, which are predominantly made of PMMA or

polycarbonate, become parasitic, and absorb strongly at wavelengths beyond 880 nm. [172,192-

194] Often the additives designed to improve various characteristics of the host matrix in the

solid state (such as altering coloration, stability, or hardness) or the presence of unreacted

monomers in the host material can have a large impact on the device’s performance.

[99,113,130] For example, an additive which shows only a small absorption when measured

through the width of the waveguide can have a severe impact on the edge output of the same

object, for the pathlength is magnified many tenfolds. [99,128] In addition, as the waveguides

age due to exposure to the elements and in particular ultraviolet light, generates brittleness,

opacity, and a host of reaction species that generate a yellow tint to the plate and act as

absorptive light ‘traps’. [195] To combat these loss mechanisms, research into copolymer

systems has demonstrated enhanced photostability over single component systems. [129]

During the transition from the excited state to the ground state of the dye molecule, electron

transfer takes place from the dye to the main copolymer (here polystyrene-co-

methylmethacrylate) chain, which may increase the photostability of the dye.

In general, the waveguide materials generally used for LSC work tend to be chosen with

economy, rather than absolute best performance, in mind. However, much optically clearer


37

materials are available in the marketplace and could be considered for use in the LSC. For

example, PMMA and perfluorinatedpolymer- based optical fibers may demonstrate

considerably enhanced transmission characteristics. [196]

An obvious way of decreasing the waveguide losses is application of waveguides using

higher refractive index materials. The most prevalent materials used for the LSC have been

PMMA and glass with refractive indexes on the order of 1.49. Higher refractive index materials

used or considered have included polycarbonate (n ≈ 1.59), [99] and special glasses (n = 1.5–

1.8). [56,69,99] Glass samples allow for potentially less absorption in the emission regions of

the dye than polymeric waveguides, [69] but many of the professed advantages of the LSC (for

example, reduced weight and ease of handling) would be nullified.

Another waveguide material that has been considered for its flexibility has been

polysiloxane. [197,198] While the durability of such materials might be a question, a range of

novel applications could be considered with the addition of flexibility to the LSC range of

characteristics. The emission of such polysiloxane-based systems has been comparable to that

of filled polymer waveguides of a certain dye concentration. This suggests that if the dyes were

more soluble, such rubbery waveguides could be a viable option.

An additional aspect of waveguide design that needs to be considered is that edge emission

from rectangular waveguide edges is non-uniform over the exit surface. Intensity of the

emission can vary 20% between the center and corner of the waveguide edge.[199,200] This

variation may also cause additional losses, as this means illumination of the attached

photovoltaic is not uniform, and non-uniform illumination of a photovoltaic cell will result in

decreased performance of the cell. [201] In order to improve the light concentration at the

edges and reduce the size of the attached solar cells, tapering of the waveguide edges has been

suggested. [202]

As one of the main goals of the LSC is to reduce the surface area of photovoltaic cells, it

would be preferable to reduce the number of edges with solar cells attached from four to fewer.

Configurations using two-edge coverage (both opposite and orthogonal cell placement) and

single-edge arrangements are common. When reducing the edge coverage of solar cells,

reflective mirrors are often added to the PV-less edges, and a fraction of this reflected light

reaches the photovoltaic cell. [69,71]

For both performance and aesthetic reasons, alternative shapes of the waveguide have

been studied. The shapes of these waveguides also influence the edge-emitted-light distribution.

[58,71,77,200,203] While the circular LSC could provide the greatest edge output, [200] the

packing limitations of such an array would be inappropriate. Rather, the hexagonal device was

calculated to be a superior design to the common rectangular device as far as light emission,

[71,200] but altering the geometry necessitates a different area of solar-cell attachment and

material usage for the waveguides, which have an effect on the price of the module.

Calculations concluded that varying the geometry type did not reduce costs significantly enough

Chapter 2

38

to be an economically viable plan: rather, it was the overall size of the object was a critical

feature. [71]

By adding a third dimension to the standard device one can also influence emission

efficiencies. Including curvature to the surface of a hexagonal waveguide was claimed to be

effective in reducing losses through the escape cone of the waveguide. [204] Expanding upon

this, a multi-cylindrical array of LSC tubes was shown to both increase the degree of light

concentration and reduce the surface reflections. [205] An LSC fiber has also been suggested

which could be bundled with other fibers and the emission light made incident on a single

photovoltaic, [206] and recently fibers with built-in focusing elements have been made. [207]

2.4 Conclusions

Over the past thirty-odd years a great amount of research has been performed to improve the

efficiency of LSCs. Most work has been performed on improving the luminophore of the LSC,

since the dye is causing the largest decrease in the efficiency of an LSC.

An estimation of the contribution of each parameter in Goetzberger’s equation (equation 2.2)

is presented in table 2.1 for LSC with one organic dye (for instance Red 305) as luminophore.

These parameters depend on both the type of luminophore and waveguide.

Table 2.1Estimated values for the parameters in Goetzberger’s euation for the optical efficieny of LSCs

Parameter Estimated value

1-R 0.96

ηabs 0.2-0.3

ηStokes 0.85-0.95

ηPLQY 0.95-1.0

ηtrap 0.75

ηhost 0.9-1

ηTIR 0.9-1

ηself Depending on ηStokes, ηPLQY, ηtrap and the number of photon

recycling events

In Goetzberger’s equation to calculate the optical efficiency of an LSC, abs is the most important

parameter. For Lumogen F Red 305, the state-of-the-art luminophore used in LSCs, this

absorption efficiency is on the order of 0.3, limiting the maximum incident photon- output

photon efficiency of the LSC to 30%. The luminophore molecules also influence the quantum

yield, the Stokes shift and self-absorption terms. Research over the past 30 years has proven

that finding a luminophore that has a combination of all the desired properties is difficult.

Another limitation in Goetzberger’s equation is the trapping efficiency. The trapping efficiency of


39

emitted photons is a parameter in that equation that is relevant in multiple events, as each

photon recycling event has trapping efficiency playing a large role. The trapping efficiency is

dependent on the spatial distribution of emitted photons in the LSC waveguide. In this thesis, I

focus on improving the trapping efficiency of the LSC where there has been minor research

conducted. A model is presented to calculate the spatial distribution of emitted photons coming

from an ensemble of dye molecules in an isotropic or liquid crystalline host. (chapters 3 and 4).

The trapping efficiency is calculated for several different types of alignments (in isotropic and

liquid crystalline hosts) of dichroic dyes and the resulting surface loss is monitored (chapters 5

and 6). In the last part of this thesis the trapping efficiency is increased by placing an organic

wavelength selective reflector on top of the LSC waveguide, which is used to reduce the escape

cone of the LSC waveguide.

3 Spatial distribution of emitted

photons from dichroic dye

ensembles

Chapter 3

42

3.1 Introduction

Photons emitted by luminophore molecules inside the LSC-waveguide will only be total

internally reflected if they encounter the waveguide-air interface at angles larger than the critical

angle. The photons with a smaller angle with respect to the normal of the waveguide surfaces

will be lost through the surfaces. The trapping efficiency of emitted photons depends not only

on the material of the waveguide, including glass, polycarbonate or PMMA, but also on the

spatial distribution of the emitted photons. This distribution is assumed to be spherical in

published literature. In this chapter a model is presented that calculates the spatial distribution

of photons emitted by dye molecules in a host material (dye ensemble). The host material is

isotropic or anisotropic such as a liquid crystal.

3.2 Spatial distribution of emitted photons from isotropically

distributed dichroic dye molecules

As already discussed in chapter 2 of this thesis, organic dyes are π−conjugated organic

molecules. In these molecules the core is mostly planar with all atoms of the conjugated chain

lying in a common plane. This leads to a cloud of π-electrons above and below the conjugated

chain. [108] The transition dipole for absorption and emission events of these molecules are

usually parallel to the conjugated plane, a phenomenon known as dichroism, which means that

absorption of linear polarized light is not isotropic for these molecules. Similarly, the spatial

distribution of emitted photons from one molecule is not isotropic. The absorption probability

of incoming light depends on the angle of the transition dipole for absorption ( ) with respect

to the polarization of the incoming light ( ie ) and the probability for emission depends on the

transition dipole for emission ( ) and the polarization of the emitted light ( fe ). The spatial

distribution of the intensity ( ,I ) of the emitted light from an ensemble of dye molecules (

) can be defined as [208-210]

2 2

, i fI e e

where and denote the polar and azimuthal angle of the emitted light and the brackets

represent the average over all positions of the dye molecules. In the remainder of this thesis it is

assumed that the direction of the transition dipoles for absorption and emission are the same

and that the molecules are static (that is, the fluorescence is faster than the molecular motions).

Using these assumptions the spatial distribution of the emitted photons will be [208]

2 2

, i fI e e . (3.1)

When there are aggregates of dye molecules formed or re-absorption and sequential re-emission

events occur, this assumption is no longer valid. In figure 3.1 a schematic definition is given for

Spatial distribution of emitted photons from dichroic dye ensembles

43

the transition dipole for absorption of a dye molecule when illuminated from the top by

collimated sunlight.

Figure 3.1 Schematic definition of the dipole, the incoming light and the emitted light for both the absorption

(left) event and the emission (right) event. In the left figure the black arrow represents the dipole moment for

absorption defined by a zenith ( ) and an azimuthal ( ) angle with respect to the axis system. In the right

picture the emitted photon ( k ) is also defined by a zenith ( ) and an azimuthal ( ) angle with respect to the

axis system. The large grey arrow in both pictures represents the direction of the incoming light which is circularly

polarzied (curved black arrow).

Sunlight can generally be treated as unpolarized. Dye molecules that lack a chiral center

have no intrinsic preference for the handedness of circular polarized light, making it possible to

mimic unpolarized light by circular polarized light, since this can be seen as a summation of all

linear polarizations. The transition dipole and the polarization of the incoming light are defined

as:

sin cos

sin sin

cos

,

11

20

ie i

leading to a probability of absorption equalling

2

2 2 2 21sin cos sin sin

2ie (3.2)

The direction of light emitted by a dye molecule ( k , equation 3.3) depends on the

polarization of this light ( fe ). This polarization of the emitted light can be described by a linear

combination of two linear polarizations ( ,1fe and ,2fe ) orthogonal to k (see figure 3.2, where

k is perpendicular to the plane).

Chapter 3

44

sin cos

sin sin

cos

k (3.3)

Figure 3.2 The polarization of the emitted light as function of two linear polarizations orthogonal to k

The two linear polarizations orthogonal to k are defined as

,1

,2

cos cos

cos sin

sin

sin

cos

0

f

f

e

e

so the polarization of the emitted light is

,1 ,2

cos cos cos sin sin

cos sin cos cos sin sin cos

cos sin

f f fe e e

leading to a probability of emission equalling

2 2 2 2 22 2

2 2 2 2 22 2

2 2 22

2 2

2

cos cos cos sin sinsin cos

2cos sin cos cos sin

cos cos sin sin cossin sin

2cos sin cos cos sin

cos cos sin

2cos cos cos sin 2cos ssin cos sin

fe

2 2

2

2

in cos cos

2cos sin cos sin 2sin cos sin

cos sin cos 2cos cos sin cos 2cos sin sin sin

cos sin sin 2cos cos sin sin 2cos sin sin cos

(3.4)

The definitions of , ,1fe and ,2fe are depicted in figure 3.2. The emitted light will have all


45

polarizations so the calculation of the intensity of this emitted light becomes

2

2 2

0

, i fI e e d (3.5)

The average over the ensemble of dye molecules can than be described by

2 2

2 2

0 0 0

, sin i fI d d d f e e (3.6)

where f is the distribution function of the transition dipoles within the ensemble. For

isotropically distributed dichroic dye molecules, the transition dipoles are isotropically

distributed and independent of the angle between the molecular and optical axis of the

molecules (see figure 3.9, black line, 2 0S ). The distribution function can be described as

f p , where p is a constant. Combining this with equation 3.2, 3.4, and 3.6, and

calculating all integrals leads to the spatial distribution of the emitted photons

2, 3 cosI (3.7)

The spatial distribution of emitted photons is depicted in figure 3.3.

Figure 3.3 Emission profile from isotropic dye ensembles illuminated from the top, both the front view (left) and

the top view (right). The axis of these emission profile have aribitrary units. The units are the same on both axis

and the emission originates from the center of the profile.

The emission profile shown in figure 3.3 for dichroic dye molecules in an isotropic host is

not isotropic, since molecules which have their optical axis perpendicular to the direction of the

incoming light absorbing more light than molecules with their optical axis parallel to the

direction of the incoming light. The molecules with their optical axis perpendicular to the

direction of the incoming light will emit photons mostly perpendicular to their optical axis,

Chapter 3

46

therefore more light is emitted in the direction of the incoming light, leading to the non-

isotropic emission profile.

3.3 Spatial distribution of emitted photons from dichroic dye

molecules in a planarly aligned liquid crystal host

In the past it was shown by several research groups (for example [210,211]) that alignment of

dichroic dyes by liquid crystal materials or stretched polymer films leads to macroscopic

dichroic behaviour of the material in both absorption and emission. The spatial distribution of

emitted photons from an aligned ensemble of dye molecules has never been calculated (to the

knowledge of the author). Control over the spatial distribution of emitted photons could

provide a toolbox to manipulate the distribution of photons in the waveguide of LSCs.

In this section a short introduction to liquid crystals and molecular dichroism is given

followed by a theoretical calculation of the spatial distribution of photons emitted by organic

dyes aligned planarly by liquid crystals.

3.3.1 Introduction to liquid crystals

Liquid crystals (LCs) can be viewed as the fourth state of matter, in addition to a solid, a

liquid and a gas. LCs exhibit orientational and/or positional order while they still flow like a

liquid. [212] For this reason the liquid crystalline phase is called a mesomorphic phase between

the liquid and the crystalline state of matter. While molecules in the crystalline phase exhibit

both positional and oriental order in three dimensions, the molecules in a liquid display no

order at all. Liquid crystalline phases can exhibit only orientational order, called the nematic LC-

phase or orientational order in combination with positional order in less than three dimensions

called the smectic LC-phase. [213] Materials exhibiting LC-phases do not have to exhibit both

types of LC-phases and they may have several different Smectic phases. LC-phases occur in

materials where the molecules have a high anisotropy in shape like rods or disk-like molecules.

Due to this anisotropy, the steric and dispersive interactions between the molecules in the

material are also anisotropic; this is the driving force for the formation of the LC-phase. These

anisotropic molecules can show order when they are molten called thermotropic LC-materials,

or in a solvent called lyotropic LC-materials. The materials discussed in this thesis are generally

thermotropic. Molecular representations of the different phases of a thermotropic material are

depicted in figure 3.4.


47

Figure 3.4 Schematic represesentation of the phases of a material exhibiting thermotropic LC behaviour. A

crystalline (left) material can undergo a transition upon heating to form a smectic phase. Heating to more elevated

temperatures the material can form a nematic phase and increasing the temperature even further induces a

transition to an isotropic liquid. A LC-material does not need to exhibit both smectic and nematic phases.

The average orientation of the LC-phases can be described by a director ( n ). The

molecules show an orientational discrepancy ( ) with this director. This discrepancy is

symmetric around the director and it is a measure of the quality of the order. To quantify the

quality of the order of LC- materials, the order parameters ( 2nS ) are generally used. The order

parameters are depending on the average angle( ) between the director and the individual

molecules and are defined as:

22

13cos 1

2S (3.8)

and

4 24

135cos 30cos 3

8S (3.9)

Nematic LC-materials exhibit an order parameter ( 2S ) typically between 0.4 and 0.8. If 2 0S ,

the material is completely disordered, so it is an isotropic liquid and if 2 1S , the material has

perfect order and a completely crystalline solid.

The order discussed in the previous paragraph is only a short range order. LC-materials

have no long range order, so the direction of the director may change over long distances in the

material. For LC-materials to exhibit long distance order a secondary driving force needs to be

applied. This external driving force can be an external field like an electrical field [214], a

magnetic field [215,216] or a shear-flow mechanical field [216], but it can also be an

anisotropically grooved surface [217], epitaxial-like transfer of molecular order in a surface (as

e.g. in surfactants) or polarized laser light [218]. In the experiments conducted in this thesis a

polymer surface with manually applied parallel grooves is used to create long distance order in

the LC-materials via anisotropic dispersive forces created by the grooves. Polymers that can be

Chapter 3

48

used to created anisotropic grooved surfaces are for example, polyimides (PIs), polyvinylalchol

(PVA) or triacetylcellulose (TAC).

These external driving forces can impose long distance order where the director is

constant parallel to the surface (called ‘homogenous’ or ‘planar’ alignment) or perpendicular to

the surface (called ‘homeotropic’ alignment), but it is also possible to change the direction of

the director gradually in one direction. In this latter case other configurations of the LC-state

can be achieved; for example splay or twisted nematic configurations, examples of both cases

are depicted in figure 3.5.

Figure 3.5 Example of different alignment configurations. a) planar or homogeneous; the molecules lay parallel

to the substrate b) homeotropic; perpendicular alignment, c) splay; the alignment changes from planar at the

bottom surface to homeotropic at the top surface; this change is gradual, and d) twisted nematic; a planar

alignment but the director is rotated by 90° over thickness of the layer.

Molecules with a liquid crystalline phase have anisotropic properties, which differ in the

direction along the director or perpendicular to the director. Due to the long distance order, the

resulting material will also have these anisotropic properties. Two examples of anisotropic

properties present in LC-materials are the refractive index and the dielectric constant. The

anisotropy in refractive index can be described by the birefringence (n ) and is defined as

e on n n (3.10)

where en (extraordinary refractive index) and on (ordinary refractive index) are the refractive

indices of the material parallel and perpendicular to the director respectively.

The LC-phases in thermotropic LC-materials all have a temperature range where they

occur.[219] To maintain the desired order of the molecules outside this temperature range, the

molecules must be fixed. This ‘freezing’ can be achieved by polymerizing the molecules in their

a) b)

c) d)


49

LC-phase. The most common method to polymerize LC-materials is by photo polymerization

of mostly (meth)acrylate functionalized molecules, also known as reactive mesogens. In photo

polymerization of reactive mesogens, ultraviolet(UV)-light is used to split a photo-initiator

which forms a reactive species, often a radical, but also ions are used. The radical attacks the

double bond of the (meth)acrylate group of the LC-material, forming a new radical on the LC-

molecule. This new radical attacks another reactive mesogen and in this way a polymer is

formed. A reaction scheme is depicted in figure 3.6.

Figure 3.6 Reaction scheme of photopolymerization of (meth)acrylate mesogens, where R1 is a hydrogen or a

methyl group and R2 is the core part of the mesogen.

The reactive mesogens can have one or multiple (meth)acrylate groups. A combination of

mesogens with one or two functional groups is used to make a network of the LC-material,

which keeps the LC-properties upon cooling and heating. A polymer made from mesogens

with only one functional group will form a LC side chain polymer. This latter polymer will still

exhibit a change of properties upon temperature change and have, for instance, a melting

temperature where the order disappears.[220] Examples of a LC-network and a LC side chain

polymer are depicted in figure 3.7.

Figure 3.7 Example of a LC side chain polymer (left) and a LC-network (right)

3.3.2 From molecular dichroism to macroscopic dichroism

Aligning dichroic dye molecules in a liquid crystalline matrix will cause the material to

exhibit macroscopic dichroism. Both absorption of incident light and re-emission of this light

by an aligned dye-liquid crystal guest-host system (aligned dye ensemble) will be influenced by

several factors: 1) the degree of order of the alignment of the dye molecules, 2) the angle of the

Chapter 3

50

transition dipoles with the director of the ensemble, and 3) the incidence angle and the

polarization of the incident light.

The conjugated plane of an organic dye molecule will not always lay in the same direction

as the molecular axis. In the liquid crystalline host the molecules are assumed to align in the

same direction as the host, but due to the angle between the molecular axis and the so-called

optical axis, this optical axis will have different order. In figure 3.8 a schematic depiction of the

alignment of the transition dipole of a dichroic molecule is shown.

Figure 3.8 The alignment of the transition dipoles of dichroic dye molecules in an liquid crystalline host. The

molecular axis (light grey line) of the dye molecule (black rectangle) is at an angle (θ) with the director of the host

( n ). The transition dipole (dark grey line) is not perfectly aligned with the molecular axis but off-set by an angle

δ.

The order of the optical axis is described by van Ewyk et al. [211] who showed that the order

parameter of the optical axis (S2,opt) can be expressed by the order parameter of the host (S2,host)

corrected for the angle between the optical and the molecular axis of the molecule:

22,

2,

2 3sin

2

host

opt

SS (3.11)

To show the effect of the order of the host and the angle between the molecular and optical

axis of the dye on the optical order parameter a plot is shown in figure 3.9 where the optical

order parameter is plotted as function of δ for hosts with different order parameter.

In figure 3.9 it can be seen that the optical order decreases with increasing angle between the

optical and molecular axis of the dichroic dye. If the host has no order ( 2, 0hostS ) the angle

between the optical and molecular axis has no influence on the resulting collective distribution

of the optical axes in the host. This supports the assumption made in the previous section: the

distribution function of transition dipoles is isotropic in an isotropic host, and independent of

the dichroism of the individual fluorescent molecules.


51

Figure 3.9 The optical order parameter of aligned dichroic dyes as function of the angle between the optical and

molecular axis (δ) for different order parameters of the host material: 0.0 (black), 0.1 (red), 0.2 (blue), 0.3

(dark green), 0.4 (magenta), 0.5 (orange), 0.6 (dark blue), 0.7 (grey), 0.8 (pink), 0.9 (light green), and 1.0

(purple).

3.3.3 Directional emission from planarly aligned luminophores

To calculate the spatial distribution of emitted photons by planar aligned dye ensembles the

same method is used as described in section 3.2 The transition dipole of the dye molecules is

now described by

sin sin

cos

sin cos

(3.12)

since the director of the liquid crystalline host is aligned parallel to the LSC waveguide and the

distribution of the transition dipoles will be described with respect to this director.

The distribution of the dye molecules is assumed to be equal to the distribution of the LC-

molecules and exhibit the same symmetry features with symmetry in rotation around the

director. In liquid crystals, the distribution of molecules around the director is described by a

summation of order parameters, which are in essence Legendre polynomials. If it is assumed

that the dye molecules exhibit the same order as the LCs, the distribution function of the dye

molecules and thus the optical axis of these molecules in the dye ensemble can be described as

[221]

Chapter 3

52

2 2

14 1 (cos )

2n n

n

f n S P d (3.13)

Combining equations 3.6, 3.12 and 3.13 and solving the integrations will lead to the spatial

distribution of emitted photons as a function of the optical order parameters ( 2,optS and 4,optS )

of the transition dipoles. By using the optical order parameter defined in equation 3.11, the

angle between the molecular and the optical axis of the dichroic dye is accounted for.

2, 4,2 2 2 2

22, 4,

2, 4,2 2 2

80 243 3 1 56cos cos sin sin

4 4 4 105 105 105

10 24142

2 105 105 105

60 24212cos sin 2cos

105 105 105

opt opt

opt opt

opt opt

S S

S SI

S S

(3.14)

Using equation 3.14 the corresponding emission profiles for 2,optS equals to 0, 0.2, 0.4, 0.6, 0.8,

and 1.0 (front and top view) are calculated and depicted in figures 3.10 and 3.11. In these

emission profiles it can be seen that with increasing order parameter the light emitted to two

sides and to the top/bottom is increasing, while the light emitted along the director is

decreasing. This leads to a change in emission profile from an oval “egg” shape to a more

“donut”-shape.


53

a)

b)

c)

Figure 3.10 Spatial distribution functions of photons emitted by planarly aligned dyes with different order

parameters: a) S2,opt=0, b) S2,opt=0.2, c) S2,opt=0.4. On the left the front view, with the director perpendicular to

the page and on the right the top view, with the director from top to bottom. The axis of these emission profiles

have aribitrary units. The units are the same on both axis and the emission originates from the middle of the

profile.

Chapter 3

54

a)

b)

c)

Figure 3.11 Spatial distribution functions of photons emitted by planarly aligned dyes with different order

parameters: a) S2,opt=0.6, b) S2,opt=0.8, c) S2,opt=1.0. On the left the front view, with the director perpendicular

to the page and on the right the top view, with the director from top to bottom. The axis of these emission profiles

have aribitrary units. The units are the same on both axis and the emission originates from the middle of the

profile.


55


molecules in a homeotropically aligned liquid crystal host

Homeotropic alignment of liquid crystalline materials can be achieved by using an appropriate

alignment layer. Due to repulsive forces between the alignment layer and the LC-molecules, it is

energetically favourable for LC-molecules to align perpendicular to the alignment layer.

Another method to achieve homeotropic alignment is by using an alignment layer which has

long molecular chains on the surface, which sterically hinder the LC-molecules from aligning

parallel to the surface. Dyes homeotropically aligned will stand perpendicular to the LSC

waveguide leading to preferential emission towards the four edges of the LSC.

To calculate the spatial distribution of emitted photons the transition dipole of the dye

molecules is described as

sin cos

sin sin

cos

and using this in equation 3.6, together with the distribution function of the transition dipoles

around the director is the same as presented by equation 3.13. The spatial distribution of

emitted photons from homeotropic dye ensembles can be described by

22, 4, 2, 4,2 2

80 24 10 2456 14cos 1 2sin

2 105 105 105 105 105 105

opt opt opt optS S S SI (3.15)

A schematic depiction of these emission profiles for S2,opt=0.2, S2,opt=0.4, S2,opt=0.6, S2,opt=0.8,

and S2,opt=0.9 are depicted in figure 3.12. For S2,opt=1.0 the dye molecules will not absorb any

light since the transition dipole for absorption of all molecules is exactly parallel to the direction

of the incoming light and perpendicular to all linear polarizations of this light.

Similar to the emission profiles from planar aligned dye ensembles, the emission profiles from

homeotropically aligned dichroic dye molecules become more like a “donut”-shape as the

optical alignment increases.

Chapter 3

56

a) b)

c) d)

e)

Figure 3.12 Emission profiles from homeotropically aligned dichroic luminophores with different order

parameters (S2,opt): a) S2,opt=0.2, b) S2,opt=0.4, c) S2,opt=0.6, d) S2,opt=0.8, and e) S2,opt=0.9. The axis of

these emission profiles have aribitrary units. The units are the same on both axis and the emission originates

from the middle of the profile.


molecules in a tilted aligned liquid crystal host

A third alignment of dichroic dyes in a liquid crystalline matrix is tilted. Here the director of the

dye ensemble is at an angle with respect to the waveguide surface. For calculating the emission

profile of tilted dye ensembles, an angle ( ) must be introduced which represents the angle

between the director of the dye ensemble and the normal of the plane formed by the LSC


57

waveguide. To simplify the calculations, the director is assumed to be coincident to the z-axis.

This means that the incoming and the emitted light, and thus also their polarizations, have to be

rotated with respect to the LSC waveguide. A schematic depiction of all angles in both the

absorption and the emission event may be seen in figure 3.13.

Figure 3.13 Schematic definition of the dipole (black arrow), the incoming light and the emitted light for both the

absorption (left) event as the emission (right) event. In the left figure the black arrow represents the dipole moment

for absorption defined by a zenith ( ) and an azimuthal ( ) angle with respect to the x,y,z axis system (black

lines). In the right picture the emitted photon ( k ) is also defined by a zenith ( ) and an azimuthal ( ) angle

with respect to the x,y,z axis system. The large grey arrow in both pictures represents the direction of the incoming

light which is circularly polarzied (curved black arrow) which is incident at angle ( ) with respect to the director

of the dye ensemble. In the emission event the LSC is depicted by the rectangle and the angle between the LSC

and both the director and the axis system is also descibed by .

When the incidence angle of the incoming sunlight is rotated by , this leads us to:

,

1 0 0 1 11

0 cos sin cos2

0 sin cos 0 sin

ie i i

i

(3.16)

and when the polarization of the emitted light is rotated by :

,

1 0 0 cos cos cos sin sin

0 cos sin cos cos sin sin cos

0 sin cos cos sin

cos cos cos sin sin

cos cos cos sin cos sin cos sin cos sin

sin cos cos sin sin sin cos

fe

cos cos sin

(3.17)

Combining equations 3.16 and 3.17 with equations 3.6 and 3.13 and calculating all the integrals

Chapter 3

58

leads to the spatial distribution function of emitted photons from dichroic dyes aligned by a

LC-material with a tilt angle ( ).

2 2 2

4 2 2 2

3

2, 4 ,2 2

2 2 2

2 2

2

3cos cos 3sin

cos 3cos sin 3cos

6cos sin cos sin sin80 2456

cos cos 1105 105 105

3cos sin sin

sin sin

2cos sin cos sin sin

cos

8

opt optS S

I

4 2

3

2 2

2 2

2 2 2

2

3

2 2

4 2

4sin

cos sin 24 cos sin sin

cos 4sin

28cos sin

cos sin 24 cos

16sin

cos sin 24 cos sin sin


sin 4 cos 4

4sin sin

2, 4 ,

4 2 2 2

2, 4 ,2 2 2

3

10 2414

105 105 105

sin 8cos sin 8cos60 2421

8cos sin sin105 105 105


opt opt

opt opt

S S

S S

(3.18)

Schematic depictions of the emission profiles from perfectly aligned tilted dye arrays (S2,opt=1)

with different tilt angles are depicted in figure 3.14. For a tilt angle of 90 degrees S2,opt=0.9 is

depicted since the dye molecules will not absorb any light if the ensemble has perfect optical

alignment, since the transition dipole for absorption of all molecules is exactly parallel to the

direction of the incoming light and perpendicular to all linear polarizations of this light.


59

a) b) c)

d) e) f)

g) h) i) j)

Figure 3.14 Frontal views of the emission profiles from perfectly aligned (S2,opt=1) dye ensembles with different

tilt angles: a) 0°, b) 10°, c) 20°, d) 30°, e) 40°, f) 50°, g) 60°, h) 70°, i) 80°, and j) a dye ensemble with a

tilt angle of 90° with an order parameter S2=0.9 The axis of these emission profiles have aribitrary units. The

units are exactly the same on both axis and the emission originates from the middle of the profile.

The pictures in figure 3.14 demonstrate that for optically perfect aligned dye ensembles the

“donut”-shaped emission profile does not change drastically in shape with changing tilt angle,

but it rotates by the tilt angle.

Chapter 3

60

3.6 Conclusions

A theoretical model is presented predicting that dichroism in absorption and emission of

organic dye molecules results in non-isotropic emission from an isotropic ensemble of these

dichroic dye molecules when illuminated with a collimated light source. Aligning dichroic dye

molecules in a liquid crystalline host leads to a change in spatial distribution of the emitted

photons. No matter the alignment direction of the liquid crystals, the emission profile from the

dye ensemble changes from “egg-shape” to “donut”-shape around the director of the liquid

crystal material, as the optical order is increased from complete isotropic to a perfect alignment.

The aligned dye ensembles emit more photons perpendicular to the director of the liquid

crystalline host. Using the model presented in this chapter it will be possible to manipulate the

spatial distribution of the photons in the waveguide of an LSC by changing the optical order of

the system or by changing the direction of the director of the liquid crystalline host.

4 Emission from planarly aligned

dichroic dyes2


P.P.C. Verbunt, A. Kaiser, K. Hermans, C.W.M. Bastiaansen, D.J. Broer and M.G. Debije,

“Controlling light emission in luminescent solar concentrators through use of dye molecules

aligned in a planar matter by liquid crystals”, Advanced Functional Materials, 19, 2714-2719,

2009.

Chapter 4

62

4.1 Introduction

Planar alignment of dye molecules has been described previously by a several research groups

(for example [208-211]). This previous work focused on measuring both macroscopic

dichroism in absorption and fluorescence. In the model presented in chapter 3 the optical order

parameter in absorption is the leady parameter. Entering this parameter in the emission profile

equation leads to a spatial distribution of photons from a dye ensemble with this specific optical

order. The model can also be used to predict the dichroism of the emitted light. In this chapter

the model as presented is validated by comparing the dichroism in fluorescence measured in the

work of van Ewijk et al. [211] In addition, the ratio in edge emission from two orthogonal edges

in an LSC with planar aligned dichroic dye molecules is measured and compared to the values

calculated using the model. Furthermore, the effect of the addition of mirrors on the edges of

the LSC and the addition of a scattering layer underneath the LSC on this edge output ratio is

measured, to determine if the spatial distribution of photons is maintained or if these additions

to the LSC will randomize the photons to a large extent.

4.2 Dichroism in fluoresencence Experimental results by van Ewyk et al. [211] are compared with results calculated with the

model presented in the previous chapter. Van Ewyk et al. describes experimental results from

aligned dye ensembles where the optical order parameter is compared with the fluorescence

parameter, the latter is described by:

2,

2fl

F FS

F F (4.1)

where F and F are the fluorescence intensities with the polarization parallel and

perpendicular to the director of the dye ensemble, respectively. For these measurements the

polarization for the incident light had a 45° angle with the director.

In the present model both the polarizations of the incoming and the emitted photons have

been varied to match the experiments of van Ewijk et al.. So the polarization of the incident light

becomes:

11

12

0

ie and the polarization of the emitted light is defined as

,

0

1

0

fe for

calculating the intensity of the light polarized parallel to the alignment direction ( I F ) and

,

1

0

0

fe for calculating the intensity perpendicular to the alignment direction ( I F ).

The results of van Ewyk et al. are not influenced by reabsorption and re-emission and can be

directly compared to the model presented in this thesis and are shown in table 4.1.

Emission from planarly aligned dichroic dyes

63

Table 4.1 The theoretical and experimental fluorescence order parameters from aligned dye ensembles

Optical order

parameter

(Absorption, 2,optS )

[211]

Fluorescence

order

parameter

(exp.) [211]

Fluorescence

order parameter

(theor.)

0.42 0.34 0.33

0.43 0.39 0.34

0.44 0.37 0.35

0.58 0.50 0.48

0.63 0.56 0.53

0.69 0.60 0.59

4,optS was approximated by using the 2cos calculated from 2,optS . Comparing the

theoretical with the experimental fluorescence order parameter shows that the theoretical

results calculated with the model presented in this thesis are in good agreement with the

experimental results. Van Ewyk et al. concluded that reduction in order parameter between

absorption and fluorescence was caused by depolarization due to a change in direction of the

transition dipole. In the model presented in this thesis the transition dipole is assumed static.

Still, the model also predicts a reduced fluorescence order parameter with respect to the optical

order parameter in absorption. This reduction is caused by the fact that the order parameter in

absorption is only dependent on 2

ie , while the order parameter in fluorescence is a

function of 2 2

i fe e .

4.3 Spatial distribution of emitted photons

4.3.1 Theoretical approach

For planar aligned dichroic dye molecules the emission profile changes with changing optical

order parameter. The change in shape of the emission profile results in a change in ratio

between light emitted to the edges of the LSC parallel to the director and perpendicular to the

director of the liquid crystalline host, for LSCs with planar aligned dichroic dye molecules. A

depiction of the parallel and perpendicular edge is shown in figure 4.1.

Chapter 4

64

Figure 4.1 Definition of the perpendicular edge (left) and the parallel edge (right). The grey cylinders

schematically depict the average alignment of the dichroic dye molecules.

The ratio ( eER ) in output between the parallel and perpendicular edges is calculated using

4

0 0

2

04

,

,

e

d d I

ER

d d I

(4.2)

and the results are shown in table 4.2. 4,optS was again approximated by using 2cos

calculated from 2,optS .

Table 4.2 The calculated ratio between the edge output from the parallel and the perpendicular edge of LSC with

planarly aligned dyes with different order parameters

S2,opt 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

ERe 1 1.02 1.10 1.18 1.27 1.36 1.46 1.57 1.68 1.80 1.93

By aligning the dye molecules planarly, the edge output of the parallel edges can become up to

93% higher than the output of the perpendicular edges if the optical axes of the dye molecules

are aligned perfectly. Nematic liquid crystals are known to have an order parameter ( 2S ) up to

0.7, so by aligning the dyes with a nematic liquid crystal 57% more photons can be sent to the

parallel edge of an LSC with respect to the perpendicular edge if the optical and molecular axis

of the dye molecules are in the same direction. In the rest of this chapter the calculated results

are compared to experimental results and the effect of the addition of a scattering back layer

and mirrored edges is experimentally determined.

4.3.2 Methods

Poly(methyl-methacrylate) (PMMA) plates of 50 x 50 x 5 mm3 were used as substrates

(waveguides). An alignment layer was spun onto some of these waveguides: the layer consisted


65

of either a planar polyimide (Nissan 130 or JSR AL-1051) or 5% poly(vinyl alcohol) (PVA,

Aldrich) in water, spun at 5000 rpm for 45 s for the polyimide or 1000 rpm for 30 s for the

PVA. Spinning was followed by a 1 hour curing period at 100°C in air. The planar polyimide

samples were rubbed by hand on a velvet cloth and the PVA samples with a rubbing machine

(Optron LCBM-6 and a YA18R rubbing cloth (Yoshikama) with a 0.35 mm pile depth, 500

rpm drum speed, and table speed of 0.9 cm/s). Dye solutions were spin coated on the prepared

surfaces at 800–1000 rpm for 30 s; the solutions were composed of 0.1–5% (by weight) dye

molecules Rhodamine B ([9-(2-carboxyphenyl)-6-diethylamino-3-xanthenylidene]- diethyl

ammonium chloride) (Aldrich), DCM (4-dicyanomethylene-2- methyl-6-(p-

dimethylaminostyryl)-4H-pyran) (Aldrich), or the coumarin dye (see figure 4.2), in a mixture of

photo reactive liquid crystals RM257 and RM82 (blended in a 4:1 weight ratio; both from

Merck; structures shown in figure 4.3), 1% photo initiator (Irgacure184, Ciba) out of a 50:50

solution of 1-ethoxy-2- propyl acetate and propylene glycol methyl ethyl acetate or xylene (all

from Aldrich). The samples also had 1% surfactant (2-(N-Ethylperfluoro-octanesulfamido)ethyl

acrylate) added to promote planar alignment at the air/liquid crystal interface. Isotropic samples

were spun from a similar solution on blank PMMA plates or from a solution of dye mixed in

dipentaerythritol pentaacrylate (Polysciences) with 1% photo initiator and 20% dried methyl

methacrylate (Aldrich) as solvent. Samples were placed on a hot plate at 80°C for about 15s (the

liquid crystal mixture was optically confirmed to be in the nematic phase by placing them

between cross polarizers at 45° with respect to the direction of the polarizers) and then photo

cured for 10 min under nitrogen by a 360 nm UV lamp to form a cross-linked, solid film. Both

the rubbed polyimide and PVA alignment layers induced a planar alignment of the liquid

crystals in these systems.

Figure 4.2 Chemical structure of the coumarin yellow dye.

The thickness of the liquid crystalline layers was on the order of 3–6 µm. Absorption

spectra of all waveguides were recorded on a Shimadzu UV- 3102 PC spectrometer with

polarized light directed both parallel and perpendicular to the rubbing direction of the

polyimide. The absorbance of the samples reported refers to the absorbance measured at the

main peak.

Chapter 4

66

Figure 4.3 Chemical structures of RM257 and RM82. The difference between the two molecules is the length of

the alkyl-spacer between the acrylate group and the mesogenic unit which is 3 and 6, respectively.

Figure 4.4 The experimental setup for measuring emission intensities at the edges of waveguides with a scattering

layer underneath, which is prevented from being in optical contact with the waveguide by an air gap.

The emission from the edges of the waveguides was measured by an SLMS 1050

integrating sphere (Labsphere) equipped with a diode array detector (RPS900, International

Light). The LSCs were placed in a custom-made sample holder with a black background with

the alignment direction of the polyimide parallel and perpendicular to the entry port of the

sphere and exposed to a collimated light source from a 300 W solar simulator with filters to

approximate the 1.5 AM (global) solar spectrum (Lot-Oriel), located at a distance of about 15

cm from the waveguide (see figure 4.4 for the experimental setup, without the scatterer or

mirror underneath the LSC waveguide). Edge emissions from the waveguides were determined


67

by placing the samples on a horizontal stage with only one edge of the waveguide entering the

integrating sphere. Illumination was constant over the whole surface by the solar simulator.

A correction for the small (~10%) polarization anisotropy in the solar simulator emission

was made for the outputs of the samples. Light output spectra and intensity from the emission

edge of the sample were recorded. Total output was determined by integrating the recorded

spectra over the range of 350–750 nm. Emission from all four edges of the samples was also

measured with a separate rear white scattering layer (a square of thick paper spray-painted

white, which acted as a Lambertian reflector). The output measurements were repeated after

application of a silver reflective foil (Scotchcal, 3M) to each individual edge of the isotropic and

aligned waveguides, with three edges of the same waveguides on a black background, and with

mirrors attached to three edges on a separate white scattering layer.

4.3.3 Experimental results and discussion

The alignment of the dye molecules is measured by determining the absorption from the

sample with linear polarized light parallel (peak absorption: A ) and perpendicular (peak

absorption: A ) to the director of the LC-material (rubbing direction). [211,222] From this

measurement the optical order parameter is calculated using:

2,

2opt

A AS

A A (4.3)

Representative absorption spectra of waveguides with each of the three dyes used as

luminophore in planar aligned LC-material, Rhodamine B, DCM, and the coumarin (see

Experimental Section) in the aligned LC-host material are given in figure 4.5.

Perfect uniaxial order of the absorbing dipoles of the fluorescent dyes would be described

as 2, 1optS , and a completely isotropic system would correspond to 2, 0optS , analogous to the

order parameter of a LC-material. Rhodamine B showed no dichroism, despite the preservation

of order of its liquid-crystal host as evidenced by its monolithic, birefringent properties. Optical

order parameter values for the DCM samples in the liquid-crystal matrix were determined to be

between 0.2 and 0.45. The coumarin dye displayed higher values for the optical order parameter

(up to 0.6).

Chapter 4

68

Figure 4.5 Absorption spectra of the coumarin (green), DCM (orange), and Rhodamine B (purple) in a planar

reactive mesogen with incident light polarized parallel (solid lines) and perpendicular (dashed lines) to the rubbing

direction. The two curves for rhodamine B are coincident.

Light emissions (both spectrum and power) from the edges of the waveguide were

determined using the equipment setup shown in figure 4.4. The light energy output from the

PMMA waveguide edge parallel to the polyimide rubbing alignment direction ( E ) was greater

than the light energy output from the edge perpendicular to the alignment direction ( E )

except for samples containing Rhodamine B as dye: see figure 4.1 for a definition of E land

E .

The relative outputs of the sample edges were described by considering a ratio of the

outputs E to E . For samples with an optical order parameter equals 0 (isotropic), the choice

of the ‘parallel’ and ‘perpendicular’ edges was arbitrary; such was the case for the samples

containing Rhodamine B. Variation in emission across the four sample edges in the isotropic

waveguides is 5%, as was the variation between emissions of the paired opposing edges of the

aligned samples. A plot of the output ratio as a function of the optical order parameter for the

three different dyes may be seen in figure 4.6. The different optical order parameters for the

same dye come from samples with a different rubbing intensity, so the order parameter of the

host is changed.


69

Figure 4.6 Ratio of the total output from the edge parallel to the polyimide rubbing direction to that from the

edge perpendicular to the rubbing direction ( eER ) as a function of the optical order parameter (S2,opt) for three

different fluorescent dyes in a reactive mesogen: Rhodamine B (black squares), DCM (dark grey circles), and the

coumarin dye (light grey triangles) measured experimentally. The calculated output ratio is displayed by the black

line.The data points of Rhodaime B containing samples are underneath the results of DCM containing samples

with an optical order parameter of approximately 0.

There is a general increase of the emission ratio with increasing optical order parameter for

fluorescent dyes aligned by a nematic LC-material. For the coumarin dye the edge output

parallel to the alignment direction was 50-60% higher than the edge output perpendicular. If

the experimental results are compared with the theoretical values (solid line in figure 4.6 and in

table 4.2), the theoretical values are seen well within the range of the experimental results. It

appears the measurements with DCM as luminophore generally show a smaller ratio in edge

output than calculated, while the measurements with the coumarin dye as luminophore show a

larger ratio, but differences are relatively small, giving support to the calculations. This suggests

that if the dye were to be aligned in a more ordered liquid crystal and if the optical axis would

be equal to the molecular axis of the dye molecule (that is the optical order parameter

approaching 1), the ratio in output could reach ~2. Aligning dyes in a planar manner in an LSC

would be a viable method two reduce the number of photovoltaic cells to two or even one,

without losing too much of the energy emitted from the two non-covered edges.

Chapter 4

70

4.4 Application of silver mirrors or a scattering background

to planarly aligned dichroic dyes in LSCs

To prevent emission from edges of the LSCs not covered by photovoltaic cells in LSCs, it is

common to attach mirrors.[49,74] Reflected light will then return into the waveguide with the

hope that it will emerge from a more desirable edge, that is, the emission edge. Silver mirrors

were attached to three edges of the planar waveguides and the outputs recorded with the

samples on a black absorbing background to determine whether the planar alignment could also

demonstrate enhanced edge emission from the parallel edge when compared to the

perpendicular edge after mirror attachment. The results of the measurements on the

waveguides with and without three mirrored edges are shown in figure 4.7.

Figure 4.7 Ratio of the output from the edge parallel to the polyimide rubbing direction to that from the edge

perpendicular to the rubbing direction ( eER ) as a function of the optical order parameter (S2,opt) , comparing

samples with three attached edge mirrors (open symbols) with samples that have no mirrors (solid symbols) for

Rhodamine B, DCM, and the coumarin dye.

Adding side mirrors to three edges of the planar samples reduced the measured anisotropy

(compare the filled and open symbols in figure 4.7). Light that normally would have escaped

the ‘side’ edges instead is redirected and exits the emission edge, resulting in a mix of E and

E light being emitted from the single edge. However, even after application of mirrors to

three edges, 25–30% more light for the parallel emission edge than the perpendicular in aligned

coumarin samples is obtained. The increase in edge emission after application of the three

mirrors to the uncovered edges is depending on the peak absorbance of the LSC and the dye


71

used. For DCM and the coumarin dye the energy leaving the edge is doubled if there are three

mirrors attached to the uncovered edges for LSCs with a peak absorbance of ~0.5.

White scattering layers are often used in conjunction with the LSC to enhance the spectral

response of the system [74,75,171,173] and increase the light output from the edge of an LSC

waveguide. The white scattering layer essentially serves two purposes: first, it may return light

that was not initially absorbed by the dye materials back through the sample, allowing a second

opportunity for absorption. Second, the scattering layer may redirect incoming light that could

not normally be absorbed, and a fraction of this light will be redirected in such a way as to

reach the exit edge of the waveguide. This is only the case when the photons reach the scatterer

close to the waveguide edge and the pathlength of the scattered photon is small than the

absorption length of the photon in the waveguide. The effect of the scatterer will therefore be

smaller for larger size LSCs.

Waveguided light is prevented from ever encountering the scatterer because an air gap

between the waveguide and the scattering elements has been employed, and thus the preferred

direction of dye emission is maintained. The scattered-in light, however, will be randomly

distributed with equal probabilities of being emitted out of any edge. Thus, there is an apparent

decrease in the output ratios of the edges, but the absolute differences in emissions remain

intact.

Figure 4.8 Ratio of the output from the edge parallel to the polyimide rubbing direction to that from the edge

perpendicular to the rubbing direction ( eER ) as a function of the optical order parameter (S2,opt) for planar

samples with a black absorbing background (solid symbols) and with a white scattering background (open

symbols) for Rhodamine B, DCM, and the coumarin dye.

Chapter 4

72

To evaluate the effect of adding a separate scattering layer to an aligned sample, the edge

outputs were determined using the aligned waveguides on top of a separate white scattering

layer, and the results on the edge output ratio are shown in figure 4.8. The increases in the

absolute emission energies were similar to the results described previously. [173]

After application of a scatterer, the ratio between the edge output of the parallel and the

perpendicular edge is decreased and it seems that this ratio levels out at approximately 1.25.

The extra light reaching the edge of the waveguide coming from the scatterer is more randomly

distributed and thus equal for all edges, which reduces the relative difference between the two

edges.

4.5 Conclusion

The model presented in chapter 3 for calculating the emission from planarly aligned dye

ensembles is compared to and supported by experimental work. There can be a significant

increase in light output from a single edge of a luminescent solar concentrator with dye

molecules aligned parallel with the emission edge compared to the perpendicular edge. In

experiments, the parallel edge emission direction is favoured over emission from the edge

perpendicular to the dye alignment by up to 60% for a relatively good alignment of the optical

axis of the dichroic dye molecules. Theoretical calculations of the anisotropy in emission match

well with these experimental results. Extrapolating the theoretical results to a higher optical

order suggests that using LC-phases with higher order, like smectic phases, or use of dyes that

show increased coincidence of the transition dipoles and molecular axis, could increase the ratio

in edge output between the parallel and the perpendicular edge to almost 2. This shows that the

macroscopic alignment of dichroic dyes is an interesting method to manipulate the distribution

of emitted photons in the LSC waveguide.

Application of either silver mirrors to the waveguide edges or a white scattering

background to the rear of the LSC demonstrates a reduced relative difference between the

parallel and perpendicular edges. However, even after application of the silver mirrors or the

white scatter there is still a difference in edge output between the parallel and the perpendicular

edge with respect to the alignment direction. This shows that there is still an effect present

from the dye alignment, which could lead to LSCs using reduced number of PV-cells.

5 Surface loss in luminescent solar

concentrators3


M.G. Debije, P.P.C. Verbunt, B.C. Rowan, B.S. Richards and T.L. Hoeks, “Measured surface

loss from luminescent solar concentrator waveguides”, Applied Optics, 47 (36), 6763- 6768,

2008.

Chapter 5

74

5.1 Introduction

Apart from the incapability of the luminophores that have been used in LSCs to absorb a large

fraction of the incident sunlight, surface loss is one of the most important loss mechanisms of

LSC as described in chapter 2. Surface loss is determined by the trapping efficiency of emitted

photons in the waveguide and by the number of photon recycling events of photons in

waveguide mode. In this chapter a theoretical approximation of the trapping efficiency of

emitted photons is given for isotropic a dichroic emitter in an isotropic host. The effect of

photon recycling events is shown and furthermore the surface loss of an LSC with BASF

Lumogen F Red 305 (hereafter referred to as Red 305) as a luminophore is measured.

5.2 Theoretical approximation

Photons that reach an interface between two materials with different refractive indices are

refracted. Snell’s law described the relation between the angle of the incident photon (1 ) and

the transmitted photon (2 ) as function of the refractive indices ( 1n and 2n , respectively) of

the materials as:

1 1 2 2sin sinn n (5.1)

If 1 2n n light will refract away from the normal to the interface, when light is transmitted

through the interface. The angle of incidence which leads to refracted light with an angle of 90°

is called the critical angle (c ).

Incident light with an angle equal or larger than this critical angle will be totally reflected

by the interface. So in an LSC only those emitted photons reaching the waveguide-air interface

with angles larger than the critical angle will be total internally reflected, while photons with a

smaller angle with respect to the normal of the interface are lost through the surface. Since the

LSC is positioned in air the critical angle depends on the refractive index of the waveguide

( waveguiden ) and can be approximated by equation 2.1. In the literature the emission of the dye

molecules has been assumed to be isotropic, and the fraction of emitted light trapped in the

waveguide (trap ) is described as [92]

12

2

1cos 1trap c

waveguiden (5.2)

Standard polymer waveguide materials used in LSCs are PMMA and PC, which have refractive

indices of approximately 1.49 and 1.58, respectively, leading to trapping efficiencies of 0.741

and 0.774.

The amount of photons lost through the surface of an LSC depends on this trapping

efficiency. Every (re-)absorption and sequential emission event, a part of the emitted photons is

lost through the surface equal to 1 trap . When the transport efficiency of the photons

through the host material of the LSC (host ) and the reflection efficiency of the waveguide

Surface loss in luminescent solar concentrators

75

determined by the smoothness of the waveguide surface (TIR ) are assumed to be unity, the

total fraction of the number of absorbed photons that is lost through the surfaces of an LSC

with an isotropic emitter can be calculated by:

1

0

1

x

i isl PLQY trap trap

i

(5.3)

where x is the average number of photon recycling events. In table 5.1 the surface loss of an

LSC with an isotropic emitter with a quantum efficiency of 1.0 is shown for several number of

photon recycling events.

Table 5.1 Theoretical surface loss from LSCs with an isotropic emitter.

x

Surface loss

PC PMMA

0 0.226 0.259

1 0.398 0.451

2 0.534 0.593

3 0.638 0.699

Results from the model presented in chapter 3 showed that the emission from LSCs with

dichroic dyes in an isotropic host illuminated with collimated light normal to the plane of the

LSC is not isotropic. Using this non-isotropic emission profile the trapping efficiency in the

waveguide can be calculated using

2

0

,0 2

0 0

, sin

, sin

c

c

trap

d d I

d d I

(5.4)

where trap is the fraction of the emitted light that is trapped in the LSC waveguide, ,I is

the emission profile, and c is the critical angle for total internal reflection. The 0 in the index

of the trapping efficiency means that this is the trapping efficiency of the light emitted after the

initial absorption and emission event. So only emission is taken into account after absorption of

the incident sunlight and no photon recycling is accounted for. The initial trapping efficiency

for a dichroic dye in an isotropic host is 0.743 in PC and 0.708 in PMMA.

Since the emission profile depends on the distribution of the light that is absorbed, the

profile will change every time a photon is reabsorbed and re-emitted. This leads to an increase

in trapping efficiency each time the photon re-absorbed and re-emitted. The surface loss is in

the case of a dichroic dye described by:

Chapter 5

76

( 1),0 ,( 1) ,

1

1 1

x

isl PLQY trap PLQY trap i trap i

i

(5.5)

where the index of the trapping efficiency denotes the number of the photon recycling events

which redistribute the photons (0 denotes the initial absorption and emission of incident

sunlight).The incident light is considered to be collimated, but the emitted light that will be re-

absorbed will have a distribution that upon increasing number of photon recycling events

becomes increasingly isotropic. For isotropic incident light the emission from isotropically

distributed dichroic dyes will be isotropic as well. This means that with each photon recycling

event the trapping efficiency will become closer to the trapping efficiency of an isotropic

emitter.

In figure 5.1 the surface loss for anisotropic and isotropic emitters in an isotropic host is

plotted as function of the number of photon recycling events, where the trapping efficiency is

kept constant at 0.743 and 0.708 for the dichroic dye in PC and PMMA respectively.

Figure 5.1 Surface loss from LSCs as function of the average number of photon recycling events for PC (grey)

and PMMA (black) waveguides containing isotropic emitters (solid lines) or dichroic dyes (dotted lines). The

lines of isotropic emitters in PMMA (solid black) and dichroic emitters in PC (dotted grey) are almost

coincident.

The surface loss for the dichroic dye is slightly overestimated and will become larger with

increasing number of photon recycling events. The difference between the surface loss from

polycarbonate LSCs containing isotropic emitters and the dichroic dyes has a maximum of 5%

for any number of photon recycling events. The trapping efficiency of the model using dichroic


77

dyes will increase with increasing number of photon events, which will lead to a smaller

increase in surface loss with increasing number of photon recycling events, which will bring the

results from the two models closer.

The surface loss from the dichroic dyes (Red 305), a perylene perinone (figure 2.4t) or a

coumarin (figure 4.2)) as dye in an isotropic host is determined experimentally using a newly

developed measuring method.

5.3 Methods

A number of filled waveguides were produced by Sabic Innovative Plastics (Bergen op Zoom,

the Netherlands) by injection moulding polycarbonate (PC) (n=1.586 at 587 nm) mixed with

various concentrations of the fluorescent dyes Red 305 (BASF, figure 2.4r), perylene perinone

(figure 2.4t) or a coumarin derivate (figure 4.2) or polymethyl methacrylate (PMMA) (n=1.49)

mixed with one concentration of Red305 into plates 50 × 50 × 3 mm3. Surface topped

waveguides were produced by spin casting solutions of Red305 in PC from THF or Red305 in

a penta-acrylate (Polysciences)/MMA (Aldrich) 80∶20 blend containing 1% photo initiator

(Irgacure 184, Ciba) on top of either clear PC (Sabic Innovative Plastics) or PMMA (Plano

Plastics) plates at 1000 rpm for 30s. The penta-acrylate systems were exposed to ultraviolet light

in an N2 atmosphere to crosslink the system. The thicknesses of the dye layers were measured

by a Zoomsurf 3D interferometer (Fogale) and were about 3 μm for the PC layers and 15–30

μm for the penta-acrylate/MMA layers.

The absorbance for all samples was determined using a Shimadzu UV-3102

spectrophotometer: the reported peak absorption values in this thesis refer to the absorbance at

the peak of the main absorption band.

Bottom surface emissions (that is, emission from the surface opposite the incident light

source) were determined by placing the 5 × 5 cm2 waveguides against the entry port of an

integrating sphere equipped with a SLMS LED 1050 light detection array (Labsphere), with a

blank waveguide used as the reference; see figure 5.2 for a depiction of the experimental setup

(in this measurement no filters are place between the entry port of the integrating sphere and

the sample). The samples were exposed to the light of a 300 W solar simulator with filters to

approximate the 1.5 air mass (AM) (global) solar spectrum (Lot-Oriel). To reduce the

background spectra from the source light, a stack of narrow pass band filters centered at 670,

710, and 750 nm were placed between the source and the sample to filter out these longer

wavelengths. The illumination area was limited to an approximately 2 cm diameter spot in the

waveguide center.

Top surface loss measurements (that is, from the surface being illuminated by the light

source) were derived from emission data taken using an Autronic DMS 703 (Melchers GmbH)

together with a CCDSpect- 2 array detector (CCD-Camera). The LSC samples were placed in a

Chapter 5

78

custom-made sample holder and exposed to a uniform light source located at a distance of

about 11 cm. Output spectra were recorded for surface emissions from 30° to 70° with respect

to the normal of the waveguide surface for both sides of the waveguide. See figure 5.3 for a

depiction of the experimental setup. The integrated outputs were determined for both “top”

and “bottom” sides of the waveguide, and a ratio of top/bottom emission was obtained. This

ratio was compared to the absolute measurements taken for the “bottom” surface from the

integrating sphere, and from this the “top” emission was calculated.

Figure 5.2 The experimental setup for measuring the surface emission from luminescent solar concentrators. The

incoming light from the solar simulator is filtered by reflectors placed in front of the sample to reduce the

background light in the emission part of the spectrum. The effect of the reflectors on the surface emission is

determined by placing the reflectors between the sample and the entry port of the integrating sphere.

Figure 5.3 The experimental setup for measuring the top surface loss relative to the bottom surface loss. The

sample is illuminated by a light source at normal incidence. The emission from the surfaces is measured by the

CCD-camera at angles between 30° and 70°.

5.4 Results and discussion

Most recent research used LSCs containing Red 305 as the organic dye.[79] This dye is used to


79

measure the fraction of the absorbed energy and photons lost through the surfaces of an LSC.

The normalized absorption and emission spectra of Red 305 were measured and depicted in

figure 5.4. Filled waveguide surface emissions were determined by placing the 5 × 5 cm2

waveguides against the opening of the integrating sphere and illuminating the top surface of the

waveguide with a solar simulator. Using a blank waveguide, we could determine the amount of

light transmitted and reflected by the waveguide itself. Absorption was essentially zero through

this small thickness of the waveguide, and the reflection from PMMA was measured to be

around 8%, and from PC about 11.5% throughout the visible range.

Figure 5.4 Normalised absorption (black) and emission (grey, excitation at 520 nm) spectra of Red 305.

Samples containing Red305 dye were then placed in the measurement position and filters

placed between the light source and the sample were used to reduce the background light in the

emission part of the spectrum where placed between the light source (figure 5.2) and the

sample, and the results recorded. Representative spectra of the blank and a filled waveguide

may be seen in figure 5.5. The blank spectrum shows the transmission spectrum of the light

from the solar simulator through a blank polycarbonate waveguide. Light with wavelengths

above ~600 nm are partially filtered by reflectors place between the light source and the

waveguide. The transmission spectrum through the dye filled waveguide shows a loss of

transmission in comparison to the transmission through the blank waveguide in the part of the

spectrum where the dye molecules absorb the incident light and some extra light leaving the

surface of the waveguide in the part of the spectrum where the dye emits light. By subtracting

the results of the blank waveguide measurement from the results of a filled waveguide, we

obtained the spectra seen in figure 5.6.

Chapter 5

80

Figure 5.5 Examples of measured spectra of light coming from the bottom side of waveguides illumninated from

the top containing no dye (black) and Red 305 (grey)

Figure 5.6 Result of subtracting the spectrum obtained from blank polycarbonate waveguide from the emission

spectrum of PC waveguides containing dye (peak absorbance of 2.5). Integration over region I gives the power

absorbed by the dye and integration of region II gives the power emitted by the dye through the bottom surface.


81

The fraction of the light absorbed by the dye that is subsequently emitted from the surface

was determined by integrating the emission spectra (the light emitted into the sphere from the

surface of the waveguide, or the positive region of the curves in figure 5.6), roughly 600 nm to

750nm, depicted as region (II) and dividing by the light absorbed by the dye (determined by

integrating over the negative region of the spectra of figure 5.6, roughly 400–600nm and

depicted as region (I).

Measurements of the surface-topped samples were done in a similar manner, with the dye

layer situated on the top side of the waveguide, facing the light source.

Measurements using the Autronic system (described in section 5.2 and figure 5.3)

established the ratio of top to bottom emission from the waveguide surfaces. Once the ratio

was determined, the absolute emission from the top surface was calculated. Several samples

were measured for output from both faces by repositioning of the waveguide with respect to

the incoming light in the Autronic setup, and the differences were determined to be less than

4%. The results from these measurements can be seen in table 5.2 and figure 5.7.

Table 5.2 Measured bottom loss and derived top loss in energy and photons from the dye-filled waveguides (PC)

Peak

Abs.

Meas.

bottom

surface

loss

(energy)

Calc.

bottom

surface

loss

(photons)

Loss

Ratio

(Top/

Bottom)

Calc.

top

losses

in

energy

Calc.

top

losses in

photons

Calc.

total

surface

loss in

energy

Calc.

total

surface

loss in

photons

(sl (%))

trap

0.05 28% 36% 0.96 27% 35% 56% 71% 0.743

0.46 21% 26% 1.04 22% 27% 42% 53% 0.743

1.01 20% 25% 1.13 22% 28% 42% 53% 0.743

1.63 18% 23% 1.14 21% 26% 39% 49% 0.743

2.36 18% 23% 1.29 23% 30% 41% 53% 0.743

>4 16% 21% 1.56 25% 32% 41% 53% 0.743

Looking at table 5.2 and figure 5.7, it becomes apparent that there is a considerable loss of

energy from the surfaces of the filled polycarbonate waveguides, and this energy loss is constant

at around 40% except in the lightly doped waveguide. This translates to a loss of 50-53% of the

absorbed photons. At low absorbance, the energy losses are greater than 50%, which translates

into 71% photon loss. Both the absorption and the surface emission are very small in this low

absorbing waveguide, which increases the error in the measurements performed.

Chapter 5

82

Figure 5.7 Calculated total surface energy loss from polycarbonate (filled symbols) and PMMA (open symbols)

waveguides with dye in the waveguide (squares) and in a thin polycarbonate (triangle) or penta-acrylate (circle)

surface layer.

The theoretical model predicted the fraction of the absorbed photons trapped in the

waveguide is 0.743, so 25.7% is emitted in surface loss mode if only the initial absorption of

sunlight is taken into account (thus no photon recycling events). Considering the error in the

experimental data the difference between these models cannot be measured. From the data in

figure 5.1 the number of photon recycling for filled PC waveguides should be around 1.4-1.6

(2.4-2.6 total dye interactions for each photon) for the dichroic dye model and approximately 2

(3 total dye interactions for each photon) for the isotropic emitter model for the experimental

data to match the models.

The number of photon recycling events is determined by the concentration of dye

molecules in the waveguide, but is limited by the degree of overlap in absorption and emission

band. Each photon recycling event results in the photon losing energy. After a number of re-

absorptions the emitted photon will not have sufficient energy to re-excite another dye

molecule and cannot be further reabsorbed. Simulations performed by the Herriot-Watt

University using software based on Monte Carlo simulations [79,223,224] have shown that the

number of dye interactions of each photon is at most 1.8. These simulations were performed

before the paper from Wilson et al. [79] that described the absorption tail of the dye molecules

for low energy photons is longer than earlier believed, thus the number of re-absorptions are

probably higher than the 1.8 predicted. For the isotropic emitted model to match these results

the number of dye interactions should almost double, for the dichroic dye model this number is

lower.


83

The surface loss of the filled PMMA waveguide measured is, as predicted, slightly higher

than a filled PC waveguide with comparable peak absorbance as a results of the lower refractive

index of PMMA, although the difference is relatively small (44,5% in compared to 42% energy

loss).

It can also be seen that for high O.D. LSCs the amount of light lost through the top

surface of the waveguide is greater than the amount of light lost through the bottom surface,

while the emission profiles show an equal amount of light emitted towards both surfaces. This

is discrepancy is probably also caused by photon recycling. In LSCs with a large absorbance the

intensity of the incident light has a large gradient through the thickness of the waveguide, since

most of the light is absorbed in the top part of the waveguide. This leads to a larger average

pathlength of an emitted photon towards the bottom surface in comparison to the photons

emitted towards the top surface and so is the number of photon recycling events. Each photon

recycling event the light is redistributed and photons have a chance to be emitted in waveguide

mode. This leads to a reduced surface loss through the bottom surface.

The experimental surface loss of the thin dye layer LSCs shown in figure 5.7 is smaller

than the surface loss of the filled waveguides with identical peak absorbance. This is probably

caused by the increase in quenching, since the concentration in these samples is much higher

than in the filled waveguides. Work by Tsoi et al. showed that for LSCs with a thin film

containing dye molecules part of its luminescence is quenched, even at low absorbance. In all

previous results and calculations a quantum efficiency of 100% is assumed for the dye

molecules. [115] In the thin layer LSCs, this is no longer the case. Each re-emission event a part

of the absorbed photons is lost due to non-radiative decay of excited electrons to their

energetic ground state. A reduction in quantum yield will reduce the surface loss as can be seen

in equation 5.5, and the influence of this reduced quantum yield increases with increasing

number of photon recycling events. Therefore, the difference between the surface loss from

filled waveguides and from the thin film samples increases with increasing peak absorbance.

The number of photons emitted from the surface of an LSC depends on the number of

photon recycling events and thus the overlap of the absorption and emission band of the dye

molecule. The surface loss of filled PC waveguides containing other dyes, namely the perylene

perinone dye (Appendix A and figure 2.4t) and a coumarin dye (figure 4.2) have also been

measured. The absorption and emission spectra are displayed in figure 5.8 The results of the

surface loss measurements have been displayed in table 5.3. These results have been corrected

for the quantum efficiency of the dye and samples with a similar peak absorbance are used

(~1.0).

Chapter 5

84

Figure 5.8 The absorption (straight lines) and emission (dotted lines) spectra of the coumarin dye (black,

excitation at 430nm) and the perinone dye (grey, excitation at 620nm)

Table 5.3 Experimentally determined fraction of the emitted photons that is lost through the surface of filled PC

waveguides

Dye Fraction of the emitted

light lost through the

surface

Red 305 0.53 Coumarin 0.46

Perylene perinone 0.325

The fraction of the emitted photons lost through the surfaces of the waveguide is a

function of the dye. In the absorption and emission spectra depicted in figure 5.8 it can be

clearly seen that the overlap between the absorption and the emission band of the Red 305 is

larger than for the other dyes, and that the overlap is the smallest for the perylene perinone dye.

This strongly suggests that photon recycling is the cause of a significant fraction of real surface

loss in comparison to the amount of surface loss calculated from the theoretical trapping

efficiency.

5.5 Conclusions

Calculations show that the trapping efficiency of isotropic and dichroic emitters in an isotropic

host leads to a trapping efficiency of 0.774 and 0.743, respectively in polycarbonate waveguides

and 0.741 and 0.708 in PMMA waveguides. The difference in trapping efficiency between

waveguides with isotropic and dichroic dyes is relatively small, which made experimental


85

verification difficult. This trapping efficiency would lead to losses of 25.7% or 29.2% of the

photons emitted by dichroic molecules in an isotropic host, for PC and PMMA, respectively.

Experimentally, LSC waveguides using Red 305 as luminophore demonstrate

approximately 50% of all absorbed photons is lost through the surfaces of the LSCs with a

peak absorbance above 0.2. The difference between the experiments and theory is a result of

photon recycling of emitted photons in waveguide mode by other dye molecules, arising from

the overlap in absorption and emission band of the molecule, which is not included in the

calculations. Using dyes with different degrees of spectral overlap results in different fractions

of photons lost through the surfaces of LSC waveguides, but always higher than calculations

predict because of this omission.

6 Reduction in surface loss by dye

alignment4


P.P.C. Verbunt, C.W.M. Bastiaansen, D.J. Broer and M.G. Debije, “The effect of dyes aligned

by liquid crystals on luminescent solar concentrator performance”, in proceedings of the 24th

European Photovoltaic Solar Energy Conference, WIP Munchen, 2009, 381-384.

Chapter 6

88

6.1 Introduction

Taking advantage of the anisotropic emission from dichroic dyes aligned in LC-matrices could

be a method to reduce the surface losses in LSCs. Since the direction of emission from an

ensemble of dye molecules orientated along a director is predominantly perpendicular to this

director, aligning dye molecules in a homeotropic fashion would lead to a preferred emission

towards the four edges of the LSC waveguide. The surface loss from homeotropic and planar

aligned dye ensembles are measured experimentally and compared to the calculated values of

the trapping efficiency. Lastly, surface losses from tilted dye ensembles, which combine the

advantages of planar and homeotropic aligned dye molecules, are calculated.

6.2 Surface loss from planar and homeotropically aligned

dye ensembles

In this section, the surface losses from LSCs are calculated for LSCs with homeotropically and

planarly aligned dichroic luminophores in nematic LC-materials and these calculations are

compared to experimental data from aligned-LSCs using different luminophores with different

degrees of alignment as measured by absorption. In addition, the amount of light leaving the

edge of the LSC waveguide is monitored.

6.2.1 Methods

PMMA plates with a size of 50 x 50 x 5 mm3 were used as waveguides for the luminescent solar

concentrators. Alignment layers were spun on top of the waveguides at 1000 rpm for 30

seconds. For planar alignment, 5% polyvinyl alcohol (PVA, Aldrich) in water and, for

homeotropic alignment, a polyimide (Nissan 5300) were used. After spinning, the alignment

layers were baked at 95°C for 30 minutes (PVA) or 165 minutes (polyimide). The PVA

alignment layer was subsequently rubbed by hand on a velvet cloth with 0.35mm pile depth.

On top of the alignment layer or the bare PMMA waveguide (for isotropic alignment), a

dye layer was spun at 1000 rpm for 30 seconds. For isotropic dye ensembles, 0.1-1.5 wt% dye

(Rhodamine B (Aldrich), DCM (Aldrich) or the Coumarin dye) was added to an 80 wt%

dipentaerythritol pentaacrylate (Polysciences) and 20 wt% methyl methacrylate (Aldrich)

mixture containing 1 wt% photo-initiator (Irgacure 184, Ciba). The dye mixture for the planarly

aligned samples contained 0.1-1.5 wt% dye, in a 4:1 mixture of two liquid crystals, RM257

(Merck) and RM82 (Merck). To this mixture, 1 wt% of the photo-initiator and 1 wt% of a

surfactant (2-(N-Ethylperfluoro-octanesulfamido)ethyl acrylate, to promote planar alignment)

were added. The material was spun from a solution containing 50 wt% xylene. Homeotropic

aligned samples were spun from a solution containing 0.1-1.5 wt% dye, 66 wt% RMM77

(Merck), 33 wt% xylene, and 1 wt% photo-initiator.

After spinning, the waveguides were placed on an 80°C hotplate for 20 seconds (the liquid

crystal mixture was optically confirmed to be in the nematic phase) and then photo cured for 10

Reduction in surface loss by dye alignment

89

min under nitrogen by a 360 nm UV lamp to form a cross-linked, solid film. The isotropic dye

ensembles were photo-cured directly after spinning. Measurements showed that the dye layers

were 3-6 μm thick for the liquid crystal layers and 20-30 μm thick for the isotropic layers.

Absorption spectra were recorded for all samples using a Shimadzu UV-3102 PC

spectrophotometer. The absorption was measured with incident isotropic light and light linearly

polarized both parallel and perpendicular to the rubbing direction. The peak absorbance

reported in this work refers to the measured absorbance at the main peak of the absorption

spectrum.

Emission from the edges of the waveguides was measured by an SLMS 1050 Integrating

sphere (Labsphere) equipped with a diode array detector (RPS9000, International light).

Illumination was provided by a collimated light source from a 300 Watt solar simulator with

filters to approximate 1.5AM (global) solar spectrum (Lot-Oriel) at a distance of about 15 cm.

The samples were placed in a holder, which positions the samples with the edge directly at the

entrance of the integrating sphere. Total output was determined by integrating the recorded

emission spectra over the range of 350-750nm. The collected data were corrected for the small

(~10%) polarization anisotropy of the light source.

Surface emission was determined using the setup described in chapter 5.

6.2.2 Results and discussion

The emission profiles of dichroic luminophores aligned planarly and homeotropically (chapter

3) have been used to calculate the trapping efficiency of emitted photons in the LSC waveguide

using equation 5.4.

Since the experimental data use different matrices for all three alignments (isotropic,

planar and homeotropic), the refractive indices of these matrices are also different. In the liquid

crystal matrix the refractive index is different for each angle of emission. A refractive index of

1.59 was selected as a reasonable average. The results of the calculations are depicted in figure

6.1.

With increasing optical order parameter, the fraction of the light lost trapped in the LSC

waveguide increases if the dichroic luminophore molecules are aligned perpendicular to the

waveguide surface. If the degree of order is high, the fraction of the emitted photons trapped in

the waveguide is more than 85%, and thus less than 15% of all emitted photons are lost

through the surfaces of the waveguide. These results match the results presented by Mulder et al.

[225] who uses a different model.

Chapter 6

90

Figure 6.1 Calculated trapping efficiency for homeotropically (filled symbols) and planarly (open symbols) aligned

dichroic dyes as a function of the optical order parameter. The trapping efficiency for both alignments with no

order is equal and only depicted as an open symbol.

If the dye molecules are aligned parallel to the waveguide surface (planar), the trapping

efficiency increases slightly if the order is increased from completely isotropic to an order

parameter of 0.1. Increasing the optical order parameter above 0.1, the fraction of emitted light

trapped in the LSC waveguide decreases with increasing order parameter. At optical order

parameters higher than 0.4 the fraction trapped photons is lower than for isotropic dye

ensembles. The initial increase in trapping efficiency may be explained by considering figures

3.10 and 3.11. Looking at the frontal view of the emission profile, it is clear that the shape of

the emission becomes more circular as the dye molecules are ordered planarly in comparison to

the completely disordered state (isotropic). So, the ratio of the light emitted towards the top

and the preferred side (perpendicular to the direction of the transition dipole) will evolve from

more light emitted towards the top to equal amounts of light emitted towards the top and

towards the side. In the top view of the emission profiles it can be seen that at low optical order

parameters the emission in the non-preferred emission direction is decreased approximately the

same at all angles of . When the order parameter increases, the decrease in the light emitted

in the non-preferred direction becomes nearly zero at large angles of , while at small this

decrease is less pronounced, due to the “donut” shape of the emission profile. This leads to the

decrease in trapping efficiency at larger order parameters.

Rhodamine B has demonstrated no optical alignment in the planar liquid crystal (see

chapter 4). The order parameter of the host will change by varying the LC-material. However,


91

since there is little difference in the molecular structure of the planar and homeotropic LCs, it is

assumed in the rest of this work that the degree of alignment of the LC-matrices will be

comparable, so Rhodamine B is assumed to have no optical order in the homeotropic LC-

material either. The results of the surface loss measurements can be seen in figure 6.2.

Figure 6.2 Measured surface loss of absorbed light energy of a Rhodamine B dye-topped LSC with isotropic

(black squares), planar (light grey triangles) and homeotropic (grey spheres) dye alignment

Due to limited solubility of Rhodamine B in the liquid crystalline material, only low

concentrations of this dye could be used for this measurement leading to low absorbance

materials. The difference in surface loss between the three alignments of the dye is very small,

less than the experimental error. Therefore, it may be concluded that there is no difference in

surface losses for the Rhodamine embedded in LCs in different orientations, which could be

expected knowing Rhodamine B has no optical order in the liquid crystal matrix.

For the isotropic thin-film dye ensembles the surface loss increases with increasing peak

absorbance. This is different from the results shown in chapter 3 which studied Red 305 in a

filled waveguide. According to equation 5.3 the surface loss increases with increasing number of

photon recycling events, leading to an increase in surface loss with optical density.

DCM demonstrates reasonable optical alignment in the nematic LC-material used for

planar alignment, reaching optical order parameters around 0.35-0.4 . For this dye, the surface

loss from thin film LSC waveguides is also measured for the three alignments and the results

are depicted in figure 6.3.

Chapter 6

92

Figure 6.3 Measured fractional surface loss of absorbed light energy of a DCM dye-topped LSC with isotropic

(black squares), planar (light grey triangles) and homeotropic (grey spheres) dye alignment

The solubility of DCM in the LC-hosts is much better than Rhodamine B, which led to samples

with a peak absorbance up to 0.6 for homeotropic alignment and almost 1.1 for planar

alignment. For all three alignments, the fractional surface loss generally decreases with increased

peak absorbance. With increasing peak absorbance the probability of re-absorption is increased

as well, and considering DCM has a quantum yield which is below unity (~0.8 in a solid matrix

[110]) the fraction of the absorbed light being lost through non-radiative events is also

increased. Therefore, the total amount of emitted energy is lower than the total amount of re-

absorbed energy, leading to a decrease in both surface loss and edge output efficiency, which

are measured as a function of total absorbed energy.

The fractional surface loss for the planar and isotropic alignments is on the same order,

while there is a reduction in surface loss when the molecules are aligned homeotropically. The

trapping efficiency calculated from the model was similar for planar and isotropic dye

ensembles, which is confirmed by the experimental, even though the experimental data show a

large degree of scatter. The experiments show an 25%-50% decrease in surface loss if the

molecules are aligned homeotropically in comparison to isotropic aligned dye ensembles.

The coumarin dye showed very good optical alignment properties in the planar LC-

material with a measured optical order parameter in absorption of 0.5-0.6. For this dye the

surface loss from thin film LSC waveguides was also measured for the three alignments and the

results are depicted in figure 6.4.


93

Figure 6.4 Measured fractional surface loss of absorbed light energy of a coumarine dye-topped LSC with

isotropic (black squares), planar (light grey triangles) and homeotropic (grey spheres) dye alignment

Clearly, there are considerable differences in surface losses between the three alignments.

The fraction of absorbed energy leaving the surface of the LSC waveguide after emission is

constant with peak absorbance, comparable with the measurements on the filled Red 305

waveguides presented in chapter 3. About 35% to 40% of all absorbed energy is lost through

the surface for an isotropic dye ensemble. Planar alignment of the dye molecules increases the

surface loss to approximately 55-60%. Aligning the dye molecules homeotropically decreases

surface loss to 5-10%.

Comparing the calculated trapping efficiency and the experimental results from the surface

loss of isotropic dye ensembles of the coumarin dye leads to an average of ~0.7 photon

recycling events. Using these 0.7 photon recycling events and equation 5.5 the theoretical

surface loss for homeotropic dye ensembles can be calculated to be ~20%. This is still higher

than measured experimentally, but in this calculation the trapping efficiency after each photon

recycling event is kept constant, while the trapping efficiency will increase with increasing

photon recycling events. Thus the 20% calculated surface loss is an overestimation.

Furthermore the absorbance of the homeotropic samples is low, which increases the error in

the measurement.

From the surface loss measurements depicted and described above using Rhodamine B,

DCM or the coumarin dye as luminophore, it can be concluded that there are some

discrepancies between theory and experiment, but the general trends are consistent.

Chapter 6

94

The goal of reducing surface losses in LSCs is to increase the emission from the edges of

the LSC waveguide and so increase the optical efficiency of the LSC. From the calculated

emission profiles the total amount of light emitted to the edge of the LSC waveguide was

determined for both planar and homeotropic aligned dye ensembles and the results normalised

by the amount of light in waveguide mode for an isotropic dye ensemble using

2

0

2

0

, sin

, sin

c

c

c

c

aligned

edge

iso

d d I

d d I

(6.1)

where edge is the fractional total amount of light emitted to the edge of the LSC waveguide,

,isoI is the emission profile of an isotropic dye ensemble and ,alignedI is the emission

profile of the aligned dye ensemble. The results of this calculation can be found in figure 6.5.

For planarly aligned dye ensembles the emission from the LSC-waveguide edge increases with

increasing order parameter; surface loss also increases at these high order parameters. The

increased edge emission results from the increased absorption of dye molecules oriented with

excitation dipoles more perpendicularly with respect to the incoming sunlight (assumed to be

normal to the device).

Despite a significant reduction in surface loss and increased emission into waveguiding

modes, the significant reduction in absorption of sunlight results in a decrease in the amount of

light emitted from the edges with increasing order parameter if the molecules are aligned in a

homeotropic fashion. The results in figure 6.5 will change if a number of photon recycling

events is taken into account. In figure 6.6 the relative edge output is plotted for both

homeotropic and planar alignment where 1, 2, or 3 photon recycling events are taken into

account. Again the trapping efficiency is kept constant after each photon recycling event

leading to an overestimation of the surface loss, but in both homeotropic and planar dye

ensembles as the reference (isotropic dye ensembles) the surface loss is overestimated.


95

Figure 6.5 Calculated normalized edge output for homeotropically (filled symbols) and planarly (open symbols)

aligned dichroic dyes. The edge output has been normalized to 1 for an optical order parameter equals zero

(isotropic). Photon recycling is not taken into account

Figure 6.6 Calculated normalized edge output for homeotropically (filled symbols) and planarly (open symbols)

aligned dichroic dyes after 1(squares), 2(triangles), or 3(circles) photon reccling events. The edge output has been

normalized to order optical parameter equals zero (isotropic) being 1.

Chapter 6

96

In figure 6.6 it can be seen that at low optical order parameters the calculated relative edge

output increases when the molecules are aligned homeotropically. The total amount of

absorbed photons is not decreased drastically in comparison to the isotropic dye ensembles,

while there is some gain in trapping efficiency. The edge output decreases drastically at larger

optical order parameters due to reduced absorption of the incident light. The effect of the

increased absorption from planar aligned dye ensembles is reduced when multiple photon

recycling events are taken into account, since these planar ensembles emit more light towards

the surface at each photon recycling event in comparison to the two other alignments

The optical efficiency of thin-film LSCs using the coumarin dye in all three alignments was

measured experimentally and the results are depicted in figure 6.7. There are no large

differences between them if the peak absorbance is the same. The samples with planar

alignment have a lower dye concentration than the isotropic samples. The homeotropic sample

has an even higher concentration of dye molecules.

Figure 6.7 The optical efficiency of a Coumarin dye-topped LSC with isotropic (black squares), planar (light

grey triangles) and homeotropic (dark grey circles) dye alignment

Aligning dye molecules in a homeotropically initially decreases the amount of energy lost

through the surface, but due to a reduction in absorption of incident light a higher

concentration of dye molecules is necessary, which has the detrimental effect of increasing the

chance of re-absorption of emitted photons. Therefore, in principle a combination of the high

absorption from planar aligned dye ensembles and the reduction of surface loss in homeotropic

dye ensembles could provide a system with optimal performance.


97

6.4 Tilted dye ensembles

Tilted dye ensembles present an opportunity to combine the advantages of planar alignment

(increased absorption) and homeotropic alignment (reduced surface loss). As suggested by the

name, tilted dye ensembles have a director which has an angle with respect to the waveguide

plane, see figure 6.8.

Figure 6.8 Schematic depiction of homeotropic (1), planar (2) and tilted (3) dye esembles. The black arrow

resembles the director of the ensemble.

Tilted aligned dye ensembles should reduce the path length of emitted photons through

the dye layer, which would decrease the chance of re-absorption while still allowing the

ensemble to emit most of the light towards the edge of the LSC. It is known that there are

smectic LC-phases which exhibit a tilted director, such as smectic C phase. It is also possible to

align nematic LC-materials with a tilt angle by using a mixture of different polyimide materials

to form the alignment layer and the processing conditions of this layer will influence the angle

of the director of the molecules. [226] Using this latter method led to samples where the LC-

molecules exhibited a tilted alignment as can be seen in the scanning electron microscopy

images depicted in figure 6.9.

Figure 6.9 SEM pictures of the cross-section from a tilted dye ensemble. The left and right picture are two

different tilted dye samples. Breaking the layer of LC-material caused fracture lines in the same direction as the

director. Optical behaviour between cross-polarizers also indicate that the samples are birefringent with the optical

axis tilted with respect to the layer.

Chapter 6

98

Unfortunately, heat degradation of the alignment layer itself caused the material to become

absorbing, preventing measurement of the effect of tilted dye ensembles on the emission of

photons and so experimental results were not obtained. Since actual measurements of the

emission from a tilted dye ensemble was not possible, the emission profile is calculated in the

same way as the calculations of the emission profile of isotropic, planar and homeotropic dye

ensembles as described in chapters 3, 4 and the previous sections of this chapter respectively.

Equation 5.4 is now used to calculate trapping efficiency from LSC-waveguides with tilted dye

ensembles described by equation 3.18. The critical angle of the device is dependent on the

refractive index of the matrix in which the dye molecules are embedded. For an LC-host, the

refractive index is dependent on the angle between the photon and the optical axis of the LC.

In the calculations it is assumed that the refractive index is constant at 1.59. The results are

depicted in figure 6.10.

Figure 6.10 Calculated trapping efficiency from LSC using tilt aligned dye ensembles with various optical order

parameters (S2,opt): 0.1 (red), 0.2 (blue), 0.3 (green), 0.4 (pink), 0.5 (orange), 0.6 (navy), 0.7 (brown), 0.8

(light blue), 0.9 (grey) and 1.0 (yellow).

For optically perfect aligned dye ensembles, the trapping efficiency decreases with increasing tilt

angle with respect to the normal to the waveguide (see figure 6.10). All the emission profiles

have a similar donut shape around the director of the ensemble (figure 3.14) and with

increasing tilt angle the amount of photons emitted in waveguide mode decreases. Contrarily to

optically perfect aligned dye ensembles, the emission profiles of optically non-perfect aligned

dye ensembles do not only change in direction but also in shape with changing tilt angle, as can


99

be seen in figure 6.11.

a) b) c)

d) e) f)

g) h) i) j)

Figure 6.11 Frontal views of the emission profiles from well aligned (S2,opt=0.8) dye ensembles with different tilt

angles: a) 0°, b) 10°, c) 20°, d) 30°, e) 40°, f) 50°, g) 60°, h) 70°, i) 80°, and j) 90°. The axis of these

emission profiles have aribitrary units. The units are exactly the same on both axis and the emission originates

from the middle of the profile.

Chapter 6

100

The change in shape of the emission profile leads to a different tilt angle where the

trapping efficiency is minimal, being 90° for perfect aligned dye ensembles and 40° for weakly

aligned dye ensembles (S2,opt=0.1). The tilt angle where the trapping efficiency is maximal is less

dependent on the optical order parameter (0°-10°).

Figure 6.12 Normalized calculated photons absorbed by the dye molecules in LSCs using tilt aligned dye

ensembles with various optical order parameters (S2,opt): 0 (black), 0.1 (red), 0.2 (blue), 0.3 (green), 0.4 (pink),

0.5 (orange), 0.6 (navy), 0.7 (brown), 0.8 (light blue), 0.9 (grey) and 1.0 (yellow). The amount of absorbed

photons in an LSC with isotropic dye ensembles is set as 1 (black line).

In figure 6.12 the relative absorption of the dye ensembles with respect to an isotropic dye

ensemble is depicted. Generally, the relative absorption increases with increasing tilt angle. The

angular dependency of the absorption increases with order parameter. For small tilt angles the

absorption of the aligned dye ensembles is smaller than the absorption of an isotropic dye

ensemble. Except for very low optical order parameters (S2,opt=0.1-0.2), a tilt angle of at least

between 50°-70° increases the absorption compared to an isotropic dye ensemble.

Both the trapping efficiency and the amount of light absorbed by the dye molecules will

influence the total amount of photons which are exiting the edge of the LSC, if the quantum

efficiency is assumed unity. In figure 6.13 the normalized amount of photons emitted in

waveguide mode is depicted, where the amount of photons leaving the edge from an isotropic

dye ensemble is set at 1. Comparing the amount of photons leaving the waveguide edge and the

absorption of the dye ensembles shows that the absorption is the key factor in the total amount

of photons leaving the waveguide edge, and not the trapping efficiency. The effect is largest

when the optical order is highest, so a perfectly aligned tilted dye system will emit the most light


101

into waveguide mode.

Figure 6.13 Normalized calculated photons leaving the waveguide edge by the dye molecules in LSCs using tilt

aligned dye ensembles with various optical order parameters (S2,opt): 0 (black), 0.1 (red), 0.2 (blue), 0.3 (green),

0.4 (pink), 0.5 (orange), 0.6 (navy), 0.7 (brown), 0.8 (light blue), 0.9 (grey) and 1.0 (yellow). The amount of

photons leaving the waveguide edge in an LSC with isotropic dye ensembles is set as 1 (black line).

All the calculations performed in this section are constrained to one absorption event with

light incident normal to the device, so no photon recycling. To show the effect of photon

recycling, the relative number of photons leaving the edge of the LSC waveguide is plotted for

the almost perfect aligned systems (S2,opt=0.9) with 0, 1, 2 or 3 photon recycling events in figure

6.14. Again the trapping efficiency if kept constant with photon recycling event.

With increasing photon recycling events the relative amount of photons leaving the

waveguide edge decreases for planar aligned dyes ensembles, while for homeotropic aligned dye

ensembles the relative amount of photons leaving the waveguide edge increases with respect to

isotropic dye ensembles (figure 6.14). The number of photons leaving the edge of the LSC

waveguide decreases with increasing number of photon recycling events (figure 6.15), but this

reduction is the largest for samples with a small tilt angle and smallest for large tilt angle

ensembles. To predict the optimal tilt angle, the exact emission profile after each photon

recycling event should be known as well as the exact number of photon recycling events, but as

can be seen in both the figure 6.14 and 6.15 the optimal tilt angle will be close to planar

alignment. Even though the trapping efficiency is much higher for homeotropically aligned dye,

the increase in absorption by planarly aligned dye molecules will stay the determining factor.

Chapter 6

102

Figure 6.14 Normalized calculated photons leaving the waveguide edge by the dye molecules in LSCs using tilt

aligned dye ensembles with optical order parameter (S2,opt) set at 0.9, including 0(black), 1(red), 2(blue), or

3(green) photon recylcing events. The amount of photons leaving the waveguide edge in an LSC with isotropic dye

ensembles is set as 1 (black line).

Figure 6.15 Number of photons leaving the edge of LSCs with perfect aligned tilted dye ensembles as funcion of

the number of re-absorption events. Tilt angle: 0° (red), 10° (green), 20° (blue), 30° (pink), 40° (yellow), 50°

(light blue), 60° (brown), 70° (purple), 80° (orange) and 90° (dark green). Dichroic dyes in an isotropic host

are depicted with the thicker black line)


103

The model as presented in this thesis should be implemented in simulation software to

derive the optimal tilt angle, and the results in this chapter should considered as a first step to

predicting the efficiency of LSCs with aligned dichroic dyes.

6.5 Conclusions

Calculations predict that aligning dichroic dyes in a homeotropic fashion increases the trapping

efficiency of the emitted photons in the waveguide, while planar alignment of these dye

molecules decreases the trapping efficiency. The surface losses measured experimentally show

identical trends. Since the number of photon recycling events is not known, theoretical surface

losses are difficult to predict. Since the preferred direction of the dipole for absorption is

parallel to the incident light, homeotropic aligned dye ensembles show drastically reduced

absorption, while absorption is enhanced when the dye molecules are aligned planarly.

Devices using dyes tilted with respect to the LSC waveguide surface were expected to

combine the advantages of homeotropic (preferred emission towards the edges of the

waveguide) and planar (enhanced absorption) alignments. Calculations show that the change in

absorption is the key factor with respect to determining the total amount of light emitted in

waveguide mode by the dye molecules, and directional emission of less importance. However,

no photon recycling is taken into account in any of these calculations, which will probably have

a significant influence on the results discussed in this chapter. Experimentally, it is shown for

the coumarin dye that the effect of aligning the dye molecules has a limited influence on the

optical efficiency of the LSC. Since the number of photon recycling events is dependent on the

dye and the size of the LSC waveguide, the effect of aligning the dye molecules will change if

another dye is used or the size of the LSC changes.

The relative difference in surface loss between large and small tilt angle dye ensembles will

increase with increasing number of photon recycling events. It is not currently possible to

calculate the effect of photon recycling analytically, since the recycling depends on the

polarization and direction of the light within the waveguide, the wavelength of the emitted light

(in principle the overlap between the absorption and emission bands of the dye molecule) and

the spatial distribution of the dye molecules. Generally the effect of multiple photon recycling

events will be the largest for dye ensembles with a large tilt angle, since the surface loss is higher

for these samples. This will probably lead to a decrease in the relative amount of photons

emitted in waveguide mode by these large tilt dye ensembles and an increase for the small angle

tilt dye ensembles. This should lead to a maximum in total photons emitted in waveguide mode

at a tilt angle smaller than 90°, making tilting of dye molecules a viable method to increase the

efficiency of an LSC. The preferred tilt angle will probably be relatively large since the amount

of light absorbed by the dye molecules remains important.

The absorption of incident light calculated in this chapter assumes light at normal

incidence to the surface of the LSC. During the day the position of the sun changes with

Chapter 6

104

respect to the LSC surface and the sun will most of the time be at a larger angle. This will

increase the absorption of the incident light and therefore the number of photons leaving the

edge of the LSC waveguide.

The calculations performed in this chapter, especially the emission profiles (which were

predicted in chapter 3) as function of the angle between the incident light and the director of

the dye ensemble should be considered as a first step in the design of a toolbox to be

implemented in a simulation software package. Such a complete package could be used to

simulate the effect of photon recycling on the directional emission from dichroic dye

ensembles.

7 Organic wavelength selective

reflectors5


M.G. Debije, M.P. Van, P.P. C. Verbunt, D.J. Broer and C.W. M. Bastiaansen, “The effect of

an organic selectively-reflecting mirror on the performance of a luminescent solar

concentrator”, in Proceedings of the 24th European Photovoltaic Solar Energy Conference,

WIP Munchen, 2009, 373-376.

M.G. Debije, M.-P. Van, P.P.C. Verbunt, M.J. Kastelijn, R.H.L. van der Blom, D.J. Broer and

C.W.M. Bastiaansen, “Effect on the output of a luminescent solar concentrator on application

of organic wavelength-selective mirrors”, Applied Optics, 49(4), 745-751, 2010

D.K.G. de Boer, C.-W. Lin, M.P. Giesbers, H.J. Cornelissen, M.G. Debije, P.P.C. Verbunt and

D.J. Broer, “Polarization-independent filters for luminescent solar concentrators”, Applied

Physics Letters, 98, 021111, 2011

P.P.C. Verbunt, S. Tsoi, M.G. Debije, D.J. Broer, C.W.M. Bastiaansen, D.K.G. de Boer and C.-

W. Lin, “Increased efficiency of luminescent solar concentrators after application of organic

wavelength selective mirrors”, Optics Express, 20 (S5), A655-668, 2012

Chapter 7

106

7.1 Introduction

The loss of photons through the surface of an LSC waveguide can also be reduced by

application of wavelength selective mirrors. Wavelength selective mirrors transmit light that can

be absorbed by the luminophore, but reflect photons that are emitted, effectively reflecting

surface directed photons back into the LSC. A picture of the working principle is shown in

figure 7.1.

Figure 7.1 The working principle of wavelength selective mirrors. Black photons in the solar spectrum are

transmitted by the reflector and absorbed by the dye molecules. Gray emitted photons are reflected back into the

device.

Wavelength selective reflectors are in principle Bragg reflectors, which consist of repeating

layers of materials with alternating refractive index. At each interface between a low and high

refractive index material a small portion of the incoming light is reflected. If the difference in

distance from which the light is reflected is such that the reflected light from different

interfaces is in phase, the photons will constructively interfere. This means that the period of

the alternating refractive indices determines which wavelength the Bragg reflector reflects. The

condition for this constructive interference is described by:

2 cosm nd (7.1)

where m is an integer denoting the order of interference, is the wavelength of the light that is

being reflected, n is the average refractive index of the multilayer, d is the periodicity of the

multilayer and is the angle of the incident light. From equation 7.1 it can be derived for reflecting light with a certain wavelength incident

normal to a Bragg-reflector, a periodicity

2

dn

is necessary. The light reflected by such a

reflector has a certain spectral bandwidth. The wavelength determined by Bragg’s law (equation

7.1) is the central wavelength of this reflection band. The bandwidth ( ) of the reflection

band is determined by the refractive indices ( 1n and 2n ) of the multilayer stack and this central

Organic wavelength selective reflectors

107

wavelength (0 ) according to:

10 2 1

2 1

4sin

n n

n n (7.2)

Bragg reflectors can be made from stacks of inorganic materials with alternating refractive

indices.[75,199,227] Since the thickness of these layers is very important for constructive

interference of the reflected photons, production of these layers has to be performed with high

accuracy. The production of these inorganic Bragg-reflectors occurs most often by deposition

methods which are very time consuming and expensive to reproduce on large scales.

Chiral nematic liquid crystalline (also known as cholesteric) materials can also act as Bragg

reflectors. [213] In chapter 3 of this thesis an overview is given of LC materials. Nematic LC

form a cholesteric phase if the liquid crystal molecule has a chiral center or if a dopant with a

chiral center is added. Due to the addition of this chiral molecule the director of the LC

material is not constant over the thickness of the layer, but it will rotate forming a helical

structure. Due to the birefringent nature of the LC-material, the helical structure will create a

periodic fluctuation in refractive index between the ordinary and extraordinary refractive index

of the LC-material over the thickness of the cholesteric layer. This rotational periodic change in

refractive index leads to a Bragg-reflector for circular polarized light with the same handedness

as the rotational direction of the helix, so a right handed helix will reflect right handed circular

polarized light for a small spectral bandwidth, while left handed polarized light is transmitted.

The wavelength of the light that is reflected by a cholesteric reflector depends on the pitch of

the helical structure, where the pitch is defined as the length of the helix wherein the director of

the LC-material is rotated by 360° (see figure 7.2). The central wavelength of the spectral band (

0 ) that is reflected by the cholesteric material is defined as:

0 n p (7.3)

where n is the average refractive index of the LC material and p is the pitch of the helical

structure.

Figure 7.2 Schematic depiction of the helical structure of a cholesteric material, defining the pitch of the material

Chapter 7

108

The resulting reflectors have a spectral bandwidth ( ) determined by the birefringence

(n ) of the LC material in unidirectional alignment and the pitch and can be described as

p n (7.4)

Commonly available liquid crystalline materials have birefringence up to 0.25, which leads

to a spectral reflection bandwidth between 50 and 100 nm for visible light. In the rest of this

thesis these reflectors are called 75 nm reflectors, or ‘narrowband reflectors’. The pitch of a

nematic liquid crystalline material, which contains a chiral dopant, can be changed by the

concentration ( c ) and the helical twisting power ( HTP ) of the chiral dopant in the mixture

used:

1

pHTP c

(7.5)

The helical twisting power of a chiral dopant is determined by the off-set in the director of

the LC-material between different layers of the material created by this dopant. A chiral dopant

with a positive HTP will lead to a right handed helix, while a negative HTP creates a left

handed helix.

As mentioned above, a cholesteric reflector will only reflect one circular polarization of

light. [213] For the application as a wavelength-selective reflector on top of an LSC to reduce

the surface losses, a reflector is desired which reflects unpolarized light, called a full reflector. A

full reflector can be created from cholesteric layers by layering a right handed and a left handed

reflector, or by sandwiching a half wave retarder between two layers with the same handedness.

A schematic description of both full reflectors is depicted in figure 7.3.

In the full reflector made from a stack of a right-handed and a left handed cholesteric

layer, the right circular polarized component of unpolarized light is reflected by the right

handed cholesteric layer and the left circular polarized component is transmitted. This left

circular polarized component is reflected by the second layer which is a left handed cholesteric

layer, creating a full reflector. In the full reflector made from a half wave retarder sandwiched

between two right-handed cholesteric layers, the right circular polarized component of

unpolarized light is reflected by the right handed cholesteric layer and the left circular polarized

component is transmitted. This left circular polarized light is transformed into right circular

polarized light by the half wave retarder and then reflected by a second right handed cholesteric

reflector. This reflected right circular polarized light is transformed back into left circular

polarized light by the half wave retarder and transmitted through the first right handed

cholesteric layer, leading to a full reflector.

In this chapter, the effect of application of these organic wavelength selective mirror on

the efficiency of LSCs is determined, both in theory and experiment.


109

Figure 7.3 The working principle of full cholesteric reflectors. a) A right handed and a left handed cholesteric

stacked on top of each other, b) a half wave retarder sandwiched between two right handed cholesterics.

7.2 Methods

Narrowband cholesteric reflectors were made by spin casting solutions of the reactive LC-

mesogens RM257 and RM82 (in a 4:1 weight ratio) (Merck), varying concentrations of the

reactive chiral dopant LC756 (BASF), 1% of the photo initiator Irgacure 184 (Ciba), and 1% of

a surfactant in xylene (50% by weight of solution). The solutions were spin cast on rubbed half-

wave retarder plates centered at 560 nm (Edmund Optics) at 1000 rpm for 40 s and placed

immediately on a hot plate at∼80 °C for ∼15 s. The samples were then cross-linked by

exposure to UV-light at room temperature in a nitrogen atmosphere for 10 min. For the

cholesterics to be capable of reflecting both left- and right circularly polarized light, a second

cholesteric from the same solution was spun on the rubbed backside of the same half-wave

retarder.

The layered broadband reflectors were made from two stacked right-handed narrowband

reflectors applied to a manually rubbed half wave retarder centered at 560 nm (Edmund

Optics). A mixture of reactive LC mesogen LC242 (BASF, figure 7.4), varying concentrations

chiral dopant LC756 (BASF, figure 7.5), 1% of photo initiator Irgacure 184 (Ciba) and 1% of

surfactant (2-(N-Ethylperfluoro-octanesulfamido)ethyl acrylate) to induce planar alignment at

the liquid crystal-air interface in xylene (1:1 by weight, Aldrich) were spin coated at 800 rpm for

30 seconds. After spin coating, the samples were immediately heated on a hot stage at 90 °C for

Chapter 7

110

30 seconds and then photo-polymerised by UV-exposure at room temperature in a nitrogen

atmosphere. Before applying the second reflecting layer with a higher concentration of chiral

dopant, the first layer was treated with an oxygen-plasma for 1 minute at 60 W, to improve the

wetting of the LC layer. A similar process was applied to the rear side of the same half-wave

retarder following an identical procedure.

Figure 7.4 Chemical structure of LC242

Figure 7.5 Chemical structure of LC756

The gradient pitch broadband reflectors were made by filling 20 µm cells with mixtures of

chiral and nematic monomers, photo initiator, and UV-absorbing dye, followed by crosslinking

with UV light. Both left and right handed reflectors were produced. The right-handed CLC

mixture was prepared by mixing 41.3 wt.% RM96 (monoacrylate chiral monomer, Merck.) with

RM257 (diacrylate nematic monomer, Merck); the left-handed cholesteric mixture was prepared

by mixing 32.4 wt.% of a diacrylate chiral monomer with monoacrylate nematic monomer

(described by Broer, et. al.[228]), respectively. In both cholesteric mixtures, 1 wt.% photo

initiator (Irgacure 651, Ciba) and UV-absorbing dye (Tinuvin 328, Ciba) were added. The

amount of chiral material was chosen so that the desired central wavelength was obtained.


111

Polyimide coated (Grade 130, Nissan Chemical), anti-parallel rubbed cells (5x5 cm2) were filled

by each mixture by capillary action. The cell gap was maintained by spacers of 20±1 μm. The

right- and left-handed CLC films were cured under nitrogen with UV radiation (λ=365 nm, 28

μw/cm2) at 105°C for 30 min. and 5 min., respectively. In this way a pitch gradient in the

cholesteric stack is made, with the smaller pitch at the top of the film. The specific UV dose

and curing time were determined to produce the desired bandwidth

The filled waveguides were produced by injection moulding (poly)carbonate doped with 5-

180 ppm BASF Lumogen F Red 305 into 50 x 50 x 3 mm3 plates (Sabic IP). The patterned

LSC waveguides (see the results and discussion section of this chapter) were produced on

PMMA plates (50 x 50 x 5 mm3) (Plano Plastics). Fluorescent dye solutions were prepared

using 0.5 wt% of Red 305, and 1 wt% photoinitiator (Irgacure 184) dissolved in a 3:1

dipentaerythritol penta-acrylate (Polysciences) and methylmethacrylate (MMA, Aldrich) blend.

The dye solutions were stirred and heated at 60°C for an hour prior to spin-coating onto the

substrates at 1000 rpm for 30 s. After spin-coating, all 100% covered samples were cross-linked

by exposing to a high-intensity UV lamp for 80 s under nitrogen flow to form a solid film. For

the fabrication of patterned LSCs, standard photolithography techniques were employed.

Uniformly coated substrates were exposed to UV light through patterned shadow masks

consisting of 10 lines with variable widths with a period of 5 mm. Line widths were varied to

cover 20 to 80% of the waveguide surface. After UV exposure, ethanol was used to etch away

the unexposed material on the PMMA substrates by placing the exposed patterned samples in

ethanol for 40 s at room temperature and the samples were continuously agitated during the

etching process. The preparation of the patterned LSC waveguides was done by Shufen Tsoi.

Transmission spectra of the manufactured reflectors and absorption spectra of the waveguides

were recorded with a Shimadzu UV-3102 spectrophotometer.

Quantitative bottom surface emissions from the waveguides (that is, emission from the

surface opposite the incident light source) were determined by use of a Labsphere spectral light

measurement system LED 1050 integrating sphere, with a blank waveguide plate used as the

reference; see figure 5.2 for a depiction of the experimental setup. The top surfaces of the

samples were exposed to the light from a 300 W solar simulator with filters to approximate the

1.5 air mass (global) solar spectrum (LOT Oriel). We use filters placed between the source and

the sample to reduce the background spectra from the source light in the spectral emission

band of the used luminophore. The illumination was limited to an approximately 6.45 cm2 area

in the waveguide center. The surface emission measurements were repeated with cholesteric

filters placed individually against the bottom surface of the waveguide. The spectra from the

Red 305-doped waveguides were subtracted from the spectrum of the blank waveguide. The

resulting spectra were integrated to determine the quantity of light absorbed by the waveguide,

and the amount of light emitted from the surface (see chapter 5 for more details).

Chapter 7

112

Representative surface emission spectra from one waveguide (absorbance 0.46) were

determined at three different angles using an Autronic 703 display measuring system (Melchers)

together with a CCD-Spect-2 (CCD camera). A 1 cm2 area of the waveguide topped by a blank

half-wave plate and of the same waveguide with a 670 nm cholesteric plate on top was

illuminated from below by a Q50MR16 daylight lamp (SoLux), and surface emissions were

recorded from a 0.5 mm diameter area centered approximately 0.5 cm from the edge of the

illumination region to avoid recording any of the source illumination. The surface emission

spectra were recorded at defined angles with respect to the surface normal.

The edge emissions from the LSC waveguides with and without cholesteric reflectors were

recorded using an SLMS 1050 integrating sphere equipped with a diode array detector. The

samples were placed with their edges in the entry port of the integrating sphere while

illuminated with a 300 W solar simulator with filters to approximate the 1.5 AM solar spectrum

(Lot Oriel). The spectrum and intensity of the edge emission were recorded. A thick paper

spray painted white placed underneath the sample acted as a Lambertian scatterer. The total

output from the edges was determined by integrating the recorded spectrum from 350-750nm.

A depiction of this setup can be seen in figure 7.6.

Figure 7.6 The experimental setup for measuring emission intensities at the edges of waveguides with a cholesteric

reflector on top and a scattering layer underneath, where both are prevented from being in optical contact with the

waveguide by an air gap

To determine the angular dependency of the reflection band of the gradient pitch

reflector, the cholesteric films were removed from the cell, combined and sandwiched between

two half-cylinders with index-matching oil. The half-lambda wave plate is a polymer film (Nitto

Denko) with a retardation of 413 nm. Transmission measurements of these samples were made

with a spectrophotometer as described in figure 7.7, where the sample is place between the half

cylinders. We also measured the transmission without the rotatable stage and the detector closer


113

to the sample. This results in a transmission increase of about 2%, because of a more complete

collection of the scattered radiation.

Figure 7.7 Experimental set-up for measuring the angular transmission spectra of gradient pitch cholesteric

reflectors: a Perkin-Elmer Lambda 800 UV-VIS Spectrophotometer with a rotatable sample holder and a

detector that includes an integrating sphere. To minimize refraction of the light, the sample was sandwiched

between two glass half-cylinders and index-matching oil was applied. The grey lines indicate light beams.

7.3 Narrowband reflectors

The purpose of the cholesteric layer is to reduce the surface losses of LSCs by selective

reflection of the emitted light. The retained light might reach the emission edges and increase

light output from the waveguide. Experiments with narrowband reflectors used right-handed

cholesterics located on either side of a half-wave retarder centered at 560 nm. Made in this way,

the right-handed circular polarized light reflecting cholesterics could be used to reflect both

handedness of light incident normal to their surface. The transmission spectra for the five

cholesteric mirrors, exposed to unpolarized light can be seen in figure 7.8.

Chapter 7

114

Figure 7.8 Transmission of unpolarized light through the two-sided cholesterics used in these experiments; onset

at 630, 650, 680, 720, and 760nm, respectively.

The surface loss of three filled waveguides containing Red 305 as luminophore was

measured as described in chapter 5, with and without the narrowband reflectors between the

waveguide and the entry port of the integrating sphere. From these measurements the

reduction of surface loss was calculated. The results are shown in table 7.1.

Table 7.1 Percentage of absorbed light emitted through the bottom of the waveguides after application of a

cholesteric reflector

Peak

Absorbance

Onset wavelength of the cholesteric reflector

None 630 650 680 720 760

0.46 16.8 10.8 10.0 9.1 12.5 13.5

1.01 15.4 10.0 8.7 9.0 11.4 12.7

2.36 15.3 11.5 9.6 10.0 12.2 13.0

In the best case, application of the 650nm cholesteric to the waveguide with absorbance

1.01 reduced bottom surface loss of the waveguide by 44%. The effect of adding a cholesteric

filter to the top of the waveguide on the surface emission spectrum as a function of emission

angle can be seen in figure 7.9.


115

Figure 7.9 Comparative surface emission spectra for a Red 305 filled polycarbonate waveguide with a peak

absorbance of 0.52. The black curve resembles the surface emission spectra measured 15° to the surface normal

with a blank half-wave plate on top. The surface emission from the same waveguide with a cholesteric reflector

centered at 670nm on the top is measured at 15° (dark gray curve), 30° (medium gray curve), and 45° (light

gray curve) to the waveguide normal.

The angular dependent surface emission spectra shown in figure 7.9, demonstrate that a

part of the surface emitted photons is reflected back into the waveguide. Upon increase of the

angle of the emitted photons with respect to the normal of the reflector, shorter wavelength

photons of the emission band of the dye are reflected. At each angle only a part of the surface

emitted photons are back-reflected, a result of the width of the reflection band being more

narrow than the spectral width of the emission band of the luminophore.

Reflectors were placed on top of LSC waveguides containing different concentrations of

the luminophore. The edge output of the waveguides with and without reflectors on top of the

waveguides was measured using the integrating sphere as described in the experimental section

of this chapter. A piece of thick paper spray painted white acted as a Lambertian scatterer and

was placed underneath the waveguides to reflect light lost through the bottom surface and light

reflected by the cholesteric reflector back into the waveguide. The results are shown in table

7.2.

The results in table 7.2 show that there is 13.7% more light leaving the edge of an LSC

with a peak absorbance of 0.05 when the 720 nm reflector is applied on top of the waveguide.

The LSCs with a higher peak absorbance (0.46-1.01) show a ~10% increase in energy that

Chapter 7

116

leaves the edge of the waveguide. In these experiments, where a white bottom scatterer is used,

the waveguide edge emission light can be divided into two types: a) that resulting from

scattering from the backscattering layer and b) light directly from the dye emission. To

determine the relative contributions, the spectrum obtained from a polycarbonate waveguide

that contained no dye with a separate white scatterer was subtracted from the spectrum of the

same scatterer underneath a polycarbonate waveguide that contained dye. This isolated the

impact of the dye. Second, the same spectrum of the clear polycarbonate waveguide was

subtracted from the spectrum measured from a dye-filled waveguide with a cholesteric layer

separated by an air gap on top, which isolates the impact of the dye with a cholesteric. Sample

spectra resulting from these manipulations can be seen in figure 7.10.

Table 7.2 Optical efficiency of the polycarbonate Red 305 waveguides. Between brackets is the relative efficiency,

where the relative efficiency of the LSC with no reflector is set at 1.

Absorbance No

reflector

Onset wavelength of the reflector (nm)

630 650 680 720 760

0.05 0.0265 0.0280

(1.058)

0.0278

(1.050)

0.0288

(1.086)

0.0301

(1.137)

0.0293

(1.108)

0.46 0.0794 0.0794

(1.000)

0.0824

(1.038)

0.0833

(1.049)

0.0858

(1.081)

0.0889

(1.120)

1.01 0.1064 0.1041

(0.978)

0.1071

(1.007)

0.1103

(1.037)

0.1118

(1.051)

0.1163

(1.093)

2.36 0.1572 0.1522

(0.968)

0.1569

(0.998)

0.1605

(1.021)

0.1622

(1.032)

0.1665

(1.059)

There are two regions in these spectra, labelled I and II in the figure. Region I is a measure

of the amount of light removed from the incoming light by dye absorption that was normally

scattered to the emission edge in a clear waveguide. Since the resultant spectra of the samples

with and without cholesteric are essentially identical in region I, we can conclude that the

presence of the cholesteric does not significantly remove light from the system at these

wavelengths (otherwise the amount of light available to be absorbed would be less). Region II is

a measure of the emission of light by the dyes only, as the additional light derived from the

scatterer has been removed. The difference between the spectra with and without the

cholesteric, then, is a measure of the addition or removal of light caused by the cholesteric. By

comparing the integrations of region II up to 750 nm for the sample with and without

cholesteric, we obtained the data displayed in figure 7.11, which shows the fractional increase in

energy emission at the edge due to the reflection of surface-directed light by the cholesteric

layers.


117

Figure 7.10 Resultant spectra of an absorbance 1.01 waveguide (graycurve) and the same waveguide with a 720

nm cholesteric separated from the waveguide by an air gap (black curve) after subtraction of the spectrum of a

blank waveguide. In all cases, a rear white scattering layer was used. The region labeled I represents additional

absorption of in-scattered light, and region II represents dye emission with scattered light removed.

Figure 7.11 Edge emission relative to the output of a bare waveguide with blank halfwave plate using cholesteric

filters with the onset wavelength at 630 nm (stars), 650 nm (circles), 680 nm (triangles), 720 nm (squares),

and 760 nm (diamonds) as a function of waveguide absorbance. Resultant emission data integrated from 350 to

750 nm.

Region I Region II

Chapter 7

118

Figure 7.11 demonstrates that the cholesteric layer can enhance the amount of dye-emitted

light that reaches the waveguide edge by up to 35%, with the relative effect larger for

waveguides with a lower dye content. The cholesterics centered at longer wavelengths

demonstrated better results than those centered closer to the emission wavelengths of the dye.

For the waveguide with absorbance of 1.01, the cholesteric with an onset wavelength of 760

nm provided 12% more dye-emitted light energy to the waveguide edge. According to our

previous measurements, 40% of the energy initially absorbed by the dye in a waveguide with

this absorbance is lost through the surface. Thus, the 760 nm cholesteric converted 30% of

light normally lost into useful output in this sample.

Measurements of the absolute energy emissions from the waveguides presented in table

7.2 describe a similar story in that the cholesterics centered at longer wavelengths demonstrate

increased performance. The absolute additional output resulting from adding the cholesteric

layer makes up a smaller fraction of the total output, which includes the contribution of

scattered light. Normally, the fractional contribution of light obtained by using a scattering layer

decreases with increases in the waveguide size [173], and so it is anticipated that the relative

contribution of the cholesteric can increase with waveguide dimensions as the contribution

from scattered light decreases. [199]

One might have expected that the optimal reflection wavelength would be much closer to

the peak emission of the Red 305 dye (630 nm). Light leaving the surface of the waveguide will

have angles from 0°-90° with respect to the normal of the reflectors. There is a blue shift of the

centre of the reflection band of a cholesteric with a change in incident light angle. The resultant

central reflection wavelength ( ) for light incident from air is generally described by

10

sincos sin

n (7.6)

where is the angle of incident light in degrees, 0 is the central reflection wavelength at

normal incidence, and n is the average refractive index of the host LC. Furthermore, separate

measurements of modulated left-circularly polarized light incident on a bare half-wave plate as a

function of incident angle demonstrated that transmitted light was decreasingly right-circularly

polarized as the incident angle increased, and thus more elliptical in nature (data not shown).

This incomplete transition of left-circularly polarized light into right-circularly polarized light

translated into reduced performance of the cholesteric and, consequently, reduced the impact

of the cholesteric in these experiments.

From the spectra in figure 7.9 it can be seen that the width of the cholesteric reflection

band of the cholesteric reflectors is not broad enough to reflect all the surface emitted light

back into the waveguide. By broadening the reflection band more light will be reflected back

into the waveguide, since a broader part of the emission spectrum of the dye is reflected. The

effect of broadband reflectors is discussed in the next section.


119

7.4 Broadband reflectors

The bandwidth of a cholesteric reflector can be broadened in three ways: 1) use of

materials with a large birefringence in accordance with equation 7.4, 2) a gradient in the pitch

over the thickness of the film and 3) stack multiple narrowband reflectors with an offset in

reflection band. The reflective properties of broadband cholesteric reflectors made by these

three methods have been calculated and the effect of these reflectors on the efficiency of LSCs

has been determined from these calculations and the results have been compared to

experimental measurements. These calculations and measurements will be displayed and

discussed in the rest of this section.

7.4.1 Theoretical approach

The reflective properties of chiral nematic liquid crystalline films can be calculated using

Berreman’s 4x4 matrix method [229] for light propagation through multi-layered homogenous

anisotropic media. [230] This method allows calculation of the optical properties of

homogenous anisotropic media at oblique incidences. The cholesteric reflector is divided in

several homogeneous slabs with a different direction for the optical axis in each slab to

calculate the reflective properties of the reflectors.

7.4.1.1 Angular dependency of the optical behaviour of broadband cholesteric reflectors

Optical properties of narrowband and broadband cholesteric reflectors, made by three

different methods (described above) have been calculated. For the broadband reflectors made

from layering two narrowband reflectors, the ordinary and extraordinary refractive indexes were

taken from the commercial liquid crystal host BASF LC242, and the pitches of the separate

layers were chosen in such a way so that the overall reflection band was continuous and the

width of the resulting reflection band was 150-200 nm. For the gradient pitch reflectors, input

data was chosen from the materials described by Broer et al. [231] and the pitch gradient was

chosen in such a way that the reflection bands were similar to the reflectors made by layering

narrow band cholesterics, and the width of the reflection band was 400 nm. For the high

birefringent material, the characteristics of the liquid crystal BASF LC1057 were chosen. This

latter material leads to reflectors with a more narrow reflection band than the two other

methods. To match the width of the reflection band of the other two broadband reflectors,

liquid crystalline materials with very large birefringence would be necessary (i.e. a n of 0.4 to

create a 175 nm broad reflector with an onset wavelength around 600 nm), but these materials

are have very low photostability. Due to the lack of available materials to produce the desired

broadbands in this way experimentally, reflectors made from high birefringent materials are not

considered in the rest of this chapter.

The simulated reflection properties of the two broadband cholesteric reflectors (stacked

right/left and stacked right/right with a half wave plate between them) for unpolarized light for

all angles of incidence in air are depicted in figure 7.12.

Chapter 7

120

Figure 7.12 The simulated reflective properties of 150 nm broad reflectors made from gradient pitch cholesterics.

Left, the reflectivity of a full reflector made by stacking a right- and a left-handed cholesteric on top of each other

and right, the reflectivity of a full reflector made by right handed cholesterics on both sides of a half wave retarder

centered a 560 nm are used to make a full reflector. The color in these plot represents the reflectivity of the

cholesteric reflectors; dark blue is 0% reflection and dark red is 100% reflection.

Figure 7.12 demonstrates that the reflectivity of a reflector made from a stack of a right-

and a left handed cholesterics is 100% over the entire width of the reflection band when the

incidence light is at small angles (up to 20 degrees). When the angle of the incident light

becomes larger, the reflective properties begin to decrease starting from the edges of the

reflection band. The right picture shows that the reflectivity of two right handed cholesteric

reflectors on both sides of a half wave retarder centered at 560 nm is not as constant over the

width of the reflection band at small angles of incidence, because the half wave retarder is not

completely converting right circular polarized light into left circular polarized light at the

wavelengths where the reflector is positioned. When a half-wave retarder is used centered in the

same wavelength regime as the reflection band the reflective properties are similar to the

properties of a reflector made from stacked right- and left-handed cholesteric reflectors.

To verify the angular dependency of the cholesteric reflectors calculated using this method

a series of measurements was performed. The calculations, measurements and sample

preparations in the rest of this section (7.4.1.1) were performed by Chi-Wen Lin and Merijn

Giesbers. In figure 7.13 and 7.14 the transmission spectra of a broadband cholesteric reflector

made via a gradient pitch are displayed as calculated and experimentally measured are depicted

for different angles of incidence. In figure 7.13 the full reflectors are made by stacking a right

and left reflector on top of each other and in figure 7.14 a stack of two right handed reflectors

with a matched half wave retarder between them was used to create a full reflector. The stacked

films are optically connected by a refractive index matching oil.


121

Figure 7.13 Transmission spectra for unpolarized light for a right-handed cholesteric reflector stacked on top of a

left-handed cholesteric reflector. The pitch varies linearly from 437 to 520 nm in the right-handed material and

from 429 to 521 nm in the lefthanded material. The refractive indices, ne=1.68 and no=1.54, are the same for

both materials. Dotted lines indicate the experimental measurements, solid lines indicate calculations. Figures a)

to f) are for incident angles in glass of 0°, 10°, 20°, 30°, 40°, and 50°, respectively.

b)

c) d)

e) f)

a)

Chapter 7

122

Figure 7.14 Transmission spectra for unpolarized light for a configuration with a half-lambda plate sandwiched

between two right-handed cholesteric layers. The pitch varies linearly from 464 to 523 nm in the layers. The

refractive indices are ne=1.68 and no=1.54. The half-wave retarder has a center wavelength of 825 nm. Dotted

lines indicate the experimental measurements, solid lines indicate calculations. Figures a) to f) are for incident

angles in glass of 0°, 10°, 20°, 30°, 40°, and 50°, respectively.

c)

a) b)

d)

e) f)


123

To fit the calculations to the measurements, the pitches of the materials were chosen such that

the left and the right sides of the individual reflection bands superimposed. From this, the

spectra for the stack are calculated for all angles, using the known refraction indices on and en

assuming a linear pitch gradient within each CLC and a constant refractive index with changing

wavelength of the incident light.

The experimental and calculated transmission spectra for unpolarized light for both

broadband reflectors overlap. The blue shift in reflection band is similar for both experiments

and calculations when the angle of incidence increases. The small mismatch in reflection band

at perpendicular incidence can be explained by a small discrepancy between the (gradient) pitch

used for the calculations and the actual one in the experimental reflectors. At high angles of

incidence the reflector made from right handed cholesterics on both sides of a half wave

retarder show reduced reflectivity in measurements than predicted in the calculations. The

reason could be the quality of the experimental reflector.

This close match between calculations and experiment makes it possible to calculate the

reflection bands of multiple cholesteric layers and calculate what the effect of these layers on

the efficiency of the LSC. This is done in the following sections.

7.4.1.2 Efficiency of cholesteric reflectors

The wavelength selective mirrors are placed on top of an LSC to reflect photons normally

escaping through the top surface of the LSC back in the waveguide. Underneath the waveguide

a perfect reflector is placed reflecting all photons normally lost through the bottom of the

waveguide. These photons are assumed to not be re-absorbed by the luminophores and instead

lost again through the top surface. To calculate the increase in LSC efficiency after application

of such a reflector, it is necessary to first calculate the reflection efficiency (refl ) of the reflector

towards dye-emitted light (equation 7.7).

( ) ( ) ( , )

( ) ( )

s p

refl

s p

E E R d d

E E d d

(7.7)

where ( )sE is the emission spectrum of the dye molecules, ( , )R is the reflection spectrum

of the reflector as a function of incidence angle of illumination and ( )pE is the angular

emission profile of the dye molecules. The efficiency of reflectors with different reflection

bandwidths has been calculated for broadband reflectors made from cholesterics with a

gradient pitch. Two gradient pitch reflectors have been considered, one twice the width of the

reflection band of the narrowband reflectors (i.e. 175 nm) and one with a reflection bandwidth

of 400 nm, approximately 5 times the width of the reflection band of narrowband reflectors.

The efficiency of reflecting surface emitted light is depicted in figure 7.15 for an LSC using Red

Chapter 7

124

305 (see figure 5.4 for the absorption and emission spectra of this luminophore). In these

calculations the emission profile of the dyes is assumed isotropic. The onset wavelength is

defined as the wavelength at the short wavelength side of the reflection band where the

reflectivity is 50% of the reflectivity within the reflection band.

Figure 7.15 The efficiency of cholesterics in reflecting light emitted from the top surface ( refl ) of a Red 305

containing LSC for narrowband reflectors (white squares), 175 nm broad gradient pitch reflectors (grey squares),

400 nm broad gradient pitch cholesteric reflectors (black squares), layered reflectors made from 2 narrowbands

(filled red circles for stacked right and left handed reflectors and open red circles for stacked right handed reflectors

on both sides of a half wave retarder centered at 560 nm) as a function of the onset wavelength of the cholesteric

reflectors.

The efficiency of all the cholesteric reflectors is equal at longer onset wavelengths and the

efficiency increases with decreasing onset wavelengths. The efficiency peaks when the

cholesteric onset wavelength is in the spectral part of the emission by the luminophore. The

efficiency of reflector with the broadest reflection band (~400 nm) peaks at the shortest onset

wavelength, which can be explained as follows: all reflectors exhibit a blue shift for high angles,

but for the broad cholesteric reflector the complete emission band of the luminophore is

within the reflection band at all angles; so the highest efficiency is reached if the onset

wavelength of the reflector is close to the onset wavelength of the emission band. The

reflectors with the smaller reflection bands are not broad enough to reflect all the emitted light

over all angles being emitted from the surface, and so the efficiency of these reflectors peaks at

slightly longer wavelengths. The maximum efficiencies with corresponding onset wavelengths

are shown in table 7.3.


125

Table 7.3 Maximum reflection efficiency of the cholesteric for surface emitted light of LSCs containing Red 305

as luminophore.

Reflector Maximum efficiency Onset Wavelength (nm)

400 nm gradient pitch 91% 560

175 nm gradient pitch 65% 620

75 nm narrowband 53% 650

The 175 nm broad reflectors made from gradient pitch cholesterics and from layered

narrowband reflectors show similar efficiencies at all onset wavelengths. From this it can be

concluded that in the cases calculated the width of the reflection band is more important for

the efficiency than the method by which the reflectors are made.

7.4.1.3 Effect of cholesteric reflectors on incoming sunlight

Reflectors placed on top of the LSC may also reflect incoming sunlight away from the device,

resulting in reduced absorption if the reflection band coincides with the absorption spectrum of

the dye. This will influence the effect these reflectors have on the total efficiency of the LSC.

The fraction of light that is absorbed by the luminophore that passes through the cholesteric

filter (EA

cholf ) can be described by

( , )(1 ( , )) ( , )

( , ) ( , )

EAchol

I R A d df

I A d d

(7.8)

where ( , )I is the intensity of the incident light (in this case, the solar spectrum), ( , )A

is

the absorption spectrum of the dye corrected for the path length if light is coming in at larger

incident angles according to Lambert-Beer’s law and ( , )R is the reflection spectrum of the

cholesteric filter. In this calculation it is assumed that only direct sunlight is incident normal to

the LSC device, so there is no angular dependency. The results are depicted in figure 7.16.

When the onset reflection wavelength is outside the absorption range of the luminophore,

the reflectors transmit approximately 90% of all the absorbable light; the 10% loss results from

the fact the cholesteric reflector is added to the LSC with an air gap creating two additional

surfaces and extra Fresnel reflections. As the onset wavelength of the cholesteric reflectors

passes into the absorption range of the luminophores, the amount of absorbable incident light

that is transmitted through the reflector decreases drastically. This decrease is approximately

equal for all reflectors and is thus not influenced by reflection band width or production

method.

Chapter 7

126

Figure 7.16 The fraction ( EAcholf ) of the incoming sunlight that can be absorbed by the luminophore (Red 305)

that passes through the cholesteric reflectors made from narrowband reflectors (white squares), 175 nm broad

gradient pitch reflectors (grey squares), 400 nm broad gradient pitch cholesteric reflectors (black squares), layered

reflectors made from 2 narrow bands (filled red circles for stacked right and left handed reflectors and open red

circles for stacked right handed reflectors on both sides of a half wave retarder centered at 560 nm).

The reflectors with a 400 nm bandwidth can block all the incident light that will be absorbed by

the dye molecules. The bandwidth of the reflector with a 175 nm bandwidth and the

narrowband reflector are not broad enough to reflect away all the absorbable sunlight, so at

shorter onset wavelengths they transmit the long wavelength part of the incident spectrum that

can be absorbed by the dye molecules. Thus the fraction of the absorbable sunlight that is

transmitted through the reflectors with a short onset wavelength is larger for the narrow

bandwidth reflectors

7.4.1.4 Increase in LSC efficiency

The maximum total increase in LSC efficiency ( ,maxLSC ) is a combination of both incident and

emitted light reflection and can be calculated from the efficiency of the reflector and the

absorbable incident light that passes through the reflector. This increase can be described by

the number of photons leaving the edge of the LSC when a cholesteric filter is added ( ,edge choln )

and the number of photons leaving the edge of the LSC without a cholesteric filter ( ,edge baren ).

, , , ,

,max

, , ,

EAedge chol chol edge bare edge SL chol

LSC

edge bare edge bare edge bare

n f n n

n n n (7.9)


127

where , ,edge SL choln is the total number of photons formerly lost through the surface that are

converted to edge emission of the LSC due to addition of the cholesteric and defined as:

, , * **EAchol PLQYedge SL chol sl cholfn (7.10)

and

, * (1 )edge bare PLQY sln (7.11)

where PLQY is the fluorescence quantum yield of the luminophore, and sl is the fractions of

emitted photons lost through the surface (i.e. within the escape cone), respectively. Although a

calculation of ,maxLSC requires a detailed knowledge of the processes in the waveguide, a rough

estimate can be obtained in the following way. Combining equations 7.9, 7.10 and 7.11 leads to

,max 1

(1 )

EA slLSC chol chol

sl

f (7.12)

For a waveguide containing Red 305 it was previously shown in chapter 5 that the number

of photons in surface loss mode was approximately 50% of all absorbed photons for 5x5 cm2

LSCs made from polycarbonate and peak absorption above 0.3 leading to

,max 1EALSC chol cholf (7.13)

In this calculation, all the light reflected back in the LSC by the cholesteric reflector is

assumed to reach the edge of the LSC. So there is only one interaction with the reflector and

there is no re-absorption of these back reflected photons, losses from parasitic waveguide

absorption, or other such events. The results are depicted in figure 7.17.

With the broadening of the reflection band, the maximum possible increase in LSC

efficiency improves. The onset wavelength of the cholesteric where the efficiency increase is

the highest is red-shifted with respect to the emission peak of the luminophore, but for the

broadest reflector the red-shift is less pronounced than for the narrower reflectors. The

maximum possible increase in LSC efficiency and the corresponding onset wavelength of the

cholesteric are shown in table 7.4.

Thus, by adding a 400 nm broad reflector at the top of an LSC, the efficiency could be

increased by up to 66%. If a reflector with a more narrow reflection band is added to the top of

the LSC, increases of 45% or 35% could be achieved for 175 nm broad reflectors and 75 nm

broad reflectors, respectively.

Chapter 7

128

Figure 7.17 The calculated maximum relative LSC efficiency ( ,maxLSC ) after application of cholesteric

reflectors to an LSC containing Red 305 as a luminophore. The reflectors are made from narrowband

cholesterics (white squares), 175 nm broad (grey squares) and 400 nm broad gradient pitch cholesteric (black

squares), layered cholesteric and reflectors made from 2 narrowbands (red circles for stacked right and left handed

reflectors and open red circles for stacked right handed reflectors on both sides of a half wave retarder centered at

560 nm).

As described in the experimental section, 175 nm broad cholesteric reflectors were made from

right-handed layered narrowband reflectors on either side of a half wave retarder centered at

560 nm and placed on top of Red 305 LSCs. The results of these measurements and a

comparison with the theoretical results described in this paragraph are shown and discussed in

the next section.

Table 7.4 Maximum calculated increase in LSC efficiency after addition of the cholesteric reflectors to LSCs

containing Red 305 as luminophore

Reflector ,maxLSC Onset wavelength (nm)

400 nm gradient pitch 1.66 620

175 nm gradient pitch 1.45 650

75 nm narrowband 1.35 650


129

7.4.2 Experimental results

Seven broadband reflectors were produced with different onset wavelengths: 620 nm, 660

nm, 700 nm, 740 nm, 780 nm, 820 nm and 880 nm. The reflection bands of these cholesterics

were measured and compared to the calculated reflection bands at perpendicular incidence. The

differences between the theoretical and experimental spectra are similar for all the different

reflectors. As an example, the calculated and experimental spectra of the reflector with an onset

wavelength of 740 nm are depicted in figure 7.18.

500 600 700 800 900 1000

0,0

0,2

0,4

0,6

0,8

1,0

Re

fle

ctio

n (

%)

Wavelength (nm)

Figure 7.18 Reflection spectra of a broadband reflector with an onset wavelength of 730 nm made from 2 layered

right handed narrowband reflectors on both sides of a half wave retarder centered at 560 nm, both experimental

(black) and calculated (gray).

The experimental spectrum shows that the width of the reflection band is the same as

calculated, but the reflectivity is somewhat lower. This reduction can be a result of reduced

layer thickness in the manufactured reflector than assumed in the theoretical calculations.

Furthermore, the first layer applied experimentally is treated with a plasma asher, which can

further reduce the layer thickness. Therefore, reflectivity on the longer wavelength side of the

reflection band is slightly lower than on the short wavelength side, since the layer with the

reflection band at the longer wavelength side of the reflection band was applied first in all cases.

These reduced reflective properties also cause the dip in the middle of the reflection spectrum.

The theoretical spectrum demonstrates less than 100% reflectivity caused by the use of the

slightly mismatched half wave retarders which do not completely convert left circularly

polarized light passing through the right handed cholesterics into right circularly polarized light,

Chapter 7

130

so the right handed cholesterics on the back side will not be capable of reflecting all remaining

transmitted light.

The cholesteric reflectors were placed on top of (poly)carbonate LSCs filled with Red 305,

and the edge emission spectra and intensity (in Watts) measured using an integrating sphere

under illumination with a solar simulator (AM 1.5). Underneath the samples, a white

Lambertian scatterer was placed. These measurements were compared with the edge output of

the same LSC with the white scatterer but without the cholesteric reflector. The ratio between

the two edge output intensities is plotted in figure 7.19.

Figure 7.19 Experimental relative LSC efficiency after application of broadband reflectors. The LSCs contain

Red 305 with different peak absorbance: calculated (black), 0.05 (green), 0.19 (light blue), 0.46 (red), 1.01

(yellow), 1.63 (blue), 2.36 (orange), and >4 (navy).

Application of cholesteric reflectors to an LSC with a peak absorbance of approximately

0.5 increased the optical efficiency by a maximum of 4.5% when the reflector with the onset

wavelength of 700 nm is added: At higher peak absorbance there is a small decrease in LSC

efficiency. At lower peak absorption (<0.1) the increase in efficiency is much higher, with a

peak increase in efficiency of 30% at 740 nm. However, this is lower than was calculated, and

the onset wavelength of the reflector with the maximum increase is red shifted in comparison

to the calculations.

There are several differences between the theoretical approach and the experimental

measurements. First, the reflectivity of the experimental reflectors is not as good as calculated

in theory. Second, the theoretical approach assumed that all light reflected back into the LSC by

the reflectors reaches the edge of the LSC. In the experiments this is not the case. Photons

reflected back into the LSC can be reabsorbed by the luminophores in the LSC due to the


131

overlap in absorption and emission spectra. Reflected photons not immediately absorbed will

not be in waveguide mode, so they will encounter the white scatterer underneath the sample,

potentially multiple times. If the reflectors have non-unity reflectivity it can result in additional

losses. Finally, the emission profile is assumed spherical for the calculations. In actual practice,

the emission profile will not be spherical due to dichroic absorption and emission of the

luminophore in combination with the collimated incident light (see chapter 3). However,

calculations using a non-spherical emission profiles show only small differences with the

calculations using spherical emission profiles. Since the LSC with the very low peak absorbance

shows a higher increase in efficiency after application of the cholesteric reflectors, photon

recycling is the primary reason behind the lower experimental increase than predicted.

Figure 7.20 The optical efficiency (opt ) after application of broadband reflectors on LSCs containing Red 305

with different peak absorbance: 0.05 (green), 0.19 (light blue), 0.46 (red), 1.01 (yellow), 1.63 (blue), 2.36

(orange), and >4 (navy). The lines denote the edge emission from the LSC without the cholesteric reflector.

In figure 7.20 the optical efficiency of the LSCs is plotted for LSC with (data points) and

without (lines) cholesteric reflectors. As is clearly show the effect of the cholesteric reflectors

on the optical efficiency is minor for LSCs with a low peak absorbance. As already mentioned

before for LSC with a high peak absorbance the efficiency of the LSC is decreased after

application of a broad bandwidth reflector.

7.4.3 Patterned waveguides

Tsoi et al. [72,105] demonstrated that producing an LSC via patterning a coating containing the

luminophore on a clear waveguide can reduce the amount of re-absorptions. To investigate the

effect of photon recycling on our previous measurements, we measured the increase in

Chapter 7

132

efficiency of the patterned-LSCs after application of the cholesteric reflectors. LSCs with a

pattern of 10 lines were used, where the widths of the lines determined the coverage of the dye

coating on the LSC. The results of these measurements are shown in figure 7.21 using a dye

coating with peak absorbance of approximately 0.6. The work presented in this section (7.4.3) is

performed in collaboration with Shufen Tsoi and the patterned waveguides were produced by

her.

Figure 7.21 Increase in patterned LSC efficiency after application of broadband reflectors. The LSCs are topped

with a coating containing Red 305 with a peak absorbance of 1.0 with different pattern coverage of the surface:

calculated (black), 20% (green), 30% (light blue), 50% (red), 70% (yellow), 100% (blue)

The sample with surface coverage of 100% shows the same results as a filled waveguide with

approximately the same peak absorbance when topped by a broadband reflector, an increase in

efficiency of 5%. Reducing the surface coverage of the coating containing the luminophore, and

thus the amount of photon recycling for emitted light, enhanced the impact of the cholesteric

reflectors on the LSC efficiency. The lower the coverage, the higher the increase in efficiency

achieved by applying the cholesteric reflectors for all reflectors sampled. The LSC with

coverage of 20% shows an increase in edge emission efficiency of up to 27%, although this is

still lower than calculated.

Furthermore, in figure 7.21 it can be seen that the application of reflectors with a longer

onset wavelength results in an increase in LSC efficiency approaching the theoretical increase

better than with a shorter onset wavelength reflectors. This could also be explained if photon

recycling is the main cause of the reduced effectiveness of cholesteric reflectors on the

experimental increase of LSC-efficiency. Re-absorption occurs mostly in the short wavelength


133

part of the emission spectrum of the dye, since the absorption in this region is the greatest.

Cholesteric reflectors with an onset wavelength close to the onset wavelength of the emission

of the dye will reflect this emitted light for most of the angles of incidence. The reflectors with

a longer onset wavelength will only reflect these photons that have a larger probability for being

re-absorbed at larger angles. Thus, the effect of photon recycling in LSCs using short onset

wavelength reflectors is larger than for longer onset wavelength reflectors. These results

demonstrate that photon recycling has a large influence on the effectiveness of the reflectors.

Since the experimental increase in the LSC with a low amount of re-absorption after application

of the cholesteric reflectors is still lower than calculated for all applied reflectors, it can be

assumed that the multiple interactions of the back reflected photons with the reflectors also

have an influence on the effectiveness of the cholesteric.

Figure 7.22 Optical efficiency(opt ) after application of broadband reflectors on LSCs topped with a coating

containing Red 305 with a peak absorbance of 0.6 with different pattern coverage of the surface: 20% (green),

30% (light blue), 50% (red), 70% (yellow), 100% (blue). The lines denote the edge emission from the LSC

without the cholesteric reflector.

In figure 7.22 the optical efficiency of the patterned LSC with (data points) and without (lines)

is plotted. The effect of the cholesteric reflector on the optical efficiency is larger for these

patterned waveguides (4.8% to 6.0% for the 20% covered LSC) compared to the fully covered

waveguide (10.9% to 11.2%).

7.5 Other luminophores

The results presented in the previous sections of this chapter only describe LSCs with Red 305

as a luminophore. One of the advantages of LSCs is the freedom in choice of colour. A change

Chapter 7

134

of luminophore will also change the specifications of the cholesteric reflectors needed to

maximize the increase in LSC efficiency, and is also determined by the fraction of the photons

that is emitted through the surface of the LSC. In this section, the maximum increase in LSC

efficiency is calculated for different luminophores.

In chapter 5 the fraction of the absorbed photons emitted trough the surface of the LSC is

determined for two other dyes beside Red 305, the perinone dye and the coumarin dye. The

fractions of emitted photons that are emitted in surface loss mode (sl ) for the perinone and

coumarin dyes are 0.325 and 0.46 respectively. The absorption and emission spectra of the

coumarin and perinone are displayed in figure 5.8. Furthermore, a dye that consists of a rare

element core ion with an organic ligand that has no overlap between the absorption and

emission band and which is an isotropic emitter is considered. The fraction of the emitted

photons in surface loss mode for this latter dye in PC as a waveguide is assumed to be 0.23. In

table 7.5 the calculated maximum relative LSC efficiency after application of 400 nm gradient

pitch and 175 nm broad layered cholesteric reflectors and the onset wavelength of the reflector

are displayed.

Table 7.5 Calculated maximum relative efficiency ( ,maxLSC ) of LSCs containing the coumarin, perinone and

rare earth complex as luminophore after application of 400 nm gradient pitch and 175 nm layered narrowband

cholesteric reflectors

Luminophore

sl

400 nm gradient pitch 175 nm layered

,maxLSC

Onset

wavelength

(nm)

Relative LSC

efficiency

Onset

wavelength

(nm)

Coumarin 0.46 1.498 520 1.434 520

Perinone 0.325 1.259 700 1.175 740

Rare earth

complex

0.23 1.163 650 1.121 700

Since the fraction of the emitted photon that are lost through the LSC surface is smaller

for the dyes presented in table 7.5 than for Red 305, the maximum relative LSC efficiency after

application of the cholesteric reflectors is expected to be significantly lower. It also can be seen

that the difference in maximum increase in LSC efficiency between using the 400 nm and the

175 nm broad reflectors for the LSC containing the coumarin dye is smaller than for all the

other luminophores. This is a result from the more narrow spectral width of the emission band

of the coumarin dye.


135

7.6 Conclusions

The addition of narrowband selectively-reflective cholesteric layers to the top of the waveguide

of a LSC containing Red 305 as luminophore using an air gap both reduces the loss of light

from the surface of the waveguide and increases the light output at the edge up to 12%, which

is a recovery of more than 30% of what is normally lost from the waveguide surface. The

spectral width of these reflectors is not broad enough to reflect all the surface emissions back

into the waveguide.

400 nm broad reflectors made from polymeric cholesteric liquid crystalline films can

theoretically reflect over 90% of all surface emitted photons back into an LSC containing Red

305 as a luminophore. However, these cholesterics reflect away a part of the absorbable

incident light if the spectral position of the reflector is matched to this maximum efficiency.

Calculations suggest the 400 nm broad reflector could increase LSC edge emission efficiency up

to 66%. Reflectors with a more narrow reflection band have reduced effect on the increase of

the LSC efficiency: 175 nm broad reflectors could increase this efficiency around 45%.

Experiments demonstrate that applying a 175 nm broad reflector to an LSC with Red 305 as a

luminophore and a peak absorbance of ~0.5 increases the efficiency of the actual LSC by only

5%. The main reason for this discrepancy is the photon recycling of the back reflected photons.

When the amount of photon recycling is decreased by lowering the peak absorption to a value

below 0.1, the relative increase in LSC efficiency becomes nearly 30%; similarly, reducing the

probability of photon recycling by reducing the coverage of dye coating increases the relative

LSC efficiency 27%. These experiments demonstrate reducing the recycling of back reflected

photons increases the effectiveness of the cholesteric reflectors.

The reflector utilized needs to match the luminophores used. The maximum performance

increase is determined by the fraction of photons emitted in surface loss mode. More

reproducible, higher-quality reflectors coupled with a reduction in the amount of photon

recycling could make the cholesteric reflectors very effective in enhancing LSC performance.

Reducing the amount of photon recycling, could be achieved by placing lenses on top of the

patterned waveguides [106] or using luminophores with no or low overlap between the

absorption and emission band, like complexes of rare-earth ions with organic ligands, [155]

quantum dots, [83] or phosphors. [82]

8 Wavelength selective reflectors

and indirect sunlight6


P.P.C. Verbunt, M.G. Debije, D.J. Broer, C.W.M. Bastiaansen and D.K.G. de Boer, "Organic

wavelength selective mirrors for luminescent solar concentrators", Proc. SPIE 8438, 843805

(2012)

Chapter 8

138

8.1 Influence of the angle of incident light on the

performance of cholesteric reflectors

In chapter 7 it was shown that organic wavelength selective mirrors made from cholesteric

polymers show an angularly dependent reflection. In the calculations, the assumption was made

that the incident sunlight always encountered the LSC normal to the surface. In reality, the

LSC is a fixed device and the position of the sun changes during the day; additionally, clouds,

surfaces like buildings, cars, trees, and other common objects will scatter sunlight and result in a

mixed environment of both direct and indirect light. In this chapter the influence of this

indirect sunlight and the angular dependence of the direct sunlight on the effectiveness of these

organic wavelength selective mirrors used in conjunction with LSC devices will be calculated. In

the second part of this chapter a solution for the angular dependent reflection band of the

organic wavelength-selective reflectors is described

Previously we calculated the effect a cholesteric reflector on the output of an LSC

containing Red 305 as a luminophore when placed on top of the LSC waveguide. The

efficiency of the LSC could be increased by 66% if all emitted photons that were reflected back

into the LSC by the cholesteric reflector could be redirected to leave the edge(s) of the LSC

without additional losses. These calculations assumed incident sunlight reaches the device

normal to the surface. This is, of course, almost never the real situation. LSCs are designed to

be static devices, and with the sun travelling along the horizon, the angular dependence of these

mirrors becomes important.

8.1.1 Influence of the angle of the incident light on the light that

passes through the reflector

At larger incidence angles, the reflection band of the mirrors shifts to shorter wavelengths and

often into the spectral absorption band of the luminophore, resulting in the rejection of useful

incident light. The angular dependence of the absorbable sunlight that passes through the

mirror for cholesterics with a reflection bandwidth of 400 nm, 175 nm and 75 nm were

calculated using equation 7.8 for LSCs using Red 305 as luminophore. The spectral reflectivity

of the mirrors was chosen on the basis of the results of the previous chapter, so the onset

wavelength of the mirrors is 620 nm, 650 nm, and 650 nm respectively. The results of the

calculations are shown in figure 8.1.

Wavelength selective reflectors and indirect sunlight

139

Figure 8.1 The calculated fraction of the absorbable incident sunlight for an LSC waveguide containing Red305

that passes through the cholesteric reflector for 400 nm (black), 175 nm (dark grey) and 75 nm (light gray)

reflectors as a function of incidence angle.

At incidence angles up to approximately 30-40 degrees there is almost no change in the

absorbable light that passes through all the filters, although the broader filter with a shorter

onset wavelength starts to reflect absorbable sunlight away at smaller angles in comparison to

the reflectors with more narrow reflection bands. If the angle of incidence is increased beyond

30-40 degrees, the reflectors start rejecting a significant fraction of sunlight that can normally be

absorbed by the dye. The reflection continues to increase with increasing incidence angle until

almost no light is passing through the mirror. The 75 nm reflector shows a slightly different

trend. Around 50 degrees the useful amount of light that passes through the reflector appears

to stabilize as the reflection band is not broad enough to reflect all absorbable light away and at

higher angles the longest wavelengths of light that can be absorbed by the dye will again be able

to pass through the reflector. At very high angles the amount of useful light that passes through

the narrowband reflector decreases again, caused by the increased Fresnel surface reflections.

This latter effect is similar for all the reflectors.

8.1.2 Dependence of maximum possible LSC efficiency with change

of the angle of the incident light

If useful light is rejected by the cholesteric reflectors, less light is available to be absorbed,

leading to less light that eventually leaves the edge of the LSC. The increase in the efficiency of

the LSC by addition of a cholesteric reflector was calculated as a function of the incoming angle

Chapter 8

140

according to the method described in the previous chapter. The efficiency of reflection of

surface emitted photons is displayed in table 8.1.

Table 8.1 Efficiency of the cholesteric reflectors towards isotropically surface emitted photons of an LSC

containing Red 305 as a luminophore.

Width of the reflector chol

400 0.87

175 0.65

75 0.53

Figure 8.2 The maximal increase in efficiency for LSCs containing Red 305 dye after application of organic

wavelength selective mirrors as a function of angle of incident light for bandwidth 400 nm (black), 175 nm

(dark grey) and 75 nm (light gray) reflectors.

The maximum achievable increase in LSC-efficiency as function of the angle of incidence

is depicted in figure 8.2. As the incidence angle becomes larger than 30-40 degrees, the

enhancement in LSC efficiency caused by the cholesteric begins to decrease due to a reduction

in useful light that passes through the reflector. When the angle of the incidence light reaches

45 degrees for the narrowband reflectors and ~50 degrees for the two broadband reflectors, the

LSC is emitting less light from the edges than when no reflector is applied. At very high angles

it appears that all light that is normally emitted from the LSC edge is lost when the cholesteric

reflectors are applied. In reality, even a bare LSC does not have much light leaving the edges

since most incident light is reflected away at the LSC surface due to Fresnel reflections. These


141

latter reflections have not been taken into account in these calculations since they are equal for

systems both with and without the reflectors (which are applied to the LSC with an air gap).

8.1.3 Indirect sunlight

To this point we have only considered the incoming sunlight to be direct, with uniform incident

angle. LSCs applied in the built environment will almost never be exposed purely to direct

sunlight. The distribution of the incoming sunlight is very different each day of the year, at

each place on earth. It is thus difficult to define a general incident light distribution for use in a

simulation, although attempts have been made to represent them with distribution equations,

for example by Hopkins[232]. Such models require inputs that vary for each weather condition,

date and location. To gain some insight into the influence of the distribution of the incidence

angles on the performance of the cholesteric-topped LSCs, the increase in LSC efficiency after

application of the cholesterics using four different distributions of incident light is calculated.

The first distribution has already been described: direct light incident at a single angle (normal

to the device). A second assumes a totally isotropic distribution of incident light, approximating

days that are completely cloudy in an urban environment. Two distributions of incident light

falling between these extremes were also selected, described by cos (Lambertian) and

2cos . These last two distribution functions are chosen somewhat arbitrarily and only vaguely

represent the actual distribution functions of incoming sunlight on semi-cloudy days, but allow

to gain insight into the effect of angular distribution of the incident light on the effectiveness of

cholesteric reflectors. Calculations were performed using cholesteric reflectors placed on top of

an LSC and illuminated with these four distributions of incidence light, each centred around the

direction normal to the LSC surface. Sunlight that encounters the LSC under larger angles has

a greater chance of being absorbed than light entering the device perpendicularly, as the light

must traverse a longer pathlength through the LSC. Using Lambert-Beer, one may derive the

angular dependent absorption:

0

cos

AA (8.1)

where A is the absorption at angle and 0A is the absorption at normal incidence. At

higher angles where the absorption is larger, the cholesteric reflectors potentially reflect away a

significant fraction of the useful incident light and the Fresnel reflections from the LSC surface

increase as well decreasing the amount of light entering the waveguide. All these events

influence the LSC efficiency after application of a cholesteric reflector, the results are shown in

table 8.2.

Chapter 8

142

Table 8.2 Calculated relative efficienc of LSCs containing Red305 after application of organic wavelength

selective reflectors with different reflection bandwidth for direct (normal to the device) and indirect incident

sunlight. Output of a bare LSC under identical lightning conditions is given as 1.

Distribution of the

incoming light

400 nm broad

reflector

175 nm broad

reflector

75 nm broad

Reflector

Direct 1.66 1.45 1.35

2cos 1.29 1.22 1.29

cos 1.09 1.06 0.90

Isotropic 0.70 0.72 0.43

As can be seen in table 8.2 the increase in LSC efficiency after application of a cholesteric

reflector decreases with increasing fraction of indirect light approaching the device. For all

three bandwidths of reflection the LSC-efficiency is decreased for isotropic incident light when

a cholesteric reflector is applied.

In the urban environment, part of the sky can be blocked by a number of objects, for

instance buildings and trees. Therefore, the amount of light reaching the Earth at ground level

at very large incidence angles can be low. To simulate these conditions the same distribution

functions described above where used, but only the angles between 0 and 30 degrees or

between 0 and 45 degrees were considered and the results are displayed in table 8.3.

Table 8.3 Calculated increase in efficiency of LSCs containing Red305 after application of organic wavelength

selective reflectors with different reflection bandwidth for indirect incident sunlight with incidence angles constricted

to 0-45 degrees and 0-30 degrees.

Distribution of the

incoming light

400 nm broad

reflector

175 nm broad

reflector

75 nm broad

Reflector

0-45

degrees

0-30

degrees

0-45

degrees

0-30

degrees

0-45

degrees

0-30

degrees

2cos 1.50 1.60 1.39 1.43 1.36 1.36

cos 1.49 1.60 1.39 1.43 1.21 1.30

Isotropic 1.47 1.59 1.38 1.43 1.07 1.23

If the angles of the scattered incident light are limited to only a 30° or 45° cone, the

increase in LSC efficiency is mostly comparable to that of direct incident light. The 75 nm

reflectors seem to suffer the most from the indirect light in comparison to the broader

reflectors. If a 30° limitation for the incident light is considered, the decrease in relative LSC-

efficiency is minimal. These results show that incident light with angles larger than 30° cause

the decrease in effectiveness of the cholesteric reflectors on top of an LSC.


143

8.2 Special dispersion cholesteric reflectors

The blue shift in reflection band of cholesteric reflectors causes a decrease in effectiveness of

the reflectors on top of an LSC, when illuminated with indirect sunlight. A large fraction of the

incident light than could be absorbed by the dye molecules is reflected from the device at larger

angles. This loss can be reduced if the reflector would have a reduced angular dependency.

8.2.1 Introduction

The reflective properties of cholesteric reflectors are a result of the birefringence of the LC-

material. This birefringence forms a periodic change in refractive index (for linearly polarized

light) when in a helical structure causing the material to act as a Bragg reflector. When

birefringence is absent, the reflector is transparent to the incident light.

The ordinary and extraordinary refractive indices of the host liquid crystals used to form

the cholesterics have been considered constant over all wavelengths up to now. As can be seen

in chapter 7, this assumption was reasonably valid for most reflectors, but in reality the

refractive indices are dependent on the wavelength of the light, especially at short wavelengths

where the change in refractive index becomes more pronounced. The wavelength dependence

of the refractive index of a material (called dispersion) is described by Cauchy’s equation:

1 20 2 4

......B B

n n

(8.2)

where n is the refractive index at a specific wavelength, 0n is a null-refractive index and

1 2, ,B B etc are coefficients. In general only the first term ( 1

2

B

) is considered, and usually is a

good approximation. The dispersion is different for both the ordinary and the extraordinary

components of the refractive index, leading to a change of birefringence with changing

wavelength of the incident light, but this variation is generally small for the materials considered

in the previous chapters.

There are materials that have a large difference between the dispersion of the ordinary and

the extraordinary refractive indices. This difference in dispersion can result in the condition that

at a specific wavelength both the ordinary and the extraordinary refractive indices are equal and

there will be no birefringence, while at shorter and longer wavelengths this material will

demonstrate birefringence. If there would be a cholesteric phase of this material the reflection

band would disappear at the wavelength where there is no birefringence, since the reflective

properties of a cholesteric reflector are based on the birefringent properties of the LC-material.

[233]

The properties of this type of material (called in this chapter ‘special dispersion material’)

has been calculated using the Berreman’s 4x4 method, [229] as in chapter 7. The extraordinary

refractive index is kept constant at 1.68, while the ordinary refractive index was chosen to

display dispersion. The dispersion was chosen in such a way that the birefringence equals zero

Chapter 8

144

at 575, 600 or 625 nm. There are two variables in equation 8.2 that determine the dispersion,

the value of the null refractive index and the first coefficient ( 1B ). Both these variables have

been varied to manipulate the dispersion of the ordinary refractive index such that the

birefringence equals zero at the desired wavelengths. The input data for the ordinary refractive

index are given in table 8.4 and depicted in figure 8.3.

Table 8.4 Dispersion parameters of the ordinary refractive index special dispersion material. (graphical

representations of the dispersion of the ordinary refractive index are indicated in the brackets which refer to figure

8.3)

Null refractive

index 1B Birefringence zero at:

1.403 0.0916 575 nm (a, black)

1.426 0.0916 600 nm (a, dark grey)

1.446 0.0916 625 nm (a, light grey)

1.377 0.1002 575 nm (b, black)

1.377 0.1097 600 nm (b, dark grey)

1.377 0.1184 625 nm (b, light grey)

Figure 8.3 Dispersion of the refractive indices. The extra ordinary refractive index (black dotted) is constant

with wavelength, while the ordinary refractive index changes. In a) the dispersion is varied by varrying the null

refractive index and in b) the dispersion is varried by changing parameter 1B .

The reflective properties of the cholesteric phase formed by these materials are calculated

according to the method mentioned in chapter 7 for 75 nm, 175 nm and 400 nm broad

reflectors with an onset wavelength of 620 nm, 620 nm and 650 nm respectively, similar as the

reflectors used in section 8.1. An example of angular dependent reflective properties from 400

nm broad reflector is depicted in figure 8.4.

a) b)


145

Figure 8.4 Example depictions of the reflective properties of a cholesteric reflector made from a special dispersion

LC material, with the birefringence set at 0 at 575 (b) nm and 625 (a) nm. The reflectivity is scaled from 0%

(dark blue) - 100% (dark red)

The difference in reflective properties between the special dispersion cholesteric (SDC)

reflectors with the same wavelength of zero birefringence, using different dispersion parameters

is relatively small. Therefore, in the rest of this chapter, only the special dispersion materials

depicted in figure 8.3a are considered. In figure 8.4b it can be seen that at, an additional

reflection band appears at the short wavelength side of the reflector, if the blue shift at larger

angles passes the wavelength where birefringence is zero.

8.2.2 Transmission of sunlight

The angular dependence of the absorbable sunlight that passes through SDCs with a reflection

bandwidth of 400 nm, 175 nm and 75 nm were calculated using equation 7.8. The results of

the calculations are shown in figure 8.5.

Due to the reduced angular dependence of the reflective properties of the SDC materials.

the light that can be absorbed by the dye molecules that passes through the filter is constant

with increasing angle until 70° for all three widths of the reflection band. At higher angles a

large fraction of the absorbable incident light is still reflected by the cholesteric filters, but this

is a result of Fresnel reflections. For all dispersion parameters and for the three widths of the

reflection band the results have similar trends. At angles larger than 30° these SDC filters

transmit more light that can be absorbed by the dye molecules than the regular cholesteric

reflectors.

Chapter 8

146

Figure 8.5 Transmittance of incident light that can be absorbed by Red 305 through a regular cholesteric (dotted

line) reflector or special dispersion cholesteric reflectors with birefringence equal zero at 575 nm (black), 600 nm

(dark grey), or 625 nm (light grey). The cholesteric reflectors have a bandwith at normal incidence of 400 nm

(a), 175 nm (b), or 75 nm (c) and the onset wavelengths is 620 nm for the reflector with a 400 nm bandwith

and 650 nm for the reflectors with a 175 nm or a 75 nm bandwidth .

8.2.3 Efficiency towards surface emitted light

The change in angular dependence of the reflection band will also have an influence on the

fraction of surface emitted photons that are reflected by the SDC layers. The efficiency of the

reflectors towards surface emitted photons is calculated according to equation 7.7 and the

results are depicted in table 8.5.

a) b)

c)


147

Table 8.5 The efficiency of the special dispersion cholesteric reflectors towards surface emitted light from an LSC

with Red 305 as a luminophore. Reflection of all surface emitted photons is given as 1.

Birefringence equals

zero at

Width of the reflection band

400 nm 175 nm 75 nm

575 nm 0.59 0.44 0.36

600 nm 0.48 0.39 0.32

625 nm 0.40 0.34 0.28

Independent of the width of the reflection band the efficiency of the SDC materials

decreases with increasing wavelength where the birefringence is zero. 625 nm is exactly the

wavelength where the emission of the dye peaks. This could be the reason why zero

birefringence at this wavelength results in minimal reflection efficiency, since all the emitted

photons with a shorter wavelength will not be reflected by the cholesteric filter. The efficiency

of all these SDC filters is lower than the same reflector made from regular nematic liquid

crystalline materials as a result of a reduced reflectivity at wavelengths close to the wavelength

where the birefringence is zero. This reduced reflectivity can be seen in figure 8.4.

8.2.4 Angular dependent increase in LSC efficiency

Increased transmission of absorbable sunlight incident on the device at higher angles along with

reduced efficiency towards surface emitted light of the SDCs compared to a regular cholesteric

reflector will have a major influence on the angular dependence of effectiveness of these

reflectors on top of an LSC waveguide. The angular dependence of the maximum relative LSC

efficiency after application of a special dispersion cholesteric is calculated and displayed in

figure 8.6.

If light incident only normal to the device encounters the cholesteric reflector, increase in

LSC efficiency is 20-40% if the 400nm broad SDCs are applied; compare this to a regular

cholesteric which increases the LSC efficiency by 66%. This drop in efficiency enhancement is

a result of the reduced amount of surface emitted light reflected by the SDC reflectors. The

enhancement in LSC efficiency decreases with increasing incidence angle for regular

cholesterics when light has an angle larger than ~30°. For the SDCs the increase in LSC

efficiency only decreases at incidence angles larger than ~70°. This results in an overall better

performance of the LSC with the SDCs in comparison to the regular cholesterics at incidence

angles above ~40°. The regular 400 nm broad reflector increase the LSC efficiency up to ~50°,

while the special dispersion cholesteric will increase the LSC efficiency up to 65-75°. At larger

angles the amount of absorbed light in the LSC decreases drastically due to an increase of the

Fresnel at these very large angles, so any decrease in LSC performance at those angles does not

influence the overall performance.

Chapter 8

148

Figure 8.6 The relative efficiency of an LSC containing Red 305 after application of a regular cholesteric

(dotted line) reflector or special dispersion cholesteric reflectors with birefringence equal zero at 575 nm (black),

600 nm (dark grey), or 625 nm (light grey). The cholesteric reflectors have a bandwith at normal incidence of

400 nm (a), 175 nm (b), or 75 nm (c).

The results of the 175 nm (figure 8.6b) and the 75 nm (figure 8.6c) broad reflectors show

similar trends to the results of the 400 nm broad reflectors described above. The largest angles

of incidence at which the cholesteric reflectors will still increase the efficiency of the LSC when

applied are shown in table 8.6

Table 8.6 The largest angle of incidence where the cholesteric filters still increase the efficiency of the LSC

Birefringence zero at Broadness of the reflection band

400 nm 175 nm 75 nm

Regular 48° 50° 36°

575 nm 74° 70° 67°

600 nm 72° 70° 67°

625 nm 67° 68° 64°

a) b)

c)


149

The results shown above demonstrate indirect sunlight will have a major influence on the

effectiveness of these cholesteric reflectors, so the relative LSC efficiency after application of

these SDC reflectors for incident light with an isotropic, cos (Lambertian) and 2cos

distribution were calculated as well. The results are depicted in figure 8.7.

Figure 8.7 The relative efficiency of an LSC containing Red 305 after application of a regular cholesteric (white)

reflector and special dispersion cholesteric reflectors with birefringence equal zero at 575 nm (black), 600 nm

(dark grey), or 625 nm (light grey) is illuminated with indirect incident light. The distributions of the incoming

light are isotropic, cos , 2cos or direct, where is with respect to the normal of the reflector.

For all three distributions of indirect sunlight the special dispersion filters show an

increase effect in comparison to the normal cholesteric reflector, except for the 75 nm broad

reflector illuminated with incident light that has a 2cos distribution. It is also clear that the

filters with zero birefringence at 575 show higher relative LSC efficiency than the other SDC

reflectors. The 400 nm broad reflector with zero birefringence at 575 nm shows an increase in

LSC efficiency even for completely isotropic incident light, resulting in a higher LSC efficiency

if applied under any weather condition. The results of the 400nm broad reflectors from figure

8.7 are also displayed in table 8.7.

a) b)

c)

Chapter 8

150

Table 8.7 The relative efficiency of an LSC containing Red 305 as a luminophore after application of a

wavelength selective reflector made from special and regular dispersion nematic liquid crystals with birefringence

equal zero at 575 nm, 600 nm, or 625 nm is illuminated with indirect incident light. The distributions of the

incoming light are isotropic, cos , 2cos or direct, where is with respect to the normal of the reflector.

The output of a bare LSC under identical conditions is given as 1 in each case.

Zero

birefringence at:

Direct 2cos cos Isotropic

Regular 1.663 1.285 1.089 0.702

575 nm 1.398 1.347 1.272 1.010

600 nm 1.281 1.250 1.182 0.938

625 nm 1.213 1.177 1.108 0.872

8.3 Conclusions

Calculations performed in this chapter imply that sunlight illuminating an LSC device with

incident angles larger than 30 degrees negatively impacts the effectiveness of cholesteric

reflectors on top of the LSC. At these larger incidence angles, a fraction of the sunlight that

normally can be absorbed by the dye is instead reflected away because of the blue-shift in

reflection band of the cholesteric reflector. However, the cholesteric remains effective in

improving overall LSC performance for incident angles up to 45-50 degrees. At increasingly

larger angles, the increased amount of incoming light that is reflected away negatively impacts

LSC performance. Sunlight that reaches the earth is never completely direct. Clouds and objects

like trees and buildings scatter, block and/or reflect sunlight leading to indirect sunlight. In

these conditions, the cholesteric shows a more subdued performance. In practice, careful

consideration as to the expected light quality should be made before the decision is made

whether to apply cholesteric reflectors.

The limitations of the cholesteric performance on LSCs in indirect light can be

dramatically alleviated by using cholesteric reflectors made from special dispersion liquid

crystalline materials. These special dispersion cholesteric reflectors have reduced angular

dependence due to zero birefringence of the liquid crystalline materials at a specific

wavelengths. If this wavelength is chosen carefully around the spectral absorption peak of the

luminophore in the LSC, the amount of absorbable light that is reflected away by the filter at

larger angles. At angles > 75 degrees the special dispersion cholesteric reflectors start to reflect

absorbable sunlight, due to increased Fresnel-reflections similar to normal cholesteric filters. If

a 400 nm broad special dispersion reflector with zero birefringence at 575 nm is applied on top

of an LSC containing Red 305 as a luminophore the efficiency of the LSC is increased in all

weather conditions, even if the device is illuminated with isotropic distributed light.

9 Technology assessment and

future possibilities

Chapter 9

152

9.1 Luminescent solar concentrator: the future

9.1.1 Energy generating applications

One challenge the LSC has to confront in order to position itself in the global solar-energy

generation space is the misunderstanding of its functionality, and difficulty in describing the

device with reference to other solar-energy generating systems. Despite its coloration and

composition, the LSC should not be viewed as just another type of organic photovoltaic (OPV)

or, for that matter, as a solar cell at all. As such, direct comparisons of electron-generation

efficiencies commonly reported for photovoltaic materials become close to meaningless. While

it is possible to tabulate the reported results for efficiency measurements of luminescent solar

concentrators over the years (such an attempt can be seen in table 9.1) it becomes obvious that

making a comparison is very difficult.

Table 9.1 Reported efficiencies of luminescent solar concentrator devices. Luminophore Cell type (# of cells) LSC size [cm] Efficiency [%] Reference

Coumarin/Rhodamine? Si ? 1.9 [91]

Not stated Si ? 2.5 [91]

DCM Si 120x100x0.4 1.3 [91]

Coumarin/Rhodamine Si 120x100x0.4 1.3 [91]

Not stated Si 40x40x0.3 2.1 [130]

Not stated GaAs ? 2.5 [130]

Not stated, 2 plates Si 40x40x0.6 3.0 [130]

Not stated, 2 plates GaAs 40x40x0.6 4.0 [130]

Coumarin/Rhodamine Si 140x140x3 3.2 [234]

CdSe/CdS QDs GaAs 140x140x3 4.5 [234]

CdSe/CdS QDs Si 5x5x0.3 2.1 [85]

Red 305 Si 5x5x0.3 3.3 [85]

Red 305 Si 5x5x0.3 2.4 [71]

Red 305/CRS040 Si 5x5x0.3 2.7 [71]

Red 305/CRS040 mc-Si (1) 5x5x0.5 2.7 [74]

Red 305/CRS040 GaAs (1) 5x5x0.5 4.6 [74]

Red 305/CRS040 GaAs (4) 5x5x0.5 7.1 [74]

BA241 GaInP (4) 2x2x0.3 5.1 [75]

BA241/BA856 GaInP (4) 2 LSCs at

2x2x0.3

6.7 [75]

BA241 GaInP (1) 5x10x0.5 2.6 [75]

Not stated a-Si 5x5x0.5 ~0.7 [178]

CdSe core/multishell QDs Si 4.95x3.1x0.4 2.8 [235]

Red 305/Perinone Si (2) 2 LSCs at

5x5x0.5

4.3 [90]

Technology assessment and future possibilities

153

The efficiencies reported in the table are entirely dependent on the nature of the attached

solar cell, dye materials used, and device size. The reported net efficiency tells little or nothing

about the features or level of performance of the waveguide itself, the most important

component of the device. Thus, it becomes increasingly apparent to this author that another

method for describing the performance of these devices needs to be adopted, perhaps some

manner of ‘total photon-in/ photon-out’ fraction, weighted in some way to account for the

standard response for a variety of solar cells.

There are a great number of improvements that can and must be made to the LSC to

make it a more viable option for use in the urban environment. The single most important

improvement before the LSC may come into general use still needs to be the luminophore,

even after more than three decades of research. There is still no luminophore with a broad

spectral absorption, high absorption efficiency over the whole absorption spectrum, a large

Stokes Shift, a high luminescent efficiency (quantum yield), a matching spectrum of the emitted

photons to the best spectral response of the PV cell (≈1.14eV for silicon), a high photostability,

and good solubility in the host-matrix material. Organic dyes still have a small spectral

absorption width, a relatively low Stokes shift, and moderate photostability. Especially in the

broadness of the spectral absorption there is a lot to gain. Red 305 the state-of-the-art dye only

absorbs approximately 30% of all incident sunlight. If this spectral coverage could be increased,

this could lead to an increase in LSC efficiency by up to three times. Quantum dots lack the

quantum yield in combination with a large Stokes shift and solubility, and for rare Earth

materials the absorption coefficient and quantum yields are still too low. Phosphors are

alternative materials that have relatively high quantum yields, good absorption properties, high

photostability, and a high Stokes shift, but the solubility in an organic matrix is still a problem.

There are also still photons in the solar spectrum that can not be used by the

aforementioned luminophores, such as photons with lower energy than the bandgap of the PV

cell. To use these types of photons up converting systems could be an option, combining

multiple low-energy photons into a single, higher-energy photon that could be used by the cell.

Another option is to include materials that may absorb a single higher-energy photon well

above the photovoltaic bandgap and emit multiple photons closer to the cell bandgap, known

as quantum-cutting. Another obstacle that must be overcome is the misrepresentation of the

capabilities of the device: for years it has been espoused as a future replacement of PV panels

for rooftops. We believe this is a mistake. On South-facing rooftops with direct lighting (free

from shading), where space and efficiency are at a premium, standard silicon-based

photovoltaics would seem the best option. However, there are huge areas that do not fall into

these categories that could be prime locations for the use of LSCs. There are a large number of

examples of luminescent objects being incorporated in the environment already, even though

the coloration performs no additional function except for visual impact: for an example,

Chapter 9

154

consider the Musac museum in Léon, Spain. These types of structures exist in our environment

anyway, and it would not appear to be such a great stretch to add energy-generating

functionality. What is necessary is to bring the architectural and building industries into the

conversation as these devices are developed. The tremendous design freedom afforded by the

devices could no doubt be well-exploited by the visions of these industries. Applications could

include sound barriers beside roadways, telephone poles, bus stop roofing, atrium panels, and

the like.

One other aspect the author finds particularly intriguing is to provide an opportunity for

the use of OPVs. One of the greatest challenges for the OPV has been the ability for the

organic molecules that make up the absorbing/transport layers efficient at utilizing the UV

portion of the spectrum, as well as survive the high energies of UV light which causes

premature degradation of OPVs through destruction of the dye materials. However, the LSC

does not illuminate the attached solar cell with a solar spectrum, but with a much more narrow

band of light, generally in the NIR. It is in precisely this range of wavelengths that OPVs

perform their best. Coupled with the lack of exposure to UV light, this could provide the OPV

with the first real niche application where it could excel.

9.1.2 Day lighting applications

Not all applications need to convert the emitted light into electricity. The LSC has also found

potential use as a day lighting device, rather than as an electricity generator. In this

configuration, an LSC consisting of a stack of dye-doped waveguides of different colours is

located on a rooftop, and the emission light from the waveguide edges is collected in a clear

polymer or glass cable and transported a distance into the interior of a building, for example. At

the end of the light pipe, the emission light is mixed to create white light, and passes through a

diffuser into the room beyond.

The LSC day lighting element displays advantages over alternative day lighting systems in

that they require no tracking or astigmatic correction. A key element in this manifestation of the

device is the necessity of long transport distances of luminophore- emitted light, and the colour

mixing of the light at the emission site. In particular, the blue portion of the spectrum is lacking

due to the unavailability of a blue emitter that is sufficiently photos table and, to compensate,

current designs have used blue light emitting diodes to provide the short wavelengths and

obtain the desired white-light emission.

9.2 Aligned luminophores

9.2.1 Static alignment of luminophores in LSCs: what can be gained?

In chapters 3 through 6 of this thesis the effect of aligning dichroic luminophores in LSCs has

been investigated to reduce the loss of photons through the surface or to enhance the emission


155

from two edges of an LSC waveguide. A model is presented to calculate the emission profile of

a dye ensemble as function of the alignment direction, the amount of order and the incident

light. Aligning the dichroic dyes in a homeotropic fashion reduced the surface loss of an LSC

drastically, but the amount of incident light that is absorbed is reduced as well when illuminated

with a collimated light source. In a planar dye ensemble the surface loss is increased, but the

absorption of incident sunlight is increased. Calculations show that the amount of light emitted

from an aligned ensemble of dye molecules is the highest for planar alignment. These results

will probably change when photon recycling is taken into account. Each photon recycling event

will increase the amount of surface loss and for planar aligned dye ensembles the amount of

surface loss is increased more than for dye ensembles with homeotropic alignment. To get

more conclusive results based on these tilted dye alignments, the model presented in this thesis

should be implemented in ray tracing software, to simulate the effect of photon recycling. In

this light, the model presented in chapter 3 should be seen as a first step towards simulation of

the distribution of photons in the waveguide of an LSC.

The reduced absorption in the homeotropic state could be improved by the addition of a

forward-scatterer on top of the LSC, so the incoming light will be distributed and there will be

more incident light which is not parallel to the director of the dye ensemble.[225] Absorption of

incident light by homeotropically aligned dye molecules will also benefit from the fact that

sunlight is indirect and the direction is changing during the day. Photons incident at larger

angles will have a larger chance of getting absorbed.

Another application of static aligned dye ensembles is in the reduction in amount of

photovoltaic cells necessary for an LSC. Aligning dichroic dyes planarly enhances the edge

emission from the two edges parallel to the alignment direction up to ~60% over the

perpendicular edge if the dye aligns well in a nematic LC material. Calculations show that with

increased alignment, for instance in Smectic LC-materials, could even enhance this preferential

emission up to almost twice the amount of light reaching the parallel edge in comparison to the

perpendicular edge. Measurements show that the emission from the parallel edge of an LSC

with a planar aligned luminophore in a nematic LC is 30% higher than the emission from any

edge of an LSC with isotropic dye molecules. The calculations indicate that with enhanced

alignment this number can be increased to almost 50%. This would mean that an LSC with

planar aligned luminophores with only 2 photovoltaic cells could have an efficiency which is

about 75% of an LSC with isotropic dyes with 4 edges covered by PV-cells. This could reduce

the cost of the device. The cost could even be more reduced by replacing one of the two PV-

cells with a silver mirror.

Chapter 9

156

9.2.2 Dynamic alignment of luminophores in LSCs: window

applications

An option for making use of dye alignment in an LSC is to use the change in absorption of the

dichroic dye when aligned in different directions and use the LSC as a window while

simultaneously generating electrical current. While a static LSC has been introduced that could

be used as a window, it has the disadvantage common to thin-film transparent photovoltaics, in

that they may generate electrical current but not alter their transparency. By the same token,

responsive systems that can change their transparency, such as photo- and thermo chromic

windows or standard blinds generate no electricity.

An alternative LSC design uses, rather than a waveguide with embedded, inflexible dyes,

two glass plates coated with a conductor, and the space between the plates is filled by a LC-host

containing a fluorescent dye species. The liquid-crystal host can be continually switched

between two states, planar and homeotropic, and all angular configurations in between.

The dichroic dye molecules mirror the alignment of the LC-host. In the planar alignment,

which is the position of maximal absorption for the dye, the dye may emit light which partially

may be trapped in the glass panes making up the ‘window’ and generate electrical current, with

preferential emission from the edges parallel to the LC alignment. This constitutes the ‘dark’

state of the device, with maximum absorption (and consequently minimum transmission

through the ‘window’) and maximum emission, and thus maximum electrical generation with

minimal power input.

By applying of a voltage across the plates, the LCs may be made to attain an angle with

respect to the glass plate, reducing the absorption by the dye, and allowing more light into the

space beyond. In this ‘light’ state, while the output of the window necessarily drops due to

reduced light absorption, it still produces current, and due to the dye alignment, the optical

efficiency is actually increased. This design still needs work to achieve the desired transmissive

properties and coloration, but could potentially provide considerable advantages over

alternative ‘smart’ windows. It is also possible to create a third state, a scattering ‘privacy glass’

condition: this work is still in progress.

9.3 Wavelength selective reflectors in luminescent solar

concentrators

Organic wavelength selective reflectors can reduce the surface loss in luminescent solar

concentrators, but their effect is limited by photon recycling of emitted photons by another

luminophore molecule. If a luminophore which does not suffer from photon recycling is used

the amount of surface loss is reduced to approximately 25% for an LSC waveguide made from

polycarbonate. The increase in efficiency of such an LSC could reach up to 33% if all surface

emitted light would be reflected back into the device and reach the edge of the LSC and the


157

wavelength selective reflector would not reflect away absorbable sunlight. Calculations show

that the efficiency of an LSC with a dye that does not suffer from photon recycling is increased

by approximately 18% when a reflector is added with a 400 nm bandwidth. In such a device the

application of organic wavelength selective reflectors would be beneficial. In an LSC with a dye

that suffers from photon recycling the effect of the mirrors are limited, as shown in

measurements, but the LSC efficiency is still increased when illuminated with direct sunlight

normal to the device. This effect can be increased by using reflectors with better reflective

properties. The reflectors used are not 100% reflective within the reflection band and were not

completely transparent outside the reflection band.

The angular dependency of the reflection band reduces the positive effect of the

cholesteric reflector when the light is not collimated and normal to the device. Especially at

large angles of incidence the angular dependency of the reflection band has a major role in the

effectiveness of the reflector. At large angles of incidence the reflector reflects so much

incoming sunlight away that the efficiency of the device is decreased after application of these

cholesteric reflectors.

The angular dependency of cholesteric reflectors can be reduced by the use of special

dispersion LC materials. Calculations show that if the dispersion is chosen correctly, the

reflector made from this LC-material can increase the efficiency of an LSC in any weather

circumstances. The availability of materials with the needed dispersion is probably the limiting

factor in the use of these special dispersion cholesterics in LSCs.

Besides the increase of LSC efficiency after application of organic wavelength selective

reflectors, aesthetics of the device are also changed. Cholesteric reflectors show a change in

reflection wavelengths with different angles of incidence. This results in a change of colour

when the sun changes position with respect to the device or the recipient of the reflected

sunlight changes position (for instance a person walking past the device). This effect could give

an architect more freedom of design when an LSC is implemented in a building.

If a cholesteric reflector should be integrated in the LSC is going to be a case by case

decision. In some instances the aesthetics are going to be the determining factor and in other

cases the efficiency of the device will be predominant. If the efficiency is the determining

factor, the position of the LSC and the luminophore used determines if the application of a

cholesteric reflector is beneficial.

Appendix A: Perylene perinone

dye7 A.1: Introduction

Application of the LSC has been curtailed by their limited ability to absorb a large enough

fraction of the incident sunlight. The most common luminophore used has been the perylene-

based dye Lumogen Red 305 from BASF [117] (see, for example, [115]). Red 305 has

demonstrated good photostability, [111,113] good solubility in both PMMA and polycarbonate,

the two workhorses for LSC research efforts, and a high measured fluorescent quantum yield

(FQY). [115] The greatest disadvantage of Red 305 has been its limited spectral coverage. With

an absorption peak of around 577 nm in polycarbonate, it is only able to absorb 16% of the

air–mass 1.5 global (AM1.5G) photons of the solar spectrum. Despite this, Red 305 has been

used in the highest efficiency LSC system. [74]

Perylene perinones, otherwise known as perylene bis-imidazoles, designed for a variety of

applications, have been described in the past. [236-239] In this work, we describe new perylene

perinone structures exhibiting extended absorption ranges, promising FQYs, and photostability

for use in a new application, the LSC. Two different types of perylene perinones are described,

3,4:9,10-bis(1,2-benzimidazole)-1,6,7,12-tetra(4-nonylphenolphenoxy)perylene (syn/anti-

isomers) and 3,4:9,10-bis(1,2-benzimidazole)-1,6,7,12-tetra(4-tert-octylphenoxy)perylene

(syn/anti-isomers), further referred to as the nonylphenol perylene perinone and the tert-octyl

perylene perinone respectively. We demonstrate the use of perylene perinone containing

waveguides in conjunction with Red 305 dye waveguides, and demonstrate significant increases

in the output of a dual-waveguide system compared to using a Red 305 waveguide alone.

This work has been performed in collaboration with Sabic IP in the Netherlands, GE

Plastics in India, and with Herriot-Watt University, Edinburgh, United Kingdom.


M.G. Debije, P.P. C. Verbunt, P.J. Nadkarni, S. Velate, K. Bhaumik, S. Nedumbamana, B.C.

Rowan, B.S. Richards and T.L. Hoeks, “Promising fluorescent dye for solar energy conversion

based on a perylene perinone”, Applied Optics, 50 (2), 163-169, 2011.

Appendix A

160

A.2 Methods

5 cm x 5 cm x 0.3 cm polycarbonate plates were prepared by the injection molding of six

different concentrations (between 15.5 and 250 parts per million (ppm)) of the perinone nonyl

phenol dye. The absorption spectra of all the plates were recorded on a Shimadzu UV-3102

spectrophotometer. The top surfaces of the samples and a reference waveguide containing no

dye were exposed to the light from a 300 W solar simulator with filters to approximate the

AM1.5G solar spectrum (Lot- Oriel) and an integrated output of 536 Wm-2 over the spectral

range 350–850 nm. Single edge emission spectra and intensity were determined using a

Labsphere spectral light measurement system LED 1050 integrating sphere. The emissions

from two orthogonal edges of each waveguide were averaged in determining the final output.

Emission spectra were integrated from 350 to 850 nm to determine the total output of the

waveguides: the variation in the integrated emissions from the two edges was <2%. The

outputs from two edges were also determined using a separate rear scatterer (an opaque piece

of white painted cardboard). Additional measurements of the edge outputs were made by laying

two waveguides on top of each other and determining the total edge output on a black

(absorbing) background. The injection-molded polycarbonate waveguides contained either the

perinone dye, Lumogen Red 305 (BASF), or no dye at all.

Weathering exposure was carried out in the Rockies Ci 4000 weatherometer from Atlas

Material Services. The protocol used continuous 0.75 Wm-2 irradiance at 30% relative humidity

with a dry bulb temperature of 35°C. Samples were compared with and without a covering of

300 μm SLX 2432- NA9A048T film (Sabic IP). All fluorescence emission and excitation spectra

were taken on a SPEX fluorolog 3 (Jobin Yvon, Edison) using double monochromators with

1200 grooves/mm grating blazed at 330 and 500 nm for excitation and emission selection,

respectively, a Hamamatsu R928-P photon counting photomultiplier tube (PMT) for emission

detection (referred to as the signal or S channel), and a 450 W continuous Xe lamp as an

excitation source. A small fraction of the excitation beam is diverted to a “reference”

photodiode just before the sample (referred to as reference or R channel) to monitor the

relative excitation intensity as a function of time and excitation wavelength. The voltage of S

channel PMT was set to 950 V for all experiments.

One set of FQY measurements was carried out by recording excitation source intensity

and calculating the area under the corrected emission spectra. The FQY value is a ratio of

“number of photons emitted” to “number of photons absorbed.” Additional FQY

measurements of similar plates were made using a FL3-211 Horiba Jobin Yvon Fluorolog-3

fluorescence spectrophotometer on an Edinburgh Instruments FS920 UV/VIS/NIR

fluorescence spectrometer with a Horiba Jobin Yvon integrating sphere, as described

previously. [115]

Perylene perinone dye

161

A.3 Results and Discussion

Polycarbonate plates containing the nonylphenol derivative were measured to have an

absorption peak at 632 nm, very close to the emission maximum of Red 305 of 620 nm. The

new dye thus expands the absorption spectra of the LSC by ∼50 nm, which translates into the

new LSC being able to harvest 30% more of the photons of the AM1.5G spectrum. See table

a.1 for additional properties of the perylene perinones.

Table a.1 Photophysical parameters and decomposition temperatures of the perylene perinones

Compound λabs (nm)a Ext. Coef.

(Lcm-1mol-1)

λem (nm)a Decomp.

Temp. (°C)

FQYa

Nonylphenol 630 63700 666 >378 1

4-tert-

octylphenol

630 61200 666 >358 0.96

a Measured in Toluene

The 5 cm × 5 cm × 0.3 cm injection-molded polycarbonate plates containing a range of

nonylphenol concentrations from 15.5 to 250 ppm were cut from samples of a slightly larger

size. The representative absorption and edge emission spectra of two of nonylphenol- filled

polycarbonate plates may be seen in figure a.1.

Figure a.1 Absorbance (solid curves) and edge emission (dotted curves) spectra of two nonylphenol-filled plates

containing 35.5 (black curves) and 250 (grey curves) ppm dye.

Appendix A

162

To evaluate the new perylene perinone dye in a LSC configuration, we measured light

emission from the edge of the filled waveguides, more completely described in the experimental

section. The top surfaces of the dye-filled waveguides and reference blank polycarbonate

waveguide were exposed to light from a solar simulator with the waveguides resting on either

black, absorbing backgrounds or white scattering backgrounds separated from the waveguide

by an air gap. The intensity and wavelength of the light emitted from the edge of the waveguide

were recorded using an integrating sphere. The integrated output energies (from 350 to 850

nm) are shown in figure a.2. Similar to previous findings, the fractional emission increase for

the samples using the scattering background was larger at lower dye content. [173] The 4-tert-

octylphenol had an output ∼90% that of the nonyl at a similar loading (100.7 to 101.8 mg

dye/kg).

Figure a.2 Integrated edge emission (350–850 nm) of the perylene perinone nonylphenol 5 cm × 5 cm × 0:3 cm

waveguides with absorbing black (filled symbols) and scattering white (open symbols) backgrounds.

Measurements of the perylene perinone nonylphenol FQY were performed in two

different solvents, resulting in values of around φ=100%±5% in toluene and φ =80±8% in

xylene. The FQY in polycarbonate plates loaded at 15.5 ppm was estimated to be φ =80±10%.

A series of edge emission measurements was made on 3 mm thick waveguides containing

either Red 305 or the perylene perinone. The samples were all measured in tandem, with one

waveguide laid on top of the other, with the ends of each inserted into the integrating sphere,

and the emission spectra were integrated to determine a total output for the system.

The results of such a series of measurements with the Red 305 waveguides lying on top of

perylene perinone waveguides of various dye content may be seen in figure a.3. A summary of


163

all the results of these measurements is found in table a.2. It would be anticipated that a two-

waveguide device, one employing Red 305 as an upper waveguide and a perylene perinone-

filled waveguide as a lower layer, will enhance emission of the Red 305 dye acting alone [75] for

several reasons. The first is because the FQY of the Red 305 dye is higher than that of the

perylene perinone. [115] Second, the perylene perinone dye will be able to absorb light that was

within the absorption range of the Red 305 dye that failed to be absorbed and absorb light with

wavelengths completely outside the absorption band of the Red 305 dye.

Figure a.3 Integrated emission (350–850 nm) determined for two waveguide systems with blank polycarbonate

(filled circles) and perylene perinones of absorbance 0.37 (filled square), 1.25 (open triangle), 2.04 (filled

triangle), and 3.55 (open circle) as the base waveguide and Red 305 waveguides of varying absorbance as the top

waveguide.

Appendix A

164

Table a.2 Integrated output for Red 305 samples on blank polycarbonate bottom waveguide and fractional

increase of Red 305 top/perinone bottom stacks compared to output of Red 305 top/blank polycarbonate

bottom waveguide stack

Integrated

output Red

305 (mW)

Perinone Absorbance (bottom)

0.37 0.93 1.25 1.36 2.04 2.86 3.55

Red 305

absorbance

(top)

0.21 17.3 0.66 1.08 1.12 1.61 1.65 1.78 1.82

0.48 25.5 0.45 0.71 0.72 1.00 1.15 1.15 1.16

1.04 36.5 0.28 0.42 0.43 0.60 0.67 0.69 0.69

1.63 45.6 0.16 0.29 0.28 0.44 0.43 0.46 0.47

2.43 50.5 0.24 0.28 0.24 0.28

Finally, the perylene perinone waveguide will be able to absorb the light escaping from the

rear surface of the Red 305 waveguide. As a significant fraction of the absorbed photons are

lost through the surfaces of the waveguide (see chapter 5), this latter effect could be significant.

To verify the advantage of using a Red 305 on top, measurements of the outputs of the dual-

waveguide system employing the perinone-filled waveguide as the top object were made. These

experiments demonstrated that, while in all cases the system performed better than the Red 305

with a blank polycarbonate plate on top, the perinone top/Red 305 bottom consistently

performed less efficiently than the Red 305 top/perinone bottom systems; see table a.3.

Table a.3 Fractional change of perinone top/Red 305 bottom stacks compared to identical waveguides with Red

305 top/perinone bottom

Red 305 absorbance

(bottom)

0.21 0.48 1.04

Perinone

absorbance

(Top)

0.37 -0.25 -0.09 -0.10

0.93 -0.18 -0.15

1.25 -0.16 -0.16 -0.21

1.36 -0.07 -0.08

2.04 -0.04 -0.12 -0.15

2.86 -0.04 -0.10

3.55 -0.06 -0.12 -0.20

As in any solar-based application, details of the photostability of the dye are of interest.

The study of the two perinone dyes in the polycarbonate matrix at a dye loading of 100 ppm

was done in an accelerated artificial weathering setup. [240] Some of the plates were covered

with a protective Lexan SLX film (Sabic IP) and compared to similar samples not employing

the protective film. The changes in fluorescent intensity as a function of 340 nm dose were


165

measured for the samples. The fluorescent efficiency of each plaque was determined after a

10000 kJm-2 exposure, equivalent to over three years of exposure in Florida [240]. The

reduction in emission intensity for the nonylphenol perinone dye monitored at the fluorescent

maximum of 678 nm was approximately 1.7% in the protected sample, while in the sample

without the protective layer, the loss was 5.4%. The fluorescent loss of the tert-octyl perinone

was monitored at 676 nm and was slightly lower than the linear chain nonyl: in the protected

system, loss was 1.2%, while the plate without the SLX film was 2.5%. This study indicates that

the photostability of the nonyl- and tert-octyl perylene perinones are at least comparable to Red

305 [111,113]and, thus, could be considered for longer term, outdoor use. This photostability is

not unexpected, as it has been shown previously that other perinones, such as Pigment Orange

43 and Pigment Red 197, are both photo and thermally stable.

A.4 Conclusions

In summary, we have prepared a novel fluorescent dye material that holds great promise for use

in solar energy research. Their use in a LSC may extend the absorption range of current dyes by

∼50 nm at a reasonable fluorescence quantum yield and photostability.

In addition, we have demonstrated a two waveguide LSC system using a Red 305 top and

perinone bottom waveguide that significantly increases the performance of the system based on

Red 305 alone. This work is an important step in making the LSC a viable option for electrical

generation from sunlight.

167

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181

Samenvatting Slinkende olie voorraden hebben er voor gezorgd dat er meer interesse is bij overheden naar

het gebruik van hernieuwde energiebronnen. Vooral in gebouwen, die in ontwikkelde landen

verantwoordelijk zijn voor 40% van al het energiegebruik, is er veel belangstelling voor het

implementeren van energie opwekkende en besparende systemen. Deze systemen zijn bij

voorkeur aanpasbaar aan de wensen van de architect, terwijl de kosten van de opgewekte

energie laag is (~€0.06/kWh). Een energiebron die in de buurt van elke gebouw aanwezig is, is

de zon. De toepasbaarheid van standaard zonnecellen in gebouwen is lastig gebleken, omdat de

kosten van de opgewekte energie te hoog zijn en de flexibiliteit naar de behoeften van de

architect beperkt is. Een alternatief dat geacht wordt de kosten van de opgewekte energie te

verlagen en de toepasbaarheid te verhogen, in vergelijk met standaard zonnecellen, is de

fluorescente licht geleider (FLG).

Een FLG bestaat uit een plastic of glazen transparante plaat, die dient als licht geleider,

waarin fluorescente kleurstof moleculen zijn aangebracht. Deze fluorescerende moleculen

absorberen het zonlicht en zenden dit uit met een langere golflengte. Een deel van dit

uitgezonden licht wordt in de lichtgeleider gevangen door totale interne reflectie en het licht

wordt hierdoor geconcentreerd aan de zijkanten van de plaat waar een zonnecel geplaatst wordt

die het licht omzet in elektriciteit. De efficiëntie van FLGs is tot op heden nog niet hoog

genoeg door verschillende verliesmechanismen. Een van deze verliesmechanismen is het licht

dat de lichtgeleider aan zowel de boven- als de onderzijde verlaat. Dit uitgekoppelde licht is een

combinatie van een gelimiteerde inkoppel-efficiëntie van de door de fluorescente kleurstof

uitgezonden fotonen en de herabsorptie van gevangen fotonen die opnieuw uitgezonden

worden, ook wel foton-recycling genaamd, maar gedeeltelijk onder een verkeerd hoek

In dit werk wordt een eenvoudig model gepresenteerd dat de ruimtelijke distributie van

uitgezonden fotonen door een collectie van dichroïtische kleurstof moleculen in een isotrope of

vloeibaar kristallijne matrix beschrijft. Het model is gevalideerd met experimenten, waarin de

mate van moleculaire orde in planaire (parallel aan de oppervlakte van de lichtgeleider) uitlijning

is veranderd. In FLGs met een planaire uitlijning van de kleurstof moleculen verlaat er tot 60%

meer licht de 2 gewenste zijkanten parallel aan de richting van kleurstof moleculen in

vergelijking met de 2 zijkanten loodrecht op de richting van de kleurstof moleculen. Deze

resultaten komen overeen met het model.

Voor FLGs met een isotrope verdeling van dichroïtische kleurstof moleculen is de

inkoppel-efficiëntie van de uitgezonden fotonen berekend. Deze is 74,3% in polycarbonaat. Dit

resulteert in een verlies van 25,7% van alle uitgezonden fotonen door de oppervlakten van de

lichtgeleider. Experimenten tonen aan dat het verlies van uitgezonden fotonen ~50% is, voor

een FLG met als kleurstof BASF Lumogen F Red 305, de standaard in FLGs. Het verschil

182

tussen de resultaten uit het model en de experimenten komt doordat een deel van de

“gevangen” fotonen nogmaals worden geabsorbeerd en opnieuw worden uitgezonden.

Het uitlijnen van de dichroïtische moleculen kan de hoeveelheid verloren fotonen door de

oppervlakten van de lichtgeleider reduceren. Zowel uit voorspellingen van het model als uit

experimenten blijkt dat het oppervlakte verlies gereduceerd kan worden tot minder dan 10% als

de dichroïtische moleculen homeotroop (loodrecht ten opzicht van de oppervlakten van de

lichtgeleider) worden uitgelijnd. Echter zorgt deze uitlijning ervoor dat de hoeveelheid zonlicht

die geabsorbeerd kan worden, wordt gereduceerd. Planair uitgelijnde dichroïtische moleculen

verhogen de kans op absorptie van het zonlicht, echter de inkoppel-efficiëntie van de

uitgezonden fotonen in de lichtgeleider wordt verlaagd. Het kantelen van de dichroïtische

moleculen ten opzichte van de lichtgeleider kan de voordelen van planaire (vergrootte kans op

absorptie) en homeotrope (hogere inkoppel-efficiëntie van de uitgezonden fotonen) kunnen

combineren. Voorspellingen over de optimale kantelhoek is moeilijk gebleken. Het

voorgestelde model dient geïmplementeerd worden in simulatie software om een voorspelling

te doen over de optimale kantelhoek.

Een tweede methode om de oppervlakte verliezen te reduceren is het toepassen van

golflengte selectieve spiegels die inkomend zonlicht doorlaten en uitgezonden licht reflecteren.

Theoretische benaderingen tonen aan dat ongeveer 90% van alle fotonen die verloren gaan

door de oppervlakten van de lichtgeleider terug gereflecteerd kunnen worden als de FLG BASF

Lumogen Red 305 als kleurstof heeft, voor een reflector met een 400 nm bandbreedte. Dit kan

leiden tot een relatieve verhoging van 66% van de efficiëntie van de FLG, terwijl een 175nm

brede reflector een relatieve verhoging van 45% geeft.. Experimenteel blijkt dat de efficiëntie

van een FLG met een piekabsorptie van ~1 maar met 5% wordt verhoogd als er een reflector

wordt toegepast met een bandbreedte van 175 nm. Wanneer de hoeveelheid foton-recycling

wordt verminderd kan de efficiëntie van de FLG worden verhoogd met ~20%, waaruit blijkt

dat de efficiëntie van golflengte selectieve spiegels wordt gelimiteerd door foton-recycling.

De golflengte selectieve spiegels hebben ook een hoek afhankelijkheid: de reflectieband

schuift naar kortere golflengtes als de hoek van inval wordt vergroot. Hierdoor wordt een deel

van het zonlicht, dat geabsorbeerd kan worden door de kleurstof moleculen gereflecteerd. Dit

reduceert de effectiviteit van de reflector and dit kan in potentie leiden tot een reductie van de

FLG efficiëntie. Simulaties tonen aan dat speciale dispersie cholesterische reflectoren (SDCs)

een verminderde hoekafhankelijkheid hebben. Theoretische berekeningen tonen aan dat het

gebruik van deze SDCs de efficientie van de FLG verhogen voor hoeken van inval tot 70°.

Zowel de uitlijning van de dichroïtische moleculen als de toepassing van organische

golflengte selectieve reflectoren reduceren de hoeveelheid licht die verloren gaan aan de

oppervlakten van de FLG. Het effect van beide methoden is experimenteel een stuk kleiner dan

voorspeld door middel van berekeningen en simulaties. Om de efficiëntie van de FLG te

verhogen zijn fluorescente kleurstoffen nodig die geen overlap hebben tussen de absorptie en

Samenvatting

183

de emissie band. Met andere woorden de hoeveelheid foton-recycling moet gereduceerd

worden. Ook zal de hoeveelheid zonlicht die door de kleurstof moleculen geabsorbeerd word,

vergroot moeten worden om de FLG tot een geschikt alternatief voor de generatie van

electriciteit te maken, aangezien de gelimiteerde absorptie van het zonlicht het grootste verlies

in FLGs is.

185

Acknowledgements Every PhD-student experiences some peaks and valleys during their project. In particular, the

valleys can be very difficult, and the help offered by individuals is necessary to proceed.

Finishing my thesis, I want to thank them for their help keeping my project on track. First of all

I would like to thank my promotor prof. Broer. Dick, thank you for allowing me to perform my

PhD-project within your group. I would also like to thank you for the inspiration and the

advice you gave me, especially in the last part of my project. When I left your office I was

always full of confidence and new ideas. Secondly, I would like to thank Kees Bastiaansen.

Kees, I really appreciate the discussions we had, even though they progressed sometimes

somewhat laboriously. Your critical view on my project kept me focusing on the main goal of

the project and I am honored to have you as my second co-promotor. I owe the most to my

first co-promoter Michael Debije. I don’t know where to start thanking you. Until the moment

you asked me to do this project, I had never considered doing a PhD. But I knew that with you

as supervisor I would be able to finish such a project. I could always drop by in your office if I

had a problem or a question. Without you these past four years would have been much more

difficult. Besides the scientific discussions we had I also enjoyed the non-scientific chats we had

on boardgames, music or any other topic. Thanks for everything. I would also like to thank

your family for opening your house to me, I especially enjoyed the boardgame nights at your

place. Half way through my project our group was extended with a very inspiring scientist,

Albert Schenning. Even though most of the topics we discussed are not in this thesis, I really

enjoyed them and I always left our office inspired and with a huge smile on my face.

I would also like to thank the core-committee for my thesis defense, prof. Janssen (TU/e),

prof. Richards (HWU, Edinburgh), prof. Urbach (TUD) and dr. De Mello-Donega (UU).

Thank you all very much in participating in the core-committee and for your comments and

suggestions on this thesis.

My project was funded by Stichting voor Technische Wetenschappen (STW) and I would

like to them, especially Monique Wiegel for the smooth arrangement of the VIDI-meetings. A

VIDI project has an external committee and I would like to thank the members for their

contribution in the project. Lenneke Slooff (ECN) and Casper van Oosten, thank you for your

contributions in the discussions we had. Besides contributions to the discussions, I would like

to thank Theo Hoeks (Sabic-IP) for the materials he provided. The fourth and last external

committee member of the VIDI-project turned out to be much more important to my project,

Dick de Boer (Philips Research). Your involvement in my work went much further than being a

committee member. You made it possible for me to take the theoretical approaches in this

thesis to another level. You taught me the more fundamental side of science and I would have

186

never guessed I would enjoy it as much as I do now. You also took a good look into my thesis

and without you I probably wouldn’t have finished this thesis by now. I am honored to have

you on my defense committee as an advisor. The meetings we had together with the people at

Philips Research I liked a lot, especially the discussions we had with Chi-Wen and Merijn.

More help with the theoretical work presented in thesis I got from Carlos Sanchez from

the University of Zaragoza. When I asked you to help me calculating the emission profiles from

tilted dyes I would have never guessed I would consume so much of your time. Thank you for

that.

I had the pleasure to work with several students during my time as a PhD-student. Duygu

and Toon, I enjoyed supervising your projects and even though the results of these project did

not make it into this thesis, your work helped us understanding some basic principles. I also

supervised a group of OGO-students. Sabrina, Stéphanie (who became a roommate when she

started her master-project within our group), Thijs and Yoran, without your help the

production of layered narrowband cholesterics would not have been so easy.

I had a lot of very enthusiastic, inspiring and helpful (PhD-)students and post-docs who

dared to share an office with me. Shufen, you were my first roommate and a we worked on the

same project. I really enjoyed our scientific discussions, the chit-chats we had and without your

help chapter 7 of this thesis would have been a lot shorter. I shared my office further with

Debarshi, Derya, Hilal, Judith, Julien, Koen, Nicole, Peter, Patrick, Stéphanie and Yang. You

were all very pleasant roommates and I hope I haven’t kept you from your work too much with

my constant talking. I also had a roommate, which was my “boss” before. Robert, it was a

delight to have you in “my” office. I already had a great time when I worked as a student-

assistant for Validus, the company you had within our group together with Thijs and Nico, but

the time we spent together during the six months you were a fellow PhD-student were great. I

loved the chats we had when everybody else had already left.

In addition to my roommates I had the pleasure to work with other great PhD-students,

post-docs and staff members. Especially in STO 0.24 I met some people that I consider as

friends. Ties, it took us a while to start chatting with each other, but we made up for that in the

second part of our time as PhD-students. I really enjoyed our coffee-breaks together, even

though we speak a different language (I speak in Chemistry and you in Physics). I learned a lot

from you. I also enjoyed to spend a lot of time with your office-mates, My and Natalia (my

dearest enemy). From boardgaming and having a beer to real science, all of this was a delight.

I would also like to thank the other people who were in SFD, but also some members

from SKT and PTG: Amol, An, Anne, Antonio, Bob (I will cheer for the Welsh rugby team),

C.K., Claudia, Danqing, Elena, Felix, Han, Helena, Huub, Iren, Ivelina, Jelle, Jeremy, Johan,

Joost, Jurgen, Katherine (the girl who scared me), Ko, Laurens, Luc, Maria, Maud, Maurizio,

Acknowledgements

187

Mian, Nick, Pauline, Peter K., Piming, Pit, Shabnam, Stefan, Sun, Tamara, Teun, Tom, Yogesh,

Youseli, Weizhen and all that I might have forgotten. We also had some very nice secretaries,

Elly, Ineke en Marjolijn, bedankt voor alles dat jullie gedaan hebben.

I would also thank one person from my professional scientific life in the past, Jens Thies

from DSM Research. When I worked as assistant scientist at DSM after I finished my Bachelor

(HBO) I had the pleasure to work with Jens. You asked me several times why I would not try

to get a Master’s. At that moment I had no interest, but your words have been in my head since

then and a couple of years later, I still remembered them and I decided to start my Master’s

study. All this lead in the end to this thesis, and without you I probably would have never

considered to start my Master’s study.

Sometimes it was necessary for me to step away from my project. I would have never done

that without some important people, all from Limburg. I would like to thank them in the

language of Limburg.

Ut laeve van eine promovendus geit neet altied euver rozen. Es ut project neet lupt zoals

ut zou motte laope, is ut good om alles aeve aan de kantj te zitten en det waor mich neet gelökt

zonger ein paar heel biezonjere miense. Es eerste wil ich mien vrunj bedanke, sommige kin ich

al bienao mien ganse laeve en angere hub ich pas later leere kinne. Martin, Bernd, Wendy (en

Joan), Patty, Bianca, Tim, Melissa en Paul bedank veur alles. Boete mien vrunj wil ich auch gaer

mien femilie bedanken veur alle sjteun. Veural mien grootelders wil ich bedanke veur alles. Ich

vinj ut jaomer det driej van heur dit neet meer mit kinne maake, mer zonger uch had ich dit

nooit kinne doon. Ein anger femilielid det ich extra wil bedanke is Patrick. Al sinds ich vreuger

nao uch in de Julianasjtraot kwaam busse meer ein broor veur mich gewaes den eine naef. De

sjpelaovende en de keren det ver zeen gaon sjtappe waren vaak precies waat ich neudig had.

Bedank veur alles.

Es allerletste wil ich de twee belangriekste miense in mien laeve bedanke. Mam en Pap,

bedank veur alles, neet allein veur de letste veer jaor, mer veur alles det ger veur mich gedaon

hub mien hele laeve. Zonger uch had ich nooit kinne bereike waat ich bereikt hub.

Paul

189

Curriculum Vitae

Paul Verbunt was born on the 30th of June 1981 in Roermond, the

Netherlands. After graduating secondary school (“HAVO”) at

Bischoppelijk College Broekhin (Roermond) in 1998, he studied

(Polymer) Chemistry at the Fontys Hogeschool TNW in Eindhoven.

After completing an internship at Dex Plastomers (Geleen, the

Netherlands) on chain-end unsaturations in polyethylene, he complete

his graduation project in 2002 at DSM Research (Geleen, the

Netherlands) on acid scavengers to improve the hydrolitic stability of

methacrylate resins.

After working almost three years in industry as a research assistant and warehouse clerk, he

started his Masters education in 2005 in the faculty Chemical engineering and Chemistry at the

Eindhoven University of Technology (TU/e). In 2008 he received his Master’s degree for his

thesis ‘Surface losses in luminescent solar concentrators’ under guidance of dr. M.G. Debije in

the group of prof. D.J. Broer.

Directly after obtaining his Master’s degree, Paul started a Ph.D. project at the Chemical

Engineering and Chemistry department of the Eindhoven University of Technology under

guidance of prof. D.J. Broer, dr. M.G. Debije and dr. C.W.M. Bastiaansen. The results of this

project are described in this thesis

190

191

List of symbols

General introduction

E Energy of a photon

h Planck’s constant

Frequency of a photon

c Speed of light

Wavelength of light

Thermodynamics of concentrators

,in outA A Area of the incoming/outgoing light in a concentrator

,in outU U Etendue of the incoming/outgoing light in a concentrator

,in out Solid angle of the incoming/outgoing light in a concentrator

max max,, , diffC C C

Concentration/maximum concentration/maximum concentration of

diffuse light by a concentrator based thermodynamics

, ,in out concn n n Refractive index of the medium of the incoming light/the medium of the

outgoing light/concentrator

Introduction to LSC/LSC in general

c Critical angle

opt Optical efficiency of the LSC

R Fresnel reflection from the LSC waveguides

trap Probability that an emitted photon is in waveguide mode

abs Fraction of the incident sunlight that is absorbed by the luminophores in

the LSC

PLQY Photoluminescent quantum yield of the luminophore

Stokes Energy efficiency of the dye molecule related to the Stokes shift

host Transport efficiency of photons through the clear host/waveguide

TIR Total internal reflection efficiency related to the smoothness of the

waveguide surfaces

self Transport efficiency of the photons through the LSC waveguide related to

photon recycling

192

Luminophore/dye parameters

af Overlap factor between the absorption and the emission spectrum of the

luminophore molecule

1 2,e e Energy of the absorbed and emitted photons

0T Ambient temperature

,PLQY FRET Energy transfer quantum yield

r Distance between the donor and the acceptor for FRET

0R Förster’s distance

abs Wavelength of the absorbed light

m Mass of an electron

L Chain length of the π-conjugated plane

N Number of electrons

Model for emission profiles

, The polar and azimuthal angle of the emitted light

,i fe e

The polarization of the incident/emitted light

Transition dipole vector

,I

Intensity of the emitted light./Emission profile

, The polar and azimuthal angle of the transition dipole

k Vector that describes the direction of the emitted photons

Angle of the polarization of the emitted light with respect to the arbitrary

polarizations of the light

,1 ,2,f fe e

Arbitrary polarizations of the emitted light, orthogonal to the emitted light

vector and each other

Ensemble of dye molecules

f

Distribution function of the transition dipoles within the ensemble.

Aligned dye molecules by a liquid crystalline host

Angle between the molecular axis and the director of a liquid crystalline

material

n Director of the liquid crystalline material

2nS The 2nth order parameter of the liquid crystalline material

List of symbols

193

,o en n The ordinary and extraordinary refractive index

n Birefringence

2,optS

Optical order parameter/macroscopic order parameter

Angle between the molecular and optical axes of a dye molecule

2nP The 2nth Legendre polynomial

2, flS

Fluorescence order parameter

,F F

Amount of emitted light with linear polarization parallel/perpendicular to

the alignment direction

eER Output ratio (see list of definitions for the definition)

,A A

Absorption of incident light with linear polarization parallel/perpendicular

to the alignment direction

sl Fraction of the emitted photons emitted in the surface loss mode.

Tilt angle of the guest-host system with respect to the LSC waveguide

General introduction to Bragg reflectors

m Integer denoting the order of interference

n Average refractive index

d Periodicity

Bandwidth of the reflection band of the Bragg reflector

0 Central reflection wavelength of the wavelength selective reflector at normal

incidence

1 2,n n Refractive indices of the materials in a Bragg reflector

Cholesteric wavelength selective reflectors p Pitch of the helix of a cholesteric liquid crystalline material

HTP Helical twisting power of a chiral dopant

c

Concentration of the chiral dopant

Central reflection wavelength of the reflector at angle of incidence

refl Efficiency of reflection of luminophore emitted light of the wavelength

selective reflector

sE

Emission spectrum of the luminophore

pE

Emission profile of surface emitted photons

194

,R

Angular dependent reflection spectrum of the cholesteric reflector

EAcholf

Fraction of the absorbable incident light transmitted by the cholesteric

reflector

,I

Solar spectrum

,A

Angular dependent absorption spectrum of the luminophore containing

LSC

,maxLSC Maximal fractional increase in LSC efficiency after application of a


,edge choln

Number of photons leaving the edges of the LSC after application of a


,edge baren

Number of photons leaving the edges of the LSC without the application of

a cholesteric reflector

, ,edge SL choln

Number of photons leaving the edges of the LSC formerly lost through the

LSC surface after application of a cholesteric reflector

0 ,A A Absorption of the luminophore containing LSC at normal incidence/at

angle

0,n n Refractive index of a material at a specific wavelength/ null refractive index

1 2,B B Dispersion coefficients

195

List of definitions Cholesteric Chiral Nematic

Dye ensemble Dye molecules acting as guests in a guest-host system.

Homeotropic

alignment

Alignment of a liquid crystalline material, where the director is

perpendicular to the substrate

Initial emission Emission from a dye ensemble after the initial absorption of the

incident light. The re-emission of light after the re-absorption of

photons in waveguide mode is not taken into account

Null refractive index The refractive index of a material at infinite wavelength as used in

Cauchy’s equation for dispersion

Peak absorbance Absorbance of the peak absorption band of a luminophore

Optical Efficiency The energy in mW leaving the 4 edges of the LSC waveguide divided

by the energy of the sunlight in mW incident to the LSC

Optical order

parameter

Order parameter of the optical axis of a dichroic dye in a host

(usually a liquid crystalline host)

Output ratio The ratio in edge emission between the edge of an LSC parallel and

perpendicular to the alignment direction in an LSC with planarly

aligned dye molecules

Photon recycling A sequential event of re-absorption and re-emission of photons

emitted by the luminophore. This leads to a redistribution of the

emitted photons

Planar alignment Alignment of a liquid crystalline material, where the director is

parallel to the substrate

196

Quenching The reduction of emission due to the formation of non-emitting

clusters of luminophores

Special dispersion

cholesteric

The cholesteric phase of a liquid crystalline material with a special

dispersion. In these liquid crystalline materials have an ordinary

refractive index which changes more with changing wavelength than

the extraordinary refractive index. At a certain wavelength the

refractive indices are equal

Surface loss The loss of emitted photons in LSCs lost through the top and

bottom surfaces of the waveguide

Tilted alignment Alignment of a liquid crystalline material, where the director is tilted

to the substrate

Top/Bottom side of

the waveguide

The surface facing towards/away from the incident light

197

List of publications Publications related to this work

P.P.C. Verbunt, S. Tsoi, M.G. Debije, D.J. Broer, C.W.M. Bastiaansen, C.-W. Lin, D.K.G. de

Boer, “Increased efficiency of luminescent solar concentrators after application of organic

wavelength selective mirrors”, Optics Express, 20 (S5), A655-668, 2012

D.K.G. de Boer, D.J. Broer, M.G. Debije, W. Keur, A. Meijerink, C.R. Ronda and P.P.C.

Verbunt, “Progress in phosphors and filters for luminescent solar concentrators”, Optics

Express, 20 (S3), A395-A405, 2012.

M.G. Debije and P.P.C Verbunt, “Thirty years of luminescent solar concentrator research: solar

energy for the built environment”, Advanced Energy Materials, 2, 12-35, 2012.

M.G. Debije, P.P. C. Verbunt, P.J. Nadkarni, S. Velate, K. Bhaumik, S. Nedumbamana, B.C.

Rowan, B.S. Richards and T.L. Hoeks, “Promising fluorescent dye for solar energy conversion

based on a perylene perinone”, Applied Optics, 50 (2), 163-169, 2011.

Dick K.G. de Boer, C.-W. Lin, M.P. Giesbers, H.J. Cornelissen, M.G. Debije, P.P.C. Verbunt

and D.J. Broer, “Polarization-independent filters for luminescent solar concentrators”, Applied

Physics Letters, 98, 021111, 2011.

M.G. Debije, M.P. Van, P.P.C. Verbunt, M.J. Kastelijn, R.H.L. van der Blom, D.J. Broer and

C.W.M. Bastiaansen, “Effect on the output of a luminescent solar concentrator on application

of organic wavelength-selective mirrors”, Applied Optics, 49 (4), 745-751, 2010.

M.G. Debije, J.P. Teunissen, M.J. Kastelijn, P.P.C. Verbunt, C.W.M. Bastiaansen, “The effect

of a scattering layer on the edge output of a luminescent solar concentrator”, Solar Energy

Materials and Solar Cells, 93, 1345-1350, 2009.

P.P.C. Verbunt, A. Kaiser, K. Hermans, C.W.M. Bastiaansen, D.J. Broer and M.G. Debije,

“Controlling light emission in luminescent solar concentrators through use of dye molecules

aligned in a planar matter by liquid crystals”, Advanced Functional Materials, 19, 2714-2719,

2009.

198

M.G. Debije, P.P.C. Verbunt, B.C. Rowan, B.S. Richards and T.L. Hoeks, “Measured surface

loss from luminescent solar concentrator waveguides”, Applied Optics, 47 (36), 6763- 6768,

2008.

Conference proceedings

P.P.C. Verbunt, M.G. Debije, D.J. Broer, C.W.M. Bastiaansen and D.K.G. de Boer, "Organic

wavelength selective mirrors for luminescent solar concentrators", Proc. SPIE 8438, 2012,

843805

M.G. Debije and P.P.C. Verbunt, “Using liquid crystals to improve the performance of

luminescent solar concentrators”, NSTI-Nanotech 2011, Vol. 1, 584-587, 2011.

P.P.C. Verbunt and M.G. Debije, Progress in luminescent solar concentrator research: Solar

energy for the built environment”, in proceedings of the World Renewable Energy Congress,

2011.

M.G. Debije, M.P. Van, P.P. C. Verbunt, D.J. Broer and C.W. M. Bastiaansen, “The effect of

an organic selectively-reflecting mirror on the performance of a luminescent solar

concentrator”, in proceedings of the 24th European Photovoltaic Solar Energy Conference,

WIP Munchen, 2009, 373-376.

P.P.C. Verbunt, C.W.M. Bastiaansen, D.J. Broer and M.G. Debije, “The effect of dyes aligned

by liquid crystals on luminescent solar concentrator performance”, in proceedings of the 24th

European Photovoltaic Solar Energy Conference, WIP Munchen, 2009, 381-384.

Conference contributions

Oral Presentation at the 2012 SPIE Photonics Europe, Brussels, Belgium entitled: “Organic

wavelength selective mirrors for luminescent solar concentrators”.

Oral Presentation at the 2012 Dutch Polymer Days, Lunteren, the Netherlands entitled:

“Luminescent solar concentrators: Another piece in the energy puzzle”.

Oral Presentation at the 2011 Dutch Polymer Days, Veldhoven, the Netherlands entitled:

“Building integrated light harvesting polymers”. Best workshop lecture award.

List of publications

199

Poster at the 2010 Liquid crystals for Photonics workshop, Elche, Spain entitled: “Liquid

crystals in luminescent solar concentrators”.

Oral Presentation at the 2010 Rolduc Polymer Meeting, Kerkrade, the Netherlands entitled:

“Harvesting light with polymers”.

Poster at the 2010 STW Jaarcongres, Nieuwegein, the Netherlands entitled: “Luminescent solar

concentrators: A solar revolution?”.

Poster at the 24th European Photovoltaic Solar Energy Conference, 2009, Hamburg, Germany

entitled: “The effect of dyes aligned by liquid crystals on luminescent solar concentrator

performance”.

Poster at the 2009 Dutch Polymer Days, Lunteren, the Netherlands entitled: “Improved single

edge emission of a luminescent solar concentrator by planar dye alignment”.

200

“The time has gone, the song is over.”

Pink Floyd

Printed at: Ipskamp Drukkers

Cover design by: Paul Verbunt

Front cover: A highly contrasted picture of a stack of luminescent solar concentrators

illuminated from above. Back cover: An illuminated red luminescent solar concentrator.

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Light management in luminescent solar concentrators : aligned organic dyes and organic

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