Structured luminescent solar energy concentrators : a new route towards inexpensive photovoltaic energy Citation for published version (APA): Tsoi, S. (2012). Structured luminescent solar energy concentrators : a new route towards inexpensive photovoltaic energy. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR724488 DOI: 10.6100/IR724488 Document status and date: Published: 01/01/2012 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected]providing details and we will investigate your claim. Download date: 22. Jul. 2020
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Structured luminescent solar energy concentrators : a newroute towards inexpensive photovoltaic energyCitation for published version (APA):Tsoi, S. (2012). Structured luminescent solar energy concentrators : a new route towards inexpensivephotovoltaic energy. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR724488
DOI:10.6100/IR724488
Document status and date:Published: 01/01/2012
Document Version:Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)
Please check the document version of this publication:
• A submitted manuscript is the version of the article upon submission and before peer-review. There can beimportant differences between the submitted version and the official published version of record. Peopleinterested in the research are advised to contact the author for the final version of the publication, or visit theDOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and pagenumbers.Link to publication
General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.
If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, pleasefollow below link for the End User Agreement:www.tue.nl/taverne
Take down policyIf you believe that this document breaches copyright please contact us at:[email protected] details and we will investigate your claim.
The solar energy market has grown considerably over the last decade due to increasingglobal awareness of environmental issues, the e�ects of greenhouse gases and fossil fuelshortages. More and more areas are now perceived as potential markets for solar energyconversion devices with the ultimate goal of replacing traditional power generating meth-ods. However, in order for electricity generated by a photovoltaic module to be viable forindustrial electricity production, the cost should be about $ 0.06/kWh according to theUS department of Energy. The lowest cost of electricity generated by standard siliconphotovoltaic modules today is still two to three times the price of conventional grid-connected electricity. An attractive alternative to the expensive photovoltaic panels andconcentrated photovoltaic systems is the simple, colorful and cost e�cient luminescentsolar concentrator (LSC).
LSCs consist of a transparent substrate embedded with �uorescent dyes or with thedyes as a thin �lm coating on top of the substrate. Generally, polymeric plates thatare commercially available in large quantities for a low price are used as the transparentsubstrates. The �uorescent dyes in the LSC act as a light converter that absorbs sunlightand emits light at longer wavelengths. A fraction of the emitted light is directed towardsthe edges of the plastic plate where photovoltaic cells are attached to convert light energyinto usable electricity. The simplicity and low material prices of LSCs potentially resultin low energy costs (∼US $0.30/Wp). However, the luminescent solar concentrator ex-hibits several potential light loss mechanisms that limit its e�ciency and practicality formany potential applications. In this work, the possibility of producing a more e�cientluminescent solar concentrator is explored by reducing the potential losses of the systemusing inexpensive and straightforward methods.
First, the behavior of standard LSCs was studied as a function of absorbance and dyeconcentration. Both theoretical and experimental results demonstrated that increases indye concentration above a critical point leads to a negligible increase in edge emission anda decrease in emitted photon to absorbed photon e�ciency. The decrease in e�ciency ismainly a result of internal re-absorption losses, which also increases the external surfacelosses. The former refers to light emitted from dye molecules that is re-absorbed byneighboring dyes, and the latter refers to light lost through the top and bottom surfacesof the �uorescent substrate. To broaden the absorption range of the LSCs, stacked LSCsystems consisting of two substrates doped with di�erent �uorescent dyes was simulatedtheoretically and compared to experimental results. The total edge emission of the stackedLSCs was found to be more than double that of the single substrate devices.
To reduce the probability of re-absorption in the luminescent solar concentrators, thethin �lm dye coating was patterned to create line and square dye arrays. The surface
v
vi Summary
area coverage of the dye coating was reduced through patterning, which in turn decreasesthe probability of emitted light encountering other dye molecules, leading to a decrease inre-absorption losses in the LSC system. This concept was con�rmed experimentally andtheoretically as the emitted photon to absorbed photon e�ciency of the patterned LSCsystems increased with decreasing dye coverage. However, a signi�cant fraction of lightwas lost through the clear regions of the patterned waveguide and the edge emission (inabsolute light intensities) decreased as dye coverage is reduced.
A possible solution for recovering light lost through the clear regions of the patternedwaveguide is to use a lens array to focus incident light on the patterned dye areas. Theobjective of the lens array design was to maximize the acceptance angle while simulta-neously minimizing the focal spot size, which dictates the size of the dye areas required.The resulting lens, designed using ray-tracing techniques, was aspherical in shape andexhibits an acceptance angle of ±30◦. Subsequently, the lens array was combined witha line patterned LSC to form an integrated system. Experimental results demonstratedthat the addition of a lens array resulted in better performing systems where the edgeemission exceeded a fully covered standard luminescent solar concentrator by more than20%.
Introducing a lens array to the patterned LSCs induces preferred emission at twoopposite edges of the substrate, and this e�ect was further enhanced by aligning the dyemolecules using liquid crystals in a guest-host system. Three di�erent dye alignments,isotopic, homeotropic and planar, were studied using patterned LSCs. The planar alignedsystems demonstrated higher emission-to-absorption power e�ciency than both isotropicand homeotropic aligned systems. In addition, emission was enhanced from two edgesin the planar aligned dye system, which exceeded the edge emission of isotropic systemsby 20%. These planar aligned dye systems are potentially advantageous for reducing thematerial cost of LSC solar modules as the preferred emission allows photovoltaic cells tobe attached to two edges of the waveguide instead of four.
A large fraction of emitted photons is lost through the top and bottom surfaces of thewaveguide in the patterned luminescent solar concentrator systems with and without alens array. To limit the surface losses and to further enhance the edge emission of the pat-terned and lens array integrated system, wavelength-selective chiral nematic (cholesteric)liquid crystal re�ectors were employed. The addition of the cholesteric re�ectors generallyincreased the edge emission of the patterned LSC system, both with and without the lensarray, when the position of the re�ection band of the cholesteric was chosen correctly.
The LSCs studied in this work primarily use inexpensive materials and fabricationmethods, and may be suitable for applications in energy generating rooftop installationsin both urban and remote areas. The relatively low cost of energy generated by LSCscompared to standard silicon PV panels makes them an attractive alternative for large-area installations. For example, the large area of industrial building rooftops allows avast number of LSC systems to be installed to produce the required amount of energy.In addition, the integrated patterned and lens system are relatively �at compared tostandard concentrating photovoltaic systems, which makes them more visually appealingin urban environments. The ability of LSCs to alter the solar spectrum, as well as the�exibility in material choices, open new potential applications in areas such as modernagriculture where LSC plates can be used to built energy generating greenhouses.
List of De�nitions
Absolute photon e�ciency:(EphotonsIphotons
), where Ephotons is the total number of photons emitted from the four
edges of the device and Iphotons is the total number of photons incident on the device.
FQY:(�uorescence quantum yield = η =
Photons EmittedPhotons Absorbed
)Integrated edge emission:(
750∑n=350
Pn
), where Pn is the power density emitted from the edges of the waveguide
in [W/nm], and n is wavelength in [nm]. Also referred to as `edge output'.
Photon-to-photon e�ciency:(EphotonsAphotons
), where Ephotons is total number of photons emitted at the edges of the
system and Aphotons is the total number of photons absorbed by the system.
Relative e�ciency:(PemissionPabs
), where Pemission is the power emitted from the 4 edges of the system
and Pabs is the total power absorbed by the system.
vii
Chapter 1
Introduction
1.1 An Overview of Solar Energy and Photovoltaic De-
vices
The sun is our greatest sources of energy and is the energy source for most other natu-ral energy resources commonly used including oil, coal, gasoline as well as green energysources such as biomass. In total, the power emitted by the sun is approximately 3.8×1026
W, and although only a small fraction of this energy (∼1.0×1017 W on average) reachesthe surface of the earth, it is about 7000 times more than the current world consumption(15×1012 W). If we can harvest the energy in solar radiation, it is su�cient for satisfyingthe world's increasing demand for energy. One of the earliest literary references to mendirectly harnessing the sun's energy was in 212BC, when the Greek scientist Archimedesmanaged to set Roman ships on �re by using hundreds of polished shields as metallicmirrors to re�ect sunlight [1, 2]. However, it was not until the 19th century that scientistAuguste Mounchout succeeded in converting solar energy into steam using a parabolictrough collector, creating the �rst ever solar powered steam engine based on concen-trated sunlight. Around the same time, the photovoltaic and photoelectric e�ects werediscovered by Alexendre Edmond Becquerel and Heinrich Hertz, respectively. In 1905,Albert Einstein described light as discrete quanta of energy, i.e., photons, and combinedthe wave-particle light theory to form his mathematical explanation of the photoelectrice�ect using the absorption of photons, which won him the Nobel Prize in Physics in1921 [3]. Combined, these �ndings provided the physical basis for modern solar cells andphotovoltaic modules.
Bell Laboratories created the �rst modern p-n junction silicon photovoltaic cells in the1950's, which for many years were only used for powering space vehicles. The energy crisisin the 1970's brought the world's attention to renewable energy and further advanced solarenergy research. The motivation for solar energy development was and still is the desireto replace the current depletable fossil fuels with renewable resources in an attempt toreduce global pollution and CO2 emissions, which signi�cantly a�ects the climate andenvironment on Earth. In the last two decades, the world has become more aware ofthe environmentally damaging e�ects of fossil fuels and the risk of exhausting the currentenergy resources [2]. Furthermore, for countries with lower supplies of natural energy
1
2 Chapter 1. Introduction
-
+ ++
N-type
P-type
-
electrode
electrode
depletion region
i
Load
Figure 1.1: Working principle of p-n junction in photovoltaic cells
resources, the switch to renewable energy allow these countries to be economically lessdependent on energy import from abroad. As a result, interest in renewable energy, inparticularly solar energy has increased, which accelerated the growth of photovoltaic (PV)markets by 30% per year for the past 5 years [4]. More and more materials, manufacturingmethods, and structures were developed for photovoltaic cells, modules, and concentratorsystems.
The most commonly used material for PV cells is silicon due to its presence in abun-dant quantities in the earth's crust. Initially, standard silicon technology was developedfor transistors and integrated circuits, and the silicon PV cells were fabricated usingmonocrystalline and polycrystalline silicon waste from the semiconductor industry [5].These were termed the �rst generation PV cells. Conventional semiconductor solar cellsconsist of layers of n-doped and p-doped silicon sandwiched together, creating a p-n junc-tion. The n-doped and p-doped silicon have a surplus of negative (electrons) and positive(holes) charge carriers, respectively. When these two oppositely doped materials areplaced together, spontaneous charge transfer occurs across the junction until equilibriumis reached. The charge transfer produces an internal electric �eld and a depletion region isformed (see Figure 1.1). The holes in n-doped silicon and similarly, electrons in p-dopedsilicon are referred to as minority carriers.
Absorption of photons promotes electrons in the silicon to jump from the valence bandto the conduction band, creating electron-hole pairs. The energy di�erence between theconduction band and valence band is a forbidden energy region de�ned as the bandgap.For silicon, this bandgap is about ∼ 1.1 eV. These electron and holes are respectivelyminority charge carriers in both p- and n-doped silicon, and they are attracted to theoppositely charged carriers on either side of the junction. Flow of the light-generatedminority carriers across the junction produces a net photocurrent, thus converting sunlightinto usable electric current. Generally, a higher light absorption creates more charge
1.1. An Overview of Solar Energy and Photovoltaic Devices 3
carriers, which leads to a higher photocurrent and electric power. Photovoltaic cellsbased on the principles of the p-n junction have a theoretical maximum photon-to-electrone�ciency limit of ∼30% [6]. For silicon, this e�ciency limit is ∼25% [7].
The solar spectrum is composed of a wide range of photon energies from nearly 0 toapproximately 4 eV, and photon energy is de�ned by the follow equation:
E = hv =hc
λ(1.1)
where E is energy (in eV), h is the Plank's constant (6.626× 10−34 Js), c is speed of lightin vacuum (∼ 3 × 108 m/s), and λ is the wavelength of light [m]. For photons to beabsorbed by the PV cell, their energy must match or be larger than the bandgap energyof the semiconductor material. When photons with higher energy than the bandgap areincident on a semiconductor, the photon energy is absorbed, but the energy exceedingthe bandgap energy is thermalized [5]. Photons with lower energy than the bandgap arenot absorbed by the semiconductor. In other words, PV cells with a certain intrinsicbandgap and impurities and/or defects will only e�ciently convert wavelengths of lightthat correspond to its bandgap levels. For a highly e�cient PV cell, it is preferable tohave either an incoming light spectrum that matches the bandgap of the PV cell, or a PVcell that can absorb a broad range of wavelengths corresponding to the solar spectrum(see Figure 1.2).
The solar irradiances is determine by the the distance sunlight has to travel throughthe atmosphere, and this distance is at a minimum when the sun is directly overhead,i.e. at the zenith position. Thus, solar irradiance is de�ned according to the optical airmass (AM), which is related to the position of the sun relative to the zenith position asdescribed by equation 1.2:
AirMass = (cosθ)−1 (1.2)
where θ is the angle of the sun with respect to the zenith. Generally, the solar irradiancejust above earth's atmosphere (i.e., the extraterrestrial spectrum) is referred to as airmass 0 (AM0), and the standard terrestrial solar spectrum, measured at an angle 48.2◦
between the sun's position and the zenith, is referred to as air mass 1.5 (AM 1.5) [8].Today, there are many types of PV cells, often referred to as �rst, second, and third
generation PV cells, and are categorized either by the type of material used for cellfabrication or the structure of the PV cell itself. First generation PV cells are singlebandgap solar cells that typically use doped silicon to form the p-n junction. Secondgeneration cells refer to cells that are fabricated using thin �lm technology based onthe working principles of the p-n junction, hence has a theoretical maximum e�ciencylimit. Some second generation cells also use polymers or low molecular weight organicmaterials as the raw material for the fabrication of PV cells. In contrast, the state-of-the-art or third generation PV cells attempts to surpass the intrinsic e�ciency limit of p-njunction cells by use of various methods and materials such as creating a multi-junctioncell. Some of the most commonly produced PV cells of today include polycrystallinesilicon [9, 10, 11], amorphous silicon (a-Si) [12, 13, 14, 15], thin �lm silicon that may bea combination of di�erent grades of crystalline silicon [11], gallium arsenide (GaAs) [16],cadmium sulphide/cadmium telluride (CdTe/CdS) [17, 18], indium gallium phosphide
4 Chapter 1. Introduction
Figure 1.2: Solar spectrum of air mass (AM) 0 (black), AM 1.5 (blue) and the directcomponent of AM 1.5 spectrum at normal incidence (red).
(InGaP) [19, 20, 21], copper indium gallium selenide (CIGS) [22, 23, 24], dye-sensitizedsolar cells (DSC) [20, 25, 26], organic/polymer cells [27, 28, 29], and organic/inorganichybrid cells [30, 31, 32]. Record e�ciencies of all the di�erent types of cells are reported byGreen et al., and a summary of those e�ciencies are given in Table 1.1 [4, 7]. The variousforms of Si solar cells still account for about 84% of the current solar energy photovoltaicsmarket, primarily due to the low cost of silicon as compared to III-V materials, and thewell-developed processing technologies of silicon.
Multi-junction cells, also known as tandem cells, are the most e�cient cells developedto date and have a demonstrated e�ciencies above 40% for a triple-junction cell [33, 34].It is possible for tandem cells to achieve high e�ciencies as they are a combination ofPV cells with di�erent bandgaps arranged such that the bandgap energies of the cellsdecrease from top to bottom. Each of the cells within the tandem cell is used to converta part of the solar spectrum at maximum e�ciency of the cell. Hence a greater amountof the solar spectrum can be absorbed by tandem cells and this ultimate theoreticale�ciency, assuming unlimited stacking of cells, is 69.9% for 1 sun illumination [35]. Onthe other hand, because these tandem cells require the stacking of multiple cells, the costof production is considerably higher than single junction cells. These highly e�cient cellsare most suited for either space applications where weight and size is more importantthan cost, or PV concentrator systems where only a small cell is required as described infollowing sections.
Solar panels, composed of arrays of solar cells, are typically evaluated by the cost ofthe energy it produces, which is given here in units of US dollars per Watt peak (Wp).The measurement unit Watt peak is uniquely used for solar panels to de�ne the power
1.1. An Overview of Solar Energy and Photovoltaic Devices 5
Table 1.1: Photovoltaic cell e�ciencies in 2011 as given by Green et al. [7].
Photovoltaic Cells Best Research-Cell E�ciencies[%]
output of the PV panel when 1000 W/m2 (equivalent to �one sun�) of solar radiation isincident on its surface at room temperature during its lifetime. The energy cost of a solarpanel is then equivalent to the total cost of the panel divided by the peak power or thecapacity of the panel. However, it is also common in the solar industry to de�ne theenergy cost of solar panels as cost per kiloWatt-hour (kWh), which is the total cost of thesolar panel divided by the total amount of energy the panel produces over its lifetime.Regardless which method is used to de�ne the cost of electricity produced by a solarpanel, it is clear that for solar energy to compete with conventional prices of electricity(3-5 cents/kWh) [4, 36], it is essential to reduce the cost of solar converted energy. Thereare several major factors that can be tuned to reduce the energy conversion cost of solarpanels, and these factors together created the current major trends in the solar energyresearch and development �eld.
One way to signi�cantly reduce cost is by increasing the energy conversion e�ciencyof photovoltaic cells. To this end, the inorganic photovoltaic industry has succeeded asdemonstrated in Table 1.1, where the most e�cient PV cells produced to date are basedon inorganic material. This increase in e�ciency has lead to considerable drop in solarenergy prices from US$ 22/Wp in 1980 to US$ 2.00/Wp in 2010 [4, 37] for commerciallyavailable residential systems. Inorganic PV modules are particularly well-suited for largeproduction capacity solar power plants in the mega-Watt range due to their stability, longlifetime (20-30 years), and high e�ciency. However, the solar energy costs are far fromcompeting with the retail electricity prices mentioned above (3-5 cents/kWh). Some ofthe best grid-connected solar power plants located in the sunniest regions on Earth areproducing energy at ∼20-25 cents/kWh [36]. This high price of solar energy convertedelectricity is mainly due to the cost of inorganic materials and processing techniques usedto fabricate solar panels. Because of the high demand for silicon from the semiconductorand the now rapidly growing PV industry, the global supply of silicon becomes tightand material prices rise, further adding to the production cost of silicon based PV panels[5, 38]. Thin �lm silicon solar cells were developed to decrease the content of the expensiveraw material such as silicon in PV cells. Aside from the production costs mentionedabove, the PV panel end user must also bear the cost of installation, daily operations andmaintenance, and energy storage (e.g., battery) for o�-grid applications.
6 Chapter 1. Introduction
Another way to reduce the cost of solar powered energy is by using inexpensive ma-terials and processing techniques. One of the major trends in the PV industry is thatresearchers are continuously seeking alternative materials for the fabrication of e�cientyet inexpensive PV cells. Consequently, organic solar cells based on polymers and organicdyes were developed. Organic solar cells have many advantages over inorganic solar cellsas they utilize cost-e�ective materials and usually can be solution processed on any typeof substrate. This implies that organic solar cells can be fabricated using e�cient tech-niques such as roll-to-roll [39], ink-jet printing and screen printing technologies. It hasbeen reported that a production line of organic solar cells can produce on the order of1000-100,000 m2 per day [40]. On the other hand, for silicon based cells of the similar area,it would take approximately a year to produce such surface area [41]. The organic solarcells are light-weight and can be �exible, which make them very suitable for applicationsin consumer products, residential and commercial systems in the built environment. Theapproximated manufacturing cost of pure organic solar cells ranges from $50-$140/m2. Ifa 5% conversion e�ciency and a lifetime of 5 years are assumed, the energy cost of organicsolar panels is ∼$1.00 - $2.83/Wp [40]. This is not signi�cantly lower in price than currentinorganic panels, and the main reason is their considerably lower conversion e�ciency andlifetime. The highest e�ciency reported to date is ∼8.3%, produced by a bulk hetero-junction polymer solar cell [7], which is far below the ∼25% conversion e�ciency achievedusing single-junction inorganic cells. Also, the lifetime (max. 10 years) and stability oforganic cells are generally lower than inorganic cells. Aside from these disadvantages,organic cells can potentially absorb a wider range of the solar spectrum than inorganicPV as the bandgap in an organic cell are determined by the molecular structure of thematerials used. For example, by changing the arrangement of molecular bonds in thepolymer, the bandgap can be shifted to higher or lower values [42]. In addition, organiccells have a wider range of materials to choose from without sacri�cing production costas most organic materials cost less than inorganic materials.
Organic and inorganic PV cells exhibit di�erent characteristics and fabrication tech-niques, and hence, they are suitable for di�erent applications. Organic cells are �exible,cost-e�cient, which makes them more suitable for portable electronic device, small pack-aging and potentially building integrated PV applications (BIPV) [42, 43]. On the otherhand, inorganic cells have the advantage of high e�ciency and long lifetime, which renderthem most suitable for solar concentrator, solar power plant, and space applications suchas supplying power to remote locations and space vehicles. The applications of inorganiccells have been well established within the last few decades, but applications for organiccells are just emerging. It is to be expected that organic cells as an emerging technologywill slowly increase their market share in the solar energy �eld, particularly in the areasof portable electronics and BIPV [43].
1.2 Photovoltaic Solar Concentrators
Yet another method that has been employed in an attempt to reduce the energy conversioncost of solar energy is solar concentrators [44, 45, 46]. Concentrating photovoltaic (CPV)systems are gaining interest and popularity in the recent years because they potentially
1.2. Photovoltaic Solar Concentrators 7
have economic advantage over current existing solar power plant technologies. CPV thatuses re�ective or focusing optics to concentrate sunlight onto a small area of solar cells havebeen developed in parallel to PV cells. By concentrating solar �ux to higher intensities, asmaller area of PV modules and thus materials can be used, which in turn lowers the costof the complete CPV system. In other words, a CPV system substitutes the expensivePV materials for more cost-e�ective optical devices such as lenses and mirrors. Thismethod also o�ers a higher system e�ciency as it increases the light intensity absorbedby highly e�cient PV cells. CPVs were �rst developed parallel to PV technology and the�rst terrestrial CPV system was built by Sandia in 1977 [46].
CPV systems are generally a joint design of a collector and a receiver (i.e., PV cells),and a wide variety of CPV systems have been proposed in the �eld [44, 45, 47, 48]. Oneof the �rst built was the point-focus system, consisting of a parabolic mirror that re�ectsdirect sunlight towards the receiver, the PV cells, which are located at the focal point ofthe parabolic mirror [49, 50]. The parabolic collector in this case uses its large surfacearea to collect light and concentrates it to a small area solar cell. Thus, the amount ofactive semiconductor material needed is reduced considerably, resulting in a lower energyconversion cost. An added advantage of the small area of PV cells is that an extremelyhigh e�ciency cell can be used to increase e�ciency of the CPV system as the price ofthe PV cells is small compared to the cost of the complete PV concentrator system.
The system discussed above uses re�ective optics to concentrate light on the receiver,but it is also possible to use focusing optics to concentrate transmitted light. Most ofthe CPV systems using focusing optics exploit the optical properties of Fresnel lenses toconcentrate direct sunlight on the PV cells [48, 51, 52, 53]. Fresnel lenses typically appearas either linear (2D type) or circular grooves (3D type) in a glass or transparent polymericplate and were developed to signi�cantly reduce the thickness of normal spherical lenses.The Fresnel lens uses step-wise, asymmetrical prism-like surfaces to simulate the curvatureof a standard, smooth lens surface, where the thickness of the surface prisms decreasesradially from the center outwards. This implies that the thickness of the Fresnel lensescan be much thinner than a standard geometric lens, but the focusing is less precise. Sinceprecise focusing is not needed for CPV systems and thinner optical devices are preferredfor light-weight and compact packaging, Fresnel lenses are well-suited for this application.In the optical system of the CPV, the Fresnel lenses are the primary optical systemused for focusing sunlight on or near the re�ective secondary optical element (SOE). Theobjective of the SOE is to increase the acceptance angle of the optical system and/or toincrease uniformity of the irradiance over the area of the PV cell.
A popular solar concentrator system in recent years is the compound parabolic con-centrator (CPC) system [54, 55, 56]. The CPC is formed by two parabolas that intersecteach other at a point below its focal point as shown in Figure 1.3a. An opening is createdat the focal point of the two parabolas and the PV module is placed at the opening.This ensures that any light ray entering the cone formed by the partial parabolas will bere�ected towards the PV module. The working principle of the CPC is based on non-imaging optics, and its concentration power is solely dependent on the acceptance angle[57, 58]. As Figure 1.3b illustrates, a compound of parabolas can focus solar radiationfrom various angles to a common focal point and does not need to track the sun. Hence,the CPC system is extremely e�cient and is often called the ideal concentrator because
8 Chapter 1. Introduction
Figure 1.3: (a) A schematic of outward facing compound parabola concentrator's work-ing principle (Source: Edmund Optics), and (b) the working principle of inward facingcompound parabola concentrator [56].
it has a large acceptance angle, high concentration power, and uses the principle of to-tal internal re�ection, which implies no metallic re�ection losses. However, the re�ectiveprinciples of the parabola produce largely non-uniform distribution of light irradiation onthe PV cell, resulting in high failure risk and instability of the PV cell. In addition, thelarge amount of expensive optical quality glass needed for the manufacturing of CPCsgreatly increases the cost of CPC, making them less attractive.
Many of the CPV systems necessarily incorporate solar trackers in their design. Ini-tially, PV concentrators were �xed in place and titled at an angle to the horizontal planeto optimize solar radiation incident on its surface. In the 1960's, Saavedra presented anelectronic automatic tracking mechanism, which allows the tracking of the sun's positionthroughout the day [59]. Solar irradiation has two components, di�use and direct light.Standard PV concentrators can only re�ect or focus the direct beam component of solarradiation. The percentage of di�use versus direct beam component of the solar radiationvaries throughout the day and is generally di�erent for di�erent locations on earth. Thisimplies that the direct beam component on the surface of a PV plate depends on its
1.2. Photovoltaic Solar Concentrators 9
location and tilt angle. For �xed PV plates or concentrators, the direct beam irradiatingits surface will vary with the sun's position throughout the day and year. Solar trackingallows the concentrator optics or the PV module to follow the sun's position, therebymaximizing the amount of direct beam irradiation on the surface of the concentratorthroughout the day [60, 61, 62]. A PV module with a solar tracker can produce up to57% more energy than the �xed system [59]. Though sun-tracking improves the energygain of PV plates and concentrators, the cost of sun-tracker including energy consump-tion and maintenance costs, greatly increases the total cost of the PV module and thegenerated solar energy. The cost and reliability considerations of the sun-tracker detractsfrom its gain in PV systems.
Photovoltaic energy concentrator systems have been demonstrated to achieve highersystem e�ciency than standard PV panels alone [63]. However, CPV systems with suntrackers require tremendous amount of space and investment capital. As a result, CPVsystems are most suitable for large solar power plant (e.g., >10 MW) applications andare typically built in deserts where there is land available and the direct beam portionof solar radiation is much greater than the di�use component. Considering the total costand complication of CPV systems including optical system, solar tracker, installation,maintenance, energy required to operate the solar tracker, and the area needed for itsbuilding, it is undoubtedly not suited for applications in the built environment. ThoughCPVs are well suited for large power plant applications and were originally designed forsuch applications, the cost of electricity produced from CPV plants (6-10 cents/kWh) isstill considerably higher than wholesale prices o�ered by conventional fossil fuel powerplants (2-3 cents/kWh) [44]. Until the price of conventional energy have risen su�cientlyto be comparable to the energy cost of CPV plants, CPV systems cannot compete withconventional energy markets. It is also di�cult for CPV systems to compete with �at PVpanels in the market of small remote area residential power as the di�erence in system costbetween a �at PV panel and a concentration system is small (∼$2/W), and thus does notgive consumers su�cient incentive to apt for a more complicated concentrating systemthan a standard �at PV panel. Furthermore, the lack of standardization and certi�cationof the CPV systems generates consumer distrust as it is di�cult for end users to determinethe reliability and economics of CPV systems. All these factors together have preventedCPV systems from become widely commercialized [44, 47].
With the continuous development of higher e�ciency PV cells, the overall systeme�ciency of CPVs are expected to increase in the near future, which will continue to reducethe price of electricity produced by CPV power plants. The electricity cost produced fromCPV power plants are in fact lower than the retail cost of electricity in many regions of theworld. For example, the average retail cost of electricity in Europe in 2011 ranges fromapproximately 7-11 ecents/kWh as reported by Europe's Energy Portal [64]. In addition,governments in many countries (e.g., Germany, Spain, UK, Australia, etc.) have o�eredlong term incentives and tax breaks for both consumers and investors in solar and otherrenewable energy. Governmental support combined with the continuous price drop ofsolar converted electricity create new markets and opportunities for the CPV systemssuch as support for power generation in remote areas.
10 Chapter 1. Introduction
Dye
PMMAWaveguide
Figure 1.4: Light rays incident on luminescent solar concentrator are absorbed andre-emitted at longer wavelengths by luminescent particles. The re-emitted light is partlywaveguided to the edge of the substrate and partly escapes the top and bottom surface ofthe substrate.
1.3 Luminescent Solar Concentrators
One way to give solar concentrator systems a more competitive edge in the current solarenergy markets is to seek other inexpensive methods for concentrating sunlight. Duringthe energy crisis in the 1970's, Weber and Lambe [65] proposed a method of concentratingsunlight by means of a luminescent solar concentrator (LSC). A few years later, Batchelderet al. [66, 67] provided a detailed theory and experimental analysis of the possible LSCe�ciencies, as well as characterization techniques of the LSC. An LSC typically consistof a transparent plate impregnated or coated with luminescent species such as organicdyes, quantum dots and/or lanthanide complexes [68, 69, 70, 71, 72, 73, 74]. When lightis incident on the LSC, it is absorbed by the luminescent particles and re-emitted at alonger wavelength (see Figure 1.4), i.e. the down conversion of light. The LSC replacesthe expensive large area solar panel for light absorption with inexpensive polymer plates,thus lowering the costs of solar energy.
The portion of the light that is emitted at an angle greater than the critical angleof the transparent plate will be trapped by total internal re�ections (TIR) and directedtowards the edge of the plate where solar cells are attached. The critical angle of anymedium can be found using Snell's law (see equation 1.3) [75].
ni sin θi = nt sin θt (1.3)
where ni is the refractive index of the surrounding medium of the incident ray, and nt isthe refractive index of the medium that the transmitted ray is traveling through. θi isthe incident angle of the light ray with respect to the normal of the surface and θt is theangle of the transmitted or refracted ray. Snell's law is the basis of the Law of Refractionand it was empirically discovered by Willebrord Snellius in 1621.
1.3. Luminescent Solar Concentrators 11
ni
nt
θi
Incident ray Refracted ray
θt
ni
nt
θc
Incident ray
Refracted rayθt
ni
nt
θi
Incident ray
Refracted rayθt
(a)
(b)
(c)
Figure 1.5: Schematics of the critical angle concept where ni and nt are the refractiveindices of material surrounding the incident ray and refracted ray, respectively, and θc isthe critical angle. In (a) θi<θc, and thus the refracted ray is transmitted; (b) illustrates thecase where θi=θc, which results in the refracted ray traveling at the surface of the medium(i.e., θt is 90
◦); and in the case of (c) θi>θc, total internal refractions occurs.
12 Chapter 1. Introduction
To calculate the critical angle of the LSC, ni is taken as the refractive index of theplastic waveguide, and nt is usually air, which has a refractive index of 1.0. An illustrationof critical angle principle is given in Figure 1.5. To trap light in the transparent waveguide,the condition for total internal re�ection must be satis�ed. When light is emitted from theluminescent species inside or on top of the waveguide, there exists a critical incident anglewhere the transmitted ray lies horizontal to the surface of the substrate. This implies thetransmitted angle θt is 90◦ and sin(θt) is 1, resulting in the critical angle equation:
θc = sin−1[1
ni] (1.4)
where θc is the critical angle, and ni is the refractive index of the waveguide.Equation 1.4 denotes that the higher the refractive index of the waveguide, i.e., ni,
the smaller the critical angle. Smaller critical angles implies more light can be trappedand waveguided to the edge of the LSC. Light emitted at angles smaller than the criticalangle escapes through the top and bottom surfaces of the plate, and the cone of anglesfor which the light escapes is referred to as the escape cone.
The escape cone losses or surface losses are but one of the several loss mechanisms inan LSC, which have lead to its low conversion e�ciencies. In addition, the e�ciency ofLSCs is strongly size dependent, which is also related to the geometric gain of the LSC.The geometric gain (Cg) of an LSC is the ratio of the area at which light is incident tothe area of the waveguide edge (i.e. Aface
Aedge) where PV cells are attached. This geometric
gain together with the absorption e�ciency of the LSC plate determines the theoreticale�ciency limit of the LSC [76, 77, 78]. For example, recent reports demonstrated theo-retically that an organic dye based LSC with a bandgap of 2 eV and a geometric gain ofmore than 5 has a power conversion e�ciency limit of ∼7%, assuming no other waveg-uiding enchancement elements are added [78]. This e�ciency limit of LSCs was recentlyachieved by Sloo� et al. [79] using an LSC with geometric gain=2.5 and GaAs PV cellsattached to all four edges, and it is comparable to highest conversion e�ciency of organicsolar cells reported (∼8.3%).
The reason for the low conversion e�ciencies of LSCs is better understood when oneexamines the photon �ux gain Gp (i.e., collection e�ciency) of the LSC as de�ned in thefollowing equation [80]:
Gp =AfaceAedge
QAηf (1.5)
where Aface is the area of the LSC for which light is incident, Aedge is the area of the LSCedge, and Aface
Aedgeis the geometric gain (Cg) of the LSC.QA is the fraction of incident photons
absorbed by the LSC plate, η is the �uorescence quantum yield, and f is the fraction ofphotons trapped inside the waveguide and transported to the edge of the waveguide wheresolar cells are attached. To obtain the highest possible photon gain, η, f, and QA needto be as close to 1.0 as possible. In reality, there are many loss mechanisms in the LSC,which lead to low conversion e�ciency of solar energy. Often the luminescent materialcan only absorb a small fraction of the solar spectrum, which leads to a small QA. It ispossible to increase QA by stacking LSCs, or placing multiple dyes and/or various sizesof quantum dots into the same waveguide. When light is incident on the waveguide, a
1.3. Luminescent Solar Concentrators 13
PV C
ell
ReflectionIncident Light
Absorption
Re-absorption TIR
Surface Scattering
Escape Cone Losses
θc
Figure 1.6: A cross-section schematic of the luminescent solar concentrator is shown.Light incident on the LSC is absorbed by a luminescent particle (◦) embedded in thesubstrate, which subsequently emits light rays in long wavelengths. The emitted light ispartially directed to the edge via total internal re�ections (TIR), and some losses are in-curred on the way including escape cone losses, re�ection losses, re-absorption and surfacescattering losses.
small fraction of the light will be re�ected from the surface of the waveguide. For PMMAand PC, the re�ection is ∼ 4-5% for normal incident light. Once the incident photonsare absorbed and re-emitted by the luminescent material, the fraction of photons thatare waveguided to the edge of the LSC (f) depends on the angle of emission as explainedabove. Any photons that are emitted below the critical angle will escape through the topand bottom surfaces of the LSC and are conssidered as escape cone or surface losses (seeFigure 1.6).
The escape cone losses account for ∼ 40% of the absorbed light energy (50%-70%photons) lost in a 50×50mm2 LSC [81]. If the waveguide surface is not optically smooth,there may also be scattering losses at the surface of the waveguide for both re-emittedand incident light. Fluorescent quantum yield (η) is de�ned as:
η =number of emitted photonsnumber of absorbed photons
(1.6)
For most common organic dyes used in LSCs, η is extremely high ranging from 0.9to 1.0. The other commonly used luminescent material, quantum dots, have somewhatlower η that ranges from 0.5 to 0.8 [82]. Assuming no re-absorption, f is directly relatedto the escape cone dictated by the waveguide material and for isotropic emission, it canbe de�ned as [80]:
14 Chapter 1. Introduction
f = cos(θc) = (1− 1
n2LSC
)1/2 (1.7)
In equation 1.7, nLSC is the refractive index of the LSC. Another crucial loss mecha-nism in LSCs is re-absorption, also known as self-absorption losses. Re-absorption occursas a result of the small Stokes-shift, which is intrinsic of organic dyes, the most commonmaterial used for LSC. A small Stokes-shift results in an overlap between the absorptionand emission spectrum of the luminescent material. This implies that photons emit-ted at wavelengths within the overlap region can be re-absorbed by another luminescentparticle. If the �uorescent quantum yield of the luminescent particle is less than unity, re-absorption can lead to a loss of photons and energy, and in particular, the �rst occurrenceof reabsorption usually results in the largest amount of energy loss compare to subsequentreabsorption events. Furthermore, in the event of re-absorption, light that was originallytrapped by waveguide will be re-emitted, resulting in re-randomization of the direction oflight. Every re-emission event produces more escape cone losses, i.e., a decrease in f , aspart of the re-emitted light is likely to be refracted out of the top and bottom surfaces.Hence, in the case of x re-absorption and re-emission events, f becomes [80]:
f = (1− 1
n2LSC
)x/2 (1.8)
For a material with refractive index n of 1.5, the fraction of photons trapped in thewaveguide without (i.e., x=0) and with two re-absorption events (i.e., x=2) is calculatedto be ∼75% and ∼56%, respectively, using equation 1.8. It is evident that non-unity η incombination with many re-absorption events signi�cantly lower LSC e�ciencies.
To avoid re-absorption losses, many have investigated luminescent materials other thanorganic dyes, e.g., quantum dots and lanthanide complexes containing luminescent rareearth ions. Quantum dots have other advantages over organic dyes in that their absorptionspectra can be tailored by changing their nanocrystal size. The spread and range ofquantum dot sizes determine the absorption properties of the LSC, and thus the widthand position of the absorption band can then be easily tuned to cover a larger portionof the solar spectrum while ensuring a large Stokes-shift. However, to date, quantumdots were reported to have relatively low quantum yield and absorption coe�cient (0.5-0.8) [68, 82]. Lanthanide complexes, though also exhibit a large Stokes-shift and nore-absorption losses, have a small absorption range and low �uorescence quantum yieldrelative to organic dyes [83]. Organic dyes on the other hand exhibit high quantum yield(near unity) compared to quantum dots and lanthanide complexes. Aside from the broadrange of colors that is available for organic dyes, new organic dye molecules with largerStokes-shift and better re-absorption properties are continuously being development.
Despite the low e�ciency and many possible loss mechanisms in an LSC, it has severaladvantages over a traditional concentrator PV systems. Luminescent materials have theadvantage of being able to absorb both direct and di�use components of solar radiation,hence a solar tracker is not required. The luminescent material in the LSC can be chosensuch that the emission wavelengths match the bandgap energy of the PV cells, hence de-creasing photon energy losses and improving the e�ciency of the PV module. In addition,multiple luminescent materials can be added to one waveguide to increase and to better
1.3. Luminescent Solar Concentrators 15
CrystallineSi cells
10 20
1.0
2.0
3.0
Thin FilmCells
OrganicSolarCells
0
Cos
t per
Wat
t [€/
W]
Efficiency [%]
LSC
30
Figure 1.7: The estimated cost per watt of various solar conversion systems as a functionof e�ciency [77, 88, 89, 40].
match the absorption of the LSC to the solar spectrum. The same e�ect can also beachieved by stacking LSCs that consist of luminescent materials with di�erent absorptionrange. On the aspects of appearance, a wide range of colors can be achieved as the colorof the LSC depends on the choice of luminescent material.
One of the greatest advantages of an LSC is that it uses cost e�ective material andmanufacturing techniques. The large transparent plate of the LSC is typically fabricatedusing polymers, and the luminescent material, in particular �uorescent dyes, can be eitherdirectly embedded into the polymer during polymer extrusion, or solution-deposited ontop of the plastic plate after fabrication. It does not require expensive vacuum thin �lmdeposition technologies nor complex clean-room processing techniques typically used forthe fabrication of inorganic PV cells. Luminescent solar collectors are typically fabricatedusing polymers such as poly(methyl methacrylate) (PMMA) and polycarbonate (PC),which have well-established manufacturing techniques and low bulk prices (∼e3/kg) [84,85, 86]. Recently, the ratio of cost-per-Watt ($/W) of standard LSC plates relative to PVpanels has been determined by Farrell et al. [78] as function of the material costs of LSCplates. According to their calculations, the cost of standard LSC plates must be 11% ofthe cost of the PV panels for the $/W of LSCs to be comparable to PV panels. The costof PV panels are approximately e600/m2 [87], and the cost of LSC plates is e30/m2 [87],which is 5% of the cost of PV panels. For a LSC plate size of 10×10×0.5 cm3, the costper Watt of the LSC plates is approximately 80% of that of PV panels according to theresults of Farrell et al. [78]. An estimated cost per Watt of di�erent solar eneregy systemsas a function of e�ciency is illustrated in Figure 1.7.
LSCs have unique characteristics of light-weight, �exibility in shape, size, and color,
16 Chapter 1. Introduction
which make them attractive for many applications aside from the established applicationsof organic and inorganic PV cells. For example, LSC systems can be used to replacesignage or posters, and windows of commercial and industrial buildings to create energygenerating devices. In the case of signs and posters, the message and images of the sign canbe screen or ink-jet printed on the surface of a transparent plate using organic dye solutionsas ink. The LSC generated energy can then be used to create self-su�cient signs thatautomatically lit up during the night. The transparency of the LSC plates are particularlyattractive for the application area known as building integrated photovoltaics (BIPV).BIPV aims to replace conventional materials used for the outer envelope constructionof a building with photovoltaic materials to partially support the electric power supplyneeded for the building. LSCs as semi-transparent, solar energy conversion units can beeasily incorporated in buildings as windows. The wide range of possible visual appearanceof LSCs give them a distinct advantage over PV modules as windows on future �green�buildings. The low cost of energy generated by LSCs also make them suitable for large-area installations such as small solar power plants in remote areas. In this case, a su�cientnumber of LSC systems can be installed to produced the required energy supply despitethe lower e�ciency of the LSCs. However, the current short lifetime (10 years) and lowe�ciency of the LSCs in comparison to silicon PV plates limit its potential applicationsin small-area rooftop installations and commercialization in the solar market.
1.4 Motivation and Objectives
Although LSCs cannot compete at the moment with �at panel PV modules or CPVsystems in e�ciency, they potentially have a great advantage over these systems in costand �exibility of appearance. The visual appearance (i.e., shape, size, and color) ofthe light-weight, semi-transparent LSCs can be easily altered, and thus they are attrac-tive to architects and designers for integration into the built environment with possibleapplications as large-area energy generating rooftop installations, windows, and self-litadvertisement signs. For these applications, the amount of energy the LSC can generateis important as the LSCs need to supply power to other devices or the device itself. Hence,if these applications are to be realized, it is essential to improve not only the stability andlifetime of the luminescent materials, but also the e�ciency of the LSC. It is expectedthat by increasing the e�ciency of the LSC system, the cost-per-Watt of the system willbe reduced.
It is well-established that to improve LSC e�ciency, the main photon loss mechanismsin the LSC such as re-absorption losses and escape cone losses must be minimized [67, 90,77, 81, 91]. Most attempts in reducing re-absorption losses have been made by replacingthe most common used luminescent material, organic dyes, with materials that exhibita large Stokes-shift. However, these materials generally have the disadvantage of eitherlow absorption, low �uorescence quantum yield, or narrow absorption band compared toorganic dyes. In order to exploit the near unity �uorescence quantum yield (FQY) inconjunction with the high absorption of the organic dyes, the re-absorption losses that isintrinsic to most �uorescent dyes used in the LSC must be limited.
It is the objective of this thesis to decrease the cost-per-Watt of thin �lm organic dye-
1.5. Organization and Scope of Thesis 17
coated LSCs by improving the system e�ciency through the use of novel methods to limitre-absorption and escape cone losses. To limit re-absorption losses, one must decreasethe probability of re-absorption of emitted light as it is waveguided towards the edges ofthe substrate. The re-absorption probability of light rays in the waveguiding mode canbe reduced via patterning the thin �lm coating of organic dyes. By patterning the dyecoating, the amount of dyes on the surface of the waveguide is greatly reduced, therebylimiting the probability of interaction between emitted light and dye molecules. However,decreasing dye coverage will create clear regions on the surface of the waveguide, leadingto a decrease in total photon absorption. Furthermore, photons incident on the clearregions will be lost as it is transmitted through the bottom surface of the waveguide. Toachieve a similar photon absorption as fully covered LSCs, an aspherical lens array wasdesigned to collect an incident cone of light and subsequently focus this cone of light ontothe dye structures in the patterned thin �lm LSC system. In this work, the performanceof various patterned thin �lm LSC system without and with the lens array is investigatedexperimentally and compared with simulation results of the two systems. In addition, thee�ect of selective wavelength cholesteric re�ectors on the pattern thin �lm LSC system incombination with the lens array was investigated. The center wavelength of the cholestericre�ectors was varied to match the emission band of the organic dyes in an attempt tominimize the surface losses of the LSC by re�ecting emitted light back into the system.Finally, the combined bene�ts and e�ects of the lens array and the cholesteric re�ectorare presented.
1.5 Organization and Scope of Thesis
The aim of this thesis is to study the possibility of reducing photon losses in luminescentsolar concentrators with perylene organic dyes as the luminescent material. In chapter2, �lled and thin �lm dye-coated LSCs are modeled using ray-tracing techniques andcompared with experimental results. The performance of LSCs using two di�erent organicdyes are examined.
Chapter 3 investigates the e�ect of patterning organic dye thin �lm coatings of theLSC. Regular square and line patterns of thin �lm dye coatings are fabricated on polymericsubstrates and the energy e�ciencies of the patterned LSC systems were studied froman experimental and theoretical viewpoint. A ray-tracing model based on the physicalpatterned LSC systems was used to simulate the performance of the patterned LSCs. Theresults of the physical system were subsequently compared with simulated results.
In chapter 4, the design and fabrication of aspherical lens arrays for patterned LSC sys-tem are discussed. The shape of the physical lens arrays is compared with the theoreticaldesign and the acceptance angle of the physical lens arrays is investigated. Subsequently,a line patterned LSC is combined with the lens arrays, and the performance of this inte-grated system is compared with the reference LSC system.
In order to reduce escape cone losses, selective-wavelength cholesteric re�ectors areintegrated with the cylindrical lens array and patterned LSC system in chapter 5. Thecholesteric re�ectors have a tunable re�ection and transmission band, which allows light inthe absorption band of the organic dye to pass through while re�ecting emitted light back
18 Chapter 1. Introduction
into the LSC system. This chapter investigates the performance of the patterned LSCsystems combined with the lens arrays and cholesteric re�ectors. The center wavelengthof the re�ection band of the cholesteric re�ectors is varied to match the emission band ofthe organic dye used in the fabrication of the patterned LSCs.
The e�ect of aligning dye molecules in the patterned LSC systems is examined inchapter 6. In this chapter, the alignment of dye molecules in the thin �lm dye coatingwas induced by liquid crystals in a guest-host system. The dye molecules were aligned ina homeotropic and planar fashion, and the edge emission output of the two aligned LSCsystems was investigated in detail. The performance of the aligned patterned systems wassubsequently compared with the isotropic patterned LSCs.
In the last chapter, chapter 7, the main conclusions of this thesis and an assessmentof the LSC technology are discussed. Recommendations for future research are also givenin this chapter.
Chapter 2
Modeling of Luminescent Solar
Concentrators
2.1 Introduction
Luminescent solar concentrators (LSCs) have attracted attention in the solar energy �eldas an alternative to standard photovoltaic concentrators primarily due to their low pro-duction costs. An LSC normally utilizes luminescent species, in particular �uorescentorganic dyes, impregnated in a polymeric waveguide to absorb sunlight and emit photonsat a longer wavelength. The photons trapped in the waveguide are directed towards thewaveguide edges where PV cells are attached. Thus, sunlight is absorbed by the largesurface area of the substrate and concentrated onto small area PV cells at the edges of thesubstrate. The polymeric LSC plates are produced by extruding or injection molding amixture of organic dyes and polymer melt. Here, the �uorescent dyes are instead dissolvedin an arcylate-based matrix used to coat the top surface of the waveguide. This coatingmethod allows the fabrication of LSCs on �exible substrates and opens up the potentialfor inexpensive roll-to-roll production of LSCs. This chapter studies the performance ofLSCs produced using these two methods experimentally and theoretically using modelsbased on ray-tracing. From here on, LSCs fabricated by impregnating the waveguide withdye molecules are referred to as �dye-�lled LSC� and those made by coating the waveguidewith a thin �lm of dyes in an acrylate-based matrix are referred to as the �thin �lm� or�dye-coated� LSCs.
It is important to understand the luminescence of organic dyes to grasp the workingprinciples of the LSC, and obtain insight on how to improve their performance. For lumi-nescence to occur, an electron in its ground state must absorb su�cient energy for it to beexcited to a higher quantum state. The energy that is required for the electron to jumpfrom the ground state to an excited state is equal to the energy di�erence between the twostates. Once the electron is in the excited state, it can relax back to the ground state inseveral ways: 1) radiative relaxation resulting in emission of photons (i.e., �uorescence);2) non-radiative or thermal relaxation where energy is internally converted to molecularvibrations or phonons; 3) �uorescence quenching where energy is directly transferred toa nearby molecule as a result of molecular interaction such as dipole-dipole interactions,spatial overlap of molecular orbitals, or stacking or aggregation of dye molecules; and 4)
19
20 Chapter 2. Modeling of Luminescent Solar Concentrators
phosphorescence in which excited electrons are converted to a triplet state and subse-quently relax by emitting a photon or heat dissipation [67, 92]. The radiative relaxationprocess determines the �uorescence quantum yield of the luminescent material (see equa-tion 1.6 of chapter 1). To have a highly e�cient LSC, it is preferable that every electronrelaxation process is radiative and results in emission of photons. Thus it is crucial tohave su�ciently low concentration of dye molecules and uniform distribution of molec-ularly dissolved dyes in the LSC to limit possible quenching of photon emission due tomolecular interactions between dyes or dye and matrix molecules.
Shortly after the proposal of LSCs in late 1970's by Weber and Lambe, and Goet-zberger et al. [65, 88], researchers used many di�erent approaches to model the perfor-mance of the LSC to gain insight into the optimal design. Monte Carlo simulations havebeen used to investigate the performance of single and double-stacked thin �lm LSCs[93], and they found that for both systems, the e�ective gain (equal to geometric gainmultiplied by the e�ciency of the system) was higher when the systems were irradiatedwith di�use light. In addition, the double-stacked thin �lm LSC system was noted tohave nearly double the gain of a single thin �lm LSC. The random-walk theory has beenused to illustrate the e�ect of re-absorption in dye-�lled LSCs where a small overlap inthe absorption and emission spectra of the �uorescent dye can signi�cantly diminish thegain of the LSC [94]. This reabsorption e�ect increases signi�cantly with increasing dyeconcentrations and/or dimensions of the LSC plate. Thermodynamic models have alsobeen used to evaluate the possible e�ciencies and losses of �uorescent material dopedwaveguides [70, 78, 95]. More recently, the re-absorption probability model proposed byWeber et al. was realized using a ray-tracing model, and the experimental re-absorptionresults of dye impregnated, thin �lm and liquid luminescent solar concentrator systemswere compared to simulation results [96]. In this modeling study, the dye-�lled LSCs werefound to behave similarly to the thin �lm LSCs.
Ray-tracing is a well-established technique that allows one to track the path of lightrays through di�erent mediums. It is generally used for designing optical components anddevices [97]. In this chapter, ray-tracing techniques are used to simulate the performanceof two di�erent LSC systems, dye-�lled and thin �lm coated. To date, a comparison ofthe edge emission and e�ciency of the two LSC systems using both experimental and ray-tracing techniques has not been investigated. The aim here is to verify the ray-tracingmodel of LSCs and to evaluate the model using experimental data from two di�erentLSCs systems. Similarities and di�erences between the two systems are discussed. Itis expected that a stacked multi-dye LSC system has the performance advantage of anextended absorption region over a single-dye system. To examine this theory in moredetail, the ray-tracing model was used to simulate the performance of a stacked LSCsystem where two LSCs, one impregnated with perinone nonyl phenol dye and the othera commercial perylene dye (perylene-1,7,8,12-tetraphenoxy-3,4,9,10 tetracarboxylic acid-bis-(2'-6'diisopropylanilide)), were placed on top of each other. The e�ect of varying dyeconcentration and waveguide dimensions on the edge output of the stacked system werestudied using this ray-tracing model and a prediction of the optimum dye concentrationwas made.
2.2. Experimental Setup 21
Figure 2.1: Molecular structure of perylene-1,7,8,12-tetraphenoxy-3,4,9,10 tetracar-boxylic acid-bis-(2'-6'diisopropylanilide) (Lumogen F Red 305, BASF) [98]
2.2 Experimental Setup
LSCs in this chapter were produced using two di�erent organic dyes: Lumogen Red 305(BASF) and perinone nonyl phenol (Sabic Innovative Plastics). The molecular structureof the Lumogen Red 305 [98], hereafter referred to as Red305, is shown in Figure 2.1.The molecular structure and synthesis of the perinone nonyl phenol dye was reported byDebije et al. in reference [99] and the structure is shown in Figure 2.2.
2.2.1 Fabrication of LSCs
50×50×3 mm3 polycarbonate (PC) waveguides �lled with various concentrations of perylene-1,7,8,12-tetraphenoxy-3,4,9,10 tetracarboxylic acid-bis-(2'-6'diisopropylanilide) (LumogenRed 305, BASF) ranging from 0.002-0.212 wt% were produced by Sabic Innovation Plas-tics via injection moulding. Another set of PC waveguides with similar dimensions andrange of dye concentrations were fabricated using perinone nonyl phenol dye (Sabic In-novation Plastics). Thin �lm coated LSC were prepared by bar-coating dye mixtureson clear polymethylmethacrylate (PMMA, Plano Plastics) or polycarbonate (PC, SabicInnovation Plastics) substrates. Both PMMA and PC substrates have an surface area of50×50 mm2. The thickness of the PMMA and PC plates were 5 mm and 3 mm, respec-tively. The dye solution consisted of a 75/25 mixture of di-pentaerythritol pentaacrylate(DPPA, Polysciences) and methylmethacrylate (MMA, Aldrich) with 1 wt% photoinitia-tor Irgacure 184 (Ciba Speciality Chemicals), and Lumogen Red 305 (BASF). MMA wasadded to the di-pentaerythritol pentaacrylate to decrease the viscosity and increase theprocessibility of the solution for succeeding procedures. The concentration by weight ofLumogen Red 305 in the dye mixtures was varied from 0.01-1.5%. The mixtures wereapplied to clear PMMA substrates at 65◦ using bars of di�erent wire thicknesses (40-120micron). Additional thin �lm LSC samples were prepared by spin-coating the mixtureson clear PMMA and PC plates at 1000 RPM for 30 s. The acrylate thin �lms werephotopolymerized in a nitrogen environment. MMA has a high vapour pressure (4 kPa
22 Chapter 2. Modeling of Luminescent Solar Concentrators
syn anti
Figure 2.2: Molecular structure of perinone phenol. The R in the structure is a nonylgroup. [99]
at 20◦) and evaporates from the dye mixture after it has been coated on the substrateand during the polymerization process. Hence, the �uorescent dyes in the thin �lm LSCsystems were most probably embedded in a poly(di-pentaerythritol pentaacrylate) matrixwith trace amounts of MMA.
2.2.2 Characterisation of LSCs
The �lm thickness of the bar-coated samples were varied to examine the e�ect of coatingthickness on the performance of the LSC samples. A Fogale Zoomsurf 3D optical pro�lerwas used to measure the thickness of the dye coating of thin �lm LSCs. A small frag-ment of the �lm was removed from the substrate prior to the measurements. The �lmthicknesses of the bar-coated LSC samples were measured to be between 31 and 106 µm,depending on the type of bar used for the coating. Images of the surface-coated thin �lmand �lled samples were taken in transmission bright�eld mode using an inverted phaseand �uorescence optical microscope (Olympus IX70) equipped with a 100x oil immersionobjective lens (Olympus, NA = 1.3, refractive index of oil n = 1.516) and a tungstenhalogen lamp source. In transmission mode, the sample was illuminated from above thestage by a tungsten halogen lamp, and any non-absorbed light, as well as �uorescencefrom the organic dyes, passes into the microscope optical train where it can be directed tothe eye piece and CCD camera system. Larger dye aggregates (on the order of 10 µm) inthe samples can be seen in the optical images taken in the bright�eld transmission modeof inverted optical microscope.
Excited state photon lifetimes of the dye-�lled and dye-coated samples were investi-gated using time-resolved �uorescence spectroscopy (Lifespec ps F900 spectrophotometer,
2.2. Experimental Setup 23
Figure 2.3: Measured spectrum of the solar simulator light source used in edge emissionmeasurements of all samples studied.
Edinburgh Instruments) based on time correlated single photon counting techniques. Thesamples were excited using a ∼400 nm pulsed laser diode (PDL-800-B, Picoquant) andemission of photons was record at wavelengths 570, 650, and 710 nm. The three measuredemission wavelengths 570, 650, and 710 nm correspond to the starting edge, center andtail of the emission spectrum of the perylene-based Red305 dye embedded in polycar-bonate substrate. For all temporal �uorescence measurements, photon count was set to5000.
Absorbance spectra of all the samples were determined using Shimadzu UV-3102 spec-trophotometer (UV-Vis): absorbance data reported in this thesis refer to the peak of themain absorption band. All absorbance measurements were executed using the absorbancespectra of clear waveguides as background, and thus the e�ect of the waveguide was elim-inated from the transmission measurements of the samples. The edge emission of thewaveguide was determined by placing a LSC sample with one edge directed towards theopen port of the integrating sphere equipped with a SLMS LED 1050 light detection ar-ray (Labsphere). The sample was illuminated at a distance of 15 cm by a 300 W solarsimulator (Lot-Oriel), with �lters to approximate the 1.5 air mass (AM) (global) solarspectrum. The spectrum of the solar simulator is given in Figure 2.3. It is noted thatthere are characteristic peaks from the light source at wavelengths above 750 nm, whichwill mask any emission signal from the sample at those wavelengths. Hence, for all theedge emission values reported here, the values were integrated from 350-750 nm.
The setup for edge emission measurement is shown in Figure 2.4. The LSC sampleholder was painted black, and acts as an absorber. The error in all measurements madeusing the integrating sphere and spectrophotometer was estimated to be ∼5%.
Edge emission spectra were converted to the number of emitted photons by dividingthe emission energy at every wavelength by the energy of a single photon (see equation1.1). The edge output power was calculated by integrating the emission spectrum from
24 Chapter 2. Modeling of Luminescent Solar Concentrators
Edge Emission
Detector
Baffle
Sample
Integrating Sphere
Light source from solar simulator
Figure 2.4: Schematic of integrating sphere setup for edge emission measurements of theLSC
350 to 750 nm. The outputs of all 4 edges from the sample were summed and a totaloutput for the sample was calculated. Total power absorbed by the samples was calculatedby multiplying the absorption spectrum of the samples by the solar simulator spectrum,and integrating the combined result from 350 - 750 nm. The relative e�ciency of the LSCsystems was calculated using the follow equation:
PemissionPabs
(2.1)
where Pemission is the sum of the power emitted from the four edges of the waveguideand Pabs is the total power absorbed by the samples. The emitted-to-absorbed photone�ciency was calculated according to the equation 2.2:
EphotonsAphotons
(2.2)
where Ephotons is total number of photons emitted at the four edges of the waveguide andAphotons is the total number of photons absorbed by the samples.
2.3 Ray-tracing Model of LSCs
Modeling of the dye-�lled and thin �lm coated LSCs was performed using a commercialray-tracing software (LightTools, Optical Research Associates). Ray-tracing simulationsof LSCs were performed using 1,000,000 incident rays with the ray power threshold set at1×10−5 W. The ray power threshold de�nes the power cut-o� limit for tracing, e.g., a raywith power below 1 × 10−5 W will be ignored by the model, and its path will no longerbe traced. This allows the user to decrease simulation time. Here, the threshold was
2.3. Ray-tracing Model of LSCs 25
set to ensure all rays with substantial power are traced while maintaining a reasonablesimulation time.
The 50×50 mm2 PMMA and PC substrates used in the experiments were simulatedby entering the transmission spectrum of the substrates for the range of wavelengths stud-ied. Thickness of the modeled substrates and thin �lm coatings were varied to match thephysical systems. For simplicity, refractive indices of PMMA and PC were assumed tobe constant for all wavelengths, and their values set to 1.49 and 1.59, respectively. Forthe thin �lm LSCs, the model assumed the dye impregnated pentaacrylate-based coat-ing to have the same refractive index as the PMMA waveguide for all wavelengths. Thisassumption is not completely accurate as the embedded dyes will change the refractive in-dex of the pentaacrylate matrix despite both dipentaerythritol pentaacrylate and PMMAexhibiting similar refractive indices of ∼1.49. Since only a trace amount of dyes is presentin the acrylate matrix, the di�erence in refractive indices is likely to be small and havenegligible e�ects on the edge emissions of the samples. The edges of the waveguide weremodeled as 100% transmissive Lambertian scatterers to approximate the edges of thephysical substrates, which were not polished after cutting. Both top and bottom surfacesof the waveguides were de�ned as optically smooth, which implies that the surfaces arenon-scattering.
The measured solar simulator spectrum was used as the light source in the model (seeFigure 2.3). The modeled light source was set to emit uniformly at normal incidence tothe surface of the substrate, mimicking the experimental setup. Fluorescence of the dyemolecules was simulated using the ray-tracing software's built-in algorithm. The measuredabsorption and emission spectra of the 0.002 wt% Lumogen F Red305 dye (BASF) in across-linked PMMA matrix with a quantum yield of 1.0 [100] were used as inputs for the�uorescence algorithm. The �uorescence algorithm in the modeling software treats dyemolecules as individual particles in a medium and uses the average absorption distanceof photons to depict the absorption of light by the dye molecules in the medium. Thisaverage absorption distance of photons l is inverse proportional to the density of particlesn (in mols per volume) and the cross-sectional area σ of the medium containing theparticles as shown in equation 2.3.
l =1
(nσ)(2.3)
In the case of �ourescent dyes, the average absorption distance of photons is wavelengthdependent. For the dye-�lled and dye-coated thin �lm LSCs, the Lambert-Beer law (equa-tion 2.4) was used to convert the transmission spectra of the samples into wavelength-dependent absorption distance as follows:
T =I
I0= e−nσx (2.4)
where T is the transmittance (i.e., the ratio of transmitted light intensity to incident lightintensity), I is the transmitted intensity, I0 is the incident light intensity, x is the pathlength of light through the medium in [m], n is the density of particles in [mol/L], σ is thecross-sectional areal of the medium in [m2]. According to equation 2.3, the nσ in equation2.4 can be replaced with l−1, and the transmittance of the dye-embedded medium as a
26 Chapter 2. Modeling of Luminescent Solar Concentrators
function of mean free path becomes:
T = e−x/l (2.5)
where l is the average absorption distance of photons in the medium and x/l is thusthe number of times photons can be absorbed by dye molecules as it travels throughthe thickness of the dye-embedded medium. The absorptance spectrum (A% [%]) wasobtained from the measured transmittance (T ) spectrum of the samples using equation2.6, where re�ection from the waveguide is accounted for by subtracting the transmittancespectrum of the clear waveguide from the sample measurements.
T = 1− A% (2.6)
Recently, Wilson et al. [101] reported that the absorption tail (630-750 nm) of theRed305 dye can become signi�cant to the performance of LSC, particularly for largesheets of LSC, though most studies of LSCs using Red305 dye prior to this report hadassumed the cut-o� of the Red305 absorption spectrum was at ∼630 nm. This absorptiontail could not be measured by our UV-Vis spectrophotometer due to its lower sensitivityand resolution. To better simulate the absorption of the Red305 dye, the absorption taildata reported in [101] were incorporated into the measured absorption spectrum of allRed305 coated samples. This was done by adding the reported absorption tail to ourmeasured extinction coe�cient. Extinction coe�cient is characteristic of a material andis calculated according to equation 2.7:
ε =A
cl(2.7)
where ε is the extinction coe�cient in [L][mol][m]
, A is the measured absorbance, c is thedye concentration in [mol/L] and l is the path length of transmitted light in [m]. Asthe extinction coe�cient is constant regardless of dye concentration or pathlength, it ispossible to extrapolate the absorption tail of the Red305 dye at di�erent concentrationsand path lengths using the extinction coe�cient. The extrapolated absorption tails wereincorporated in all the simulations of LSCs.
The �uorescence in the model is described by the probability of photons encounter-ing a dye molecule as determined by the wavelength-dependent mean free path of thephotons. Photons encountering a dye molecule are either absorbed, or continue to followan undeviated path. Subsequently, the probability of dye molecules emitting a photon iscalculated based on the �uorescence quantum yield and the emission spectrum given bythe user. In this case, the emission spectra given to the program prevented up-conversionof light, i.e. the wavelength of emitted light must be longer than the wavelength of ab-sorbed light. The error in all the model measurements was estimated to be ∼5%, whichwas determined by the spatial sampling interval of the model.
2.4. Results and Discussion 27
2.4 Results and Discussion
2.4.1 Dye-�lled LSC systems
The performances of the PC waveguides doped with varying concentrations of Red305�uorescent dyes were examined both experimentally, and theoretically. Edge emissions of50×50×3 mm3 Red305 �lled LSC systems were measured and simulated for absorbanceranging from ∼0.05 to ∼2.4 and the results are shown in Figure 2.5a. Figure 2.5b givesthe photon-to-photon e�ciency (equation 2.2) of the dye-�lled systems where the numberof edge emitted photons are divided by the total number of absorbed photons.
The simulated results in Figure 2.5a demonstrated that the number of photons emittedat the edges increased proportionally with increasing absorbance for absorbance < 1.0.When absorbance is >1.0, the rate of increase in edge emission was reduced. The reduc-tion in this rate of increase is partially due to the absorbance being logarithmic, implyingthat the increase in photon absorption for peak absorbance above 1.0 is small, i.e., peakabsorbance of 1.0 and 3.0 corresponds to an absorption (A%) of 90.6% to 99.9%, respec-tively. At absorbance values below 1.0, edge emission was found to be nearly porportionalto absorption. However, the relationship between edge emission and absorption becomesmore non-linear at absorbance values above 1.0. This non-linearity may be a result ofincreased re-absorption and probability of dye aggregation at higher dye concentrations.Edge emissions measured from the experimental system showed similar trends as thesimulated results with the greatest di�erence between the two sets of data being ∼10%,which is within the accumulated experimental and model error. This suggests that theperformance of dye-�lled LSCs is well simulated by the model.
For absorbances <1.0, the photon-to-photon e�ciency was noted to increase propor-tionally with increasing absorbance values in both measured and simulated results. Athigher absorbances (>1.0) and dye concentrations (∼0.04 wt%), the e�ciency plateaus,and even slightly decreases. The further increase in light absorption at higher dye con-centrations is o�set by increased reabsorption in the system, resulting in smaller increasesin photon emission and lowering of the system e�ciency. Therefore, increasing dye con-centration produces proportional increases in the edge emission and e�ciency of 50×50mm2 dye-�lled LSC systems given its absorbance is <1.0.
To con�rm that reabsorption increases with increasing absorbance, the edge emissionspectra of both the model and the physical systems are plotted in Figure 2.7. The shape ofall the simulated edge emission spectra matched well with the experimentally measuredspectra. The emission peaks in both the model and the physical systems were notedto red-shift as absorbance (and dye concentration) of the samples increased. It is wellknown that re-absorption process in LSC systems lead to red-shifting of emission peaksas photons and photon energy is lost during this process [94]. In addition, re-absorptionand subsequent re-emission result in increased surface losses in the LSC system as afraction of the re-absorbed photons are re-emitted within the escape cone and is lost tothe system. Therefore, it is not surprising that the edge emission plateaus and photon-to-photon e�ciencies decreases at high absorbance values where probability of re-absorptionis the highest. The tail of the simulated emission spectra were noted to be di�erent thanthe measured spectra and this is partially due to the dye emission spectra used in the
28 Chapter 2. Modeling of Luminescent Solar Concentrators
(a)
(b)
Figure 2.5: An experimental (•) and modeling (�) comparison of (a) total photon edgeemission, or the sum photons emitted at all four edges of the LSC and (b) photon-to-photon e�ciency (equation 2.2) as a function of absorbance of 50×50×3 mm3 Red305�lled polycarbonate systems.
2.4. Results and Discussion 29
Figure 2.6: Experimental (solid symbols) and simulated (open symbols) total photonedge emission at all 4 edges for the 50×50 mm2 Red305 �lled polycarbonate systems withthicknesses ranging from 1 (4), 2 (�) and 3 mm (◦).
model. The spectrum of the light source (Figure 2.3) showed a large characteristic peak atwavelengths above 750 nm and this peak will likely mask the emission signal of the LSCsystems beyond this wavelength. Hence, the emission spectrum of the dye in the modelwas cuto� at 750 nm, where the emission of the dye was not completely diminished. Thismay have led to the large tail observed in the simulated emission spectra, which was notapparent in the measured data.
To further verify the ability of the model to simulate the edge emissions of the dye-�lledLSC systems, the waveguide thicknesses of the dye-�lled systems were varied from 1-3 mmand the model was used to simulate the edge emissions of these systems. The simulatedand experimental results are plotted in Figure 2.6 as a function of absorbance. For allthe waveguide thickness studied, the simulated results were observed to be in agreementwith the experimental data. Small di�erences between the simulated and experimentaldata may be attributed to waveguide imperfections as well as measurement errors. Forexample, at extremely high and low absorbances, the sensitivity of the spectrophotometerwas not su�cient for precise measurements of the sample's absorption spectrum. Thismay have led to greater errors in the experimental results, which in turn increase errorsin the simulations as the model uses experimental absorption spectra as its input.
2.4.2 Thin �lm LSCs
Performance of thin �lm dye-coated LSCs were simulated using a similar model as the oneused above for dye-�lled LSC waveguides. In this model, the dye molecules were insteadembedded in a thin �lm atop the waveguide rather than impregnating the waveguide.Again, the model used the experimentally measured absorption and emission spectra ofthe thin �lm samples as parameters in the �uorescence algorithm. The edge emissionof bar-coated samples with dye layer thicknesses varying from ∼30 µm to ∼106 µm,
30 Chapter 2. Modeling of Luminescent Solar Concentrators
(b)(a)
Figure 2.7: (a) Measured and (b) modeled emission spectra of LSC systems embed-ded with various concentration of Luomogen Red305 resulting in absorbance values 0.05(black), 0.19 (red), 0.46 (blue), 1.0 (pink), 1.63 (green), and 6.4 (dark blue).
and absorbances ranging from 0.05 to 2.2, were studied in detail and results are shownin Figure 2.8a. Figure 2.8b compares the experimental photon-to-photon e�ciencies ofthese dye-coated samples with simulated results as a function of absorbance and dyeconcentration.
The simulated edge photon outputs exhibit a similar trend as the experimentallymeasured data in Figure 2.8a. In both cases, the number of photons emitted from theedges of the waveguide increases proportionally with increasing absorbance when sampleabsorbance values are <1.0. At absorbance values >1.0, the positive slope of the edgeemission plot begins to plateau, i.e., the edge emission approaches a maximum value. Thisis similar to the edge emission trend observed in the dye-�lled systems (Figure 2.5a), whichcon�rms that increasing dye concentration and absorbance of LSCs will only produce aproportional increase in edge emission at absorbance values <1.0. Continued additionof dyes to the sample beyond the absorbance of 1.0 will diminish the increase in edgeemission.
The simulated edge emissions at absorbances >1.0 were observed to be higher thanthe experimental values, which may be attributed to the model underestimating surfacelosses of photons in the system. The �uorescent dye used was dichoric, absorbing andemitting photons anisotropically [102], which is not accounted for in the model, resultingin miscalculations of reabsorption and surface losses in the simulations especially at higherabsorbances [81, 103]. The ray-tracing software (LightTools) does not allow the user tospecify the emission pro�le of the dye, and the default de�nition for dye emission pro�lein the model is isotropic.
There is some scatter in the experimental photon-to-photon e�ciency data at lowabsorbances (< 0.5), likely due to thin �lm coating thickness di�erences of the samples.Representative values of measured e�ciencies of the dye-coated systems and corresponding�lm thickness and absorbance values are given in Table 2.1. Samples 1 and 2 in Table 2.1have similar absorbance values but the coating thickness of sample 2 is more than doublethat of sample 1. However, the photon-to-photon e�ciency of sample 2 is lower than
2.4. Results and Discussion 31
(a) (b)
Figure 2.8: (a) Total photon edge emission (sum of photons emitted at all four edgesof the waveguide) and (b) photon-to-photon e�ciency (equation 2.2 of Red305 thin �lmcoated 50×50×5 mm3 PMMA waveguides plotted as a function of absorbance and dyeconcentration (in the inset graph). A comparison of the experimental (•) and modeled(�) results are presented in both plots.
Table 2.1: Measured photon edge emissions (integrated from 350-750 nm) and photon-to-photon e�ciencies of dye-coated LSC systems with varying absorbance values and �lmthicknesses.
Sample Number Absorbance Film Thickness Edge Emission Photon-to-Photon[µm] [# of photons] E�ciency[%]
that of sample 1. This strongly suggests that the scatter of data points in the e�ciencyplot (see Figure 2.8b) at absorbance <0.5 is a result of the di�erence in �lm thicknesses.Generally, at absorbances >0.5, a thicker dye-coating produces more e�cient systems,which is evident if one compares the e�ciency and �lm thickness of samples 3 and 4, aswell as samples 5 and 6 in Table 2.1.
A small decrease in photon-to-photon e�ciency was observed in both experimental andsimulation data with increasing absorbance and dye concentration (see Figure 2.8). Thisdecrease in e�ciency is more evident when sample absorbances are above 1.0. The combi-nation of photon edge emission and e�ciency plots (Figure 2.8) suggests that increasingdye concentration in the thin �lm systems at absorbances >1.0 produces minimal increasein edge emission at the expense of decreasing e�ciency as denoted by the negative slopeof the e�ciency plot. These results indicate that it is important to �nd the optimum dyeconcentration for a particular LSC con�guration to realize the optimum LSC system.
32 Chapter 2. Modeling of Luminescent Solar Concentrators
50 μm
50 μm
(a) (b)
(d) (e) (f) (g)(c)
Figure 2.9: Fluorescence microscopy images of (top) dye-�lled samples containing (a)0.078 wt% and (b) 0.212 wt% and (bottom) thin-layer samples containing (c) 0.1 wt%,(d) 0.25 wt%, (e) 0.5 wt%, (f) 0.75 wt%, and (g) 1.0 wt% of Red305 dye.
The measured photon-to-photon e�ciency values of all dye-coated samples for ab-sorbance >0.25 appeared to be slightly lower than the simulated values for the range ofabsorbances studied (see Figure 2.8b). The average di�erence between the model and ex-perimental values in the photon-to-photon e�ciency plot was ∼15% (see Figure 2.8b). Aprobable explanation for this di�erence is that the dye molecules may stack or aggregatein thin �lm at high concentrations where the dye molecules are less likely to be uniformlydispersed due to the lack of space. Aggregated molecules can lead to quenching of �u-orescence, and thereby lowering the apparent �uorescence quantum yield (FQY) of thephysical systems [104, 92]. To verify aggregation in the thin �lm samples, optical �uores-cence microscopy was used to image both �lled and surface-coated devices. The resultsmay be found in Figure 2.9. It is possible that there are many smaller dye aggregatesin the samples than presented in Figure 2.9 as they would be undetectable at the lowmagni�cations of the optical microscope. However, the photon-to-photon e�ciency re-sults of the dye-�lled samples show no signs of quenching, consistent with previous works[100] where no quenching was noted in dye-�lled LSCs up to a concentration of 1000 ppm(∼0.42 wt%). If dye aggregates are present in the �lled LSCs, their size and number maybe too small to have a noticeable e�ect on the performance of the LSC. For the thin �lmsystems, the optical images show obvious dye aggregates and they appear to increase innumber and size as dye concentration increases, being most evident at dye concentrations≥ 0.75 wt%. This suggests that at very high dye concentrations, the coated samples maybe working in the quenching regime resulting in a lower edge emissions and e�cienciesthan the simulated values from the model.
The possibility of aggregations in both dye-�lled and dye-coated samples were furtherinvestigated using time-resolved �uorescence spectroscopy. The �uorescence decay of both
2.4. Results and Discussion 33
(a)
(c)
(b)
(d)
Figure 2.10: Examples of �uorescence decay of perylene-based Red305 dye-�lled (leftgraphs) and dye-coated (right graphs) LSCs. Plots (a) and (b) show the decay as a functionof optical densities (OD), where both are recorded at emission wavelength 710 nm. Plot(c) and (d) illustrate the respective �uorescence decays of OD 2.36 dye-�lled and OD 1.46dye-coated samples recorded at emission wavelengths 570, 650, and 710 nm.
dye-�lled and dye-coated LSCs were recorded at wavelengths 570, 650, and 710 nm. Theoptical density (OD) of both LSC systems were varied to studied the in�uence of OD onthe �uorescence lifetime of the di�erent systems. Examples of these measurements aregiven in Figure 2.10.
The average lifetime of the emitted photons were obtained by �tting the tail of the de-cay curves in Figure 2.10 using a non-linear least-squares procedure given by the followingequation: [92]
I = C + a1et/τ1 + a2e
t/τ2 (2.8)
where C, a1, a2 are constants, t is the measurement time in [ns], and τ1 and τ2 are thelifetimes of the photons in [ns]. The fraction of photons that exhibit the �tted lifetime (τ1and τ2) can be obtained using the relative pre-exponential factor (B), where the intensityamplitudes a1 and a2 are divided by the total intensity of the system (i.e., a1τ1+a2τ2).This relative intensity factor B allows one to examine the change in photon lifetimes of
34 Chapter 2. Modeling of Luminescent Solar Concentrators
Table 2.2: Lifetimes (τ), relative pre-exponential intensity factors (B), and χ2 residualparameter for the �tting of �uorescence decays of Red305 dye either embedded or coated onPMMA waveguides with various optical densities at di�erent emission wavelengths withexcitation at 400 nm.
τ [ns] B [%] χ 2 τ [ns] B [%] χ 2 τ [ns] B [%] χ 2
the LSC devices as a function of optical density (which is equivalent to absorbance andproportional to absorption) and emission wavelength.
Given that the dye molecules are completely isolated in the matrix, and no energytransfers between the dyes molecules occur after excitation, the �uorescence decays canbe �tted using a single lifetime constant τ irrespective of dye concentration. Moreover, thevalue τ should be similar at all emission wavelengths for samples with no dye aggregations.However, this was not the case for both the dye-�lled and the dye-coated samples as shownin Table 2.2.
At emission wavelength 570 nm, the �uorescence decay of the dye-�lled samples canbe accurately described by a single lifetime, but this lifetime was found to increase withincreasing optical density. The increase in time constant τ suggests that dye aggregatesare present at higher optical densities (and dye concentration) as energy transfers withinaggregates result in longer-lived emissions. In contrast, the �uorescence decays at 650 and710 nm were better described by two lifetime constants. Again, both lifetime constantswere noted to increase with increasing optical density (OD).
Previous literature has described average lifetimes of perylene bisimide derivativesas ∼4 ns and ∼12-17 ns [92, 105], where the shorter lifetime (4 ns) is characteristic ofemissions from isolated or 'monomeric' perylene bisimide molecules, and the longer photonlifetime (12-17 ns) is attributed to dye aggregates and excimer emissions. Similar theoriesmay be applied to the perylene-based Red305 dye doped polycarbonate substrates. Hence,the shorter �uorescence lifetimes ∼4 ns observed in the studies of dye-�lled devices may beattributed to isolated perylene-based Red305 molecular emission, and the longer lifetimesmay be ascribed to excimer-type emissions. In addition, the values of τ were observed
2.4. Results and Discussion 35
to be similar for both emission wavelengths 650 and 710 nm at the same optical density.This suggests that the �uorescence lifetime changes little at wavelengths >650 nm, andemissions at these longer wavelengths can be attributed to similar fractions (see valuesof 'B' in Table 2.2) of isolated and aggregated dye molecules in the dye-�lled systems.The longer lifetimes of 7-10 ns that described the �uorescence decay at OD 2.36 stronglysuggests that only dye aggregates are present in the polymer matrix at such high opticaldensities.
The �uorescence decays of dye-coated systems at OD 0.18 were �tted with two lifetimeconstants, ∼3.3-4 ns and ∼8 ns. This shorter lifetime (4 ns) is in agreement with the �uo-rescence lifetime of dye-�lled LSCs, and is likely attributed to isolated molecular emission.The longer lifetimes at 8 ns is most probably characteristic of aggregated emission and itcorresponds to the lifetime noted at OD 2.36 of the dye-�lled samples. Again, as opticaldensity of the samples increased, the emission lifetimes of all the dye-coated devices alsoincreased, with the exception of the OD 0.76 sample. Interestingly, a shorter (2-2.5 ns)and longer (∼10 ns) lifetimes than the other two dye-coated samples were observed in theOD 0.76 sample. One possible explanation for the uncharacteristically short time constantof ∼2 ns is e�cient energy transfer from an excited isolated molecule to an aggregatedmolecule. Moreover, the longer lifetime constant ∼10 ns may indicate larger aggregatesare present in the OD 0.76 sample. The �uorescence lifetime of the dye-coated sampleswere generally longer than the dye-�lled samples at emission wavelengths >650 nm, whichsuggests that the dye-coated devices may exhibit more and larger aggregates than dye-�lled systems due to the lack of space for molecular distribution. This accounts for thelower photon-to-photon e�ciency of the thin �lm systems compared to the theoreticallysimulated values as dye aggregates increases the probability of �uorescence quenching inthe experimental LSC system, which was not incorporated into the modeling. The overallresults of the �uorescence decay time studies above strongly suggests that dye aggregatesare present in the used acrylate matrix of the dye-�lled and thin �lm LSC systems evenat very low optical densities. Furthermore, the emissions at longer wavelengths (above650 nm) are mostly attributed to dye aggregates.
2.4.3 A Comparison of Dye-Filled and Thin Film LSCs
The theoretical performance of two di�erent thin �lm LSC systems were simulated andcompared with the simulated results of dye-�lled systems to better evaluate the two typesof LSC systems and to investigate the e�ect of refractive index of the dye-coating on theperformance of thin �lm LSCs. The thin �lm device 1 consisted of PC plate coated withRed305 dyes in a PC matrix, and system 2 comprises of PC plate coated with Red305dyes in a PMMA matrix. PC was also used as the waveguide in the dye-�lled systems.The edge emission of system two LSC con�guration and dye-�lled LSC are reported inFigure 2.11, which clearly demonstrates that thin �lm coated LSCs can produce similaredge emissions to dye-�lled waveguides when refractive index of the coating matches thatof the waveguide in the thin �lm systems. When absorbance <0.5, emission values of alldi�erent LSCs were within experimental error of each other. As absorbance increases,the e�ect of refractive index mismatch in device 2 becomes more apparent and the edgeemission decreases compare to system 1 and the dye-�lled LSC. The overall lower edge
36 Chapter 2. Modeling of Luminescent Solar Concentrators
Figure 2.11: The simulated total edge emission (sum of emission at all 4 waveguide edge)integrated from 350 nm-750 nm of Red305 dye-�lled (•) and dye coated (50×50×3 mm3)polycarbonate waveguides are compared. Two thin �lm dye coated systems are studied, inwhich the dye coating consist of dyes embedded in a polycarbonate (�) or PMMA (4)matrix.
emission of system 2 may also be in part due to the lower refractive index of the PMMAdye matrix (∼1.49) as compared to the PC dye matrix of 1.59 in system 1, leading to lessTIR of dye emissions in the dye matrix. Comparing the results of Figure 2.12 with Figure2.11 indicate that the refractive index of the waveguide has considerably more e�ect onthe edge emission of the LSC than the thickness of the waveguide.
The experimental performance of thin �lm coated LSCs were compared to dye-�lledLSCs by plotting their edge emissions as a function of absorbance in Figure 2.12. Notethat the thickness of the PMMA waveguides used in the thin �lm devices was 5 mm, 2mm thicker than the PC plates used in the dye-�lled LSCs. With 67% thicker waveguide,one would expect the edge emission of the PMMA thin �lm system to be higher than thePC dye-�lled systems. However, the edge emission of the thin �lm LSCs were found tohave lower edge emission than the dye-�lled LSCs in the absorbance range of 0.05-2.5.The bene�ts of increased thickness were at least partially o�set by the reduced refractiveindex of PMMA (1.49) compared to PC (1.59), which allows more photons to be trappedby total internal re�ection (TIR). Overall, the thin �lm LSCs performed less well thanthe dye-�lled LSCs, despite the simulations which predicted that thin �lm and dye-�lledsystems have similar behaviors and comparable performances as demonstrated in Figure2.11 and reference [96]. This is due to more and perhaps larger dye aggregates in the thin�lm LSCs as suggested by the longer �uorescence decay lifetime compared to the dye-�lledLSCs (see Table 2.2). Again, a greater number of dye aggregates increases the probabilityof quenching, thereby resulting in lower edge emission from the thin �lm systems.
2.4. Results and Discussion 37
Figure 2.12: A comparison between measured total edge emission (sum of emission at all4 edges of the waveguide) integrated from 350 nm-750 nm for dye-�lled (•) and thin �lmcoated (�) LSCs. Dye-�lled LSCs were fabricated as 50×50×3 mm3 polycarbonate platesdoped with Red305, and the thin �lm coated LSCs were produced by bar-coating 50×50×5mm3 PMMA plates with Red305 dye mixture.
2.4.4 Stacked LSCs
In the previous section, I established that the ray-tracing model can be used to simulatethe performances of dye-�lled and thin �lm coated LSC systems. The model can now beused to gain insight in the performance of LSCs with various size, thickness, absorbanceand waveguide material. In this way, an optimum absorbance and size can be determinedfor LSCs and/or stacks of LSCs doped with di�erent organic dyes. Stacked LSCs havethe advantage of increasing absorption and emission of the LSC system without losingphotons in the waveguide mode (i.e., undergoing total internal re�ection TIR) to energytransfers between organic dyes. In addition, photons that are not absorbed and/or escapethe bottom surface of the upper LSC can enter and be absorbed by the dyes embeddedin the lower LSC plate. Adding multiple species of organic dyes to the same waveguidealso increases the absorption range of the LSC, but this leads to potential quenchingof luminescence. Moreover, it has been demonstrated through ray-tracing calculationsthat adding a dye with lower �uorescence quantum yield (i.e.,50%) to a multi-dye dopedwaveguide reduces the overall quantum e�ciency of the device [77]. This is due to energytransfers from dye molecules with higher �uorescence quantum yeild (FQY) to ones withlower FQY, which increases photon losses substantially. The increase in photon losses andpotential of quenching incurred in the single waveguide doped with multiple dyes are notpresent in stacked LSCs as di�erent species of dyes are contained in separate substrates.Therefore, adding a waveguide doped with low FQY dyes to the stacked LSCresults in anincrease in system e�ciency [77, 91].
The stacked LSC system here uses two di�erent species of dyes, perylene-1,7,8,12-tetraphenoxy-3,4,9,10 tetracarboxylic acid-bis-(2'-6'diisopropylanilide) and perylene peri-none nonylphenol [71], which will be referred to as Red305 and perinone dye, respectively.
38 Chapter 2. Modeling of Luminescent Solar Concentrators
Figure 2.13: Representative absorption (line) and emission spectra (dash line) of peri-none nonyl phenol(black) and Lumogen F Red 305 (grey) dye.
Absorption and emission spectra of the two dyes are shown in Figure 2.13. The two dyeswere separately embedded in PC waveguides, which were stacked and separated by an airgap to prevent out-coupling of total internal re�ected photons. The absorption band ofthe perinone nonylphenol dye coincides with both the emission and the absorption bandof the Red305 dye, and so can absorb both photons that are not absorbed by the Red305dye as well as those emitted into the bottom escape cone of the top waveguide. This lattere�ect is important to the performance of the stacked system as a signi�cant fraction ofabsorbed photons are lost through the surfaces of the waveguide. It is thus more advan-tageous to use the perinone LSC as the lower layer of the stacked LSC system. Anotheradvantage of placing the Red305 doped LSC on top is that the quantum e�ciency of theRed305 is ∼1.0, which is higher than that of perinone nonylphenol dye ∼0.8 [71, 100]. Byplacing the Red305 doped LSC on top, the quantum e�ciency of the stacked LSC systemwill not be reduced by the lower FQY of the perinone nonylphenol dye in initial photonabsorption and re-absorption.
This stacked system was evaluated both experimentally and theoretically using theray-tracing model for the single dye-�lled LSC device. A list of the absorbance valuesof perinone and Red305 dye-�lled LSC examined in this section are given in Table 2.3.The performance of stacked LSCs was �rst examined by placing Red305 dye-�lled LSC,in particular samples with absorbance 0.19, 1.63 and 2.36, on top of the perinone dye-�lled LSCs with various absorbances. Subsequently, the position of the Red305 andperinone dye-�lled LSCs were switched and the performance of the di�erent stacked LSCarrangements were evaluated and compared.
The setup of the model used to evaluate the performance of the stacked LSC systemsis illustrated in Figure 2.14. Simulated edge emissions of stacked LSC systems with threedi�erent Red305 LSCs, each lying on top of perinone LSCs with a variety of dye con-
2.4. Results and Discussion 39
Table 2.3: Absorbance and Reference Names of Two Dye-Filled Polycarbonate LSCs
Figure 2.14: An example of (a) side view and (b) angled view of the stacked LSC modelcreated in ray-tracing software (LightTools). In the model, a light source is placed 10 cmabove the stack of two LSCs, perinone nonyl phenol (top plate) and Red305 (bottom plate)dye-�lled polycarbonate plate separated by an air gap. Receivers are placed on top, bottomand at all 4 edges of the stacked LSC to collect all light rays exiting the LSCs.
40 Chapter 2. Modeling of Luminescent Solar Concentrators
(a) (b)
Figure 2.15: Modeled (open symbols) and experimental (solid symbols) (a) total edgeemission (sum of emission at all four edges) integrated from 350 - 850 nm, and (b)relative e�ciency (equation 2.1) of stacked LSC systems consisting of three Red305 dye-�lled waveguides, with absorbance values: 0.19 (�), 1.63 (4), and 2.36 (◦), each on topof perylene perinone nonylphenol dye-�lled LSC of various dye contents.
tents are reported in Figure 2.15a. Speci�cally, the three Red305 LSCs studied exhibitedabsorbance values of 0.19, 1.63, and 2.36, which will be referred to as R0.19, R1.63, andR2.36, respectively. Similar to the LSC systems studied in the previous sections of thischapter, the model edge emissions were found to increase with increasing absorbance ofthe perinone LSCs for absorbance below 1.36. For perinone absorbance above 1.36, theedge emission of the three stacked systems plateaus. Not surprisingly, the highest edgeemission was produced by the stacked system consisting of Red305 LSC with the highestabsorbance (2.36) on top of perinone LSCs of various absorbances. The edge emission ofthe R1.63 top/perinone bottom system was found to be similar to the R2.36/perinone stackarrangement, which suggests that the increase in absorbance from 1.63 to 2.36 does nothave an signi�cant e�ect on the edge emission of the stacked system. This again con�rmsour previous theory that at su�ciently high absorbances, the edge emission approachesan maximum, and continual increase in dye concentration (and absorbance) producesnegligible increases in edge emission.
Similar edge emission trends were observed in the experimental data of the samethree stacked LSC systems. The measured edge emissions were found to be, on average,15% lower than the simulated values. This di�erence partially can be accounted forby considering the anisotropic emission pro�le of the dye molecules in the experimentalsystem compared to the isotropic emission pro�le of the model, in which the former mayresult in more escape cone losses.
The simulated and measured relative e�ciencies (equation 2.1) of the three stackedLSC systems are displayed in Figure 2.15b. For all stacked systems studied, the e�cien-cies were found to decrease with increasing absorbance of the perinone LSC. This is tobe expected since the probability of re-absorption increases with increasing dye concen-tration and absorbance. By replacing a low absorbance (0.19) Red305 LSC with a higher
2.4. Results and Discussion 41
absorbance (1.63) sample in the stacked system, the e�ciency increased by an average of∼16-27% for all doping concentrations of the perinone LSCs studied. Clearly, it is bene-�cial to the performance of the stacked system to have a higher absorbance Red305 LSCon top of the perinone LSCs. However, the e�ciency of the R2.36 top/perinone bottomsystem exhibited similar simulated e�ciencies as the R1.63(top)/perinone(bottom) sys-tem. The e�ciency of the system appear to be una�ected by the increase in absorbanceof the R2.36 sample compared to R1.63, which is similar to the trend observed in the edgeemission of the two systems. Therefore, in this stacked LSC system, neither the edgeemission nor the e�ciency of the complete system increases noticeably when absorbanceof the Red305 LSC on top is above 1.6.
Experimental results of the system e�ciency showed that the e�ciency increased by16-28% when R0.19 was replaced with R1.63 in the stacked system for the range of perinoneLSC absorbances studied, except at the lowest absorbance value of the perinone LSC. Atthe lowest perinone absorbance value (0.37), the increase in e�ciency when R0.19 was re-placed with R1.63 was negligible. The experimental e�ciencies of both R0.19 top/perinonebottom and R1.63(top)/perinone(bottom) were observed to decrease with increasing ab-sorbance, but a sudden increase in e�ciency was noted at perinone absorbance of 1.36 inboth systems. For perinone LSCs with absorbance ranging from 0.37-1.25, the trend ofdecreasing e�ciency as absorbance increased is obvious. However, when absorbance wasincreased to 1.36, the e�ciency jumped to a higher value. This behaviour was unexpected,but the lower e�ciencies at absorbance of 0.93-1.25 may be partially due to imperfectionsin the LSC plates, and partially, the greater measurement errors incurred for samples withlower absorbances as a result of sensitivity range of the spectrophotometer and integratingsphere.
The amount of increase noted in the experimental e�ciencies were similar to the sim-ulated e�ciencies under the same system arrangements. Also, the general trend of the ex-perimental e�ciencies of both the R0.19 (top)/perinone (bottom) and R1.63 (top)/perinone(bottom) system were similar to the trends observed in the simulated systems except thee�ciencies in the experimental data appear to increase suddenly at perinone absorbanceof 1.36. Generally, the simulated e�ciencies from the model were found to be higher thanthe experimental data by ∼15% for all perinone LSC absorbances studied. Again this is atleast partially due to imperfections in the physical LSC plates such as scratched surfacesand defects that may cause additional scattering and out-coupling of the total internallyre�ected light. Also, as discussed before, the emission pro�le of the dichoric perylene dyesis typically anisotropic, and this may result in more surface losses than predicted in theray-tracing model as the model assumes the emission of the dyes to be isotropic.
From both the experimental and simulated performance of the stacked LSC system,it is apparent that there are competing factors: edge output increases while e�ciency ofemission decreases with increasing absorbance. The optimum absorbance, which may bede�ned as the sample absorbance value that produces the highest edge output withoutcompromising e�ciency, for the studied stacked system was found to be ∼1.63 and ∼1.36for the Red305 (top) and perinone (bottom) LSC, respectively. In both experimentaland simulated results, the edge emission plateaus after the optimal absorbance valuesof the Red305 and perinone dye-�lled LSCs are reached. The relative e�ciencies werealso noted to decrease considerably when absorbance of the top and bottom LSCs were
42 Chapter 2. Modeling of Luminescent Solar Concentrators
Figure 2.16: Experimental (solid symbols) and model (open symbols) total edge emissionintegrated from 350 - 850 nm of LSC stacks arranged in 1) (4, black) perinone nonylphe-nol doped LSCs with various dye content atop of Red305 LSC with absorbance of 0.19(R0.19) and 2) (◦, grey) R0.19 is placed on top of perinone nonylphenol LSC with variousdye concentrations.
increased above the optimal values stated. Thus, for the stacked system of Red305 LSCatop perinone LSC, the best performance of the system was achieved using absorbancevalues of 1.63 (R1.63) and 1.36 (P1.36), respectively.
To better understand the e�ect of the arrangement of the LSCs in the stack on theperformance of the system, the position of Red305 LSC and perinone LSC was switched,i.e., perinone LSCs exhibiting various dye content were placed on top of R0.19. The per-formance of perinone LSCs on top of Red305 LSC with absorbance of 0.19 were comparedwith the inverse arrangement of LSCs as shown in Figure 2.16.
Theoretically, the simulated edge output of the system where Red305 plate was placedon top of perinone plate was very similar to the edge output obtained in the peri-none(top)/Red305(bottom) arrangement. Only at perinone absorbances >1.36 can oneobserve a slight di�erence between the edge outputs of the two arrangements where theRed305(top)/perinone(bottom) system generated higher edge emission. The measurededge outputs of the two stacked arrangements showed higher edge emission for Red305top/perinone bottom arrangement for all absorbances studied (see Figure 2.16). At ab-sorbances above∼1.3, the di�erence between the measured edge outputs of the two stackedarrangements was small and generally within measurement error. When the Red305 platewas placed on top of the perinone LSC, the perinone dyes can absorb photons escapingthe bottom surface of the Red305 waveguide as well as photons that were not absorbedby the Red305 dyes atop. However, photons emitted within the top surface escape cone of
2.4. Results and Discussion 43
the perinone waveguide cannot be absorbed by the upper Red305 waveguide as the emis-sion band of the perinone is outside the absorption band of the Red305 dye (see Figure2.13). For the perinone top/Red305 bottom arrangement, the bottom Red305 waveguidecan absorb photons that failed to be absorbed by the perinone waveguide, and the peri-none waveguide on top is able to absorb photons escaping the top surface of the Red305waveguide. This latter process is possible because the emission band of the Red305 dyeis within the absorption band of the perinone dye (see Figure 2.13). The photon absorp-tion and emission processes in both stack arrangements are very similar with primarydi�erence being the quantum yield of the two dyes used. As the perinone dye had alower �uorescence quantum yield than Red305, it was anticipated that at higher perinoneabsorbances, the initial photon energy loss would be greater for the perinone top/Red305bottom system, resulting in a lower edge output. The measured edge emission was ingood agreement with this theory as the Red305 top/perinone bottom stacked systemconsistently produced higher edge emission than perinone top/Red305 bottom system.
The simulated edge outputs of the two stack system arrangements were found to be ingood agreement with the experimental results at absorbances above 1.36. An increasingdeviation of the simulated from the experimental values was noted at absorbances below1.3 in Figure 2.16, and the simulated values were higher than the experimental valuesby 25-30%. This large di�erence may be partially due to greater measurement errorsat low absorbance values. Moreover, the model may predict lower surface losses in thestacked system than experimentally measured. Again, the model uses isotropic emissionpro�le for the dyes and thus it most likely underestimates the surface losses of waveguidesthat contain dichoric anisotropic emitting dyes. Despite the shortcomings of the modelin predicting the edge output of the stacked LSC system at lower absorbance values, itdoes provide a good qualitative estimate of the edge emission trend for all absorbances,and it can predict the performance of stacked system with high absorbance LSC plates.
Comparing the edge emission of the best performing double stacked LSC waveguidesto a single Red305 and perinone LSC plate (see Figure 2.17) showed that the edge emissionof the stacked LSC system was double that of the single waveguides. Clearly, by stackingwaveguides doped with dyes exhibiting di�erent absorption spectrum on top of each other,photon absorption was increased, resulting in higher edge emission than single waveguidesystems. However, the relative e�ciencies of the stacked system are similar to that of thesingle waveguide systems. The additional dye-�lled waveguide in the stacked system didnot attribute to any noticeable increase in e�ciency. Contrastingly, the drop in e�ciencywith increasing absorbance was also smaller in the stacked systems than the single LSCplates. In other words, the e�ciency dependence on absorbance is less in a stacked systemthan single LSCs. This con�rms that double stacked LSC systems perform better thansingle LSC waveguides.
Simulated performance of large area LSCs
The model discussed above was extended to predict the performance of 10×10×0.3 cm3
stacked LSC systems and to evaluate the optimal absorbance values. The simulated edgeoutput results of the 10×10×0.3 cm3 stacked system are shown in Figure 2.18 and theywere compared with the simulated edge outputs of the 5×5×0.3 cm3 stacked system
44 Chapter 2. Modeling of Luminescent Solar Concentrators
(a) (b)
Figure 2.17: Measured (a) total edge emission (integrated from 350 - 850 nm) and (b)relative e�ciency (equation 2.1) of single Red305 (◦) dye-�lled plate, single perinone (4)dye-�lled waveguide, and double stacked LSCs (�) composed of Red305 dye-�lled plate(absorbance=1.63) on top of perinone dye-�lled waveguide with varying dye content. Allwaveguides used were 5×5×0.3 cm3.
discussed above.It was observed that the edge emission of the 10×10 cm2 stacked system approaches a
maximum at absorbance >1.0, and a similar plateau was observed in the 5×5 cm2 stackedsystem at the absorbance value of 1.36 (see Figure 2.18a). This trend matches well withprevious edge output trends observed in dye-�lled and thin �lm LSC systems and againcorroborates that the increase in edge output of an LSC system diminishes at su�cientlyhigh absorbances. The edge emissions of the stacked system with a high absorbance(1.63) Red305 plate on top were found to be signi�cantly higher than the stacked systemconsisting of a lower absorbance (0.19) Red305 plate. The di�erence in edge emissionbetween the two 10×10 cm2 stacked systems were considerable greater than the di�erenceseen between the two 5×5 cm2 stacked system using similarly doped Red305 plates. Thissuggests that the e�ect of dye concentration, and indirectly, absorbance on edge emissionbecomes more predominant when waveguide size of the LSC is doubled.
As expected, the relative e�ciency of the 10×10 cm2 stacked LSCs shown in Figure2.18b is lower than the 5×5 stacked systems for all absorbances studied. Undoubtedly,the lower e�ciency of the 10×10 system is due to the increased light path length, which inturn increases re-absorption and surfaces losses in the system. The e�ciency of the 10×10system was noted to decrease rapidly at absorbance >1.0, which reinforces the importanceof limiting re-absorption, especially at high dye concentration. Moreover, the absorbancevalue (1.0) at which the 10×10 system's e�ciency starts to decline is lower than that ofthe 5×5 system, as the 5×5 system e�ciency decreases at absorbance >1.36. This againdemonstrates that the increased light path in the 10×10 waveguides greatly enhances theprobability of reabsorption and thus results in an e�ciency drop at a smaller absorbancevalue. The best performing absorbance values for the 10×10 cm2 stacked LSC system wasdetermined to be ∼1.63 and ∼ 1.0 for Red305 and perinone nonylphenol dye doped PC
2.5. Conclusions 45
(a) (b)
Figure 2.18: Simulated (a) total edge emission integrated from 350 - 750 nm, and (b)relative e�ciency (equation 2.1) of 10×10×0.3 cm3 (4) and 5×5×0.3 cm3 (�) LSCs ar-ranged in a stack where Red305 dye-�lled waveguides with absorbance 0.19 (open symbols)and 1.63 (solid symbols) were each placed on top of LSC doped with various concentrationsof perinone nonylphenol dyes.
waveguides, respectively. At these absorbance values, the edge emission generated by thestacked system is similar to the values obtained at higher absorbances while the declinein e�ciency is not yet obvious.
2.5 Conclusions
In this chapter, the performances of dye-�lled, thin �lm coated, and stacked LSCs aresimulated using commercial ray-tracing software (LightTools), and the simulations areveri�ed with experimental measurements. The edge emissions for all types of 5×5 cm2 LSCsystems were observed to increase proportionally with increasing dye concentration untilan absorbance of 1.36 is reached. At absorbance >1.36, further addition of dyes to the LSCwaveguide produces insubstantial increase in the edge emission and signi�cant decrease inphoton-to-photon e�ciency, particularly in the thin �lm coated LSCs. The thin �lm LSCswere found to contain macroscopic dye aggregates, which increase in size and number withincreasing dye concentration. The presence of dye aggregates suggests quenching in thethin �lm LSC system, which would partially account for the e�ciency decrease at highdye concentrations. Aggregation of dye molecules and quenching were not accounted forin model systems, resulting in a di�erence between the simulated and experimental thin�lm LSC results. The other primary source of the small increase in edge emission anddecrease in e�ciency is re-absorption as LSCs containing high concentration of dyes havea greater probability to be reabsorbed. The greater reabsorption and surface losses appearto counter the increase in edge emission from increasing dye concentration. Comparingthe simulated results of the thin �lm systems to the dye-�lled LSCs demonstrated thatthe thin �lm systems can perform as well as the dye-�lled system if the refractive indexof the coating matches that of the waveguide. A mismatch in refractive index leads to
46 Chapter 2. Modeling of Luminescent Solar Concentrators
a reduction in edge emission in the thin �lm LSC. In contrast, dye coating thicknesseswhere observed to have little e�ect on the edge emission of the LSC.
The performance of stacked LSC studied in this chapter illustrated the importanceof choosing the optimal dye concentration and LSC arrangement for a multi-stack. ForLSCs doped with Red305 and perinone nonylphenol dyes, a stacked arrangement withRed305 dye-�lled LSCs as the top waveguide was noted to be preferable. The highest edgeemission in the studied stacked LSC system was found at absorbance values ∼1.6 and ∼1.4for the Red305 and perinone nonylphenol doped polycarbonate waveguides, respectively.Furthermore, the edge emission of the best performing stacked system was doubled thatof the single dye-�lled waveguide systems, though it exhibited similar relative e�ciency(edge emission normalized by absorption) as the single waveguides. Doubling the sizeof the dye-�lled waveguides in the stacked LSC system lead to considerable decreases ine�ciency, but the edge emissions of the larger stacked system was generally four timesthat of the smaller systems.
Chapter 3
Patterned LSCs: An Approach to
Limiting Reabsorption
3.1 Introduction
In the previous chapter, simulations of the thin �lm coated and dye-�lled LSCs werediscussed and compared with experimental measurements. The simulations and experi-mental results both demonstrated that edge emission of the LSC reaches a plateau regionat an absorbance of 1.5. Further increases in the dye concentration after an absorbanceof 1.5 has been reached produces a negligible increase in edge emission, and a decreasein photon-to-photon e�ciency. The decrease in photon-to-photon e�ciency with increas-ing absorbance limits the photon to electron conversion e�ciency of the single waveguideLSC modules. Recently, moderate e�ciency of ∼7.1% with GaAs PV cell attached to fouredges of the waveguide has been found [79], but improvement is still necessary for LSCsto become economically attractive with conventional silicon based photovoltaic cells.
A main factor limiting LSC e�ciency is internal losses due to re-absorption of lightemitted by the dye molecules. The small Stokes-shift of the most commonly used �uores-cent dyes result in an overlap between the absorption and emission spectra, and signi�cantre-absorption of emitted light (see chapter 2). The reabsorbed photon is potentially lostin one of two main ways: 1) re-emission at angles outside the LSC waveguiding modes, or2) transferred to heat due to less than unity quantum yield of the �uorophore [94, 106].For an 5×5 cm2 LSC with a refractive index of 1.5 (typical of many polymers) it hasbeen reported that reabsorption may account for ∼ 25% of light loss [83]. To limit reab-sorption, attempts have been made to increase the Stokes shift of the emitting materialby replacing the �uorescent dyes with quantum dots [68, 70, 107, 108] or rare-earth com-plexes [83, 101]. In the case of quantum dots, its absorption property is controlled bythe size of the nanocrystals, and moreover, the Stokes shift of the quantum dots is quan-tatively related to the distribution of nanocrystals sizes in the sample [70, 109]. Thus,it is possible to eliminate the overlap between luminescence and absorption by carefullychoosing the spread of quantum dot sizes. Rare-earth complexes containing organic lig-ans and rare-earth ions exhibit zero reabsorption due to the energy transfer mechanismbetween the absorbing ligans and emitting ions [83, 101, 110]. These complexes generallyhave large Stokes shift where the absorption peak lies in the range of ultraviolet or visible
47
48 Chapter 3. Patterned LSCs: An Approach to Limiting Reabsorption
light, and the emission is in the red or near-infrared wavelengths. While these alternateluminophores allow better control of the Stokes-shift of the luminescent material, the ma-terials su�er from either low quantum yields or poor light absorption when compared toorganic �uorescent dyes such as perylenes [100].
To exploit the advantages of the �uorescent dyes, reabsorption losses must be limited.This chapter demonstrates reduced reabsorption losses of �uorescent dye coated LSCs viapatterning of the dye layer. Normally, when light is incident on a uniformly coated LSC,it is absorbed and re-emitted at a longer wavelength by the dye molecule. Part of thisre-emitted light propagates through the waveguide and has a high probability of beingreabsorbed. As discussed in previous chapters, reabsorption leads to loss of photons as aresult of non-unity �uorescence quantum yield (FQY) of the dye and the re-randomizationof the light direction. The lower the FQY of the organic dyes, the more signi�cant thee�ect re-absorption has on the performance of the LSC.
In the patterned LSCs, the surface area coverage of the dye molecules is reduced sincethe dye coating is deposited as line patterns separated by regions of clear waveguide.Decreasing surface area coverage reduces the probability of interaction between re-emittedphotons and dye molecules, and thereby minimizing re-absorption losses (see Figure 3.1 forthe concept behind the patterned layers). Also, this reduces the amount of light escapingfrom the surfaces of the waveguide due to the re-distribution and emission of light afterre-absorption occurs. To better evaluate the e�ect of patterning on the performance ofLSCs, a �uorescent dye (coumarin yellow) with less than unity �uorescence quantum yield(FQY) was chosen for this study. A ray-tracing model was used to predict the performanceof the coumarin yellow patterned LSCs and its results were veri�ed against experimentaldata. Finally, the e�ect of the FQY of the dyes on the e�ciency of patterned LSCs wasinvestigated by comparing the performance of coumarin yellow (FQY∼=0.95) patternedsystems to Lumogen F Red305 patterned LSCs (FQY∼=1.0).
3.2 Experimental Procedures
3.2.1 Substrate Preparation
Glass (50 x 50 x 3 mm3) and PMMA (50 x 50 x 5 mm3) substrates were used as waveg-uides. A polyimide (Nissan 130 or JSR AL-1051) adhesion layer was spun onto the glasssubstrates at 5000 rpm for 60 s. The adhesion layer was needed to reduce the interfacialtension and prevent peeling of the dye coating during the later stages of the experiments.The polyimide coated glass substrates were baked at 90◦C for 10 min in air, then placedin an oven at 190◦C for 90 min under vacuum. After baking of the polyimide coatedsubstrates, they were allowed to cool in air overnight.
3.2.2 Sample Fabrication
Two types of �uorescent dyes were chosen for this study because they exhibit di�erentabsorption range, and hence can cover a greater range of the solar spectrum when the twodyes are used together in an stacked LSC con�guration. Fluorescent dye solutions wereprepared using 0.5% wt of �uorescent dye molecules coumarin yellow (DFSB-K160, Risk
3.2. Experimental Procedures 49
PV Cell
Dye Layer
Incident Light
Waveguide
(a)
PV Cell
Incident Light
(b)
Waveguide
Dye Layer
Escaped Light
Escaped Light
Figure 3.1: a) Light incident on a uniformly coated LSC is absorbed and re-emitted at alonger wavelength by a dye molecule. Part of this re-emitted light propagates through thesubstrate in the waveguiding mode (solid light rays), and has a high probability of beingreabsorbed by another dye molecule. Each time re-absorption occurs, there is a potentialloss of photons due to < 100% quantum yield of the dyes and the redirecting of light. Hencethe intensity of the re-emitted light from a reabsorption event decreases (dotted light rays);b) The dye layer is patterned into line structures which extend into the plane of the paper.By decreasing the area coverage of dyes on the surface of the waveguide, the probabilityof emitted light encountering other dye is reduced, thereby minimizing reabsorption andenergy losses as light travels across the waveguide.
Reactor) or Lumogen Red 305 (perylene-1,7,8,12-tetraphenoxy-3,4,9,10 tetracarboxylicacid-bis-(2'-6'diisopropylanilide, BASF), and 1% photoinitiator (Irgacure 184, Ciba) dis-solved in a 3:1 dipentaerythritol penta-acrylate (Polysciences) and methylmethacrylate(MMA, Aldrich) blend. The molecular structure of the coumarin yellow dye is given inFigure 3.2.
The dye solutions were stirred and heated at 60◦C for an hour prior to spin-coatingonto the substrates at 1000 rpm for 30 s. After spin-coating, all 100% covered sampleswere crosslinked by exposing to a high-intensity UV lamp (OmniCure S2000 UV spotcuring lamp) for 80 s under nitrogen �ow to form a solid �lm. For the fabrication ofpatterned LSCs, standard photolithography techniques were employed. Uniformly coatedsubstrates were exposed to UV light through patterned shadow masks consisting of 1)
Figure 3.2: Molecular structure of coumarin yellow dye from Risk Reactor
50 Chapter 3. Patterned LSCs: An Approach to Limiting Reabsorption
5 lines with variable widths with a period of 10 mm, 2) 10 lines with variable widthswith a period of 5 mm, and 3) 90 squares of variable dimensions, arranged in a 9 × 10rectangular pattern. Line widths and square sizes were varied to cover 14 to 80% of thewaveguide surface. After UV exposure, ethanol was used to etch away the unexposedmaterial on PMMA and glass substrates. The exposed patterned samples were placed inethanol for ∼40 s at room temperature and the samples were continuously agitated duringthe etching process. The thicknesses of the �uorescent dye coatings and microstructureswere measured to be 14-20 µm using a Fogale Zoomsurf 3D optical pro�ler.
3.2.3 Sample Characterization
Quantum yield of the Lumogen Red 305 and coumarin yellow dye have been measured tobe 1.0 and 0.95, respectively [100]. Absorbance spectra of all samples were measured usinga Shimadzu UV-3102 PC spectrophotometer. Average peak absorbance was found to be0.63, 0.6, and 0.47 for the coumarin yellow doped 5 line, 10 line, and square patternedPMMA waveguides, respectively. The absorbances of the Red305 doped line and squaredpatterned PMMA waveguides were measured to be 0.34. Edge emission of the waveguideswas measured by a SLMS 1050 integrating sphere (Labsphere) equipped with a diode arraydetector (RPS900, International Light). The LSC samples were placed in a custom-madesample holder, which is connected to the entry port of the integrating sphere and preventssurrounding light from entering the port. The LSCs were exposed to a collimated lightsource from a 300 W solar simulator with �lters to approximate the 1.5 AM global solarspectrum (Lot-Oriel) located at a distance of 15 cm from the top surface of the waveguide.Light output spectra and intensity from all four emission edges of the LSC were measuredand recorded. Total edge emissions were determined by integrating the recorded spectraover the range of 350-750 nm. Total absorbed power of the 100% covered samples wascalculated by multiplying its absorption spectrum with the solar simulator spectrum, andsubsequently integrating it from 350 - 750 nm. The power absorbed by the dye moleculesin the patterned waveguides was calculated as shown in equation 3.1:
Apattern = Dcoverage × A100% (3.1)
where Apattern is the total absorbed power of the patterned sample, Dcoverage is the surfacearea of the substrate covered by dye structures, and A100% is the total absorbed powerof the 100% covered sample. For example, the energy absorbed by a sample with 30%coverage is 0.3×A100%. At high dye coverages, the absorbance of the dye structures canbe measured using the spectrophotometer and it was found to be similar to the 100%covered sample. Hence, the absorption of the patterned LSCs were calculated using theabsorption of the fully covered sample. Relative e�ciency of the system was determinedby dividing the total edge emission (summation of all 4 edges) by the absorbed power ofthe sample (equation 2.1). The measurement error in edge output of the low (< 30%)and high (> 30%) coverage patterned samples is ∼ 10% and 5%, respectively.
3.3. Patterned LSC Model Parameters 51
3.3 Patterned LSC Model Parameters
The performance of rectangular square patterned coumarin yellow coated PMMA sub-strate was modeled using commercial ray-tracing software (LightTools, Optical ResearchAssociates). A similar model to the one described in chapter 2 was used to simulate theperformances of the patterned LSCs. To simulate luminescence of the coumarin yellow,the experimental absorption and emission spectra were used for the �uorescence algo-rithm in the model. The �uorescence quantum yield (FQY) was set to 0.95 in the modelaccording to previous unpublished measurements performed by the B. Richards groupat Heriot Watt University. Fluorescence modeling was set such that the probability ofphoton-to-dye molecule interactions were expressed by the wavelength dependent meanfree path as de�ned by equation 2.5 in section 2.2 of chapter 2. Once photons are absorbedby the dye molecules, re-emission is de�ned by the experimental emission spectra. Thee�ect of dye area coverage on the performance of the LSCs was invested by varying theedge length of the squares in the pattern from 1.5 to 5 mm, corresponding to a surfacearea coverage of ∼8 to 100%. The thickness of the dye pattern coating in the model wasdetermined by the measured thicknesses for all experimental samples.
The optical properties of the PMMA substrate were simulated by entering its trans-mittance, measured using air as the background, in the model and setting the refractiveindex to 1.49. The refractive index was set as constant for all wavelengths to reducecalculation time. All four edges of the substrate were de�ned as Lambertian scattererswith 100% forward transmission. This appears reasonable as the edges of the physicalsubstrates were not polished after cutting. The transmitted rays are captured by receiversattached to all four edges and analyzed. A light source, which emits the measured spec-trum of the solar simulator, is placed above the substrate (see Figure 2.3). It is notedthat there are characteristic peaks from the light source at wavelengths above 750 nm,which will mask any emission signal from the sample at those wavelengths. Hence, for allthe edge emission values reported here, the values were integrated from 350-750 nm.
3.4 Results and Discussion
A series of samples were produced that consisted of ten equally spaced lines of acrylatesdoped with 0.5 wt% of the �uorescent dye K160 deposited on top of clear glass and plasticwaveguides. The line widths were varied to produce regular patterns that, in total, coveredfrom 30 to 100% of the waveguide surface. The edge emissions from all four edges of eachwaveguide were measured to determine the e�ect of dye coverage on the edge emissionand e�ciency of the LSCs. Representative edge emission and absorption spectra of theK160 dye are given in Figure 3.3. The relative e�ciency (equation 2.1) of the patternedLSC system, de�ned as the ratio of total edge emission (integrated from 350-750 nm) tothe total energy absorbed by the sample, increased with decreasing area of dye coverage(Figure 3.4a). At 30% dye coverage, the relative light emission e�ciency of the patternedLSC on glass was more than double the e�ciency of the 100% covered sample. Thissuggests that reabsorption losses can be limited by reducing surface coverage of the dyemolecules, and the e�ciency of the system signi�cantly improved. Similar trends were
52 Chapter 3. Patterned LSCs: An Approach to Limiting Reabsorption
Figure 3.3: Representative absorption (black) and edge emission (grey) spectra of 0.5wt% coumarin yellow (line) and Lumogen Red 305 (dash dot line) doped thin �lm on glasswaveguides.
noted on PMMA substrates: the relative e�ciency at 30% coverage on the PMMA wave-guides increased by ∼70% compared with the 100% coverage control sample. To furtherverify the decrease in reabsorption losses is a result of the patterning, the emission spectrafrom one waveguide edge for all waveguide surface coverages were plotted in Figure 3.5.The emission peak of the patterned LSC is noted to red-shift with increasing coverage,which implies that reabsorption increases with coverage. This �nding is similar to themeasurements performed by Sholin et al [108].
The absolute edge emissions of the micro-patterned LSCs were noted to be lower thanthe 100% covered LSC (Figure 3.4b). The line patterns allow a considerable fraction ofincident light to pass through the clear regions of the waveguide without encounteringdye molecules, and this light is lost: as dye coverage decreases (that is, the line widthsdecrease), the amount of absorbed light also decreases, leading to a reduction in totaledge emission despite the increased e�ciency. It is possible to reduce these light lossesby increasing light absorption through focusing incident light onto the dye regions via amicrolens array. This method of increasing light absorption of microstructured LSC hasbeen investigated and will be presented in chapter 4.
The slightly higher edge emission observed in the dye patterned glass substrates showsthat higher refractive index and higher transmittance substrates can trap more light,which is in general agreement with the �ndings of Kastelijn et al. [111] There are onlysmall di�erences observed in the emissions from the two di�erent edges, which are eitherperpendicular or parallel to the lines of the structured LSC. Since the dye molecules inthe acrylate host are isotropic, it is not surprising that the di�erence in edge emission of
3.4. Results and Discussion 53
(a) (b)
Figure 3.4: (a) Calculated relative e�ciency (equation 2.1) and (b) total edge emis-sion integrated from 350-750 nm of dye-coated PMMA (�) and glass (�) substrates as afunction of surface area covered by line structures of 0.5% coumarin yellow dye.
Figure 3.5: Measured edge emission spectra of coumarin yellow doped 10 line patternLSC system with varying line widths to cover 100% to 20% of the substrate surface. Theposition of edge emission peaks are increasingly red-shifted (as indicated by the verticalline) as �uorescent dye coverage increases from 20% to 100%
54 Chapter 3. Patterned LSCs: An Approach to Limiting Reabsorption
a) b)
3 mm2 mm 2 mm
3 mm
Figure 3.6: Schematic of a) �ve equally spaced (dark grey) acrylate lines and b) rectan-gular pattern of 9×10 acrylate (dark grey) squares doped with 0.5 wt% coumarin yellowdyes coated on 50×50 mm2 substrates. The distances given in a) and b) are examples ofspacing and width of the dye doped lines and square patterns.
the four waveguide edges is small [112].Possible dimensional and geometric e�ects on the edge emission and e�ciency of the
micro-patterned LSC were investigated by preparing a series of samples consisting of1) �ve equally spaced acrylate lines, and 2) 90 squares arranged in a 9×10 rectangulararray doped with 0.5 wt% coumarin yellow were deposited on top of PMMA waveguides(see Figure 3.6). The line widths and the area of the squares were varied to produceregular patterns that covered 20 to 100% of the waveguide surface. The patterned samplescomposed of 5 and 10 lines with varying surface coverage will be referred to as 5 line and10 line samples hereafter. Relative e�ciency as a function of surface area coverage ofthe dye measured from the 5 line samples were similar to the 10 line (Figure 3.7a), andthe edge emission was the same for both systems (Figure 3.7b). The slight di�erence( 5%) in relative e�ciency between the line patterned samples are likely due to a greaternumber of re-encounters with the dye in the broader-lined 5 line patterns. In addition,the measured peak absorbance of the 5 line samples (0.63) was slightly higher than the 10line (0.6). This again suggests that reabsorption is the reason the 5 line samples exhibitthe same edge emission but a greater absorbed power compared to the 10 line samples.The combined results signify that variations in the width of the lines have little e�ect onthe edge emission of the line patterned LSC system.
The e�ect of geometry on the performance of patterned LSCs was studied by exam-ining the square patterned samples and comparing with the 5 line and 10 line samples.The area of the squares was varied to cover 14-100% of the waveguide surface. The seriesof square patterned waveguides showed similar trends as the 10 line and 5 line systems(Figure 3.7). Table 1 reports total edge emission values as a function of waveguide sur-face area coverage for two di�erent (square and 10 line) patterned LSC systems. Bothedge emission and relative e�ciency values appear to be very similiar for all the coumarinyellow coated patterned LSC systems examined. However, the square patterned samplesexhibit lower absorbance value (0.47) than the 5 line (0.63) and 10 line (0.6) patternedLSC samples. From the results of chapter 2, the edge emission and relative e�ciency were
3.4. Results and Discussion 55
(a) (b)
Figure 3.7: a) Relative e�ciency (equation 2.1) and b) total edge emission of patternedwaveguides composed of equally spaced 5 (◦) and 10 lines (4), and a rectangular patternof 9×10 squares (�) doped with 0.5 wt% coumarin yellow as a function of dye patterncoverage.
observed to increase with increasing absorbance for coated thin �lm LSCs for absorbance<1.0. This suggests that the performance of the square pattern LSCs will improve whenits absorbance is increased from 0.47 to 0.63. In other words, the square pattern LSCscan perform better than the line patterned samples when it is fabricated at the sameabsorbance and dye area coverage. The square patterns have a lower reabsorption prob-ability than line patterns because it lacks the long length of the lines where any lightray travelling along the length of the line would have the same reabsorption probabilityas a fully covered sample. Thus in a line pattern, reabsorption is only reduced in onedimension, i.e., perpendicular to the length of the lines. In a square pattern LSC, theregular spacing of clear waveguide regions between the squares of dye coating allows thereduction of reabsorption losses in two directions (Figure 3.8). Hence, reabsorption lossesare more limited in squared patterned LSCs and it is not surprising that they shouldperform slightly better than the line patterned samples.
To verify the e�ect of geometry patterns on the output of the LSC, similar squareand 10 line patterned samples doped with 0.5 wt% Lumogen Red 305 were prepared.Analogous to the patterned systems doped with coumarin yellow dye, total edge emission(i.e., sum of emission at all four edges of the waveguide integrated from 350-750 nm) ofthe two Red305 doped patterned LSCs were similar in both values and trend (Table 3.1).Figure 3.9 illustrates the relative e�ciencies of the Red305 doped square patterns andthe 10 line pattern systems. Again, by decreasing the dye surface area coverage of thewaveguide, relative e�ciencies of the patterned samples increased.
Only small di�erences (within measurement error) were noted in relative e�ciencyand edge emission of the Red305 square and 10 line patterned samples, where all thesamples have similar peak absorbance values. Geometry of the pattern appears to beirrelevant to the performance of thin �lm LSCs coated with Red305 dye. This �ndingis somewhat di�erent from the results observed in the coumarin yellow doped patterned
56 Chapter 3. Patterned LSCs: An Approach to Limiting Reabsorption
(a)
(b)
solar celldye
Square pattern solar concentrator
Line pattern solar concentrator
solar cell
Figure 3.8: Schematic showing (a) cross-section of square patterned LSC where twodimensional reduction of reabsorption losses is possible, and (b)cross-section of line pat-terned LSC along the length of the line where reduction of reabsorption losses is in one di-rection only. Light incident on dyes in the patterned structures is absorbed and re-emittedin random directions (grey arrows). In (a), the square pattern decreases the probability ofre-absorption as indicated by the black arrows, which represents one of the possible pathsof re-emitted light in waveguide mode. In contrast, the re-absorption probability along thelength of the line structure in (b) is the same as a fully covered LSC. Note, the thicknessof the black arrows in both (a) and (b) indicates the relative energy of the total internallyre�ected light rays.
Figure 3.9: Relative e�ciency (equation 2.1) of line (4) and square (�) patternedwaveguides doped with 0.5 wt% of coumarin yellow (solid black symbols) and Red305 dye(open grey symbols).
3.4. Results and Discussion 57
Table 3.1: Measurement results of di�erent �uorescent dyes and 50×50 mm2 patternedwaveguides
Dye Pattern Area of Area Integrated RelativeType Type Individual Features Coverage Edge Emission E�ciency
58 Chapter 3. Patterned LSCs: An Approach to Limiting Reabsorption
systems. The square pattern in the coumarin yellow doped samples showed potential forbetter performance compared to the 10 line patterns. A reasonable explanation for thedi�erence in performance observed from the Red305 and coumarin yellow doped squarepatterned systems may lie in the �uorescence quantum yield (FQY) of the dyes. TheFQY for Red305 and coumarin yellow were measured as ∼1.0 and ∼0.95, respectivelyby the group of B. Richards at Heriot-Watt University [100]. Unlike the well-establishedquantum yield of the Red305 dye [113, 100], which has been measured by various groupsover time, the quantum yield and molecular structure of the coumarin yellow dye havenot been studied extensively. Reabsorption has signi�cant in�uence on edge emission ande�ciency of LSCs doped with low FQY organic dyes as low FQY implies that photonlosses are higher for each reabsorption that occurs in the waveguide. Hence, the ability ofthe patterns to reduce these reabsorption losses becomes more apparent. This results in ahigher gain in relative e�ciency, especially at low dye surface coverages where probabilityof reabsorption is the lowest. On the other hand, reabsorption losses are lower in LSCscoated with near unity FQY dyes, leading to a smaller observed e�ect of patterning on theperformance of the Red305 coated devices. This supports our theory that the geometryof the pattern has a greater in�uence on the performance of LSCs coated with lower thanunity FQY organic dyes, but little e�ect on thin �lm devices coated with near unity FQYdyes. Within the di�erent patterned devices studied here, square patterned dye structuresproduced the most e�cient LSC devices, particularly at dye coverages lower than 50%.
3.4.1 Modeling Results
The performance of the coumarin yellow doped square patterned LSCs was simulatedusing a ray-tracing program (LightTools) and the results were compared with experimentalmeasurements. Figure 3.10a shows the number of photons emitted at the edges of thewaveguide as simulated by the ray-tracing model in comparison with measured values.The simulated edge emission was observed to be similar to the measured data for dyecoverages >20% (see Figure 3.10a). At high dye coverages, the absorbance of the squaredye structures can be measured using the spectrophotometer, and it was found to besimilar to the absorbance of the 100% covered LSC. Therefore, the absorbance spectrumof the 100% covered sample was used to calculate the absorption of the patterned LSCs(see equation 3.1). However, at lower dye coverages (<40%), the absorbance of the samplescannot be accurately measured using the UV-Vis spectrophotometer as the spot size ofthe incident beam in the spectrophotometer is larger than the size of the squared dyestructure. Hence, the average absorption path length of photons entered in the model,which was calculated from the measured absorbance of the samples, has an accumulatederror of ∼15%. This error is subsequently propagated through the model, resulting in theconsiderable di�erence noted between the simulated and measured values of photon edgeemission. The total number of photons absorbed by the samples was generally found to bemuch greater in the model than the experiment for all dye coverages (Figure 3.10b). Thisoverestimation of photon absorption maybe due to the assumption that absorption of alldye molecules is isotropic in the model, whereas in practice, absorption of the dichoricdyes is anisotropic [92]. Therefore, both the measured and the simulated absorptions ofthe patterned LSC devices are not very accurate. More investigation is needed for a better
3.4. Results and Discussion 59
(b)(a)
Figure 3.10: Experimental (N) and model (�) results of (a) total number of photonsemitted from the four edges of the waveguide, and (b) the total number of photons absorbedby the square patterned 0.5 wt% coumarin yellow LSCs as a function of dye coverage.
understanding of the absorption di�erences between the experimental and model data ofthe patterned LSCs.
In an attempt to gain some insight into the possible di�erences between the modeland the physical system, the edge emission spectra of patterned LSCs with various dyecoverages are examined from both the model and the experimental systems (Figure 3.11).In both the measured and modeled emission spectra, the vertical line indicates the peakposition of the 8% coverage emission spectrum, which clearly demonstrates that the emis-sion spectra red-shift as dye coverage increases from 8 to 100%. A larger red-shift isnoted in the measured spectra compared to the model results. Moreover, the emissiontail in the model is much higher than that of the measured results. Red-shifting of theemission peak is typically a result of reabsorption. Photons reabsorbed by dye moleculesare re-emitted at longer wavelengths, thereby shifting the emission peak to longer andlonger wavelengths as reabsorption increases in the sample.
The large emission tail apparent in all modeled emission spectra indicates that morephotons were emitted at longer wavelengths in the model. This may be due to theoverestimation of photon absorption at longer wavelengths in the model. The dye emissionspectrum entered in the model was directly obtained from the measured values, and itis likely that the intensity of the emission tail was too high due to small amounts oflight scattered into the port of the integrating sphere during measurements. Hence, moreemphasis is given to the emission tail in the model, leading to more emitted photons atlonger wavelengths and overall larger emission tail than experimentally measured.
As discussed, absorption is greatly overestimated in the model compared to the ex-perimental measurements, and thus cannot be used to calculate the photon-to-photone�ciency (equation 2.2) of the patterned LSCs. However, it would be interesting to in-vestigate the maximum achievable edge emission from the patterned LSCs assuming thatthe patterned LSCs can absorb the same amount of photons as a fully covered samplewhile retaining its e�ciency (i.e., photon absorption is proportional to photon emission).
60 Chapter 3. Patterned LSCs: An Approach to Limiting Reabsorption
(b)(a)
Figure 3.11: (a)Measured and (b) simulated spectra of number of photons emitted fromone edge of the waveguide obtained from square patterned coumarin yellow LSC with dyesurface coverage ranging from 8 - 100%. The vertical line added to both plots correspondsto the approximate position of the peak emission of squared patterned LSC at 8% dyecoverage.
Figure 3.12: The measured (N) and simulated (�) total number of photons emitted fromall four edges of the waveguide are summed and divided by the dye area coverage of thesamples to obtain the maximum possible photon edge emission for the samples with variousdye coverage. The photon edge output in this plot depicts the total number of edge emittedphotons assuming all the patterned samples can absorb the same amount of photons as afully covered sample, and photon emission is directly proportional to absorption.
3.5. Conclusions 61
Hence, the total number of edge emitted photons was divided by the dye area coverage toobtain the �nal photon edge emission of the sample, and the results are plotted in Figure3.12. The trend of the simulated results follows the trend of the experimental valuesfor dye coverage >30%. Below 30% dye coverage, the simulated photon edge emissiondoes not increase with decreasing dye coverages as it would be expected judging from thetrend of the measured results. In contrast with the measured values, which suggest thate�ciency of the system continues to increase with decreasing dye coverage, the modelsuggests that no e�ciency can be gained at dye coverages below 30%. The di�erencebetween the model and the experimental results at low coverages may be due to measure-ment errors in the absorption spectrum, leading to more errors in the calculation for theaverage absorption path length of photons. Since the average absorption length of thesample in combination with the emission spectrum of the dye determines the amount ofabsorption, emission and reabsorption in the model system, any measurement error in thespectra will propagate to the photon emission output of the model. If the dye absorptionspectrum can be more accurately measured for the lower dye coverage patterned samples,the results of the model may be improved to better match the measured values, renderingit more useful for performance prediction of patterned LSCs.
3.5 Conclusions
We have demonstrated that the relative e�ciency of light emission from the edges of anLSC can be signi�cantly improved by reducing the dye surface coverage of the waveg-uide via patterning. The increase in emitted-to-absorbed photon e�ciency supports ourhypothesis that patterning the dye coating on a LSC limits reabsorption losses. Similaredge emission and relative e�ciency were observed for both 5 and 10 line pattern LSCsystems with similar dye surface coverage. Hence, the width and spacing of lines in aline pattern have little e�ect on the performance of the LSC. The square patterned LSCswere noted to have similar performance as the line patterned LSCs despite having a lowerabsorbance value. This indicates that square patterns have the potential of performingbetter than line patterned LSCs as a result of its two dimensional reabsorption reductionin comparison to the one-dimensional reduction of line patterns.
The comparison of K160 and Red305 patterned LSCs showed that the K160 patternedLSCs generally exhibit higher e�ciency compared to the Red305 systems at dye coverageabove 50%. Patterning and the geometry of the patterns appear to have a greater in�uenceon LSCs coated with dyes exhibiting lower �uorescent quantum yield. For LSCs dopedwith dyes exhibiting near unity FQY, both dimension and geometry of the patterns havelittle e�ect on the performance of the patterned system.
Simulation of square patterned LSC performance using ray-tracing showed consider-able disagreement between the photon absorption of the model and the measured values.Thereby, photon-to-photon e�ciency predictions and calculations of square patternedLSCs using the model were not possible. However, the model was able to reproduce theperformance of the squared patterned LSC for dye area coverages above 30%. Thus, moreaccurate measurements of absorption and emission spectra of the samples may lead tobetter matched model and experimental results, especially at low dye coverages.
62 Chapter 3. Patterned LSCs: An Approach to Limiting Reabsorption
Chapter 4
Patterned LSCs with an Integrated
Lens Array
4.1 Introduction
In the previous chapter, patterning of the dye layer has been demonstrated as a method forlimiting reabsorption losses and increasing the emission e�ciency of LSCs. However, oneof the primary issues of the patterning method is that it creates alternating clear and dye-coated regions. Light incident on the clear regions does not encounter dye molecules; thusa considerable fraction of the light passes through the substrate without being absorbed.This results in a large light loss, and ultimately low edge emissions. Obviously, e�cientsolar converting modules require high edge emission from the concentrator in order toproduce more electric energy from the attached PV cells. This reason alone rendersthe patterned LSC system impractical as it exhibits lower edge output than a standard,fully covered LSC despite its ability to increase emission e�ciency. The patterned LSCswill only become practical if its light absorption is comparable to the absorption of fullycovered LSCs. Therefore, to take advantage of the patterned concentrators, it is essentialto develop a method for enhancing light absorption.
Since their development in the 1960's, large non-imaging Fresnel lenses have been usedas concentrators for focusing sunlight on relatively small photovoltaic cells [114]. Theyhave been applied to various solar concentration systems including standard PV concen-trators [115, 116, 117], solar thermal conversions [118], space concentrated PV systems[119], and more recently, solar lighting [52]. Non-imaging Fresnel lenses imitate the con-ventional planar-convex lens shape, which allows them to achieve a high concentrationratio and short focal length. This is similar in concept to the application of a lens array tothe top of the patterned LSCs, and thus planar convex or Fresnel lenses are also suitablefor focusing light on dye structures in the patterned LSC system. However, consideringFresnel lenses use asymmetric grooves or prisms to simulate the shape of planar convexlenses, it would be more advantageous to use a conventional planar convex lens as it ismore straightforward to fabricate and scatters less light due to its smooth surface.
Therefore, I propose the use of cylindrical lens arrays based on converging convexlenses to focus sunlight directly onto the patterned dye structures. The objective of thelens array design is to maximize the absorption of the dye structures while maintaining
63
64 Chapter 4. Patterned LSCs with an Integrated Lens Array
the high emission e�ciency of the patterned LSCs. As demonstrated in chapter 3, thee�ciency of emission of the patterned LSCs tends to increase with decreasing dye coverage.Hence, the lens should be designed to exhibit a large acceptance angle for maximizingabsorption of the dyes, while simultaneously minimizing the focal spot size of the lensto maintain low dye coverage. If the goals of the design can be achieved, integrating thelens array with a pattern LSC should result in a more e�cient LSC module with higheredge photon output. In this case, ray-tracing was used to design the lens array to meetthe requirements mentioned above, and the array was realized using both compressionmolding and polymer resin casting. The lens array was subsequently integrated with linepattern LSCs and its performance was investigated using integrating sphere techniques.
4.2 Brief Background on Geometrical Optics
The lens array was designed using the fundamental theories of geometric ray optics andray-tracing software. Geometric ray optics is based on the law of re�ection and Snell'slaw of refraction (see equation 1.3). A lens is a refracting optical device typically usedto converge or diverge light rays, and consists of two optical surfaces. The curvatures ofthe optical surfaces determine the classi�cation of the lens as concave, convex, or planar.For example, a lens with one �at or planar surface and one convex surface is referred toas a planar convex lens, whereas a lens exhibiting two convex surfaces is referred to asbiconvex. The optical axis of the lens passes through the two centers of curvature of thelens surfaces, and de�nes the path that is perpendicular to both surfaces of the lenses.In other words, a light ray entering the lens along the optical axis of the lens emergesunchanged. A few of the types of simple lenses are illustrated in Figure 4.1.
Concave lenses are thinner at the center than at its edges and are known as diverginglenses as they tend to diverge collimated light rays at normal incidence if the refractiveindex of the lens is higher than the medium it is immersed in. A concave lens is clearlynot suitable for the focusing purposes of the lens array in the LSC system. Convex lensesare said to be converging lenses because light rays incident parallel to the optical axis ofthe lens is converged or focused to a point along its optic axis. Convex lenses are thickerat the center than the edges. Converging lenses have found applications in many imagingsystems such as cameras, microscopes, telescopes, as well as concentrator photovoltaicmodules.
The optical properties of the lens are primarily determined by the curvature of itsoptical surfaces and the refractive index of the lens material as described by the lensequation 4.1, which is used to calculate the geometric shape needed to achieve the requiredfocal length of the lens:
1
f= (nl − nm)
[1
R1
− 1
R2
+(n− 1)d
nR1R2
](4.1)
where f is the focal length of the lens measured from the lens axis (see Figure 4.2 forde�nition), nl is the refractive index of the lens, nm is the refractive index of the mediumsurrounding the lens, d is the thickness of the lens, and R1 and R2 are the radii of thecurvature of the two surfaces that form the lens, respectively.
4.2. Brief Background on Geometrical Optics 65
Bi-convexConcave Lenses
Convex Lenses
Planar convex Meniscus convex
Bi-concave Planar concave Meniscus concave
Figure 4.1: Examples of simple optical lenses. Top from left to right: biconvex, planarconvex, meniscus convex; and bottom, from left to right: biconcave, planar concave, andmeniscus concave. The dashed line at the geometric center of all the lenses de�nes theoptical axis of the lens.
66 Chapter 4. Patterned LSCs with an Integrated Lens Array
d
R2
R1
Fi
Figure 4.2: The focal length fi, radius of curvature R1 and R2, and thickness, d, of alens as described in the Lensmaker's equation is illustrated.
For a thin lens, i.e., a lens with thickness d much smaller than R, and assuming thesurrounding medium is air (nm is 1.0), the thick lens equation can be approximated as:
1
f= (n− 1)
[1
R1
− 1
R2
](4.2)
This equation is better known as the Lensmaker's equation. The Lensmaker's equationis a �rst order approximation for the paraxial region, also known as the small angleapproximation. The focal length of the lens is measured from the axis of symmetry of thelens to the focal point of the lens, which is de�ned as converging point of collimated raystransversing the lens. There are many focal points to a lens as each bundle of collimatedrays with a di�erent incident angle produces di�erent focal points on a curved surface asshown in Figure 4.3. If the cone angle of the bundles of light rays entering the lens issmall, i.e., with in the paraxial region, the collection of focal points at the curved surfacecan be approximated by a non-curved focal plane. As the lens has two optical surfaces, italso has two focal planes, one in the object space of the lens called the front focal plane,and one in the imaging space called the back focal plane. If the image created by the lensis real, the focal points of the lens will lie on the back focal plane of the lens.
The most common type of lenses are spherical and conic lenses, where the two opticalsurfaces are parts of a sphere for the former lens type and sections of a cylinder for thelatter. Spherical and conic lenses each exhibit several types of aberrations or distortionsthat have a considerable e�ect on the imaging quality of the lens. The two main typesof aberrations are chromatic and monochromatic. Chromatic aberrations are related tothe dispersion of light as it transverses the lens. Di�erent wavelengths of light travelat di�erent speeds and refract at slightly di�erent angles through the lens because therefractive index of the lens is a function of wavelength (see equation 4.2), and the focallength of the lens is dependent on its refractive index. As refractive index is wavelength(or color) dependent, there will be di�erent focal lengths for each wavelength of light after
4.2. Brief Background on Geometrical Optics 67
σ σFocalPlane
f
FiOF0
(a) (b)
Figure 4.3: Comatic aberrations refers to a) the focal points of a convex lens form acurved surface (σ) when bundles of collimated rays are incident at di�erent angles; and b)the approximation of σ as a non-curved focal plane can be used given the angle variationof the bundles of rays are small, i.e., paraxial.
Fi
Paraxialfocus
Figure 4.4: A schematic illustrating spherical aberrations where the focal point of axialrays is di�erent from the focal point of the paraxial rays Fi.
it passes through the lens.Monochromatic aberrations are distortions that arise even when using monochromatic
or single wavelength light and it includes some of the most commonly known aberrationsas spherical, coma, and astigmatism. Spherical aberrations occur because the focal lengthof the lens is dependent on the vertical height deviation of the normal incident light rayfrom the optical axis of the lens (see Figure 4.4. Third order expansion of the �rst orderLensmaker's equation 4.2 con�rms this dependence [75]. For a converging lens, this impliesthat light rays incident near the edge of the lens is bent more (i.e., shorter focal length)than light rays incident near the optical axis of the lens. Similar to spherical aberrations,comatic aberrations arise due to the paraxial approximations of the focal length and focalpoints of the lens from the simple lens equation. As shown in Figure 4.3, when light raysare incident at an angle with respect to the optical axis of the converging lens, especiallyat oblique angles, the focal points of the rays form a curved surface. This coma e�ect
68 Chapter 4. Patterned LSCs with an Integrated Lens Array
increases with increasing angle of incidence, and the focal plane approximation can onlybe used for rays within the paraxial region. Another e�ect related to oblique or o�-axislight rays is astigmatism. The astigmatic e�ect generally elongates the cross-section of thelight beam from circular to an elliptical shape as it travels away from the lens after passingthrough it. Similar to the coma e�ect, the more oblique the incident angle of the lightbeam, the greater the astigmatism. Images formed from lenses exhibiting astigmatismare usually observed to be asymmetric.
The e�ect of aberrations on the focal length and focal spot of the lens provide sig-ni�cant insight for proper design of a lens array for the patterned LSC system, keepingin mind that the lens array should have a large acceptance angle and minimal focal spotsize. In the case of focusing light on the patterned dye structures, the clarity of the imageformed by the lens array does not necessarily matter as it is more important to capture allincident light within the acceptance angle of the lens. This implies that the focal lengthof the lens is not critical to the lens array design as long as the small dye structures canabsorb all the light emerging from the lens array. Aberrations tend to distort the focalspot and vary the focal length of the lens, and this distortion increases with increasingincident angle. For example, a lens exhibiting strong comatic aberrations produces a largefocal spot with the slightest deviation of incident light from its optical axis. Thus it isimportant to limit aberrations in the lens design in order to minimize the focal spot size.
A common method for limiting or correcting aberrations is the use of aspherical mirrorsand lenses. Aspherical lenses, as the name suggests, have optical surfaces that are neitherparts of a sphere nor cylinder. In theory, aspherical lenses can take the form of anysurface shape; however, several mathematical equations are commonly used to designatethe optical surface of an aspherical lens. To gain a better understanding of the equationsused to describe the aspherical surfaces, one must �rst study the geometric equations usedto prescribe a spherical and conic surface. The curve of a conic surface is given by theequation:
z =r2
R(1 +√
1− (1 + κ) r2
R2 )(4.3)
where z describes the displacement of the surface from the vertex of lens (where a vertexis de�ned as the point where the surface crosses the optical axis of the lens), r is thedistance from a point located on the surface z to the optical axis and r = x2 + y2, wherex and y describe the position of the point; and R is the radius of curvature (reciprocal ofcurvature). κ is known as the conic constant and its value describes the surfaces obtainedfrom di�erent sections of a cone. For example, when κ is zero, equation 4.3 becomes theequation for a spherical surface. A list of possible κ values and the conic section thosevalues describe is give below:
To describe an aspherical surface, several di�erent equations can be used, and one ofthe most common equations used to design an aspherical lens is the polynomial asphericalequation:
z =r2
R(1 +√
1− (1 + κ) r2
R2 )+
10∑n=2
C2nr2n (4.4)
4.3. Aspherical Lens Design 69
Table 4.1: Conic constant values and conic sections it represents
where the same parameters as equation 4.3 are used, and the new parameters C and n arethe polynomial constant and the order of the polynomial, respectively. C and n can bevaried to obtain the desirable lens shape that ful�lls the design requirements. Ray-tracingallows one to study the optical properties of the lens by tracking the path of all the lightsrays incident on the lens. Thus, it is straightforward to design the optical surface ofan aspherical lens using ray-tracing where all parameters of the lens can be varied andtested.
4.3 Aspherical Lens Design
To design a lens array to focus sunlight directly on the patterned dye structures, one must�rst decide on the acceptance angle necessary for the lens as this would de�ne the shapeof the lens. In this case, the acceptance angle of the lens is directly related to the relativeposition of the sun to the earth surface throughout the year, which in turn de�nes theamount of sunlight that can be focused by the lens and absorbed by the dye structures.However, the azimuthal (east-west) and elevation (north-south) angles of the sun changethroughout the day and are di�erent for di�erent geographic locations on earth. Allpositions of the sun quoted in this chapter will be relative to the city of Eindhoven in TheNetherlands. Studying the sun's position throughout the year at the geographic locationof Eindhoven (51.26◦, 5.28◦) using a sun position calculator (Forster Engineering Services,Australia) showed that changes in the azimuthal and elevation angle of the sun can be> 200◦ and ∼ 60◦, respectively, from sunrise to sunset in the month June. The monthof June was chosen for the study because it is month at which the changes in the sun'sposition are the largest.
It is unlikely that a single lens can be designed to have an acceptance angle of 100◦,which would be required if the lens were to focus direct sunlight throughout the day inJune. An alternative is to use a cylindrical lens such that the length of the cylindricallens lies in the east-west direction as depicted in Figure 4.5. This allows the azimuthal(east-west) path of the sun to be aligned with the optical axis of the lens, resulting in theappearance of light rays along this path as normal incident light to the lens. By omittingthe azimuthal angles of the sun from the acceptance angle design considerations, ourattention can now be placed on designing a lens to focus light rays along the north-southpath of the sun. The angle of the sun varies a maximum of ∼ 60◦ in the north-southdirection, implying that the acceptance angle of the lens need to be 30◦ to focus direct
70 Chapter 4. Patterned LSCs with an Integrated Lens Array
West
East
North
South
SunSummer solar path
Winter solar path
Figure 4.5: Cylindrical lens with its length aligned with the azimuthal (east-west) pathof the sun such that light rays emitted from the azimuthal path appears as normal incidentrays to lens.
sunlight on the dye structures from 05:00 to 20:00 of a summer day if the LSC is positionedproperly.
Initially, the lens array for the patterned LSCs was designed using the simple planarconvex lens in the shape of a conic section. Ray-tracing was used to investigate the focus-ing abilities of the lens in order to �nd the optimal lens surface that would ful�ll the designrequirements of maximum acceptance angle in combination with minimum focal spot size.Due to the large distance between the sun and the earth, the direct beam component ofsunlight appears as collimated rays to objects on earth. Hence, direct sunlight in theray-tracing model was simulated by light sources producing bundles of collimated lightrays with incident angles ranging from 0◦ to ±30◦, where 0◦ corresponds to the opticalaxis of the lens. The di�used solar radiation was not examined as geometric lenses cannotfocus di�used light. The shape de�ning parameters (radius of curvature, conic constant,thickness and diameter) of the conic lens were adjusted to decrease the focal spot sizeof the lens within the 30◦ acceptance angle. After testing di�erent parameter values, itwas noted that the comatic aberrations of the lens were signi�cant, resulting in a largecurved focal surface and large focal spot. To decrease the size of the focal spot, comaticaberrations must be limited.
Aspherical surfaces are well-established for their ability to correct and minimize aber-rations, and hence they are the optimal choice for the lens array design. The lens typechosen was planar convex to ensure light rays converge on to the dye structures and oneof the optical surfaces must be planar to be placed on top of the planar transparent sub-strate. The polynomial aspherical equation 4.4 was used to describe the convex optical
4.3. Aspherical Lens Design 71
Figure 4.6: A schematic of the ray-tracing model used to design and verify the opticalcharacteristics of a lens. Collimated light rays simulating the direct component of thesunlight are incident on the sample at ±30◦ and 0◦ with respect to the normal of thesample surface. A receiver is placed in the image plane of the lens to determine the focalspot size of the lens.
surface of the lens and it was executed in the ray-tracing software (LightTools, OpticalResearch Associates) accordingly. An example of the ray-tracing model used for designingand testing the lens array is illustrated in Figure 4.6.
Light sources emitting collimated light rays with 5600 K black body radiation spectrumare placed at ±30◦ and 0◦ with respect to the normal of the sample surfaces to simulatethe direct component of solar radiation. The lens was modeled using polycarbonate andPDMS exhibiting refractive indices of 1.42 and 1.59, respectively. A receiver is placed inthe image plane of the lens to capture the converging light rays. A built-in optimizationfunction in the ray-tracing software was used to vary the geometrical parameters andthe polynomial constants of equation 4.4 to achieve the minimal focal spot size whilecapturing the maximum illumination energy. The optimization tool allows the user tode�ne the type of function (e.g., focusing, collimating light, maximizing captured energyor intensity functions) used for the evaluation of a speci�c optical device. Evaluation
72 Chapter 4. Patterned LSCs with an Integrated Lens Array
Table 4.2: Optimized lens parameters from ray-tracing program
radius of curvature R 0.487polynomial constant C2 1.236polynomial constant C4 3.366polynomial constant C6 5.863
Figure 4.7: Illustration of the resulting lens from entering the optimized lens parametersinto the ray-tracing program.
functions are basically error functions that evaluate the di�erence between the currentand the targeted value of the function as variables of the lens are modi�ed. A functionvalue of zero implies there is no di�erence between the actual and targeted value, and theoptimization objective was achieved. Once the evaluation function, the capturing receiver,and the variables to be modi�ed are de�ned, the optimization function will automaticallyalter the values of the variables in an attempt to achieve the set target. Here, the focusingevaluation function was used, which attempts to focus light rays through a lens to achievethe minimal spot size on the receiver plane. For the lens design, the variables used inthe optimization function were the conic constant, radius of curvature and the �rst threepolynomial constants in equation 4.4. After several optimization runs, the �nal optimizedvalues of the various lens parameters are given in Table 4.2:
Using the optimized variables in the polynomial lens equation 4.4 resulted in the lensshape shown in Figure 4.7 with a focal length of ∼1 mm. The focal length of the lenswas not a signi�cant designed factor as an air gap is placed between the lens array andthe dye structures, and the distance of the air gap is variable within the range of 0.5-3mm depending on the focal length of the lens. A plot of the light rays refracted throughthe optimized lens and incident on the receiver plane is shown in Figure 4.8. The dense
Figure 4.8: A scatter plot of the receiver plane capturing the refracted light rays from theoptimized aspherical lens made of (a) PDMS and (b) polycarbonate. The dense clutter ofdots in the center of the plot are rays focused by the lens onto the receiver plane and dotsin the outer region of the plot are light rays that missed the lens altogether. Three lightsources producing bundles collimated light rays at incident angles 0◦ and ±30◦ were usedfor this study of lens focal spot. The receiver plane was placed at 0.5 mm from the planarsurface of the lens.
74 Chapter 4. Patterned LSCs with an Integrated Lens Array
clutter of black dots in the center of the plot are the converged rays and the dots at theouter edge of the plot are rays that missed the lens altogether. From the scatter plot,one can retrieve the radius of the focal spot size, and it was observed to be ∼0.30 mmand ∼0.27 mm respectively for lens with a refractive index of 1.42 and 1.59 at a receiverdistance of 0.5 mm from the lens. This suggests that an increase in refractive index ofthe optical lens by 12%, the focal spot size of the lens decreases by ∼10%.
4.4 Lens Array Fabrication and Experimental Setup
4.4.1 Lens Array Fabrication
To fabricate a cylindrical lens array with an aspherical lens surface as designed in thelens model, the lens shape, described by rectangular coordinate points (x, y, z), was fedinto a computer and was subsequently milled into a copper mold by the machine shopof Gemeenschappelijk Technische Dienst (GTD) at Eindhoven University of Technology(TUE). To minimize scattering from the lens surface, the mold was chemically polishedto attain optical smoothness. Inevitably, this polishing process has an e�ect on the shapeof the lens mold, but as optical smoothness is required, an e�ort was made to limit itse�ect by not over polishing.
After obtaining the lens mold, two methods were used to fabricate the lens arrays,compression molding and resin casting. For compression molding, the copper lens moldwas �rst preheated in the press to 90◦C for 15 minutes to limit temperature gradientsin the mold. Subsequently, 50×50×2 mm3 transparent polycarbonate plates (PC, SabicInnovative Plastics) were placed in the mold, and heated for 15 minutes at 90◦C betweenthe two pressing plates with no applied pressure. This step ensures uniform heating of thePC plate. The temperature was chosen to be lower than the glass transition temperature(Tg) of PC to prevent the substrate material from �owing, and thereby decreasing thechance of bubble formation during the pressing process. Next, the substrate is pressedinto the mold with a pressure of 100 mBar for 5 minutes in a nitrogen environment. Lastly,the mold and the PC lens array were cooled down to room temperature prior to releasingthe lens array from the mold. Figure 4.9a displays one of the PC aspherical lens arrayproduced using this method.
Fluorescent dye solutions were prepared using 0.5 wt% of perylene-1,7,8,12-tetraphenoxy-3,4,9,10 tetracarboxylic acid-bis-(2'-6'diisopropylanilide) (Lumogen Red 305, BASF), and
4.4. Lens Array Fabrication and Experimental Setup 75
(a)
(b)
Figure 4.9: Cylindrical lens array fabricated using (a) compression molding of 50×50×2mm3 polycarbonate plate, and (b) thermal crosslinking of poly(dimethyl siloxane) (PDMS).
1% photoinitiator (Irgacure184, Ciba) dissolved in a 3:1 dipentaerythritol penta-acrylate(Polysciences) and methylmethacrylate (MMA, Aldrich) mixture. The dye solutions werestirred and heated at 60◦C for an hour prior to spin-coating onto 50×50×3 mm3 PMMAsubstrates at 1000 rpm for 30 s. After spin-coating, the fabricated lens arrays were usedto focus UV light (OmniCure S2000 UV spot curing lamp) onto the samples for 60 s undernitrogen �ow at 55◦C. The areas of dye coating exposed to UV light were cross-linked andform patterns of dye structure. Unexposed material on the patterned PMMA substrateswas etched away by placing the samples in ethanol and agitating it continuously for ∼40seconds. 100% uniformly covered control LSC samples were fabricated using the samedye mixture and similar methods as the patterned samples with the exception that the100% covered sample was directly exposed to the UV lamp. The thicknesses of the dyecoatings and line patterns were measured to be 8-10 µm using a Fogale Zoomsurf 3Doptical pro�ler.
4.4.3 Lens Array and Line Pattern Sample Characterization
The shape of the lenses produced using both compression molding and resin castingmethods were photographed (see Figure 4.9) and the images of the lenses were digitalizedinto rectangular (x,y) points using MATLAB (MathWorks). A polynomial was used to�t the digitalized points describing the shape of the lens as demonstrated in Figure 4.10.
The transmission of the lens arrays was measured using a SLMS 1050 integratingsphere (Labsphere) equipped with a diode array detector (RPS900, International Light)and the measurement setup is shown in Figure 4.11. The lens array was placed in a custom-made holder with a 50×50 mm2 opening and the opening was positioned to face the entryport of sphere. A prism was used to direct collimated light at the lens surface within theopening of the holder. The size of the prism was designed to ensure that the re-directedlight is uniform within the 50×50 mm2 opening of the holder. Assuming the measured
76 Chapter 4. Patterned LSCs with an Integrated Lens Array
50 100 150 200 250 300 350
−50
0
50
100
150
20050 100 150 200 250 300 350 400
−50
0
50
100
150
200
250
(a) (b)
X [a.u.]
Y [a
.u.]
Y [a
.u.]
X [a.u.]
Figure 4.10: Polynomial �t of the resulting lens shape of the (a) polycarbonate, and (b)PDMS lens array.
source spectra to be 100% transmission for wavelengths 350-1000 nm, the transmittance(% transmission) of the lenses can then be calculated by dividing the transmission spectraof the PC and PDMS lens by the optical spectrum of the light source.
Transmission spectra of the fully covered samples and the two lens arrays (PC andPDMS) were measured using a Shimadzu UV-3102 PC spectrometer. The peak ab-sorbance calculated from the transmission spectra was found to be ∼0.42 for the 100%thin �lm dye-coated PMMA waveguides. Edge emission of the patterned and fully coveredwaveguides was measured using the integrating sphere technique. The fully covered andline patterned LSC with and without the lens array were placed in a custom-made (blackpainted) rotating sample holder, which is connected to the entry port of the integratingsphere and prevents surrounding light from entering the port. All samples were exposedto a collimated light source from a 300 W solar simulator with �lters to approximate the1.5 AM global solar spectrum (Lot-Oriel) located at a distance of ∼15 cm from the topsurface of the sample. For the integrated system, the lens array was placed on top of thepatterned LSCs in a way that focused the incident light directly onto the line patterns ofthe sample. Note that an air gap was present between the lens array and the line patternedLSCs, and hence they were not in optical contact. Edge emission was then recorded every10◦ as the sample rotates from -30◦ to +30◦ with a black (painted) absorbing background.This procedure was repeated for edge emission spectra measurements of all four substrateedges. Subsequently, a white scattering layer (i.e., white cardboard paper) was placedunderneath the samples with an air gap, and the edge emission measurements were re-peated as above. Similar edge emission measurements were performed on all samples todetermine and compare the e�ect of the angle of incidence and the scattering layer on theperformance of fully covered, line patterned, and integrated (lens array combined withpatterned LSC) systems. To examine the e�ect of the two lens arrays on patterned LSC,the patterned sample that was fabricated using the polycarbonate lens array is used forall edge emission measurements of the integrated pattern and lens array system. Thisreduces the in�uence of variations in dye mixture and fabrication procedures on the per-
4.4. Lens Array Fabrication and Experimental Setup 77
TransmissionSpectrum
Detector
Baffle
Lens Array
Integrating Sphere
Prism
Figure 4.11: A schematic of the experimental setup for transmission measurements ofa lens array using the integrating sphere.
formance of the integrated systems, thereby allowing us to better assess the e�ect of thetwo di�erent lens arrays.
At incident angles deviating from the substrate normal, the intensity of light (i.e.,the amount of photons) illuminating the surface of the substrate is reduced. To correctfor the reduced photon �ux, edge emissions measured at an angle were multiplied by anangle-dependent correction factor. Assuming that the quantum yield of the �uorescentdye is ∼1.0 and emission is proportional to absorption, the number of photons emittedat the edges was scaled by the same correction factor. This factor was calculated byprojecting the concentrator area on a plane that is perpendicular to the light source,which is a cosine function of the incident angle (see Figure 4.12). Hence, all integratededge emission values were multiplied by 1
cosθ, where θ is the incident angle of light to the
surface plane of the waveguide. Edge emission spectra were converted to the number ofemitted photons by dividing the emission energy at every wavelength by the energy of asingle photon (see equation 1.1).
The total edge emissions were determined by integrating the recorded spectra overthe range of 350-750 nm. The total absorbed power of the 100% covered samples wascalculated by multiplying its absorption spectrum with the solar simulator spectrum, andsubsequently integrating it from 350 - 750 nm. The absorption of the patterned waveguideswas calculated by multiplying the power absorbed by the 100% covered sample with thefractional area coverage of the patterned samples, e.g., for a sample with 30% coverage, thepower absorbed would be 0.3×Powerabs of 100% covered device. Subsequently, the relativee�ciency (equation 2.1) of the system was determined by dividing the total edge emission(summation of all 4 edges) by the absorbed power of the sample. The measurement errorin edge output of the integrated systems and line patterned samples is ∼ 10% and 5%,respectively.
78 Chapter 4. Patterned LSCs with an Integrated Lens Array
θ
Ao
Aocosθ
Incident Light
Figure 4.12: A representative schematic of the correction factor calculation based on theprojection of the concentrator area to the plane perpendicular to the radiation direction.
4.5 Results and Discussion
4.5.1 Lens Array Comparison
The modeled cylindrical lens array was fabricated by both compression molding polycar-bonate (PC) plates and resin casting PDMS. A photograph was taken of the resultinglens arrays and digitalized using MATLAB programming to compare with the design lensshape (see Figure 4.13). Both PC and PDMS lenses were noted to exhibit nearly identicalshapes. On the other hand, both physical lenses deviated considerably from the shapeof the theoretical design lens. The di�erence between the theoretically designed and theactual lens array is possibly due to a number of factors: polymer shrinkage, fabricationprecision, and chemical polishing of the lens mold. The change in the curvature of thephysical lenses as compared to the simulated lens will a�ect the focusing properties of thelens. The in�uence of this di�erence between the actual and the theoretical predictionswill be discussed later.
Transmissions of the PC and PDMS lens arrays were measured using the integratingsphere with the light source at normal incidence, and its transmittance values were cal-culated by normalizing the transmission spectra of the lens arrays by the spectrum of thelight source. As reported in Figure 4.14, the PC and PDMS lenses exhibit a transmittanceof 85% and 88%, respectively. The di�erence in the measured transmittances between thetwo lenses is partly due to the higher refractive index of PC (∼1.59) compared to PDMS(∼1.42) and partly the di�erence in the quality of the lens, i.e. amount of scattering onthe surfaces and within the lens array. From the lens transmittance, one can calculate thecombined e�ect of re�ection, absorption, and scattering of the lenses using the formula1-T, where T is transmittance. This calculation produced ∼15% and ∼12% for the PCand PDMS lens array, respectively, which represents the precentage of light lost throughthe lens arrays. Clearly, it is desirable to have the maximum possible amount of light to betransmitted by the lens array to enhance the absorption of the patterned dye structures,which in turn increases the edge emission of the LSC system. The amount of light lost
4.5. Results and Discussion 79
Figure 4.13: A graphical comparison of the model (solid line) lens shape with the resultinglens shape from compression molding polycarbonate plate (dash line) and resin casting ofPDMS.
through the lens will have an negative e�ect on the edge output of the patterned LSCswhen the lens array is added to the system.
4.5.2 Integrating Lens Array with Patterned LSCs
The PC and PDMS lens arrays were used directly to fabricate the line patterned LSCs viaphotolithographic techniques. This allows the resulting line pattern to be aligned withthe focal areas of the lens array, thereby incident light can be focused directly on the linepattern dye structures. Care was taken to place an air gap between the lens array and thepatterned LSCs to ensure waveguided light will not be coupled out of the substrate by thelens array. The combination of the lens array with the patterned LSCs will be referredto as the 'integrated system' hereafter, and the integrated systems will be classi�ed bythe type of lens array, PC or PDMS, used in the system. To evaluate the performance ofthe integrated systems, edge emission spectra were recorded at incident angles of −30◦ to+30◦ and compared with the edge emission of patterned (without lens array) and 100%covered LSC.
PC Integrated System
The edge emissions of the integrated system and line patterned LSCs were divided intotwo groups, parallel and perpendicular (see Figure 4.15), where the classi�cation solelydepended on the waveguide edge at which the measurement was recorded. The recordededge emission of the PC integrated system with an black absorbing background, i.e., PClens array combined with line pattern LSC, were compared to the edge emission of line
80 Chapter 4. Patterned LSCs with an Integrated Lens Array
Figure 4.14: Transmittance of PC (dash) and PDMS (solid) microlens array calculatedby dividing the transmission spectra of the lens arrays by the spectra of the light source.
pattern and 100% covered LSC in Figure 4.16. Anisotropy was noted in the edge emissionof the PC integrated system as di�erent outputs were produced by orthogonal edges ofthe system. It was observed that the edge emission from the parallel edges was relativelyhigher than that recorded at the perpendicular edges, and the emission remained nearlyconstant over the range of incident angles studied. Edge emissions at the perpendicularedges also remained nearly constant from −20◦ to +20◦. This implies that the lens arraywas able to focus light directly onto line pattern structures at incident angles within therange of ±20◦, and similar edge emission values can be achieved regardless of the positionof the light source within the acceptance angle of 20◦. However, this is 10◦ smaller thanexpected as the lens array was design to have an acceptance angle of 30◦ as discussed inthe �Aspherical Lens Design� section of this chapter. Furthermore, at ±30◦, a 25% drop inedge output was noted for the emission at the perpendicular edges, which nearly matchesthe edge emission of patterned LSC system without the lens array. In other words, raysrefracted at ±30◦ with respect to the optical axis of the lens are not converging ontothe dye patterns, resulting in edge emission decrease. This is primarily a result of theobserved di�erence in lens curvature between the designed lens and the fabricated lensarray. The changes in the lens shape, particularly at the edges of the lens (see Figure4.13), have an signi�cant e�ect on the focus and comatic aberration of the lens, especiallyat oblique incident angles. Light rays incident at oblique angles tend to arrive at the edgeof the lens surface, and hence the curvature of the lens at the edges determine where theoblique rays are focused. Considering the di�erence in lens curvature, it is not surprisingthat the acceptance angle of the actual lens arrays was not as large as designed, andthe oblique light rays (at ±30◦) are not converging onto the dye pattern. In addition,the PC lens array tend to slip out of position in the sample holder during the angular
4.5. Results and Discussion 81
{ {θi
E||
E⊥
{ {
E||
E⊥
(a) (b)
Figure 4.15: Schematic displaying parallel (E‖) and perpendicular (E⊥) edge emissionof both (a) PC integrated and (b) line patterned LSC systems, where parallel emissionrefers to emission at the substrate edge that is parallel to the length of the cylindrical lensarray as well as the line pattern.
measurements, which may lead to slight misalignment between the lens array and the linepatterned substrate, resulting in lowered edge output. At the parallel edge, the lens arrayalso slipped within the sample holder, but the e�ect on edge emission is negligible. Thisis because in the parrallel emission measurement con�guration, light is always incidentparallel to the optical axis of the lens as the incident angle is varied along the length ofthe cylindrical lens array, and thus resulting in near constant emission.
By adding a lens array to focus light directly on the line patterns, edge emissionincreased by ∼60% compared to the line patterned LSCs for all angles studied (Figure4.16)up to ±20◦. This con�rms that the lens array indeed increases the absorption ofthe line patterned dyes and decreases the light lost through the clear substrate regions.Without the lens array, the di�erence in emission from the parallel and perpendicularedges of the line patterned substrate is trivial. However, the added lens array only in-creased the parallel edge mission of the integrated system to ∼90% of the fully coveredsample. This suggests that the improved e�ciency of the pattern system is overcome bythe losses incurred from the addition of in the lens array. As discussed above, the PClens array exhibit only ∼85% light transmittance, implying that ∼15% of incident lightis lost as a result of re�ections, absorption, and scattering in lens array. Furthermore, theair gap between the lens array and the patterned LSC induces additional re�ections andscattering losses for the transmitted rays. The line pattern LSCs were assumed to havesimilar peak absorbance values as the 100% dye covered sample since the same solutionwas used for the fabrication of both systems. However, this may not be true as dyes notcompletely crosslinked in the line pattern may be rinsed away during the etching process,leading to a lower peak absorbance than the 100% covered sample. It was di�cult toaccurately determine the absorbance of the pattern samples due to the line width of thedyes structures being smaller than the beam size of the UV-Vis spectrophotometer used
82 Chapter 4. Patterned LSCs with an Integrated Lens Array
Figure 4.16: Photon emission of PC integrated (Eint, solid black symbols) and linepattern (Epat, open symbols) LSC systems were recorded from parallel (‖, ◦) and perpen-dicular (⊥, 4) edges of the substrate as a function of incident angles (−30◦ to +30◦),and compared with edge emission of 100% covered LSCs (E100%, grey square) measured atsimilar angles. A line connecting the data points were added to aid the eye of the reader.
for absorbance measurements.It has been established that adding a scattering layer improves the performance of
LSCs [121, 122]. Hence a scattering layer was placed at the bottom of the various pat-terned and 100% covered LSC systems with an air gap in an attempt to enhance theedge emissions of line patterned and integrated systems. The edge outputs of the varioussystems measured with a scattering layer are reported in Figure 4.17. Similar to the edgeoutputs measured with an absorbing background, anisotropy was observed in the edgeoutputs of the integrated system, and the di�erence between the parallel and perpendic-ular edge outputs was much greater than that measured using an absorber background.The parallel edge exhibits ∼12% higher emission than the perpendicular edge for all in-cident angles measured (Figure 4.17), even at normal incidence, which was not observedin the measurements using an absorbing background (Figure 4.16). A similar di�erencebetween the parallel and perpendicular edge emission was noted in the line patternedsystems incorporating a scattering layer in Figure 4.16.
The lower perpendicular emission noted in both the integrated and patterned LSCsystems may be partially due to the di�erence in reabsorption probability of photons asthey travel to the di�erent edges of the waveguide. A fraction of the emitted photonsmeasured at the perpendicular edge must traverse the lines of dye coating, which increaseits chance of reabsorption. At the parallel edge, photons have mainly traveled throughalternating regions of dye pattern and clear substrate, which is designed to decrease theprobability of reabsorption. When an absorber was placed at the bottom of the LSCsystems with an air gap, a large fraction of photons incident on the clear regions of the
4.5. Results and Discussion 83
Figure 4.17: Parallel (‖, ◦) and perpendicular (⊥, 4) edge emitted photons of PCintegrated (Eint, solid black symbols), and line patterned (Epat, open symbols) LSC systemswere measured at incident angles ranging from −30◦ to +30◦, and compared with edgeemission of 100% covered LSCs (E100%, solid grey squares) measured at similar angles.All edge photon emissions were measured with a scattering layer placed at the bottom ofthe system with an air gap. Lines connecting the data points were added to aid the eye ofthe reader.
84 Chapter 4. Patterned LSCs with an Integrated Lens Array
Figure 4.18: The ratio of photons emissions from parallel (•) and perpendicular (N)edges of the PC integrated system using a scattering background was normalized by theedge emission of the 100% covered LSC as a function of incident angles −30◦ to +30◦. Aline connecting the data points were added to aid the eye of the reader.
substrate were lost through the bottom surface of the waveguide and absorbed by the blackabsorber. Hence only a small fraction of photons undergoing TIR in the waveguide canbe reabsorbed. Photons reaching the perpendicular edge may undergo more reabsorptionthan the parallel edge, but the di�erence in reabsorption is too small to be detectablewhen an absorber is used. However, when the black absorber was replaced by a scatteringlayer, photons exiting the bottom surface of the substrate are scattered back into thesubstrate at di�erent angles, and it can now be absorbed and reabsorbed multiple timesby the lines of dye. This increases the number of reabsorption events along the lines of dyeand in turn lowers the emission of photons at the perpendicular edge, leading to a morenoticeable di�erence between the parallel and perpendicular edge outputs. Therefore,the addition of the lens array to the line pattern LSCs in combination with a scattererinduced anisotropy in the edge emission of the system. This is potentially advantageousfor decreasing the number of PV cells attached to LSC system, i.e., one can attach justtwo PV cells to the two opposite edges exhibiting highest edge emission and place mirrorsat the two other edges to re�ect light back into the system, thereby reducing the numberof PV cells.
The edge output from the perpendicular edges using a scattering background decreasedby ∼25% at incident angles of ±30◦, which is similar to the results obtained using anabsorbing background. This con�rms that the altered lens shape in the physical lensarray decreased the acceptance angle of the lens from the designed 30◦ to 20◦. If a 30◦
acceptance angle is to be achieved, more precise but expensive methods must be employedfor the making and polishing of the lens mold.
4.5. Results and Discussion 85
Discounting the perpendicular edge emission at ±30◦ incident angles, both the paralleland perpendicular edge emission of the integrated system using a scattering backgroundwas greater than the 100% fully covered LSC. The emission from the parallel edges of theintegrated system exceeded that of the fully covered LSCs (Figure 4.18) by 20%. Thiscon�rms that the lens array was able to increase the absorption of the dye structures whileexploiting the enhanced e�ciency of the patterned LSCs observed in chapter 3, resultingin higher absolute edge outputs than the standard fully covered LSC despite a 15% lightloss due to re�ections, scatterings and absorptions in the lens array. The absolute photone�cency ηabs (equation 4.5) of the integrated lens and patterned LSC systems was de�nedas follows:
ηabs =EphotonsIphotons
× 100% (4.5)
where Ephotons is the total number of photons emitted from the four edges of the waveguideintegrated from 350-750 nm and Iphotons is the total number of photons incident on thesample integrated over the same range. ηabs was noted to be approximately 5.7% (atincident angle of 0◦) for the integrated lens array and patterned LSC system, which is∼0.6% higher than that of the fully covered LSC system (5.1%). This again con�rms thatthe integrated system outperforms the standard fully covered LSCs. A larger increasein edge emission and absolute e�ciency may be achieved if an anti-re�ection coating isapplied to the lens array and/or an lower refractive index layer is used to introduce opticalcontact between the patterned LSC and the lens array to reduce the 15% light loss.
By utilizing a scattering layer to return photons to the LSC systems, the edge outputwas increased for all LSC systems studied as shown in Figure 4.19. It was found thatthe scatterer increased the parallel and perpendicular edge emission of patterned and in-tegrated LSCs by > 70% and > 60%, respectively, and a maximum of 28% increase wasnoted for the 100% covered LSC for all angles studied. Clearly, the patterned and inte-grated systems bene�t more from the back scatterer than a standard fully covered system.This is not surprising as the scatterer returns light into the system, which increases thenumber of absorption and reabsorption events in the LSC. In a standard fully coveredLSC, an increase in reabsorption leads to more surface losses and eneregy losses. How-ever, because the probability of reabsorption is reduced by decreasing the surface coverageof dyes in the patterned LSCs, the increase in reabsorption is limited in these systems.Therefore, the scattering layer only produced a small improvement in the edge output ofthe fully covered system, whereas the improvement in both the patterned and integratedsystems was considerably higher. Again, increase in emission from the parallel edges wasnoted to be greater than the perpendicular edges. As aforementioned, this di�erence islikely due to photons reaching the perpendicular edge of the system su�ering from morereabsorption along the lines of dyes than photons reaching the parallel edges.
PDMS Integrated System
To investigate the e�ect of the lens array's refractive index on the performance of theintegrated LSC system and to further reduce the production cost of an integrated system,a cylindrical lens array was also fabricated using PDMS (refractive index∼=1.42). Similar
86 Chapter 4. Patterned LSCs with an Integrated Lens Array
Figure 4.19: The increase in parallel (‖, ◦) and perpendicular (⊥, 4) edge photonemission of PC integrated (Eint, solid black symbols), line patterned (Epat, open symbols),and 100% covered (E100%, solid grey squares) LSC systems using a scattering layer withrespect to the edge emitted photons of the same systems measured using an absorbingbackground are plotted. The lines connecting the data points were added to aid the eye ofthe reader.
to the PC lens arrays, the PDMS integrated system was formed by placing the PDMSlens array on top of the same line-patterned LSCs, separated by an air gap. Due tothe �exibility and adhesiveness of the PDMS lens array, it was di�cult to prevent thearray from coming into optical contact with the line patterned LSC, and thus a smallpart of the lens array was generally in optical contact with the patterned substrate for allmeasurements.
Edge emission of the PDMS integrated system was measured with a black absorber anda scattering layer at the bottom of the system, and compared with the edge emission ofline patterned and fully covered LSCs measured using a similar setup. The edge outputresults of various LSC systems with an absorbing back is given in Figure 4.20. Theedge emission of the various LSC systems remained nearly constant for incident anglesranging from −30◦ to +30◦. No signi�cant di�erence was noted between the parallel andperpendicular edge emission of the line pattern system. The addition of the PDMS lensarray increased the line pattern edge emission by ∼ 25%. For the integrated system,the near constant perpendicular edge output suggests that the PDMS lens array wasable to focus light incident within ±30◦ onto the line pattern and thus no decrease inedge emission was noted. This suggests that the acceptance angle of the lens is 30◦ astheoretically designed. The emission from the parallel edges of the integrated systemwas noted to be higher than the perpendicular emission by ∼ 10% for all incident anglesstudied. Again, the reason for the perpendicular edge to exhibit lower edge emission thanthe parallel edge is likely due to the photons at the perpendicular edge su�ering from
4.5. Results and Discussion 87
Figure 4.20: Photon emission from parallel (‖, ◦) and perpendicular (⊥, 4) edges ofPDMS integrated (Eint, solid black symbols), and line patterned (Epat, open symbols) LSCsystems were measured with a back absorber at incident angles ranging from −30◦ to+30◦, and compared with edge emission of 100% covered LSCs (E100%, solid grey squares)measured at similar angles. Lines connecting the data points were added to aid the eye ofthe reader.
more reabsorption along the lines of dye pattern. The highest edge emission of the PDMSintegrated system with an absorber background is merely ∼70% of the fully covered LSC(see Figure 4.20). This suggests that the photon absorption of the line-patterned dyestructures integrated with a lens array is still insu�cient compared to the absorption ofa fully covered LSC, resulting in a considerably lower edge emission from the integratedsystem.
To increase the amount of photons returned to the integrated system, a scatteringlayer was placed at the bottom of the PDMS integrated system with an air gap and theperformance of the LSC systems were re-measured. The performance of the three LSCsystems with a scatterer back layer is reported in Figure 4.21. All recorded edge emissionsof the di�erent LSC systems were found to be relatively constant for incident anglesranging from −30◦ to +30◦ and all systems exhibit similar trends in edge emission. Theaddition of the lens array with a scattering layer increased both parallel and perpendicularemission of the integrated system by ∼35% compared to emission from similar edges ofthe line patterned LSC without the lens array. By adding a scatterer to the bottomof the devices, the parallel edge output of the PDMS integrated system was increasedto ∼95% of the fully covered system. Theoretcially, the PDMS integrated system witha back scatterer was expected to perform at least as well as the fully covered LSC asthe integrated system should have higher e�ciency than the 100% covered LSCs due toreduced reabsorption losses in the line-patterend waveguide. Moreover, light absorptionin the integrated system was assumed to be comparable to the 100% covered LSCs if
88 Chapter 4. Patterned LSCs with an Integrated Lens Array
Figure 4.21: Photons emitted from the parallel (‖, circles) and perpendicular (⊥, tri-angles) edges of PDMS integrated (Eint, solid black symbols) and line pattern (Epat, opengray symbols) LSC systems were measured with a scattering layer at incident angles rang-ing from −30◦ to +30◦, and compared with edge emission of 100% covered LSCs (E100%,solid gray squares) measured at similar angles. Lines connecting the data points wereadded to aid the eye of the reader.
the lens array was able to focus all incoming light on the dye structures. However, itwas not possible for the integrated system to achieve the same absorption as the fullycovered LSCs as 12% of the incident photons are lost due to scattering, re�ections, andabsorption from the lens array. Furthermore, the partial optical contact between the lensarray and the patterned substrate may cause waveguided photons to be coupled out ofthe substrate through the lens array. Of course, the lower refractive index of the PDMScompared to polycarbonate also changes the focal length of the lenses. Given that theair gap distance between the PDMS lens array and the line patterned sample was similarto that of the PC lens array and patterned waveguide, the patterned substrate here ismost probably not placed at the optimal focal plane of the lens array. These mechanismstogether provide at least a partial explanation for the lower than expected edge output ofthe PDMS integrated system.
The e�ects of the scattering layer on the measured edge outputs of all three LSCsystems were examined more closely by plotting them with respect to the edge outputsof the systems measured using a black absorber (see Figure 4.22). Similar to the PCintegrated system, the percentage increase in the edge output of the integrated system asa result of the scatterer ranged from ∼58%-∼72%. This is approximately 10% higher thanthe edge output increase noted in the line patterned LSC system and more than doublethe increase of a fully covered LSC with the scattereing layer. Evidently, the scattering
4.5. Results and Discussion 89
Figure 4.22: The increase in emission from the parallel (‖, ◦) and perpendicular (⊥,4) edges of PDMS integrated (Eint, solid black symbols) and line patterned (Epat, opensymbols) LSC systems, as well as the edge emission of fully covered LSC (E100%, solid greysquares) all using a scattering background were plotted with respect to the edge emissionof the same systems measured using a black absorber. Lines connecting the data pointswere added to aid the eye of the reader.
Table 4.3: Absolute photon e�ciencies of di�erent LSC devices.
LSC Device Absolute Photon E�ciency ηabs[# of photons]
layer is more advantageous to the line patterned and integrated LSC systems than thefully covered LSC, which is most probably a result of the reduced reabsorption in bothsystems.
To attain a better insight into the e�ect of the di�erent lens arrays on patterned LSCs,the edge emissions of the PC integrated systems were compared with the PDMS integratedsystem. As demonstrated in Figure 4.23, the PC lens array appears to produce ∼13%greater edge outputs than the PDMS lens array for almost all incident angles studied, withthe exception of the emission from the perpendicular edges at ±30◦. The PC integratedsystem o�ers better performance because of its higher refractive index compared to PDMS.Higher refractive index materials allow the lens to bend the light rays more, particularlyimportant at oblique incident angles, leading to a smaller focal spot size for all angles ofincidence deviating from the normal. Hence, it is not surprising that the PC integratedsystem produced the highest edge output among all the LSC systems studied as higher the
90 Chapter 4. Patterned LSCs with an Integrated Lens Array
Figure 4.23: Photon emissions from parallel (•) and perpendicular (N) edges of twointegrated LSC system consisting of polycarbonate (black symbols) or PDMS (grey symbols)aspherical shaped cylindrical lens array on top of line patterned LSCs. The lines connectingthe data points were added to aid the eye of the reader.
refractive index, better the focusing power of the lens array. A summary of the absolutee�ciencies calculated for the various LSC devices stuided in this chapter is presented inTable 4.3. The ηabs values con�rm that the PC lens array integrated with line patternedwaveguide is the best performing LSC system studied.
4.6 Conclusions
Two aspherical lens arrays, polycarbonate and PDMS, designed to focus solar radiationon dye structures were realized and integrated with patterned LSCs. The acceptanceangle of the lens array was investigated by examining the performance of the integratedLSC system in a range of ±30◦ incident angles. In addition, an absorber and scatteringlayer were each added separately to the integrated line pattern and fully covered systemsto study the e�ect of each of the back layers on the performance of the LSC systems.Both lens arrays were able to focus light incident within the range of ±20◦ directly onthe lines of dye structure as con�rmed by a near constant edge output of the integratedsystem observed with both an absorber and a scatter back layer for the range of incidentangles studied. The addition of the PDMS and PC lens array increased the line patternedLSC edge output by ∼35% and ∼60%, respectively. However, the edge output of theintegrated system with a black absorber at the bottom was less than the output of a fullycovered LSC. By replacing the black absorber with a white scatterer, the edge emissionof the polycarbonate integrated system was noted to exceed the performance of the fullycovered LSC by > 20%. The absolute photon e�ciency achieved by this polycarbonate
4.6. Conclusions 91
integrated system was 5.7%, which is 0.6% higher than the standard LSC. The higherrefractive index of the polycarbonate lens array combined with the advantages of the backscatterer signi�cantly enhances the photon absorption of line pattern LSCs while retainingbene�ts of the line pattern to reduce reabsorptions. As a result, the performance of thePC integrated system with a back scatterer was found to be the best among the LSCsystems studied.
92 Chapter 4. Patterned LSCs with an Integrated Lens Array
Chapter 5
Aligned Dye Molecules in Patterned
LSCs
5.1 Introduction
In chapter 3, the concept of patterning �uorescent dye coated LSCs to reduce reabsorptionlosses was discussed. It was shown that the photon-to-photon e�ciency of the systemgenerally increased with decreasing dye coverage of the waveguide surface. Here, linepatterned thin �lm LSCs are fabricated using aligned �uorescent dyes in a guest-hostliquid crystal matrix. By using liquid crystals (LCs) to induce a macroscopic order in thedye molecules, an additional enhancement of the patterned LSC's may be achieved.
Fluorescent dye molecules used in our studies are typically dichroic, and it has beenestablished that dichroic dyes exhibit anisotropic absorption and emission [81, 102, 123,124, 125]. For example, the maximum light absorption for most commonly used dichroicdyes in an LSC occurs when incident light is propagating normal to the long axis (orthe dipole axis) of the dye molecule (see Figure 5.1). In addition, the emission fromdichroic dyes forms a lobe-shaped distribution about the long axis of the molecule [102].This suggests that the amount of absorption and emission per dye molecule is at leastpartly dependent on the position and arrangement of the dye molecules when irradiated atnormal incidence. In other words, the dye molecules will absorb little light if the incidentlight is propagating parallel to the long axis of the dye molecule as shown in the leftillustration of Figure 5.1. Low absorption in turn leads to low emission. In contrast, thesame dye molecule with its long axis positioned normal to the propagation direction ofincident light will resulting in greater absorption and emission. This preferred absorptionand emission directions can be used to control the spatial emission of the dyes to anextent, and potentially decrease the escape cone losses in the LSCs.
Alignment of dichroic dye molecules in LC guest-host systems has been previouslyreported [112, 126, 127] and results show that macroscopic and monolithic alignmentof the dye molecules are possible. Guest materials in an LC mixture tend to follow thealignment of its host material (the LCs) given that it has an anisotropic form. In addition,it has been demonstrated that using homeotropic aligned dyes in an LC guest-host systemresults in the reduction of surface losses and better waveguide coupling in comparison toisotropic and planar aligned dye LSCs [128, 129].
93
94 Chapter 5. Aligned Dye Molecules in Patterned LSCs
Incident Light
Emission
Incident Light
Emission
Figure 5.1: Schematic representation of a dichroic �uorescent dye molecule with its longaxis positioned parallel (left) and normal (right) to the propagation direction of incidentlight absorbs and emits anisotropically. The reduced length of the incident light and emis-sion arrows indicates the relative amount of absorption and emission for the di�erent longaxis positions of the dye molecule.
The aligned dye LSC system o�ers us control over the spatial distribution of dyeemitted light. It is always desirable to decrease the coverage of PV material in an LSCas it would lower the cost of the LSC module, making it a more attractive and a�ordablealternative to standard PV panels and PV concentrators. This goal can be achieved byarranging dye molecules along the lines of the patterned LSC such that emitted light isdirected to two preferential edges rather than four. Thus, only two edges of LSC need tobe attached with PV cells.
To direct the dye emitted light to two opposite edges of the LSC, the preferred emis-sion of dye emitted light is exploited. Here, by aligning the dye molecules in a planarcon�guration with its long axis lying along the length of the lines in a patterned LSCas shown in Figure 5.2a, the spatial distribution of dye emitted light is favored at twoedges of the waveguide. The resulting performance of the line patterned LSC with dyemolecules aligned in a planar fashion is subsequently compared with the application ofhomeotropic aligned dyes (Figure 5.2b) in LSCs, as well as isotropic LSC samples.
5.2 Brief Background on Liquid Crystals
To better understand how liquid crystals can be used to induce order in dye molecules,one must �rst understand the basic principles and properties of liquid crystals. Liquidcrystal (LC) molecules are typically rod (e.g., nematic LCs) or disc-like (e.g., smecticLCs), and they can either have orientational order with no speci�c positional order (e.g.,
5.2. Brief Background on Liquid Crystals 95
(a) (b)RubbingDirection
Figure 5.2: Schematic of guest �uorescent dye molecules (yellow cylinders) aligned in(a) planar and (b) homeotropic con�guration using liquid crystals (green cigar-shapedmolecules) as the host. The aligned dye systems are applied to line pattern LSCs.
nematic phase) or both orientational and positional order (smectic phase). A liquidcrystalline polymer (LCP) is thus a polymer where the molecules are mesogenic, i.e., themolecules can form a liquid crystalline phase. Orientation of LCs and LCP are drivenby anisotropic steric and dispersion interactions, and it is formed spontaneously as itis the equilibrium state for the anisotropic molecules. The liquid crystalline phases forthermotropic LCP systems forms at temperatures between the melting temperature (Tm)and upper transition temperature (Tc) of the LC [130]. The general phase transitions ofLCs with increasing temperature (and in some cases, decreasing pressure) starting froma solid are: crystalline solid → smectic → nematic → isotropic. However, the nematicor smectic phase may not be present in some LCs. Examples of the molecular orderof di�erent LC phases is given in Figure 5.3. The local preferred orientation of the LCmolecules at any given point is denoted by a director, n. For a distribution of moleculesthat has rotational symmetry in its orientation such as a chiral nematic LC, then thedirector is the axis of rotational symmetry.
Liquid crystals in the nematic phase possess long range orientational order, but shortrange or local positional order [131]. The molecules are aligned with respect to thedirector, n, at random positions as depicted in �gure 5.3b. The orientational order orthe quality of alignment of the nematic phase is quanti�ed by the order parameter S̄(equation 5.1), which is a function of the angle β, where β is the angle between the longmolecular axis of the molecules and the director.
S̄ =
⟨1
2(3cos2β − 1)
⟩(5.1)
For an isotropic liquid, S is 0, and S=1 denotes a perfect crystal structure or perfectorientational order. A nematic phase LC typically has a S value ranging from 0.4 to 0.8.As only nematic phases are dealt with in this chapter, the focus here will be on nematicliquid crystals.
Polymers that exhibit liquid crystallinity are referred to as liquid crystals polymers.
96 Chapter 5. Aligned Dye Molecules in Patterned LSCs
nβ
(b)(a) (c)
TS/N TN/I
Figure 5.3: Representations of the molecular order in di�erent types of LC phases: (a)smectic A phase, (b) nematic phase, and (c) isotropic. The transition temperatures of thephases are shown in between the schematics of the phase molecular arrangement, whereTS/N and TN/I represents the transition temperature from smectic to nematic phase andnematic to isotropic phase, respectively. The temperature increases from left to right.
Liquid crystal polymers can be classi�ed into 3 main groups: LC main chain polymers,LC side chain polymers and LC networks. A representation diagram of all the di�erenttypes of LCPs is given in Figure 5.4.
Since this chapter mainly deals with LC networks, most attention will be given to thistopic, particularly LC networks created by photopolymerization. LC networks can be fab-ricated either by cross-linking side chain LCPs or polymerization of LC monomers. Bothmethods improve the stability of the liquid crystalline phase. The quality of alignmentin a LC network is again denoted by the order parameter S. Cross-linked LCPs will nolonger �ow in the liquid crystalline phase, but can still undergo di�erent phase transitions(e.g., from nematic to isotropic state) provided that the length of the backbone betweencross-linked points is su�ciently large to allow the molecules to re-orientate. Di�erentfrom the cross-linked LCPs, polymerized LC networks cannot undergo phase transitionto the isotropic state since the cross-linked density is much higher in a polymerized LCnetwork and hence, less room for molecular movement. To photopolymerize LC networks,a photoinitiator is mixed with LC monomers to produce reactive groups via dissociationwhen ultraviolet (UV) light is absorbed. The reactive groups are subsequently attachedto the ends of the LC monomers to initiate polymerization. The advantage of photopoly-merization is that many monomers can be blended to optimize the modulus and strengthof the polymeric state, as well as the viscosity of the monomer mixture. Moreover, pho-topolymerization ensures the orientation of the LC monomers by stabilizing the positionof the LC monomers once they have been aligned in a desirable direction.
Alignment of LCs can be induced by either applying an external �eld (electrical [132],magnetic [133], and shear-�ow mechanical �elds [134] are commonly used), or using
5.2. Brief Background on Liquid Crystals 97
(a) (c)(b)
Figure 5.4: Schematics of di�erent types of rod-like liquid crystal polymer structures:(a) LC main chain polymers, (b) LC side chain polymers, and (c) LC networks. Similarstructures are possible with disk-like liquid crystals.
grooved or anisotropic surfaces [135], as well as polarized laser light [136]. The shearstress induced by external �elds on the LC molecules combined with its anisotropic formtend to order the molecules along the lines of the external �eld. In this way, macroscopicorientation can be obtained using relatively little energy. Another method of orientat-ing LC molecules uses alignment layers such as polyimide derivatives or polyvinylalcohol(PVA). The deposited alignment layer is rubbed with velvet or velvet-like polyester clothto produce nano-groove structures in the rubbing direction. LC molecules coated on topof the alignment layer will preferentially align with the groove as induced by anisotropicdispersive interactions with the oriented chain segments of the molecules. Some of thecommonly produced alignment con�gurations of LCs using oriented surfaces are: planar,homeotropic, splay, tilted, twisted nematic and chiral nematic. Note that homeotropicalignment of LCs is achieved using surfactants to induce molecular order rather than rub-bing the alignment layer. Examples of LC alignment con�gurations are shown in Figure5.5.
This chapter primarily uses planar and homeotropic alignment of liquid crystals, whichcan be determined by examining the LC samples between crossed polarizers with anambient light source illuminating through one of the polarizers as illustrated in Figure 5.6.The ambient light upon entering the �rst polarizer emerges linearly polarized, in this case,horizontally. If a planar aligned LC sample is positioned immediately following the �rstpolarizer such that the LCs are aligned parallel to the �rst polarizer (see Figure 5.6), lightemerges from the LC sample oscillating horizontally. Subsequently, this horizontal lightentering the second (vertical) polarizer will be completely absorbed and one observing tothe right of the second polarizer perceives complete blackness. Similar observations weremade when the LC sample was placed such that the LCs were aligned perpendicular tothe �rst polarizer. When the LC sample was rotated (±)45o to the polarization directionof the �rst polarizer, a bright �eld is observed. In this case, the planar aligned LCs
98 Chapter 5. Aligned Dye Molecules in Patterned LSCs
(a)
(d)(c)
(b)
Figure 5.5: Representative examples of alignment con�gurations of nematic LC usingorientating surfaces: (a) planar, (b) homeotropic, (c) twisted nematic, and (d) splay. Thearrows in the schematic indicate the possible rubbing direction of the alignment layer.
5.3. Experimental Setup 99
Horizontal Polarizer
Vertical Polarizer
Incident Light
Liquid Crystal Sample
Figure 5.6: Schematic of liquid crystal alignment characterization setup using crosspolarizers. The planar aligned LC on a line patterned waveguide is placed in between twolinear polarizers with ambient light source on the left of the horizontal polarizer.
changes the phase of the incident horizontally polarized light, and light emerging fromthe LC sample is now ±45o linearly polarized. Approximately half of the ±45o polarizedlight propagates through the vertical polarizer. Therefore, the bright �eld one observesis the vertically linear polarized light exiting from the second polarizer. In contrast,if homeotropic aligned LCs were placed in between the cross polarizers with the samepolarizer setup, only complete blackness would be observed. Ideally, light passing throughthe homeotropically aligned LCs remains una�ected, resulting in complete absorptionof the horizontal polarized light emerging from the LC sample by the second verticalpolarizer. These characterization methods were used for the studies in this chapter todetermine the alignment of the LCs.
5.3 Experimental Setup
5.3.1 Sample Preparation
50 x 50 x 5 mm3 glass substrates were cleaned by sonicating alternately in acetone, distilledwater, and isopropanol. For the planar and isotropic samples, a polyimide (Nissan 130or JSR AL-1051) alignment layer was spin-coated at 5000 rpm for 60 s on one side of the
100 Chapter 5. Aligned Dye Molecules in Patterned LSCs
Figure 5.7: The molecular structure of LC242.
substrates. The polyimide coated glass substrates were baked at 90◦C for 10 min in air,then placed in an oven at 190◦C for 90 min under vacuum. For the homeotropic samples,a di�erent polyimide (PI5300, Sunever, Nissan) was spin-coated at 1000 rpm for 30 s onone side of the substrate, and placed on a hot plate at 90◦C for 30 s in air. Subsequently,the PI5300 polyimide coated samples were placed in an oven at 95◦C for 45 min. Afterbaking, all the polyimide coated substrates were allowed to cool in air overnight.
For isotropic samples, a dye solution composed of 0.13 wt% dye molecules DFSB-K160(Risk Reactor), and 1% photoinitiator (Irgacure184, Ciba) dissolved in a 3:1 dipentaery-thritol penta-acrylate (Polysciences) and methylmethacrylate (MMA, Aldrich) mixturewas spin-coated on top of the alignment layer at 1000 rpm for 30 s. Prior to spin-coating,the dye solutions were stirred and heated at 60◦C for an hour. The alignment layer inthis case acted as an adhesive between the dye mixture and the glass substrate becausethe dye coating was noted to peal o� the substrate after crosslinking.
For planar alignment, the samples were rubbed by hand on a velvet cloth to createnano-grooves in the polyimide layer. The dye solution used to produce the planar alignedsamples consist of 0.5 wt% dye (DFSB-K160, Risk Reactor), 1% surfactant, 1% pho-toinitiator (Irgacure184, Ciba) in a 1:1 reactive liquid crystal (LC242, BASF) to xylenemixture. The molecular structures of LC242 is shown in Figure 5.7. The surfactant wasused to promote planar alignment. This mixture was �rst stirred and heated at 60◦C foran hour before spin-coating on the rubbed samples at 1000 rpm for 30 s.
To fabricate homeotropic samples, a similar dye mixture as the planar aligned sampleswere used, but LC242 was replaced with RMM77 (Merck), an LC mixture containing 4-(6-acryloyloxyhexyloxy)-benzoic acid-4-(cyanophenylester). No rubbing was needed forthe homeotropic samples as the polyimide alignment layer promotes spontaneous verticalalignment of RMM77. After spin-coating of the dye mixture, the samples containing liquidcrystals were placed on a 80◦C hotplate for 20 s. All 100% covered samples regardlessof alignment were photocured by exposing it to a high-intensity UV lamp (OmniCureS2000 UV spot curing lamp) for 80 s under nitrogen �ow to form a solid �lm. Forthe fabrication of patterned LSCs, standard photolithography techniques were employed.
5.3. Experimental Setup 101
Uniformly coated samples were exposed to UV light through patterned shadow masksconsisting a regular pattern of 10 lines with variable widths and a period of 5 mm. Linewidths were varied to cover 20 to 80% of the waveguide surface. After UV exposure, thecoated glass substrates were submerged in xylene and continuously agitated for 30 s toetch away the unexposed material.
5.3.2 Sample Characterization
The thicknesses of the �uorescent dye layers were measured to be 25-35 µm for the isotropicsamples containing 0.13 wt% of dyes, and 2-3 µm and 5-10 µm respectively for the planarand homeotropic aligned LSCs, both containing 0.5 wt% of dyes. All thickness mea-surments were performed using a Fogale Zoomsurf 3D optical pro�ler. The absorptionspectrum was recorded using a Shimadzu UV-3102 PC spectrophotometer with incidentlight linearly polarized both parallel and perpendicular to the rubbing direction. All ab-sorbance reported in this chapter refers to the absorbance measured at the main peak ofthe absorption spectrum. E�ective absorbance of the samples was calculated by averag-ing the absorbance measured with light polarized in the parallel and the perpendiculardirections. The e�ective absorbance found for isotropic, planar, and homeotropic aligneddye LSCs are ∼0.15, ∼0.12, and ∼0.2, respectively. The order parameter in absorption,Sa, is de�ned by equation 5.2 [137, 138, 139]:
Sa =A‖ − A⊥A‖ + 2A⊥
(5.2)
where A‖ is the measured peak absorbance of the sample for incident light polarizedparallel to the rubbing direction, and A⊥ is the peak absorbance measured with incidentlight polarized perpendicular to the rubbing direction. Both A‖ and A⊥ were determinedfor all samples. The Sa values found for most of the planar aligned samples were between0.35-0.6, indicating reasonable alignment as the liquid crystals exhibit Sa values ∼0.6to 0.7. Planar and homeotropic alignment of the dye molecule embedded liquid crystalpolymers were also con�rmed optically using cross-polarizers. For the isotropic samples,the Sa values were determined to be nearly 0, showing no alignment in the plane of the�lm as expected.
Edge emission of all the samples were measured using SLMS 1050 integrating sphere(Labsphere) equipped with a diode array detector (RPS900, International Light). Colli-mated illumination was provided by a 300 W solar simulator with �lters to approximatethe 1.5 AM global solar spectrum (Lot-Oriel) at distance of ∼15 cm above the sample.The measurement setup for edge emission is shown in Figure 2.4 in section 2.2. Lightemissions from the edges of the aligned samples were measured parallel and perpendicularto the rubbing direction with a white scattering layer (white painted cardboard) placedwith an air gap at the bottom of the samples. Descriptions of the emissions from paralleland perpendicular edges of the planar LSCs with respect to the lines of dye structure aregiven in Figure 5.8. All four edges of the samples were measured to determine the totaledge emission and the anisotropy between the emissions perpendicular and parallel to therubbing direction. A correction for the small polarization anisotropy (∼10%) of the lightsource was done for edge emission measurements of all samples. Total edge emission was
102 Chapter 5. Aligned Dye Molecules in Patterned LSCs
RubbingDirection
E
E⊥
Figure 5.8: Schematic of planar aligned dyes in a thin �lm LSC where the long axis of thedye molecule is aligned parallel to the rubbing direction of the polyimide layer underneaththe dye coating. The emissions of this aligned dye patterned LSC were measured at edgesparallel (E‖) and perpendicular (E⊥) to the alignment direction of the liquid crystals anddyes as indicated by the large arrows at the edges of the waveguide.
determined by integrating the measured emission spectra from 350-750 nm for each edgeand subsequently summing the integrated values for all four edges.
5.4 Results
The K160 �uorescent dye molecules were aligned in a planar fashion along the rubbingdirection in a 10 line-patterned LSC using a guest-host system, where dye molecules arethe guest and liquid crystals are the host. Area coverage of the line pattern on the 50×50mm2 PMMA substrate was varied from 20 to 100% to study the e�ect of reducing areacoverage in an aligned dye LSC system. For the planar LSC system, the line pattern wasfabricated with the length of the lines parallel to the rubbing direction as depicted inFigure 5.9a, and dyes are also aligned parallel to the rubbing direction. Emission of thedyes is then distributed about the long axis of the dye molecules, and a fraction of thisemission is primarily directed to the two parallel edges of the waveguide. By controling theemission direction, the ability of the line pattern to reduce reabsorption can be furtherexploited as most of the emitted light will encounter the reduced dye coverage regionsinstead of traversing the length of the line where reabsorption probability is the same as auniformly covered sample. For reasons of clarity, the planar and homeotropic aligned dyesin the line patterned LSC will be referred to hereafter as planar LSC and homeotropicLSC, respectively.
Figure 5.9b shows the edge emission measurements of planar aligned K160 dyes in a linepatterned LSC system. The edge emission measured at the parallel edges was considerablyhigher than the perpendicular edges for all dye coverages studied. The e�ect of orderingdye molecules in an LSC is further examined by comparing the performance of planar
5.5. Discussion 103
(a) (b)
Figure 5.9: (a)A schematic of the anisotropic emission of K160 �uorescent dye alignedin a planar fashion and patterned in an array of lines on glass substrate, and (b) emissionof planar aligned dyes in line pattern LSC measured at edges parallel (N) and perpendicular(•) to the alignment of dyes as a function of dye area coverage.
aligned LSC to homeotropically aligned LSC systems. The results of the comparisonbetween planar and homeotropic aligned LSC systems are shown in Figure 5.10. Forboth systems, the edge emission increased with decreasing dye coverages. Comparing thehomeotropic to planar aligned dye LSCs, the edge emission of the homeotropic alignedsamples was higher at dye coverages less than 40% and greater than 70%. However, inthe midrange dye coverages (40-70%), emission from the parallel edge of planar alignedLSCs is comparable to the homeotropic aligned LSC edge emissions.
To evaluate the performance of aligned dye LSCs, both the planar and homeotropicaligned patterned LSC systems were compared to isotropic pattern LSCs as a function ofdye area coverage. The emission from the parallel edge and the total emission from allfour edges were measured for all samples and the results are provided in Figure 5.11. Thesingle edge relative e�ciencies of the aligned and isotropic LSC samples were calculatedby normalizing the single edge emission by the absorption of the sample. Similarly, totalrelative e�ciency of the samples was calculated by normalizing the total edge emissionfrom all four waveguide edges by the absorption of the sample. The result of the e�ciencycalculations is shown in Figure 5.12.
5.5 Discussion
The di�erence between parallel and perpendicular edge emission displayed in the planaraligned LSCs con�rms that the dye molecules indeed exhibit emission anisotropy and thefavored emission was in the direction parallel to the alignment direction of the liquidcrystals. The results imply that the guest dye molecules conform to the alignment of thehost liquid crystals, which is ordered along the line pattern. The large di�erence between
104 Chapter 5. Aligned Dye Molecules in Patterned LSCs
(a) (b)
Figure 5.10: (a) Total edge emission (sum of emission from all four waveguide edgesintegrated from 350-750 nm) and (b) relative e�ciency (equation 2.1) measured at theparallel (Eparallel, 4) and perpendicular(E⊥, ◦) edges of the planar (solid symbols) andhomeotropic (open symbols) aligned dye LSC systems exhibiting a peak absorbance of ∼0.12and 0.2, respectively.
(a) (b)
Figure 5.11: Edge emission (integrated from 350-750 nm) of isotropic (�), planar (Nfor E‖ and • for E⊥) and homeotropic (�) aligned LSC samples measured at (a) a singleedge and (b) sum of all four edges as a function of dye area coverage.
5.5. Discussion 105
(a) (b)
Figure 5.12: Relative e�ciency (equation 2.1) of isotropic (�), planar (N for E‖, and• for E⊥) and homeotropic (�) aligned LSC samples calculated from (a) a single edgeemission and (b) sum of all four edge emissions as a function of dye area coverage.
parallel and perpendicular edge output supports the hypothesis that the direction of dyeemission can be controlled by ordering them using liquid crystals in a guest-host system.
In contrast, the edge outputs at both the parallel and perpendicular edges of thehomeotropic aligned samples were nearly identical (Figure 5.10a). Homeotropic alignmentimplies that the long axis of the dye molecules is ordered perpendicular to the substratesurface, and light is emitted in a lobe-shape normal to long axis of the dyes. In otherwords, the dye emitted light is uniformly distributed to all edge of the waveguide in ahomeotropic aligned dye system, and thus no preferential edge emission was observed.
It was anticipated that the homeotropic LSCs have lower edge emission than planarLSCs as incident light is propagating parallel to the long axis of the dye molecules inhomeotropic alignment, which suggests low light absorption by the dyes and hence, lowemission. However, homeotropic aligned samples also have low surface losses as the dyeemission is in the plane of the waveguide surface (i.e., normal to the long axis of the dyemolecules), which implies that only a small fraction of emitted light is directed towards thetop and bottom surfaces of the waveguide [128]. Conversely, dye molecules aligned planarto the surface of the substrate have a large fraction of the emission directed at the topand bottom surfaces of the waveguide, and thus incurring higher surface losses. The lowersurface losses of the homeotropic aligned LSCs provide at least a partial explanation forthe higher edge emission observed in Figure 5.10a. In addition , absorbance of the samplealso plays an important role in both absorption and edge emission of the samples. Themeasured absorbance of the homeotropic and planar aligned samples are ∼0.2 and ∼0.12,respectively, due to the di�erent thicknesses of the dye coating in the samples. Higherabsorbance of the homeotropic samples implies greater absorption of photons comparedto planar aligned samples, which in turn leads to higher edge emission as noted in Figure5.10a.
106 Chapter 5. Aligned Dye Molecules in Patterned LSCs
Aside from surface or surface losses, re-absorption losses also have a considerablee�ect on the edge emission and e�ciency of the aligned dye systems. Re-absorption inhomeotropic systems is assumed to be greater than planar LSC systems as emissionsfrom homeotropic aligned dyes are primarily directed into the waveguide (i.e., emissionis normal to the long axis of the molecules), which implies that a fraction of the emittedphotons will be directed along the lines of dye structure. As the lines of dye structurestretches from one edge of the waveguide to the other, the probability of photon re-absorption greatly increases along the length of these lines. Contrastingly, the emissionof the dye molecules in the planar LSC is favored in the direction of the alternatingdye structure and clear regions of the waveguide. Hence, in the planar LSC system,the probability of emitted photons encountering other dye molecules is lowered and re-absorption losses are limited. This suggests that the planar aligned systems was able tofully exploit the advantages of the line pattern to reduce re-absorptions, and it o�ers anexplanation for the planar LSCs producing edge emissions comparable to the homeotropicaligned systems despite lower absorbance at dye coverages ranging from 40 - 70% (seeFigure 5.10a).
The relative e�ciencies of the samples as determined by the ratio of edge emission toabsorption provided some insight into the e�ect of patterning on reabsorption losses inthe planar and homeotropic LSCs. As in chapter 3, the general trend found for all thesamples studied was that the relative e�ciency increased with decreasing dye coverage asshown in Figure 5.10b. Clearly, patterning the dye coating reduces reabsorption losses inall the samples, resulting in higher relative e�ciency than a fully coated LSCs. Thoughthe homeotropic samples have higher absorbance and were assumed to have lower surfacelosses, the relative e�ciencies at both parallel and perpendicular edges were found to belower than the planar samples for dye coverages above 30%. This suggests that at highdye coverages, the energy loss due to reabsorption in the homeotropic system is greaterthan the energy gain from reduced surface losses, and thus resulting in lower e�cienciesin comparison to the planar LSCs. At dye coverages <30%, the amount of dyes coveringthe surface of the waveguide have decreased considerably leading to signi�cant reduc-tion in reabsorption losses for both homeotropic and planar LSC systems. Subsequently,the energy gained from reduced surface losses take e�ect and relative e�ciencies of thehomeotropic systems become higher than the planar samples. The planar systems exhibithigher relative e�ciency than the homeotropic systems for high dye coverages because theline pattern is more e�ective at reducing the reabsorption losses in the planar systems asa result of its dye molecule alignment. Therefore, the planar alignment is more suitablefor producing e�cient LSCs, particularly when taking into consideration that fewer solarcells are desired for lower production costs.
The performances of the two aligned dye LSCs were evaluated by comparing their edgeoutput and relative e�ciencies with isotropic LSC systems. It was found that aligning thedye molecules in a planar fashion resulted in an enhanced edge emission at the paralleledge and decreased emission at the perpendicular edge when compared to the edge outputof the isotropic LSCs. These results and trends are in agreement with the ones previouslyreported by Verbunt et al. [112]. Furthermore, similar trends were noted for all three(isotropic, planar, and homeotropic) LSC systems in the range of dye coverages studied(see Figure 5.11a). The parallel edge emission from the planar aligned systems exceeds
5.6. Conclusions 107
the isotropic emission by ∼20%. Comparing the total edge outputs, i.e., edge emissionfrom all 4 edges of the waveguide, the planar LSC system displays nearly identical valuesas the isotropic system (see �gure 5.11b). This implies that the dye alignment controlsthe emission direction of the dye molecules, but the total amount of energy emitted isequivalent to a standard isotropic LSC system. The highest single and total edge outputsfor all dye coverages were produced by homeotropic aligned LSCs due to its lower surfacelosses and higher absorbance values than both isotropic and planar systems.
Relative e�ciency of the isotropic dye systems were noted to be the lowest among thethree LSC systems. The homeotropic system has lower surface losses due to its alignment,resulting in slightly higher e�ciency than the isotropic LSCs. Relative e�ciency of planaraligned LSCs measured at the edge perpendicular to the rubbing direction is comparableto the homeotropic system, and the e�ciency at the parallel edge was found to be thehighest of all LSC systems. Similar e�ciency trends were noted in the calculation of thetotal e�ciency of the LSC system. By ordering the dye molecules in the planar systemsuch that its long axis is parallel to the length of the lines in the line patterned LSC, theability of the line patterns to limit reabsorption losses is fully exploited. From both edgeemission and relative e�ciency studies, the planar aligned dyes in line patterned LSCsexhibit the highest edge output and relative e�ciency at dye coverages >30%.
5.6 Conclusions
Three di�erent types of dye alignment, isotropic, planar, and homeotropic, were appliedto line pattern LSCs using liquid crystals in a guest-host system. For all three types ofalignment, it was observed that relative e�ciency increased, and edge emission decreasedwith decreasing area coverage of dyes. The planar aligned dye LSCs were observed toexhibit higher emission at edges parallel to the alignment direction than the perpendicularedges. Maximum parallel edge emission for the planar system exceeded the perpendicularedge emission by ∼30%. By aligning the dye molecules parallel to the line pattern, theparallel edge emission exceeds the isotropic system by ∼20% and was comparable to thehomeotropic LSC system despite having lower peak absorbance. Both the isotropic andhomeotropic system exhibited lower relative e�ciencies (edge emission to absorption ratio)than the planar system, and the homeotropic systems have higher relative e�ciencies thanthe isotropic system. However, at dye coverages <30%, the homeotropic system exhibithigher e�ciency than the planar system, rendering it the most e�icent system at lowdye coverages. At higher dye coverages, the planar system is the most e�cient with thehighest edge emission at the two parallel edges of the waveguide. The planar system alsoreinforces the preferred edge emission observed in the integrated system in Chapter 5,where emission at two opposite edges of the patterned LSC was favored. This preferredemission o�ers the possibility of attaching only two solar cells to the two highest emittingedges, thereby reducing the production cost of the LSC solar module. A combination ofthe planar aligned dye system with the lens array in Chapter 4 may have an synergistice�ect that further increases emissions at the preferred edges.
108 Chapter 5. Aligned Dye Molecules in Patterned LSCs
Chapter 6
Wavelength-Selective Re�ectors for
Patterned LSCs
6.1 Introduction
It has been established that a large fraction of the photons emitted by �uorescent dyemolecules in an luminescent solar concentrator escape through the top and bottom surfacesof the substrate and this loss is typically referred to as surface losses or escape-cone losses[77, 81]. Previous reports demonstrated 40%-55% surface energy losses in dye-�lled LSCs,corresponding to a loss of 50%-70% of the total absorbed photons with more photons lostfrom the top than the bottom surface [81]. To reduce the bottom surface loss, a whitescattering layer or a mirror can be placed underneath the LSC waveguide to re�ect orscatter emitted light back into the waveguide. In chapter 4, it was shown that a scatteringlayer back-scatters escaped light into the waveguide, and thereby increases edge emissionof the LSC systems by more than 30%. The top surface losses also can be reduced usingwavelength selective mirrors or distributed Bragg re�ectors. The operating principle of aBragg re�ector is based on the phenomenon that light waves re�ected at multiples of 2πfrom surfaces within a multilayer stack will interfere with each other constructively, andthe condition for constructive interference is given by Bragg's law (equation 6.1):
mλ = 2ndcos(θ) (6.1)
where λ is wavelength of light in vacuum, n is the average refractive index of the multilayerstack, m is an integer that de�nes the order of interference, θ is the incident angle of thelight, and d is the spacing at which the layers within a multilayer stack is repeated (i.e.,the periodicity of high and low refractive index layers in a Bragg re�ector).
Consider the simplest case at which the incidence angle is 0◦ to the surface normal, andfor �rst order interference (m=1), the period d must be equal to λ/2n. Bragg re�ectorsusually consist of periodic alternating layers of high and low refractive index material. Oneof the most straightforward ways to obey equation 6.1 is to ensure the optical thicknessof each layer in the Bragg re�ector is one quarter of the wavelength (λ/4n) for whichthe re�ector is designed. Thus, the most commonly used design for the Bragg re�ectoris a stack of quarter-wave layers. At each of the interfaces between the high and low
109
110 Chapter 6. Wavelength-Selective Re�ectors for Patterned LSCs
refractive index layers, Fresnel re�ection occurs. These re�ections interfere with eachother constructively to produce high re�ectance. The re�ectivity of a Bragg re�ector,assuming the originating and exiting medium of the re�ector is air (refractive index=1),is given by the following equation:
R =n2p2 − n
2p1
n2p2 + n2p
1
(6.2)
where n1 and n2 are respectively the refractive indices of the low and high refractive indexmaterial of the Bragg re�ector, p is the number of repeated pairs of high/low refractiveindex layers, and R is the intensity of re�ectivity. From equation 6.2, it is evident thatthe greater number of the repeated high/low refractive index pairs (p), the higher there�ectivity. Re�ectivity also increases if the refractive index di�erence between the highand low layers increases. The refractive index di�erence between the high/low layers isvery important in designing Bragg re�ectors as it does not only determine the re�ectivity,but also the bandwidth (BW) of the re�ector, which is de�ned as:
BW =4λoπsin−1(
n2 − n1
n2 + n1
) (6.3)
where λo is the center wavelength of the re�ector's stopband, and n1 and n2 are again thelow and high refractive indices of the alternating layers in a Bragg re�ector.
According to equation 6.1, the re�ected wavelength of a Bragg re�ector is dependenton the angle of incidence and the period of alternating layers in the re�ector. Hence thecentral wavelength of the re�ector's stopband can be tuned by varying the period of there�ector layers. This implies that the Bragg re�ector can be used to decrease surface lossesin the LSC systems by matching the stopband of the re�ector to the emission band ofthe dyes. However, the re�ection band has an angular-dependence where as the incidentangles increase, so does the shift of the re�ection band to shorter wavelengths.
For the LSC system, a good Bragg re�ector must completely re�ect the emission bandof the dye at all emission angles smaller than the critical angle of the waveguide. Inaddition, to ensure no light of interest is blocked from entering the LSC system, one mustverify that at incident angles smaller than the critical angle, the re�ection band of there�ector does not shift into absorption wavelengths of the dye.
Previous studies have described Bragg re�ectors produced using multilayers of inor-ganic material that selectively re�ect the emission wavelengths of �uorescent dyes embed-ded in a plastic waveguide while allowing light within the absorption band of the dyesto enter the system [91, 140, 141]. Light retained inside the concentrator system canbe directed to the edges of the waveguide via total internal re�ections. Good inorganicBragg re�ectors often require deposition of many tens of layers using vacuum technology,which is a di�cult and expensive process to up-scale for mass production of large-areaLSC systems. A more practical and inexpensive alternative is to use chiral nematic liquidcrystals to form a wavelength selective mirror.
Nematic liquid crystals, as described in chapter 5, have long range orientational order,and the anisotropic LC molecules tend to align with micro-grooves rubbed in an alignmentlayer such as polyimide. Hence the alignment of nematic liquid crystals is controlled bythe alignment layer. A cholesteric, or chiral nematic LC, is a special class of nematic liquid
6.1. Introduction 111
crystals where a chiral dopant is added to induce helical rotation in the planes of orderedmolecules to be twisted relative to the planes above and below, and these resultant helicescan be either right handed (RH) or left handed (LH).
It has been demonstrated that cholesterics re�ect a narrow bandwidth of circularpolarized light of the same handedness as the cholesteric helix and transmits the oppositehandedness [131]. For example, a right-handed cholesteric helix will re�ect right-handedcircular polarized light. Non-polarized light is composed of both right and left-handcircular polarized light. Hence, to produce a Bragg re�ector that completely re�ects non-polarized light emitted by �uorescent dyes, it is necessary to have both right- and left-handed cholesterics. However, it was di�cult to obtain chiral dopants that produced left-handed cholesterics as they are not widely available, so an alternative method was soughtto replace the LH cholesteric while still completely re�ecting non-polarized light. Since thephase di�erence between the right and left-handed circular polarized light is π, a half-wave(λ/2) plate can be used to change the phase of left-handed circular polarized (LCP)lightinto a right-hand circular polarized (RCP)light and vice versa. When unpolarized lightis incident on a right-handed cholesteric, RCP is re�ected and LCP light is transmitted.This LCP light subsequently interacts with a half-wave plate, which changes the phaseof LCP by π, resulting in RCP light. This latter right-handed polarized light is thenre�ected by the RH cholesteric on the other side of the half-wave plate. Thus by placinga half-wave plate in between two right-handed cholesterics, both LCP and RCP polarizedlight can be completely re�ected. This principle is illustrated in Figure 6.1.
It is possible for a cholesteric to mimic the high and low refractive index layers ofan inorganic multi-stack Bragg re�ector because liquid crystals are birefringent, i.e. theordinary refractive no index is di�erent than its extraordinary index ne, and ne is typicallygreater than no. A light wave linearly polarized perpendicular to the optical axis of the LCmolecule encounters the ordinary refractive index, whereas a light wave polarized parallelto the optical axis experiences the extraordinary refractive index.
By rotating the anisotropic LC molecule to form a helix, incident light experiences thedi�erence of refractive indices no and ne as it passes through the helix, which acts to changethe phase and the polarization of the light wave. Hence, in the case of cholesterics, no andne replaces n1 and n2 in equation 6.2, and the di�erence between no and ne determinesthe re�ectivity and has an a�ect on the bandwidth of the stopband of a cholesteric Braggre�ector. The bandwidth of the cholesteric Bragg re�ector is de�ned by equation 6.4[142]:
∆λ = ∆nP = (ne − no)P (6.4)
where ∆λ is the bandwidth and P is the pitch (i.e., 2π rotation) of the helix, which isequal to half of the period d. The di�erence between no and ne is relatively small (∼0.2)for most common liquid crystals, which implies that the bandwidth of the stopband islimited and the pitch plays an important role in the resulting bandwidth. The pitchof the helix P is also important in tailoring the center re�ection wavelength (λ0) of thecholesteric re�ector as given by the following equation:
λ0 = nP (6.5)
112 Chapter 6. Wavelength-Selective Re�ectors for Patterned LSCs
Unpolarized Incident light
Right-hand Cholesteric
Half-wave Plate
(1) LCPtransmit
Right-hand Cholesteric
(2) RCPtransmit (2) RCPreflected
(3) LCPreflected(1) RCPreflected
Unpolarized Incident light
LCPreflectedRCPreflected
Right-hand Cholesteric
Half-wave Plate
Right-hand Cholesteric
(3) LCPreflected
Figure 6.1: The structure (left) and working principle (right) of a cholesteric Braggre�ector consisting of two right-handed cholesterics separated by an half-wave plate isillustrated. The e�ect of the right-hand cholesterics and half-wave plate on incident un-polarized light within the re�ector's stopband is given in 3 steps in the right schematic:(1) the right-hand cholesteric re�ects right circularly polarized light (RCPreflected, bluehelix) and transmits left circularly polarized light (LCPtransmit,orange helix); (2) WhenLCPtransmit encounters the half-wave plate, its phases changes by π and becomes righthand circular polarized light (RCPtransmit, which is subsequently re�ected by the secondlayer of right-handed cholesteric; (3) this RCPreflected encounters the half-wave plate, andbecomes LCP light (LCPreflected), which is then transmitted out of the complete re�ectorthrough the �rst layer of right-hand cholesteric. By this method, both RCP and LCP arere�ected from the cholesteric re�ector.
6.2. Experimental Setup 113
where n is in-plane average refractive index of the cholesteric. The pitch of the helixis generally determined by both the concentration and the helical twisting power of thechiral dopant in a dilute solution and it is described by the equation 6.6:
P ∼=1
c×HTP(6.6)
where c is the concentration of the chiral dopant and HTP is the helical twisting powerof the chiral dopant, which is a function of both the molecular structure of the chiraldopant and the host LC. The helical twisting power is inversely proportional to the pitchof the helix, which implies the higher the HTP of a chiral dopant, the smaller the pitchof the resulting helix. In other words, the HTP determines how �tight� a helix twistsabout an axis [143, 144, 145]. The ease at which the center re�ection wavelength andthe bandwidth of a cholesteric re�ector can be tuned by changing the pitch of the helixmakes them well-suited for wavelength selective mirror applications. Furthermore, theinexpensive and straightforward solution deposition of the cholesterics is ideal for roll-to-roll large-scale production, giving them an distinct advantage over inorganic Braggre�ectors that require expensive vacuum technology.
In this chapter, narrowband cholesteric wavelength selective re�ectors are applied toline patterned thin �lm LSCs to re�ect light escaping through the top surface of thewaveguide back into the system. The main purpose of the cholesteric layer is to reducesurface losses of the LSC, and allow the contained light to be either reabsorbed by dyemolecules or directed to the edge of the waveguide, and ultimately increasing the usefuloutput of light as described in Figure 6.2. To increase the bandwidth and the performanceof the cholesteric re�ectors, broadband cholesteric re�ectors were created by stacking twonarrowband re�ectors with di�erent pitches, and thus di�erent centering wavelengths andbandwidths, on top of each other. By ensuring the stopbands of the stacked re�ectorsoverlap, the two stopbands merge to form one broad re�ecting band. The stopbandcentering wavelength that best matches the emission band of the �uorescent dye (LumogenF Red 305) commonly used in LSC systems was investigated using sets of narrow- andbroad-band re�ectors with varying centering wavelengths. The e�ect of these narrow-and broad-band cholesteric re�ectors on thin �lm line patterned LSCs were studied asa function of edge emission and varying dye coverage. Finally, both narrow and broad-band re�ectors were combined with the integrated system of PC lens array and patternedwaveguide to evaluate the e�ect and possible bene�ts of the di�erent re�ectors on theintegrated LSC system. All work presented in this chapter was done in collaboration withPaul P. C. Verbunt.
The cholesteric layers were made by spin-casting solutions of the reactive LC mesogensRM257 and RM82 in a 4:1 weight ratio (Merck, Southampton, United Kingdom), varyingconcentrations of the reactive chiral dopant LC756 (BASF), 1% of the photoinitiatorIr-gacure 184 (Ciba, Basel, Switzerland), and 1% of a surfactant in xylene (50% by weight
114 Chapter 6. Wavelength-Selective Re�ectors for Patterned LSCs
Scattering layer
Waveguide
Cholesteric reflector
PV Cell
Line patterns of dye
Figure 6.2: Working principle of the wavelength-selective cholesteric re�ector in a linepatterned LSC system. Light emitted by the dye molecules directed at a angle smaller thanthe critical angle of the waveguide and directed at the top surface is re�ected back intothe waveguide by the cholesteric layer at the top. This re�ected light can be re-absorbedby another dye molecule or exits the bottom surface of the waveguide, and encounters thescattering layer. The scatterer changes the direction of the light and scatters it againinto the waveguide. A fraction of this scattered light is directed towards the edge where aphotovoltaic cell can attached.
of solution). The molecular structures of the various liquid crystals and chiral dopantsare given in Figure 6.3. The solutions were spin-coated on hand-rubbed half-wave plates(Edmund Optics, York, United Kingdom) at 1000 rpm for 40 s and placed immediatelyon a hotplate at ∼80◦C for ∼15 s. The samples were then crosslinked by exposure to UVlight at room temperature in a nitrogen atmosphere for 10 min. For the cholesterics tobe capable of re�ecting both left and right circularly polarized light, a second cholestericfrom the same solution was spun on the rubbed backside of the same half-wave plate.These narrowband cholesterics were fabricated by Paul P. C. Verbunt.
To create broadband re�ectors, two right-handed narrowband re�ectors were stacked onboth sides of a half wave retarder (Edmund Optics, York, United Kingdom) with centeringwavelength of 560 nm, which was rubbed manually. On both sides of the half-wave re-tarder, solutions were spin-casted from a mixture of reactive LC mesogen LC242 (BASF,Germany), varying concentrations of chiral dopant LC756 (BASF, Germany), 1% of pho-toinitiator Irgacure 184 (Ciba, Switzerlands) and 1% of surfactant in a 1:1 ratio of xylene(1:1 in mass) at 800 rpm for 30 seconds. After spincasting, the samples were immediatelyheated on a hotplate at 90◦C for 30 s. Subsequently, the samples were photo-polymerisedin a nitrogen atmosphere. Before applying the second re�ector layer with a lower con-centration of chiral dopant, the �rst layers were treated with an K1050X oxygen-plasmaasher (Quorum Technologies) for 1 minute at 60 W to improve the wetting of the LClayer. The second relfecting layer was then fabricated using similar methods as the �rstand the procedure was repeated for the otherside of the half-wave retarder. Aagain, thebroadband cholesteric samples used in this thesis were fabricated by Paul P. C. Verbunt.
6.2. Experimental Setup 115
(a)
(b)
(c)
Figure 6.3: Molecular structure of (a) RM257, (b) RM82 and (c) chiral dopant LC 756.
116 Chapter 6. Wavelength-Selective Re�ectors for Patterned LSCs
6.2.3 Line Pattern and Thin Film Coated LSC Fabrication
Fluorescent dye solutions were prepared using 0.5% wt of Lumogen Red 305 (BASF)�uorescent dye molecules , and 1% photoinitiator (Irgacure184, Ciba) dissolved in a 3:1dipentaerythritol penta-acrylate (Polysciences) and methylmethacrylate (MMA, Aldrich)mixture. 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, 100% coveredsamples were crosslinked by exposing to a high-intensity UV lamp (OmniCure S2000 UVspot curing lamp) for 80 s under nitrogen �ow to form a solid �lm. For the fabrication ofpatterned LSCs, standard photolithography techniques were employed. Uniformly coatedsubstrates were exposed to UV light through patterned shadow masks consisting of 10lines with variable widths with a periodicity of 5 mm. Line widths were varied to cover 20to 80% of the waveguide surface. After UV exposure, the dye-coated PMMA substrateswere continuous agitated in ethanol for ∼40 s to etch away the unexposed and non-crosslinked material. The thicknesses of the �uorescent dye coatings and microstructureswere measured to be 14-20 µm using a Fogale Zoomsurf 3D optical pro�ler.
6.2.4 Integrated System Fabrication
To produce the cylindrical aspherical-shaped lens array, the copper lens mold was �rstpreheated in the press to 90◦C for 15 minutes to limit temperature gradients in the mold.Subsequently, 50×50×2 mm3 transparent polycarbonate plates (Sabic IP) were placed inthe mold, and heated for 15 minutes at 90◦C between the two pressing plates with noapplied pressure. This step ensures uniform heating of the PC plate. The temperaturewas chosen to be lower than the glass transition temperature (Tg) of PC to prevent thesubstrate material from �owing and thereby decreasing the chance of bubble formationduring the pressing process. Next, the PC substrate is pressed into the mold with apressure of 100 mBar for 5 minutes in a nitrogen environment. Lastly, the mold and thePC lens array were cooled down to room temperature prior to releasing the lens arrayfrom the mold.
Fluorescent dye solution was prepared using similar methods and concentrations asdescribed above in section 6.2.3. The dye solutions were stirred and heated at 60◦C foran hour prior to spin-coating onto 50×50×3 mm3 PMMA substrates at 1000 rpm for 30. After spin-coating, the PC lens array was placed on top of the coated sample separatedby an air gap. Subsequently, samples were cross-linked by focusing the high-intensity UVlight (OmniCure S2000 UV spot curing lamp) through the PC lens array for 60 s undernitrogen �ow at 55◦C to form a solid patterned �lm. Unexposed material on the patternedPMMA substrates were etched away by continuously agitating the samples in ethanol for∼40 s. 100% uniformly covered LSC samples were fabricated using the same solutions,but the samples were cross-linked using direct exposure to UV light for 70s under nitrogen�ow at 55◦C. The thicknesses of the thin �lm coating of the patterned and fully coveredsamples were found to be ∼10µm using a Fogale Zoomsurf 3D optical pro�ler.
6.2. Experimental Setup 117
6.2.5 Sample Characterization
Transmission spectra for all samples were measured using a Shimadzu UV-3102 PC spec-trophotometer. Absorbance as a function of wavelength was calculated using Lambert-Beer law:
A = −log(T ) (6.7)
where T is the fractional transmission of light through the sample, and A is the ab-sorbance. The peak absorbance of the fully covered and line patterned LSCs measuredwas ∼0.57 for all samples. Quantitative edge emission of the waveguides was measuredusing SLMS 1050 integrating sphere (Labsphere) equipped with a diode array detector(RPS900, International Light) as illustrated in Figure 6.4. The patterned and fully cov-ered LSC samples were placed in a custom-made black sample holder, which is connectedto the entry port of the integrating sphere and prevents surrounding light from enteringthe port. A white scattering layer was placed at the bottom of the LSC systems withan air gap to scatter light exiting the bottom surface back into the system. A half-waveretarder, narrow- and broad-band cholesterics were placed atop the patterned and fullycovered LSC samples, also separated by an air gap. The air gap was placed in betweenthe wavelength selective cholesteric re�ectors and the LSC system to ensure waveguidedlight was not coupled out of the LSC system through the cholesteric layer. For the edgeemission measurements of the complete integrated system, a cylindrical aspherical PClens array (described in chapter 5) was placed above the cholesteric layers. Therefore, thetopological structure of the complete system used in the edge emission measurements fromtop to bottom was as follows: PC lens array, cholesteric layers, ∼40% Red305 dye-coveredline patterned waveguide, and white scattering layer. An air gap was present between allthe elements in this system.
All LSC systems were exposed to a collimated light source from a 300 W solar simulatorwith �lters to approximate the 1.5 AM global solar spectrum (Lot-Oriel) located at adistance of ∼15 cm from the top surface of the waveguide. Edge output spectra ofpatterned and fully covered LSCs with and without a half-wave plate were �rst recordedand the results were considered as the control values of the experiment. Subsequently,narrow and broadband cholesteric re�ectors were placed on top of the patterned and 100%dye-covered samples and all four waveguide edge emissions were measured. Total edgeemissions were determined by integrating the recorded spectra over the range of 350-750nm and summing the emission from all 4 edges. The integrated edge emssion values of thewaveguides with cholesterics atop were divided by the edge emission values of the samewaveguides with a blank half-wave plate atop. This allows us to eliminate the e�ect ofthe half-wave plate on the patterned waveguides, and focus our attention on the e�ect ofthe cholesterics on the performance of the LSCs.
118 Chapter 6. Wavelength-Selective Re�ectors for Patterned LSCs
Edge Emission
Detector
Baffle
Sample
Integrating SphereIncident light
Cholesteric reflector
scatterer
Figure 6.4: The experimental setup for measuring emission intensities at the edges ofwaveguides with cholesteric re�ector on top and a scattering layer at the bottom, whereboth are prevented from being in optical contact with the waveguide by an air gap.
6.3 Results and Discussion
6.3.1 Narrowband Wavelength-Selective Re�ectors
Wavelength selective re�ectors were made using two right-handed cholesterics on eitherside of a half-wave plate to re�ect unpolarized light at normal incidence. The half-waveretarder converts left-hand circular polarized light into right-hand and vice versa, and thusallows both handedness of circularly polarized to be re�ected when placed in between tworight-handed cholesterics. The re�ection spectra for the four narrowband cholestericsexposed to unpolarized light at normal incidence are given in Figure 6.5. All four of thenarrowband cholesterics exhibit a bandwidth of ∼90-95 nm.
The set of narrowband cholesterics were then placed on top of the line patternedwaveguides and their performances were recorded to determine the e�ect of the cholester-ics on line patterned waveguides. Edge emission intensity of the patterned waveguidesdepends on the dye coverage of the patterns, i.e., the width and spacing of the lines inthe pattern. Here, the width of the lines of dye structure was varied from 1 - 5 mm,corresponding to an area coverage of 20 - 100%, and the edge outputs were measured asa function of the varying dye coverage as well as the varying center wavelength of thecholesterics. The results presented in this chapter are primarily focused on the relativeincrease or decrease in edge emission of the waveguides as a result of the application ofcholesteric re�ectors compared to the systems without a cholesteric re�ector. Hence,theedge emission of the line patterned waveguides with the cholesteric re�ector atop wasdivided by the edge emission of the same waveguides measured with a half-wave retarderon top. In other words, the increase in edge emission attributed to the cholesteric re�ec-tor is 0% if a patterned waveguide combined with a cholesteric re�ector produce similaredge emission as the same patterned waveguide with a half-wave plate. By using the edgeemission of the systems with a half-wave plate on top as the reference measurment, weeliminate the e�ect of the half-wave plate in the cholesteric measurements, and it allows
6.3. Results and Discussion 119
Figure 6.5: Transmission spectra of narrowband two-sided cholesteric re�ectors withstopband onset wavelengths of 630, 680, 720, and 760 nm. All spectra were measuredusing unpolarized light at normal incidence to the surface of the cholesterics.
Table 6.1: Center and corresponding onset wavelength of the two-sided cholesteric re-�ectors
Center Wavelength (λ0) Onset Wavelength[nm] [nm]670 630730 680770 720810 760
us to focus better on the e�ect of the cholesteric layers.Figure 6.6 reports the relative increase in edge emission of patterned waveguides with
the application of cholesteric re�ectors as a function of varying onset wavelengths of thecholesterics. Onset wavelength of the re�ector is de�ned as the shorter wavelength atwhich the transmittance (% transmission) of the cholesteric drops to approximately halfof its peak transmission value. This was found to be more convenient for determiningthe e�ect of the cholesterics than the center wavelength as it provides information on thestarting point of the stopband, and thus shall be used instead of the center wavelengthhereafter. The corresponding values of the onset wavelength to the center wavelength ofthe cholesteric re�ectors is provided in Table 6.1.
To understand the e�ect of the cholesteric layers on the performance of patternedwaveguides, let us �rst examine the edge emission of the patterned waveguides with onecholesteric re�ector as a function of varying dye coverage. By applying the cholesteric re-�ector with an onset wavelength of 630 nm to the patterned waveguides, the edge emissionwas noted to increase with decreasing dye coverage. The same trend was observed when
120 Chapter 6. Wavelength-Selective Re�ectors for Patterned LSCs
Figure 6.6: The relative increase in edge emission of patterned waveguides with nar-rowband cholesteric re�ectors (with onset wavelengths of 630, 680, 720, 760 nm) on topcompared to the same patterned waveguides with a half-wave retarder on top are presented.The edge emissions were measured as a function of dye coverages of the patterned waveg-uides: 20% (�), 30% (•), 50% (4), 70% (H), and 100% (♦).
6.3. Results and Discussion 121
cholesterics with di�erent onset wavelengths were applied to the same set of patternedwaveguides. This suggests that patterned samples bene�t more from cholesteric re�ectorsthan fully covered waveguides regardless of its onset wavelength. The primary di�erencebetween the fully covered and line patterned waveguides is the e�ectiveness of the linepatterns in reducing re-absorption as demonstrated in chapter 3, which implies that thecholesteric re�ector is more bene�cial for LSC systems with less reabsorption.
Furthermore, the 100% covered sample in combination with cholesterics of variousonset wavelengths produced edge emissions that were, at best, equivalent to the edgeemission of the sample without the cholesteric layers. Similar results were also observedfor the 70% dye covered line patterned waveguide. Except the cholesteric layer with anonset wavelength of 760 nm, all other re�ectors appear to have an negative a�ect on theedge emission of the fully covered and 70% dye covered sample. As discussed, the addedcholesteric returns light to the LSC systems where it can be absorbed and emitted againby the dye molecules. However, this dye emitted light can be subsequently re-absorbedby other dye molecules, thereby increasign the probability of re-absorption. At higherdye coverages, this increase in probability of re-absorption may o�set the light gainedfrom possible reduction of surface losses as a�orded by the combination of the scattererand the re�ector. As a result, no relative increase in edge emissions were observed at100% and 70% dye coverages of the combined patterned and cholesteric system respectto the same patterned systems without the cholesteric layer. The highest increase in edgeemission was achieved after the application of each of the four narrowband cholestericson the 20% dye-covered LSC sample, which is consistent with the above explanation asthe 20% dye-covered sample exhibits the lowest probability of reabsorption.
Within the set of four narrowband cholesterics investigated, the re�ector with an onsetwavelength of 760 nm (center wavelength=810 nm) o�ered the highest relative emissionincrease compared to the system without the cholesteric re�ectors according to Figure 6.6.This is surprising as one would expect the optimal cholesteric to be the one exhibitinga center wavelength that best matches the peak emission of the Red305 dye (∼610 nm).However, the cholesteric re�ectors were separated from the waveguide by an air gap, whichserved the purpose of preventing the out-coupling of waveguided light. As air has a muchlower refractive index (1.0) compared to that of the PMMA waveguide (∼ 1.49), lightexiting the surface of the waveguide (at angles lower than the critical angle of 42◦ forPMMA) was bent away from the normal, and thus is encountering the cholesteric layer athigher angles relative to the surface normal of the cholesteric [79]. The center wavelengthof cholesteric re�ectors varies with incident angle and its angle dependence is generallydescribed by equation 6.8 [146]:
λθ = λ0cos[sin−1(
sinθ
n)] (6.8)
where λθ is the angle dependent center wavelength, λ0 is the center wavelength of there�ector at normal incidence of light, θ is the incident angle of light, and n is the averagerefractive index of the cholesteric. According to equation 6.8, the center wavelength de-creases or blue shifts with increasing incident angle θ. The incident angles of emitted lightoriginating from the waveguide are likely to be greater than 0◦, and its angle increasesas light rays travel from a higher refractive index waveguide to the lower refractive index
122 Chapter 6. Wavelength-Selective Re�ectors for Patterned LSCs
air/cholesteric interface. Hence, from the thin �lm LSCs' perspective, the center wave-length and the onset wavelength of all the cholesteric re�ectors appear to be blue-shiftedcompared to the values reported in Figure 6.5.
The fact that the cholesteric with the longest center wavelength provided the highestedge emission gain for the line patterned LSC system (see Figure 6.6) suggests that alarge fraction of the dye emitted light is encountering the surface of the cholesteric atoblique angles. For oblique incident angles, the stopband of the cholesteric re�ector withlower onset wavelengths may have blue-shifted to such a degree as to partially overlapthe absorption band of the Red305 dye, resulting in the blockage of a fraction of theuseful light from entering the waveguide. This would account for the lower increase,and for samples with higher dye coverage, even decrease of the recorded edge emissionwhen the cholesteric with an onset wavelength of 630 nm was applied. In addition, itwas previously reported that at larger incident angles, the transmission of left circularpolarized light through a half-wave plate was decreasingly right-circular, i.e. the phasechange of the incident light wave as induced by the half-wave plate is less than π, leadingto a more elliptical transmitted wave [99]. This demonstrates an incomplete transitionfrom left to right-handed circularly polarized light, which further hinders the performanceof the cholesterics at high incident angles.
Also, a slight drop of ∼2% in edge emission was observed at the cholesteric onsetwavelength of 720 nm for patterned waveguides with lower dye coverage (20 - 50%).One would expect the percentage increase in edge emission to continue increasing as theonset wavelength of the cholesteric red-shifts given the trend of the curves. However, thisdrop was unnoticable in the relative edge output curve of 100% dye-covered waveguide(see Figure 6.6), but as the dye converage decreased, the drop becomes more and moreevident. As the same patterned waveguides were used with each cholesteric re�ector, andno experimental parameters were change during the measurements, it is rather puzzlingwhat this drop in the edge output curve at 720 nm indicates. One could argue thatthis unexpected decrease may be caused by the lower re�ectivity (∼5%) of the 720 nmcholesteric (see Figure 6.5). The re�ectivity of the cholesterics tend to decrease withincreasing incident angle [99]. Given that the 720 nm cholesteric initially exhibited lowerre�ectivity than other cholesterics within the set, its re�ectivity may decrease su�cientlyat oblique angles to produce the noted drop in edge outpt. The veri�cation of thishypothesis remains to be investigated.
Taking into account that the cholesteric stopband blue-shifts with increasing incidentangle, it is reasonable that the highest increase in edge output was produced by thecholesteric exhibiting the longest center (810 nm) and onset wavelength (760 nm) withinthe studied set. By applying a cholesteric re�ector with the center wavelength of 810 nmto a 20% dye-covered line patterend LSC, a high of 12% increase in total edge output(sum of all 4 edges) can be achieved. However, the increase in edge output here is limitedas the bandwidth (∼95 nm) of the narrowbands is less than half of the bandwidth (∼200nm) of the Red305 dye's emission band. Thus, the cholesterics only re�ect a fractionof the emitted light back into the system and the other fraction is still lost through thetop surface of the waveguide. To improve the performance of the cholesterics and furtherdecrease the top surface losses, the bandwidth of the re�ectors must be increased to nearlycover the dye's emission band.
6.3. Results and Discussion 123
Table 6.2: Center and corresponding onset wavelength of the two-sided cholesteric re-�ectors
A set of broadband cholesteric Bragg re�ectors were fabricated by stacking two right-handed cholesterics of di�erent pitches on either side of a half-wave retarder. As the pitchdetermines the center wavelength of the re�ector, the bandwidth of the cholesteric can beincreased by calculating the pitch of the cholesterics in order to form overlapping narrowstopbands. The resulting center and onset wavelengths as well as the bandwidth of thebroadband cholesterics used in this study are listed in Table 6.2. Figure 6.7 displaysthe transmission spectra of the cholesterics measured at normal incidence. Note thatfor all the broadband cholesterics, the overlap of the narrow stopbands was not prefectand small decreases in re�ectivity were observed within the stopband of the broadbandre�ectors. This may increase the transmission of emitted photons through the stopbandof the re�ectors, in particular for the 735 nm cholesteric as it has the largest (amplitudeand bandwidth) drop in re�ectivity within the set of cholesterics studied. In addition, the656 nm cholesteric exhibited a narrower stopband bandwidth than the other cholester-ics within the set by ∼20 nm, which reduces the amount of emitted light it can re�ectback into the system, and may result in lower edge outputs from the combined 656 nmcholesteric and patterned systems. The spectrophotometer used to measure the trans-mission spectra changes light source at ∼850 nm, which may account for the additionalspikes noted in the transmission spectrum of the 820 nm cholesteric.
To compare the performance of the broadband to narrowband cholesterics, the broad-band re�ectors were placed atop the same line patterned waveguides as described above,and similar edge output measurements were carried out to evaluate the performance ofthe patterned LSC with and without the broadband cholesteric. Each of the broadbandcholesterics were combined with the set of patterned LSCs, and the increase in each edgeoutput was calculated using identical patterns with a blank half-wave retarder atop asreference (see Figure 6.8). All broadband re�ectors produced a positive increase in edgeoutput for all the patterned waveguides, including 100% coverage, contrary to the nar-rowband cholesterics. This demonstrates that increasing the bandwidth of the re�ectorsproduces higher edge emissions, which can be attributed to the stopband of the broad-band re�ectors nearly covering the emission band of the dye, and hence re�ecting moreemitted light back into the patterned waveguide.
The relative edge emissions of the broadband re�ectors on patterned systems as a
124 Chapter 6. Wavelength-Selective Re�ectors for Patterned LSCs
Figure 6.7: Transmission spectra of broadband two-sided cholesteric re�ectors with stop-band onset wavelengths of 656, 696, 735, 783, and 820 nm. All spectra were measuredusing unpolarized light incident normal to the surface of the cholesterics.
Figure 6.8: The increase in edge emission of patterned waveguides with broadbandcholesteric re�ectors (exhibiting onset wavelengths of 656, 696, 735, 783, and 820 nmand bandwidth of ∼160-200 nm) on top relative to the same patterned waveguides with ahalf-wave retarder on top are presented. The edge emissions were measured as a functionof dye coverages of the patterned waveguides: 20% (�), 30% (•), 50% (4), 70% (H), and100% (♦).
6.3. Results and Discussion 125
function of dye coverage were observed to have similar trends as the narrawband systems,i.e., the relative edge emission increased with decreasing dye coverage. The largest increasein edge output was noted when broadband cholesterics were applied to the patternedwaveguide with 20% coverage for all onset wavelengths studied. This suggests that evenwith increased bandwidth of the broadband re�ectors, reabsorption continues to hinderthe performance of the higher dye coverage samples. As the dye coverage decreases inthe patterned waveguides, the probability of reabsorption also decreases, which allowsthe LSC systems to bene�t more from the increased light retained in the waveguide.This again supports the conclusion that cholesteric re�ectors are optimal for LSC systemsexhibiting little to no reabsorption, i.e. waveguides embedded or coated with �uorescentmaterials with a large Stokes-shift such as phosphors, rare earth complexes, or quantumdots.
Examining the increase in edge output data as a function of onset wavelength of thecholesterics showed that the peak increase occurred at an onset wavelength of 783 nm forall patterned waveguides. As the onset wavelength increased beyond 783 nm, the increasein edge output began to decrease. This optimal onset wavelength again di�ers greatlyfrom the peak emission wavelength (∼610 nm) of the Red305 dye. As explained above,a large fraction of the dye emitted light is likely to be incident at an angle to the surfacenormal of the cholesteric layer. From the perspective of the angular light encountering thesurface of the cholesteric, the re�ection band of the cholesteric blue-shifts to shorter wave-lengths. The blue-shifting of the stopband results in a longer optimal onset wavelengthfor the cholesterics than anticipated. From the series of broadband cholesteric re�ectorsstudied, the greatest increase in edge output was obtained by applying a cholesteric layerwith an onset wavelength of 783 nm to the 20% dye-coated line patterned waveguide. Theedge output of the 20% patterned sample increased by ∼22% as a result of the broad-band cholesteric layer, which is 10% higher than the value obtained using narrowbandcholesterics on the same waveguide.
A drop in the relative edge emission increase was again noted at the onset wavelengthof 735 nm for the patterned waveguides with a dye coverage of 20 - 50%. For the samplesexhibiting dye coverage of 70 and 100%, this decrease in edge output was not observed.Here, this sudden drop in relative edge output is unlikely to be caused by the lowerre�ectivity of a speci�c cholesteric as the re�ectivity of 735 nm broadband cholesterics wascomparable to the other broadband cholesterics in the set (see Figure 6.7). However, thereare imprefections in the overlap of the two narrow stopbands used to form the broadbandre�ector, which were particularly obvious for the cholesteric with an onset wavelength of735 nm. These imprefections lead to drops in the re�ectivity of the stopband (see Figure6.7), which can in turn increase the top surface transmission of emitted light, resulting ina decrease in relative edge output.
6.3.3 Broadband Cholesterics in Integrated LSC System
Broadband cholesteric re�ectors have been demonstrated to increase the edge output ofpatterned waveguides. The broadband cholesterics are now applied to the integratedsystem of cylindrical lens array and a 40% dye-covered line patterned waveguide in anattempt to reduce top surface losses and increase edge output. This combination of the
126 Chapter 6. Wavelength-Selective Re�ectors for Patterned LSCs
Figure 6.9: Transmission spectra of a two-sided broadband cholesteric Bragg re�ectorwith stopband onset wavelength of 735 nm, 783 nm, 820 nm, and 874 nm, correspondingto center wavelengths of 824nm, 882 nm, 910 nm, and 960 nm, respectively.
lens array with patterned waveguides is referred to as the integrated LSC system hereafter.The transmission spectra of the set of cholesteric re�ectors used here is illustrated in Figure6.9. The center wavelength and the bandwidth of the stopband in the 874 nm cholestericis 960 nm and 172 nm, respectively.
The broadband cholesterics were placed between the PC lens array and the line pat-terned waveguide of the integrated LSC system to evaluate the e�ect of the broadbandre�ectors on the integrated system. An air gap was introduced on either side of thecholesterics for two main reasons. First, the air gap between the cholesterics and thepatterned waveguide prevents out-coupling of waveguided light from the substrate to thecholesteric. The second reason is more practical. Without a contact �uid to induce op-tical optical at the cholesteric/lens and cholesteric/waveguide interfaces, the same set ofcholesterics can be reused for each sample measurement, minimizing variations due to thecholesterics.
The absolute edge emission values of both the integrated and line patterned systemwith cholesteric layers exhibiting various onset lengths are displayed in Figure 6.10. Byadding the PC lens array on top of the cholesteric and line patterned system (system2), the edge emission increased by approximately 30% compared to the system withoutthe lens array (system 1), for all onset wavelengths of cholesteric layers except 735 nm.The emission increase at onset wavelength of 735 nm was ∼15%. This lowered emissionmay be a result of the lens array changing the angle of normal incident light, causingthe light to arrive at angle at the surface of the cholesteric re�ector. As the cholestericstopband blue-shifts with increasing incident angle, it may have lead to a partial blockageof incoming angular light at an onset wavelength of 735 nm, which in turn lowers the edgeemission of system 2.
6.3. Results and Discussion 127
Figure 6.10: The total edge emission of system 1 (�) consisting of broadband cholestericsplaced on Red305-coated line patterned waveguides, and system 2 (•), which comprises ofthe same patterned waveguides with broadband cholesterics and cylindrical PC lens arrayon top, as a function of the cholesteric's stopband onset wavelength. The edge emissionof the systems were integrated from 350-750 nm.
To distinguish the in�uence of the lens array from the in�uence of the cholesterics inthe integrated LSC system, the edge outputs of the integrated system with cholestericsplaced on top were divided by the edge outputs of the same integrated system with anhalf-wave plate as shown in Figure 6.11. This allows the normalization of both the e�ectsof the half-wave plate used in the making of the cholesteric and the lens array on theLSC system. Therefore, the results will mainly focus on the relative increase/decrease inthe edge output of a speci�c system with respect to the reference system. In this case,the reference system is the integrated system with a blank half-wave plate. The e�ectof the broadband cholesterics exhibiting various onset wavelengths on the line patternedwaveguide with and without the PC lens array are shown in Figure 6.12. The patternedsystem combined with cholesteric re�ectors (without the lens array) will be referred toas �system 1�, and the same patterned waveguide integrated with the lens array andcholesteric re�ectors will be referred to as �system 2� hereafter.
After normalizing the contribution of the lens array and half-wave plate to the edgeoutput, the cholesteric layer was found to increase the edge output of the integrated (lensarray and patterned waveguide) system by a maximum of ∼5% at the onset wavelengthof 783 nm. For the patterned waveguide combined with the cholesteric re�ector system,the maximum increase in edge emission (∼11%) was obtained at the cholesteric onsetwavelength of 735 nm. The onset wavelength that produced the maximum edge outputincrease in system 1 (without a lens array) is di�erent than that observed for system2, with a lens array. This di�erence may be a result of the angular dependence of thecholesteric stopbands. The angle of incidence on the cholesteric re�ector is di�erent forlight rays transmitted through air (system 1) and a lens array (system 2). As discussed, the
128 Chapter 6. Wavelength-Selective Re�ectors for Patterned LSCs
Lens Array
Patterned LSCCholesteric Reflector
(a)
(b)Lens Array
Patterned LSC
Half-wave plate
Measured Edge Emission
Control SystemEdge Emission
Figure 6.11: Schematic of the experimental setup to calculate the relative increase in edgeoutput where the edge output of (a) integrated LSC system with broadband cholesterics arenormalized by each of the edge outputs of (b) the reference system where the cholestericre�ector is replaced by the half-wave retarder.
Figure 6.12: The increase in edge output of system 1(�), consisting of Red305 linepatterned waveguides combined with broadband cholesterics, and system 2 (•), composedof Red305 line patterned waveguides combined with broadband cholesterics and cylindricalPC lens array, as a function of the cholesteric's stopband onset wavelength. The edgeoutput of system 1 and system 2 were normalized by each of the system edge outputs witha half-wave retarder replacing the cholesteric re�ector.
6.3. Results and Discussion 129
(a) (b)
Lens Array
Patterned LSC
Cholesteric Reflector
IiIR
IRIi
Figure 6.13: When incident light encounters the interface between two mediums, afraction of the incident light is re�ected by the new medium. In the case of (a) system1, re�ection losses are incurred at two interfaces, whereas in system (2), an additionalinterface is present as a result of the lens array. Thus, the re�ection losses are greater inthe system 2 than system 1.
lens array bends normal incident light rays in order to focus them to a point, resultingin angular incidence at the cholesteric re�ector. The angular incidence of light mayhave encoutered a su�ciently blue-shifted stopband of the cholsteric, resulting in partialre�ection of the incident light at shorter onset wavelength of 735 nm. Hence, the optimalstopband onset wavelengtgh was longer when a lens array was applied to system 1 to formsystem 2.
At the optimal onset wavelengths, the absolute increase in edge emission of the in-tegrated system was ∼5% lower than the line patterned waveguide. One might expectthe integrated system to fully exploit the advantages of the cholesteric layer, and as aresult, the gain in edge emission of the integrated system should at least match the gainof the line patterned system. However, the added cholesteric/lens array interface in theintegrated system induces more re�ection losses as illustrated in Figure 6.13, which par-tially accounts for the lower edge emission gain in the integrated system. The addedcholesteric re�ector also increases the distance between the lens array and the patterneddye structures. This implies that the dye structures are no longer at the optimal posi-tion for absorbing the light emerging from the lens array, which undoubtly contributesto the low increase in edge emission observed from system 2. Moreover, the surface ofthe cholesteric re�ectors was slightly curved, making it di�cult to properly align the lensarray to the patterned waveguide when the cholesterics were placed between them. Allthese factors attribute to the lower than anticipated increase in edge emission from theintegrated system of lens array, cholesteric re�ector and patterned waveguide.
Figure 6.14 shows the combined e�ect of the PC lens array and the cholesteric re�ectorat an onset wavelength of 783 nm on a 40% dye-covered patterned waveguide with respectto patterned system with a blank half-wave retarder on top. A maximum of∼37% increasein edge emission of the cholesteric and lens array combined system was achieved. Thisedge output gain would likely be higher if the interfacial re�ections can be eliminatedby placing all three elements, lens array, cholesteric layer, and patterned waveguide, inoptical contact using a low refractive index material.
130 Chapter 6. Wavelength-Selective Re�ectors for Patterned LSCs
Figure 6.14: The increase in edge emission of the integrated system consisting of cylin-drical PC lens array, broadband cholesterics of various onset wavelengths, and 40% dye-covered line patterned waveguide with respect to the identical line patterned waveguide witha blank half-wave plate on top. The edge emission values of line patterned and integratedsystem were integrated from 350-750 nm.
6.4 Conclusions
The addition of narrowband cholesteric Bragg re�ectors to line patterned waveguidesresulted in an edge output gain of a maximum ∼12% when a cholesteric with an onsetwavelgnth of 760 nm was used. By increasing the bandwidth of the cholesteric re�ectorsfrom 95 nm to 200 nm, a further 10% increase in edge output gain was achieved. Themaximum edge output gain of ∼22% was produced with a broadband cholesteric with 735nm onset wavelength placed on top of a line patterned waveguide with 20% dye coverage.Both narrow and broadband cholesterics were observed to be more bene�cial for 20-50%dye-coated line patterned waveguides as lower dye coverage results in lower reabsorptionprobabilities. The probability of reabsorption increases as cholesteric layers re�ect theemitted light back into the LSC system and may in turn lead to lower edge output gain ofsamples with higher dye coverage. When broadband cholesterics where applied betweena cylindrical PC lens array and a 40% dye-covered line pattern waveguide with air gapson either side of the cholesteric, the edge emission of the system was noted to increaseby ∼5% compared to the system without the cholesteric re�ectors. The combinationof the lens array, which increases light absorption of the patterned waveguide, and thebroadband cholesteric, used to reduce top surface losses of the LSC, resulted in a ∼37%increase in edge emission compared to a line patterned waveguide with nothing on top.This gain can be improved by eliminating re�ection losses at the lens/cholesteric interfacewith the use of contact �uid to induce optical contact between the two elements.
Chapter 7
Conclusions and Technology
Assessment
7.1 Conclusions
Though the cost per Watt of energy produced by standard �at solar photovoltaic panelsand concentrators has decreased dramatically in the last two decades, it is still too high tocompete with conventional methods of electricity generation. As a result, various di�erentmaterials and concepts for solar energy conversion have been investigated to further reducethe cost per Watt of solar devices. Luminescent solar concentrators have advantages overstandard PV panels and PV concentrator systems, particularly for applications in thebuilt-environment as they are visually appealing and presumably inexpensive to fabricate.This thesis aims to produce more cost e�cient LSCs by improving the photon-to-photone�ciency and edge emission using inexpensive and straightforward methods in an attemptto increase their feasibility for future commercialization.
The performance of standard 5×5 cm2 LSC systems were �rst investigated both ex-perimentally and theoretically using ray-tracing models to gain a better understanding ofthe e�ect of the loss mechanisms involved in an LSC. In both the model and simulateddata, there exists a maximum edge emission achievable using the basic LSC systems atwhich any additional increase in dye concentration or sample absorbance has negligibleadditional e�ect. Moreover, the edge emission and photon-to-photon e�ciency of thesystem appear to be competing factors: increasing the dye concentration of the systemincreases edge emission, but also leads to higher reabsorption and surface losses in theLSC. To obtain the best performing LSC system, one must �nd the optimum dye concen-tration that balances edge emission and e�ciency. The model was subsequently extendedto predict the performance and the optimum absorbance for the LSC systems in whichtwo waveguides doped with di�erent organic dyes were placed in a stack to increase theabsorption and, in turn, the edge emission of the LSC system.
Well-established photolithographic techniques were employed to structure thin �lmsinto di�erent patterns as a way to limit re-absorption losses in the system. Re-absorption,which is intrinsic to �uorescent dye molecules exhibiting small Stokes-shifts, induces pho-ton (and energy) losses (i.e., reabsorption) in the LSC system. By decreasing the surfacecoverage of the dye-coating via patterning, the probability of re-absorption was signi�-
131
132 Chapter 7. Conclusions and Technology Assessment
cantly reduced, resulting in a higher absorbed photon to emitted photon e�ciency. Gener-ally, the smaller the area of the dye structures, the higher the photon-to-photon e�ciencyof the system and the lower the edge emission of the system. Experimental data werecompared against the simulated results of the patterned LSC ray-tracing model. It wasdemonstrated that the model can correctly predict the edge emission and absorptiontrends of the patterned LSC system, but large discrepancies were observed between thequantitative e�ciency values of the experimental and the model system, particularly forsmall dye structures. The discrepancies may be attributed to additional measurementerrors at low dye coverages and the incorrect assumption that emission and absorptionof the dye molecules are isotropic. Commercial ray-tracing software packages thus mayhave limitations in simulating complex �uorescent optical devices.
While structuring the dye-coating of a thin �lm LSC increased the photon-to-photone�ciency of the system, the emission at the edges of the system was reduced as a largefraction of incident light is not absorbed and lost through the clear regions of the waveg-uide. To rectify the initial light loss and maintain the light absorption of dye structures onthe order of a fully covered LSC, an aspherical lens array was designed using ray-tracingto focus incident light directly on the dye structures. The lens array was subsequentlyfabricated and combined with the patterned LSC to form a new integrated LSC system.Studies of the performance of the integrated system showed the lens array o�ered ∼60%increase in edge emission compared to the patterned LSC alone. Further increases in theedge emission of the integrated system was achieved by placing a scattering layer at thebottom of the system and the resulting edge emission exceeded that of the fully coveredLSCs by more than 20%. The best performing integrated system consisted of polycar-bonate lens array, line patterned LSCs and a scattering layer. This system achieved anabsolute e�ciency of 5.7%, 0.6% higher than standard fully covered LSCs. Evidently,the integrated LSC system allows the exploitation of the advantages of patterned dyestructures while increasing the absorption of the dye structures to a similar absorptionlevel as a fully covered waveguide. The aspherical lens array also induced a preferred edgeemission e�ect where emissions were favored at edges parallel to the length of the linestructures.
It is desirable to reduce the amount of PV materials needed for an LSC, and this ispossible if the emission of the LSC is directed to one or two edges of the waveguide atwhich PV cells are attached. To gain control over the emission direction of dye molecules,liquid crystals were used to induce order in the dye molecules in a guest-host system.Two di�erent dye alignments, planar and homeotropic, were studied and the e�ect of thetwo dye alignments on the performance of the line patterned LSC was investigated andcompared with the isotropic patterned LSCs. Line patterned LSC with dyes aligned in theplanar fashion was anticipated to show preferential edge emission as the emission directionof the dyes was restrained by the alignment of its emission dipole. This was con�rmedin the studies as emissions at two edges of the planar aligned systems were observed tobe 30% higher than the orthogonal edges. The planar aligned systems utilize the linepatterns to reduce re-absorption losses in the preferred emission directions of the dye,resulting in better performances than the isotropic systems. Thus, the planar-aligned dyeLSCs is most suitable for integration with the previously developed lens array, which maygenerate signi�cant emission increases at two edges of the waveguide due the synergistic
7.2. Recommendations for Future Research 133
favored edge emission e�ects of the dye alignment and the lens array.The integrated system of lens array and line patterned LSC has been demonstrated to
reduce re-absorption losses and improve the performance of the LSC system. However, asigni�cant portion of the emitted photons in the integrated system are still lost throughthe surfaces of the waveguide. To limit surface losses in the integrated system, wavelengthselective re�ectors were fabricated using cholesterics and placed in between the lens arrayand line patterned waveguide. A series of narrow and broad-band re�ectors with di�erentcentering wavelengths were investigated in the integrated system and the edge emissionswere evaluated as a function of dye coverage to �nd the stopband centering wavelengththat best matches the emission band of the dyes. The addition of broadband re�ectorsto line patterned LSCs increased the edge emission of the system by ∼20%, 10% higherthan the increase obtained using narrowband re�ectors. When the optimal broadbandre�ector was combined with the lens array and patterned waveguide, a 30% increase inedge emission was achieved compared to the same system without the lens array. Furtherincreases in edge emission of the integrated system may be possible if re�ection losses atthe interfaces between the cholesteric re�ector and lens array are eliminated. Finally, theperformance of the line pattern system in combination with the lens array and broadbandcholesteric re�ectors demonstrated a 37% improvment compared to the line pattern systemalone.
7.2 Recommendations for Future Research
This research has demonstrated that the performance of LSCs can be improved by in-tegrating a cylindrical lens array with patterned thin �lm LSC, in which the lens arraywas used to reduce light lost through the clear regions of the patterned waveguide. Theintegrated lens array and patterned waveguide LSC device achieved the highest edgeemission and absolute e�ciency among all the LSC systems studied here. Yet anothermore cost-e�ective and straightforward method to limiting the initial light loss of thepatterned devices may be the stacking of two patterned waveguides with an air gap wherethe patterns complement each other and the sum of the dye coverages of the two pat-terned waveguides is nearly 100%. A schematic of this system is shown in Figure 7.1.This stacked arrangement of complementary patterned waveguides allows the light ab-sorption of the system to approach that of a fully covered waveguide while exploiting thehigher luminescent e�ciency (edge emission normalized by absorption of the system) ofpatterned waveguides. Moreover, the double patterned waveguides are considerably easierto fabricate than a lens array. Thus, the production cost of the double patterned systemis expected to be lower than the integrated system of lens array and patterned waveguide.
Preliminary results showed that the edge emission of the double stacked patternedLSCs exceed that of the double stacked fully covered LSCs by ∼17% (see AppendixA for experiemtnal results). Hence, double patterned systems have demonstrated theirpotential in enhancing the performance of luminescent solar concentrators as comparedto a double stack of fully covered systems. In the future, the author suggests to improvethe performance of double patterned LSC system by reducing the re�ection losses at theinterface of the two patterned waveguides. Also, the patterns on the waveguide can be
134 Chapter 7. Conclusions and Technology Assessment
Figure 7.1: Schematic of two patterned waveguides on top of each other such that thepatterns complement each other and full dye coverage is achieved.
fabricated with more precision to achieve better alignment of the patterns when the upperand lower waveguides are combined in order to obtain full dye coverage. More accuratelight absorption measurements of the patterned waveguides are also needed to betterquantify the e�ciency and the performance of the patterned systems.
The advantages of using �uorescent dyes with di�erent absorption and emission spectrain separate waveguides and a double stacked arrangement were demonstrated in chapter 2,and a similar concept can be applied to the patterned waveguides. In this case, alternatingline patterns (or patterns of other geometric structures) coated on a single waveguide canbe created from di�erent species of dyes to form a fully covered waveguide. This eliminatesthe initial light loss through the clear regions of a patterned waveguide and increase theabsorption range of the patterned LSCs. Subsequently, the patterned substrates coatedwith structures of two or more species of dye can also be stacked to further enhance theabsorption of the patterned LSC system.
It would also be interesting to investigate the performance of a square-patterned waveg-uide with a rectangular patterned aspherical lens array (as designed in chapter 4). In thiscase, each of the aspherical lens in the array would correspond to an squared dye struc-ture on the waveguide. As discussed in chapter 3, the square patterned waveguide canpotentially out-perform the line-patterned waveguides as the square dye structures re-duces re-absorption losses in two directions instead of one. Hence, the integration of theaspherical lens array with the square pattern should result in higher edge emission andphoton-to-photon e�ciency than the combined system of line-patterned waveguide andcylindrical lens array. Broadband cholesteric wavelength-selective re�ectors can also beapplied to the squared patterned integrated system.
Other possible methods for further improving the performance of luminescent solarconcentrators may involve the re-design of the lens array for the integrated system ofcholesteric re�ector, patterned waveguide and lens array. The design of the lens arrayin this case should take into account the optical properties of the cholestetric layer asthe cholesteric is positioned between the patterned waveguide and the lens array. There-fore, the optical properties (i.e., re�ectivity and position of the re�ection band) of thecholesteric will have considerable in�uence on the amount of focused light emerging fromthe cholesteric layer and the focal spot size on the patterned waveguide. A new design
7.3. Technology Assessment 135
of the lens array incorporating the cholesteric layer should improve the performance ofthe three component integrated LSC system. In addition, each lens within the lens arraycan be down-sized to the order of microns. Patterns of micron-sized structures can sub-sequently be created by exposing through the micron-size lens array. The e�ect of thisdown-sizing may be interesting as it can potentially decrease light lost through the clearregions of the pattern and lower scattering losses in the lens array.
The focus of the various concepts mentioned above is to enhance the overall perfor-mance of luminescent solar concentrators as an extension of the work in this thesis. Tohave a better insight into the performance of the integrated lens array and patterned LSCsystem, the system should be tested outside the laboratory in realistic environments. Thelocation of the tests should be in areas where there is a large amount of direct sunlightfor most of the year. This condition is favorable to the integrated system, and simul-taneously tests the performance durability of the system under rising temperatures andcontinuous exposure to solar radiation. Many applications for the LSC such as large-arearooftop installations, windows, and self-lit signs will only be feasible if the luminescentsolar concentrator can generate su�cient energy to provide power to auxiliary devices andits performance is reliable over a long period of time. Thus, improving the e�ciency andreliability of the LSCs are essential to its success in applications outside of the laboratory.By continuing our investigations in the enhancement of luminescent solar concentratorsusing various techniques, we can promote its applications in the `real' world and open thedoors to other areas where luminescent solar concentrators can be utilized.
7.3 Technology Assessment
The current solar market is still dominated by silicon based technologies, as it was threedecades ago. As global markets come to recognize the high cost of silicon as raw materialfor PV panels, other photovoltaic technologies that reduces the use of silicon such as thin�lm and organic PV cells, as well as concentrators for photovoltaics are gaining interest.Most of the photovoltaic and concentrator photovoltaic technologies developed to date areprimarily intended for roof-top and/or solar power plant related applications. The lumi-nescent solar concentrator is a cost-e�ective alternative to standard expensive PV panelsand sun-tracking PV concentrator systems, and is particularly well suited for urban usedue to its simplicity, appealing visual appearance, and cost-e�ectiveness. The followingsections provides a brief evaluation of the possible applications of patterned luminescentsolar concentrators integrated with lens arrays as energy generating installations in boththe urban and remote areas.
7.3.1 Rooftop Applications
Most people today are aware of the roof-top applications of solar energy converting deviceswhere photovoltaic concentrators or panels convert solar energy to another form of usefulenergy such as electricity or thermal energy. The integrated lens array and patternedluminescent solar concentrator (described in chapter 3 and 5) was designed with roof-topapplications in mind. There are two possible types of rooftop applications, residential
136 Chapter 7. Conclusions and Technology Assessment
(small-area) rooftops and industrial (large-area) rooftops. From a consumers' point ofview, the non-tracking integrated LSC systems are light-weight, cost-e�ective, and likelymore straightforward to maintain than standard PV concentrators or panels. Moreover,the combined patterned and lens array LSC system remains relatively �at even after theintegration of the lens array, which is more visually appealing than the large sun-tracking,awkwardly-shaped PV concentrators. However, due to the relatively short lifetime ofLSCs compared to standard inorganic PV panels, it is still di�cult for the integratedLSC systems to compete with inorganic PVs as no one would �nd it appealing to climbup the roof to change their solar energy converting system regularly (approximately every10 years compare to ∼30 years lifetime of PV panels).
One way to improve the stability of dyes is the use of optical �lters to �lter outultra-violet (UV) light (at the expense of increasing device cost), which is the main causefor the degradation of �uorescent dye emissions [148]. Another method for battling theshorter lifetime (∼5 years [77, 149]) issue of LSCs may be to apply the �uorescent dyecoating to an adhesive tape, which is adhered to the clear waveguide. The adhesive tapecan be peeled o� in exchanged for a new dye-coated adhesive �lm as the performance ofthe �uorescent dyes degrade over time. This allows the re-use of the waveguide and onlyrequires a simple changing of adhesive �lms to prolong the usage and functioning of theLSC.
Aside from short lifetime issues, the low e�ciency of LSC systems is also an issue forresidential rooftop applications. For an average household in developed countries, theelectricity consumption per year is ∼6000 kWh [150]. This translates to a consumptionrate of approximate 10 kWh per day (assuming 365 days in a year). In other words, thesolar rooftop modules need to produce approximately 16 kWh per day to fully supportthe electricity consumption of an average household. Given that the current e�ciency ofLSCs with silicon PV cells attached is ∼5%, and assuming an active LSC module area of20 m2 with continual solar irridation of 1000 W/m2 for 5 hours a day, the LSC modulecan produce only 5 kWh per day. Clearly, this is not enough energy to completely supportthe power consumption of households on a daily basis. Furthermore, this calculation forthe LSC generated was performed using highly favorable assumptions, i.e., 5 hours ofdirect sunlight at one sun. In more realistic situations and depending on the location,the amount of energy produced by the LSC module will likely be less than calculated.Hence, for the integrated patterned LSC systems to be successful in residential rooftopapplications, methods must be developed to stabilize the photoluminescence process oforganic dyes over long periods of time and further enhance the e�ciency of the systems.
Conversely, it may be possible to use the integrated lens array and patterned LSCsystem for industrial large-area rooftop applications. The large-area industrial rooftopsallow the installation of a greater number of LSC systems compared to the residentialhousing roofs, which implies that the total amount of energy produced will be higher. Forexample, using the calculation described above and by simply tripling the rooftop areacovered by LSC modules with 5% e�ciency to 60 m2 , the resulting energy produced perday is 15 kWh. Though this is likely to be below the daily energy consumption of industrialand commerical buildings, but it will provide notable support in energy consumption andlower the annual cost of electricity. For very large area installations in more remote areas,it may be possible to use a su�ciently large number of LSC systems to produce the
7.3. Technology Assessment 137
required energy supply since the cost of LSCs is considerably lower than standard PVconcentrators or modules. In the case where the operating lifetime of the integrated lensarray and patterned LSCs can be improved, or the cost of changing adhesive �lms coatedwith �uorescent dyes is su�ciently low, it would be plausible to use the LSC systemsdescribed in this thesis for large-area rooftop applications.
7.3.2 Greenhouse Applications
Greenhouses are commonly built using transparent glass or polymeric plates as roof andwalls to allow the plants growing inside to absorb sunlight, and to retain thermal energywithin the greenhouse. In 2010, there were approximately 10,000 hectares of greenhousesin the Netherlands alone [151]. This is potentially a large market for the solar energy�eld where glass or polymeric roofs and walls can be replaced by solar energy conversionsystems to generate electricity or additional thermal energy as support for heating sys-tems. The light-weight and inexpensive luminescent solar concentrators are particularlysuited for such an application. As LSCs are fabricated using PC or PMMA polymericplates, which is the same material currently used in plastic greenhouses, it can be directlyincorporated into the building of new greenhouses without additional changes to the in-frastructure. The roof and walls of the greenhouse can be constructed from the moree�cient integrated (lens array and patterned) and the patterned LSC system, respec-tively. The latter is used as walls to prevent the obstruction of view. An added bene�t forusing LSC systems for the building of the greenhouse is the potential of the �uorescentdyes (or other luminescent materials) to convert the unused wavelengths of light to usefullight that the plants can absorb. Initial trials of using such colored plates in the growthof particular crops (e.g., tomatoes and lettuce) have lead to an increase in yield up to20% [148]. The long-term e�ect of an altered solar spectrum on di�erent types of cropshas yet to be investigated. Detailed investigation in the growth of di�erent crops using angreenhouse built from LSC systems will be essential to determining the plausibility andpracticality of integrating LSCs in greenhouses.
Another concern of applying LSC systems to greenhouses is, again, the lifetime ofthe LSCs as one would not want to invest in the re-built of a greenhouse when theperformance of LSCs drops after 5 years. A possible solution to this a longstanding issue isthe aforementioned method of applying a dye-coated adhesive �lm to the polymeric plate.This eliminates the need of re-constructing the greenhouse at the end of the LSC lifetimeas the dye-coated adhesive �lm can be removed and changed as needed. In addition, thisadhesive �lm method o�ers the possibility of converting existing greenhouses to energyproducing greenhouses without the re-purchasing of roof and wall material as the adhesive�lm can be attached to any transparent plate to form an LSC. Thus, the initial investmentcosts in this case would be lower than attaching thin �lm or organic photovoltaic panels tothe greenhouses. Re-structuring may be needed as photovoltaic cells need to be attachedto the edge of the glass or polymeric plates that make up the roof and walls of thegreenhouse in order to complete the LSC module.
In conclusion, the characteristics of luminescent solar concentrators make them at-tractive for many applications where high e�ciency is not the primary goal. However,continual improvement in both e�ciency and lifetime of the LSC devices are essential for
138 Chapter 7. Conclusions and Technology Assessment
taking the LSC from laboratory environments to real applications. Some methods for im-proving e�ciency and battling the relative short lifetime issue have been discussed here.Further developments and performance testing is needed to verify the feasibility of thesesuggestions. Additionally, the implementation of luminescent solar concentrators in realapplications requires standardized testing and quantifying methods for e�ciency, stabil-ity, and durability among researchers and future businesses. Thus, the key to successfultranslation of luminescent solar concentrators from the laboratory to real applications liesin a synergistic combination of e�ciency, stability and low cost, as well as standardizationof quanti�cation methods.
Appendix A
Double stacked patterned LSCs
A.1 Experimental Procedures
A.1.1 Sample Fabrication
50×50 mm2 PMMA substrates with thickness of 3 mm and 5 mm were used to fabri-cate patterned LSCs. Fluorescent dye solutions were prepared using 0.5% wt of perylene-1,7,8,12-tetraphenoxy-3,4,9,10 tetracarboxylic acid-bis-(2'-6'diisopropylanilide) �uorescentdye molecules (Lumogen Red 305 , BASF), and 1% photoinitiator (Irgacure184, Ciba) dis-solved 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 hourprior to spin-coating onto the 50×50 mm2 PMMA substrates at 1000 rpm for 30 s. Afterspin-coating, all 100% covered samples were crosslinked by exposing to a high-intensityUV lamp (OmniCure S2000 UV spot curing lamp) for 80 s under nitrogen �ow to form asolid �lm. For the fabrication of patterned LSCs, standard photolithography techniqueswere employed. Uniformly coated substrates were exposed to UV light through patternedshadow masks consisting of 10 lines with variable widths and a period of 10 mm. Linewidths were varied to cover 30%, 50%, and 70% of the waveguide surface. After UV expo-sure, the exposed patterned samples were placed in ethanol for ∼40 s at room temperatureand the samples were continuously agitated to etch away the unexposed material. Thethicknesses of the �uorescent dye coatings were measured to be 15-20 µm using a FogaleZoomsurf 3D optical pro�ler.
A.1.2 Sample Characterization
Abosrbance spectra of all samples were measured using a Shimadzu UV-3102 PC spec-trometer. The peak absorbances of the Red305 coated line patterned PMMA waveguideswere measured to be 0.34. Edge emission of the waveguides was measured by a SLMS1050 integrating sphere (Labsphere) equipped with a diode array detector (RPS900, In-ternational Light). The LSC samples were placed in a custom-made black sample holder,which is connected to the entry port of the integrating sphere and prevents surroundinglight from entering the port. The LSCs were exposed to a collimated light source froma 300 W solar simulator with �lters to approximate the 1.5 AM global solar spectrum
139
140 Appendix A. Double stacked patterned LSCs
(a) (b)
(c)
(d)
Figure A.1: The total (sum of 4 edges) emission of various double stacked line patterned50×50×3 mm3 PMMA waveguides coated with Lumogen F Red 305 was normalized by(d) the total edge emission of double stacked fully covered dye-coated waveguides. Thedye-coating area coverage of the upper and the lower waveguides in the stacked systemsare respectively: (a)50% and 50%, (b) 30% and 70%, and (c) 50% and 70% (d) 100% and100%.
(Lot-Oriel) located at a distance of 15 cm from the top surface of the waveguide. Lightoutput spectra and intensity from all four emission edges of the LSC were measured andrecorded. A white scattering layer (white painted cardboard) was placed at the bottomthe samples with an air gap for all edge emission measurements. Total edge emissionswere determined by integrating the recorded spectra over the range of 350-750 nm.
A.2 Results
Three combinations of dye coverages, 50% and 50%, 30% and 70%, and 50% and 70% inthe double stacked line patterned system were studied. Preliminary results showed thatthe edge emission of the double stacked 50% and 70% dye covered patterned LSCs exceedthat of the double fully covered LSCs by ∼17% (see Figure A.1). This strongly suggeststhat the e�ciency of the double stacked patterned LSCs is higher than the double stackedfully covered LSC system. Other double stacked patterned systems (i.e., ones where thesum of dye coverages is 100%) showed slightly lower increase in edge output relative tothe fully covered double waveguides. This is due to a slight misalignment between theline patterns of the upper and lower LSC resulting in a less than 100% dye coverage ofthe waveguide surface.
References
[1] Anderson, B. Solar energy: fundamentals in building designs. New York, McGraw-HillBook Co., (1977).
[2] Kalogirou, S. A. Prog. Energy Combust. Sci. 30(3), 231 � 295 (2004).
[3] Einstein, A. Am. J. Phys. 33, 367�374 (1965).
[4] Price, S. and Margolis, R. Technical report, US National Renewable Energy Laboratory(NREL) Report TP-6A-46025, (2010).
[5] Goetzberger, A., Hebling, C., and Schock, H.-W. Mater. Sci. Eng. R Rep. 40(1), 1 � 46(2003).
[6] Shockley, W. and Queisser, H. J. J. Appl. Phys. 32(3), 510�519 (1961).
[7] Green, M. A., Emery, K., Hishikawa, Y., Warta, W., and Dunlop, E. D. Prog. PhotovoltaicsRes. Appl. 19(5), 565�572 (2011).
[8] Bird, R. E., Hulstrom, R. L., Kliman, A. W., and Eldering, H. G. Appl. Opt. 21(8),1430�1436 Apr (1982).
[9] Sarma, K. R., Legge, R., Gurtler, R. W., and Baghdadi, A. J. Electron. Mater. 8(5), 717(1979).
[10] Soderstrom, T., Wang, Q., Omaki, K., Kunz, O., Ong, D., and Varlamov, S. Phys. StatusSolidi RRL 5(5-6), 181�183 (2011).
[11] Gordon, I., Dross, F., Depauw, V., Masolin, A., Qiu, Y., Vaes, J., Van Gestel, D., andPoortmans, J. Sol. Energy Mater. Sol. Cells 95(Sp. Iss. SI Suppl. 1), S2�S7 (2011).
[12] Carlson, D. E. J. Non-Cryst. Solids 35-36(Part 2), 707 � 717 (1980).
[13] Williams, R. and Crandall, R. S. RCA Rev. 40(4), 371�389 (1979).
[14] Yoon, J.-H., Song, J., and Lee, S.-J. Sol. Energy 85(5), 723�733 (2011).
[15] Del Gobbo, S., Castrucci, P., Scarselli, M., Camilli, L., De Crescenzi, M., Mariucci, L.,Valletta, A., Minotti, A., and Fortunato, G. Appl. Phys. Lett. 98(18), 3 (2011).
141
142 References
[16] Chu, S. S., Chu, T. L., and Firouzi, H. Sol. Cells 20(3), 237�243 (1987).
[17] Batzner, D. L., Oszan, M. E., Bonnet, D., and Bucher, K. Thin Solid Films 361, 288�292(2000).
[18] Morales-Acevedo, A. Sol. Energy Mater. Sol. Cells 90(6), 678�685 (2006).
[19] Le Donne, A., Acciarri, M., Gori, G., Colletto, R., Campesato, R., and Binetti, S. Sol.
Energy Mater. Sol. Cells 94(12), 2002�2006 (2010).
[20] Miles, R. W., Hynes, K. M., and Forbes, I. Prog. Cryst. Growth Charact. Mater. 51(1-3),1�42 (2005).
[21] Mohr, N. J., Schermer, J. J., Huijbregts, M. A. J., Meijer, A., and Reijnders, L. Prog.
Photovoltaics 15(2), 163�179 (2007).
[22] Chandramohan, M., Velumani, S., and Venkatachalam, T. Mater. Sci. Eng., B 174(1-3),205 � 208 (2010).
[23] Dhere, N. G. Sol. Energy Mater. Sol. Cells 95(1), 277�280 (2011).
[24] Faraj, M. G., Ibrahim, K., and Salhin, A. Optoelectron. Adv. Mater., Rapid Commun.
4(12), 2092�2096 (2010).
[25] Kuang, D., Comte, P., Zakeeruddin, S. M., Hagberg, D. P., Karlsson, K. M., Sun, L.,Nazeeruddin, M. K., and Graetzel, M. Sol. Energy 85(6), 1189�1194 (2011).
[26] Wei, D. Int. J. Mol. Sci. 11(3), 1103�1113 (2010).
[27] Bijleveld, J. C., Verstrijden, R. A. M., Wienk, M. M., and Janssen, R. A. J. J. Mater.
Chem. 21(25), 9224�9231 (2011).
[28] Galagan, Y., de Vries, I. G., Langen, A. P., Andriessen, R., Verhees, W. J. H., Veenstra,S. C., and Kroon, J. M. Chem. Eng. Process. 50(5-6), 454�461 (2011).
[29] Wei, H. X., Li, J., Cai, Y., Xu, Z. Q., Lee, S. T., Li, Y. Q., and Tang, J. X. Org. Electron.12(8), 1422�1428 (2011).
[30] Chandrasekaran, J., Nithyaprakash, D., Ajjan, K. B., Maruthamuthu, S., Manoharan, D.,and Kumar, S. Renewable Sustainable Energy Rev. 15(2), 1228 � 1238 (2011).
[31] Ong, P.-L. and Levitsky, I. A. Energies 3(3), 313�334 (2010).
[32] Kim, T., Jeon, J. H., Han, S., Lee, D.-K., Kim, H., Lee, W., and Kim, K. Appl. Phys.
Lett. 98(18), 3 (2011).
[33] King, R. R., Law, D. C., Edmondson, K. M., Fetzer, C. M., Kinsey, G. S., Yoon, H., Sherif,R. A., and Karam, N. H. Appl. Phys. Lett. 90(18), 183516 �183516�3 (2007).
[34] Futer, W., Schone, J., Philipps, S. P., Steiner, M., Siefer, G., Wekkeli, A., Welser, E.,Oliva, E., Bett, A. W., and Dimroth, F. Appl. Phys. Lett. 94, 223504�1 � 223504�3(2009).
143
[35] Martí, A. and Araújo, G. L. Sol. Energy Mater. Sol. Cells 43(2), 203 � 222 (1996).
[37] SolarByTheWatt.com. Retrieved from: http://solarbythewatt.com/2009/10/21/lowest-solar-panel-prices-per-watt-2009-10-20/, August (2011).
[38] Grätzel, M. Philos. Trans. R. Soc., A 365(1853), 993�1005 (2007).
[39] Krebs, F. C., Gevorgyan, S. A., and Alstrup, J. J. Mater. Chem. 19(30), 5442�5451(2009).
[40] Kalowekamo, J. and Baker, E. Sol. Energy 83(8), 1224 � 1231 (2009).
[41] Krebs, F. C., Spanggard, H., Kjær, T., Biancardo, M., and Alstrup, J. Mater. Sci. Eng.,
B 138(2), 106 � 111 (2007).
[42] Hoppe, H. and Sariciftci, N. S. J. Mater. Res. 19, 1924�1945 (2004).
[43] Pagliaro, M., Ciriminna, R., and Palmisano, G. ChemSusChem 1(11), 880�891 (2008).
[44] Swanson, R. M. Prog. Photovoltaics 8(1), 93�111 (2000).
[45] Antón, I., Pachón, D., and Sala, G. Prog. Photovoltaics 11(6), 387�405 (2003).
[46] Makrides, G., Zinsser, B., Norton, M., Georghiou, G. E., Schubert, M., and Werner, J. H.Renewable Sustainable Energy Rev. 14(2), 754�762 (2010).
[47] Muñoz, E., Vidal, P. G., Nofuentes, G., Hontoria, L., Pérez-Higueras, P., Terrados, J.,Almonacid, G., and Aguilera, J. Renewable Sustainable Energy Rev. 14(1), 518 � 523(2010).
[48] Davies, P. A. Appl. Opt. 31(28), 6021�6026 (1992).
[49] Hassan, K.-E. and El-Refaie, M. F. Sol. Energy 15(3), 219 � 244 (1973).
[50] Ja�e, L. D. Sol. Energy 42(2), 173 � 187 (1989).
[51] Leutz, R. and Annen, H. P. Sol. Energy 81(6), 761�767 (2007).
[52] Tripanagnostopoulos, Y., Siabekou, C., and Tonui, J. K. Sol. Energy 81(5), 661 � 675(2007).
[53] Yeh, N. Renewable Sustainable Energy Rev. 14(9), 2926 � 2935 (2010).
[54] Fraidenraich, N., Tiba, C., Brandão, B. B., and Vilela, O. C. Sol. Energy 82(2), 132 � 143(2008).
[55] Jing, D., Liu, H., Zhang, X., Zhao, L., and Guo, L. Energy Convers. Manage. 50(12),2919 � 2926 (2009).
144 References
[56] Hatwaambo, S., Hakansson, H., Roos, A., and Karlsson, B. Sol. Energy Mater. Sol. Cells
93(11), 2020 � 2024 (2009).
[57] Welford, W. T. and Winston, R. The optics of nonimaging concentrators: light and solar
energy. Academic Press, New York, (1978).
[58] Gleckman, P., O'Gallagher, J., and Winston, R. Nature 339(6221), 198�200 (1989).
[59] Mousazadeh, H., Keyhani, A., Javadi, A., Mobli, H., Abrinia, K., and Shari�, A. RenewableSustainable Energy Rev. 13(8), 1800 � 1818 (2009).
[60] Lorenzo, E., Pérez, M., Ezpeleta, A., and Acedo, J. Prog. Photovoltaics 10(8), 533�543(2002).
[61] Dickinson, W. C. Sol. Energy 21, 249�251 (1978).
[62] Nafeh, A. E.-S. A. Int. J. Numer. Model. Electron. Network. Dev. Field. 17(4), 385�395(2004).
[63] Karni, J. Nat. Mater. 10(7), 481�482 (2011).
[64] Europe's Energy Portal. Retrieved from: http://www.energy.eu, August (2011).
[65] Weber, W. H. and Lambe, J. Appl. Opt. 15(10), 2299�2300 (1976).
[66] Batchelder, J. S., Zewail, A. H., and Cole, T. Appl. Opt. 20(21), 3733�3754 (1981).
[67] Batchelder, J. S., Zewail, A. H., and Cole, T. Appl. Opt. 18(18), 3090�3110 (1979).
[68] Shcherbatyuk, G. V., Inman, R. H., Wang, C., Winston, R., and Ghosh, S. Appl. Phys.
Lett. 96(19), 191901 (2010).
[69] Bakr, N. A., Mansour, A. F., and Hammam, M. J. Appl. Polym. Sci. 74(14), 3316�3323(1999).
[70] Chatten, A., Barnham, K., Buxton, B., Ekins-Daukes, N., and Malik, M. Semiconductors
38, 909�917 (2004).
[71] Debije, M. G., Verbunt, P. P. C., Nadkarni, P. J., Velate, S., Bhaumik, K., Nedumbamana,S., Rowan, B. C., Richards, B. S., and Hoeks, T. L. Appl. Opt. 50(2), 163�169 (2011).
[72] Earp, A. A., Smith, G. B., Swift, P. D., and Franklin, J. Sol. Energy 76(6), 655 � 667(2004).
[73] Reisfeld, R. and Jørgensen, C. Sol. Energy Mater. 49, 1�36 (1982).
[74] Rowan, B. C., Wilson, L. R., and Richards, B. S. IEEE J. Sel. Top. Quantum Electron.
14(5), 1312�1322 (2008).
[75] Hecht, E. Optics, International Edition (4th edition). Addison Wesley, (2002).
[76] Rau, U., Einsele, F., and Glaeser, G. C. Appl. Phys. Lett. 87(17), 171101 (2005).
145
[77] Van Sark, W. G., Barnham, K. W., Sloo�, L. H., Chatten, A. J., Büchtemann, A., Meyer,A., et al. Opt. Express 16(26), 21773�21792 (2008).
[78] Farrell, D. J. and Yoshida, M. Prog. Photovoltaics (2011).
[79] Sloo�, L. H., Bende, E. E., Burgers, A. R., Budel, T., Pravettoni, M., Kenny, R. P.,Dunlop, E. D., and Büchtemann, A. Phys. Status Solidi RRL 2(6), 257�259 (2008).
[80] Hermann, A. Sol. Energy 29(4), 323�329 (1982).
[81] Debije, M. G., Verbunt, P. P. C., Rowan, B. C., Richards, B. S., and Hoeks, T. L. Appl.Opt. 47(36), 6763�6768 (2008).
[82] Peng, X., Schlamp, M. C., Kadavanich, A. V., and Alivisatos, A. P. J. Am. Chem. Soc.
119(30), 7019�7029 (1997).
[83] Rowan, B. C., Wilson, L. R., Richards, B. S., Jones, A., Moudam, O., and Robertson, N.In 24th European Photovoltaic Solar Energy Conference Proceedings, 346�349, (2009).
[84] Komiya, K., Fukuoka, S., Aminaka, M., Hasegawa, K., Hachiya, H., Okamoto, H., Watan-abe, T., Yoneda, H., Fukawa, I., and Dozono, T. In Green chemistry - designing chemistry
for the environment, Anastas, P. and Williamson, T., editors, volume 626 of ACS Sympo-
sium Series, 20�32. American Chemical Society (1996).
[85] Strathearn, H. Retrieved from: http://www.icis.com/v2/chemicals/9076048/methyl-methacrylate/pricing.html, June 28 (2011).
[86] Victory, M. Retrieved from: http://www.icis.com/v2/chemicals/9076145/polycarbonate/pricing.html,June 28 (2011).
[87] Kennedy, M., McCormack, S. J., Doran, J., and Norton, B. In Proceedings of ISES
World Congress 2007, Goswami, D. Y. and Zhao, Y., editors, 1484�1488. Springer BerlinHeidelberg (2009).
[88] Goetzberger, A. and Greube, W. Appl. Phys. A: Mater. Sci. Process. 14, 123�139 (1977).
[89] Chopra, K. L., Paulson, P. D., and Dutta, V. Prog. Photovoltaics 12(23), 69�92 (2004).
[90] Roncali, J. and Garnier, F. Appl. Opt. 23(16), 2809�2817 (1984).
[91] Goldschmidt, J. C., Peters, M., Bösch, A., Helmers, H., Dimroth, F., Glunz, S. W., andWilleke, G. Sol. Energy Mater. Sol. Cells 93(2), 176 � 182 (2009).
[92] Ford, W. E. and Kamat, P. V. J. Phys. Chem. 91(25), 6373�6380 (1987).
[93] Carrascosa, M., Unamuno, S., and Agullo-Lopez, F. Appl. Opt. 22(20), 3236�3241 (1983).
[94] Olson, R. W., Loring, R. F., and Fayer, M. D. Appl. Opt. 20(17), 2934�2940 (1981).
[95] Smestad, G., Ries, H., Winston, R., and Yablonovitch, E. Sol. Energy Mater. 21(2-3),99�111 (1990).
[96] Meyer, T. J. J., Hlavaty, J., Smith, L., Freniere, E. R., and Markvart, T. In Proc. SPIE,volume 7211, (2009).
146 References
[97] Burgers, A. R., Sloo�, L. H., Kinderman, R., and Van Roosmalen, J. A. M. In Presented
at the 20th European Photovoltaic Solar Energy Conference and Exhibition, volume 6, 10,(2005).
[99] Debije, M. G., Van, M.-P., Verbunt, P. P. C., Kastelijn, M. J., van der Blom, R. H. L.,Broer, D. J., and Bastiaansen, C. W. M. Appl. Opt. 49(4), 745�751 (2010).
[100] Wilson, L. R. and Richards, B. S. Appl. Opt. 48(2), 212�220 (2009).
[101] Wilson, L. R., Rowan, B. C., Robertson, N., Moudam, O., Jones, A. C., and Richards,B. S. Appl. Opt. 49(9), 1651�1661 (2010).
[102] Van Gurp, M. and Levine, Y. K. J. Chem. Phys. 90, 4095 (1989).
[103] McDowall, S., Johnson, B. L., and Patrick, D. L. J. Appl. Phys. 108, 053508 (2010).
[104] Ebeid, E. Z. M., El-Daly, S. A., and Langhals, H. J. Phys. Chem. 92(15), 4565�4568(1988).
[105] Neuteboom, E. E. Photoinduced processes of functionalized perylene bisimides. PhD thesis,Eindhoven University of Technology, (2004).
[106] Sóti, R., Farkas, ., Hilbert, M., Farkas, Z., and Ketskeméty, I. J. Lumin. 68(2-4), 105 �114 (1996).
[107] Gallagher, S. J., Rowan, B. C., Doran, J., and Norton, B. Sol. Energy 81(4), 540 � 547(2007).
[108] Sholin, V., Olson, J. D., and Carter, S. A. J. Appl. Phys. 101, 123114 (2007).
[109] Barnham, K., Marques, J. L., Hassard, J., and O'Brien, P. Appl. Phys. Lett. 76(9),1197�1199 (2000).
[110] Moudam, O., Rowan, B. C., Alamiry, M., Richardson, P., Richards, B. S., Jones, A. C.,and Robertson, N. Chem. Commun. (43), 6649�6651 (2009).
[111] Kastelijn, M. J., Bastiaansen, C. W., and Debije, M. G. Opt. Mater. 31(11), 1720�1722(2009).
[112] Verbunt, P. P. C., Kaiser, A., Hermans, K., Bastiaansen, C. W. M., Broer, D. J., andDebije, M. G. Adv. Funct. Mater. 19(17), 2714�2719 (2009).
[113] Ahn, T.-S., Al-Kaysi, R. O., Muller, A. M., Wentz, K. M., and Bardeen, C. J. Rev. Sci.Instrum. 78, 086105�1 � 086105�3 (2007).
[114] Xie, W. T., Dai, Y. J., Wang, R. Z., and Sumathy, K. Renewable Sustainable Energy Rev.
15(6), 2588�2606 (2011).
[115] Leutz, R., Suzuki, A., Akisawa, A., and Kashiwagi, T. Sol. Energy 65(6), 379 � 387 (1999).
[116] Lorenzo, E. Appl. Opt. 20(21), 3729�3732 (1981).
147
[117] Araki, K., Uozumi, H., Egami, T., Hiramatsu, M., Miyazaki, Y., Kemmoku, Y., Akisawa,A., Ekins-Daukes, N. J., Lee, H. S., and Yamaguchi, M. Prog. Photovoltaics 13(6), 513�527(2005).
[118] Andreev, V. M., Vlasov, A. S., Khvostikov, V. P., Khvostikova, O. A., Gazaryan, P. Y.,Sadchikov, N. A., and Rumyantsev, V. D. In Photovoltaic Energy Conversion, Conference
Record of the 2006 IEEE 4th World Conference on, volume 1, 644 �647, (2006).
[119] Piszczor, M. F. and O'Neil, M. J. Technical report, National Aeronautics and SpaceAdministration, United States., (1987).
[120] Dow Corning Corp. Retrieved from: http://www.dowcorning.com, August (2011).
[121] Debije, M. G., Teunissen, J.-P., Kastelijn, M. J., Verbunt, P. P., and Bastiaansen, C. W.Sol. Energy Mater. Sol. Cells 93(8), 1345 � 1350 (2009).
[122] Roncali, J. and Garnier, F. Sol. Cells 13(2), 133�143 (1984).
[123] Natarajan, L. V., Stein, F. M., Blankenship, R. E., and Chang, R. Chem. Phys. Lett.
95(6), 525 � 528 (1983).
[124] Sánchez, C., Villacampa, B., Cases, R., Alcalá, R., Martínez, C., Oriol, L., and Piñol, M.J. Appl. Phys. 87(1), 274�279 (2000).
[125] Thulstrup, E. W. and Michl, J. J. Phys. Chem. 84(1), 82�93 (1980).
[126] MacQueen, R. W., Cheng, Y. Y., Clady, R. G. C. R., and Schmidt, T. W. Opt. Express18, A161�A166 (2010).
[127] Mulder, C. L., Reusswig, P. D., Beyler, A. P., Kim, H., Rotschild, C., and Baldo, M. A.Opt. Express 18(S1), A91�A99 (2010).
[128] Verbunt, P. P. C., Bastiaansen, C. W. M., Broer, D. J., and Debije, M. G. In 24th EuropeanPhotovolatic Solar Energy Conference Proceedings, 381�384, (2009).
[129] Mulder, C. L., Reusswig, P. D., Velázquez, A. M., Kim, H., Rotschild, C., and Baldo,M. A. Opt. Express 18(S1), A79�A90 (2010).
[130] Rathi, P. and Kyu, T. Phys. Rev. E 79(3), 031802 (2009).
[131] Oseen, C. W. Trans. Faraday Soc. 29(140), 883�899 (1933).
[132] Barberi, R., Ciuchi, F., Durand, G. E., Iovane, M., Sikharulidze, D., Sonnet, A. M., andVirga, E. G. European Physical Journal E (EPJ E), The - Soft Matter 13(1), 61�71 (2004).
[133] Boamfa, M. I., Lazarenko, S. V., Vermolen, E. C. M., Kirilyuk, A., and Rasing, T. Adv.Mater. 17(5), 610�614 (2005).
[134] Forest, M., Zheng, X., Zhou, R., Wang, Q., and Lipton, R. Adv. Funct. Mater. 15(12),2029�2035 (2005).
[135] Nishikawa, M. Polym. Adv. Technol. 11(8-12), 404�412 (2000).
148 References
[136] Gibbons, W. M., Shannon, P. J., Sun, S.-T., and Swetlin, B. J. Nature 351(6321), 49�50(1991).
[137] Ewyk, R. V., O'connor, I., Mosley, A., Cuddy, A., Hilsum, C., Gri�ths, J., Blackburn, C.,and Jones, F. Electron. Lett. 22(18), 962�963 (1986).
[138] Bauman, D., Chrzumnicka, E., Mykowska, E., Szybowicz, M., and Grzelczak, N. J. Mol.
Struct. 744-747, 307 � 313 (2005).
[139] Stolarski, R. and Fiksinski, K. J. Dyes Pigm. 24(4), 295 � 303 (1994).
[140] Goldschmidt, J. C., Peters, M., Prönneke, L., Steidl, L., Zentel, R., Bläsi, B., Gombert,A., Glunz, S., Willeke, G., and Rau, U. Phys Status Solidi 205(12), 2811�2821 (2008).
[141] Richards, B. S., Shalav, A., and Corkish, R. P. In Proceedings of the 19th European
Photovoltaic Solar Engergy Conference, (2004).
[142] Broer, D. J., Mol, G. N., Haaren, J. A. M. M. v., and Lub, J. Adv. Mater. 11(7), 573�578(1999).
[143] Osipov, M. and Kuball, H.-G. Eur. Phys. J. E 5, 589�598 (2001).
[144] Nakagiri, T., Kodama, H., and Kobayashi, K. K. Phys. Rev. Lett. 27(9), 564�567 (1971).
[145] Wilson, M. R. and Earl, D. J. J. Mater. Chem. 11(11), 2672�2677 (2001).
[146] de Gennes, P.-G. and Prost, J. The physics of liquid crystals. Oxford University Press,(1993).
[147] Melchert, L. Build. Environ. 42(2), 893 � 901 (2007).
[148] Seybold, G. and Wagenblast, G. Dyes Pigm. 11(4), 303�317 (1989).
[149] Reisfeld, R., Shamrakov, D., and Jorgensen, C. Sol. Energy Mater. Sol. Cells 33(4),417�427 (1994).
[150] Firth, S., Lomas, K., Wright, A., and Wall, R. Energy and Buildings 40(5), 926 � 936(2008).
[151] International, N. E. Retrieved from: http://www.hollandtrade.com, September (2011).
Samenvatting
De laatste jaren is de markt voor duurzame energie sterk gegroeid door toegenomen zorgenover de e�ecten van broeikasgassen en de afname van fossiele brandstofvoorraden. De zonis een betrouwbare energie bron die dagelijks genoeg energie levert om de hele wereld tevoorzien in zijn energiebehoeften. Echter, energie opgewekt met behulp van zonnecellenis nog altijd twee tot drie keer zo duur als de energie geleverd door het conventioneleelektriciteitsnet, welke voornamelijk is opgewekt uit fossiele brandsto�en. Om de kostenvan zonne-energie te verlagen, kan zonlicht geconcentreerd worden op kleine zonnecellen.Een makkelijke, kleurrijke en kosten-e�ciënte manier om dit te doen is door gebruik temaken van een zogeheten �uorescerende golfgeleider (FGG).
Een FGG bestaat uit een transparante plastic of glazen plaat, die als golfgeleider di-ent, waarin �uorescerende kleursto�en zijn aangebracht. Deze �uorescerende kleursto�enabsorberen zonlicht dat op de golfgeleider valt en zenden het vervolgens weer uit. Eendeel van dit uitgezonden licht word naar de zijkant van de golfgeleider getransporteerdwaar een kleine zonnecel is aangebracht die het licht omzet in elektriciteit. Het eenvoudigeontwerp en het gebruik van relatief goedkope materialen voor de golfgeleider, kunnen ervoor zorgen dat de elektriciteit opgewekt door de FGG kan concurreren met elektriciteitvan het elektriciteitsnetwerk. Echter, een deel van het door de FGG geabsorbeerde lichtgaat verloren voordat deze de zonnecel bereikt, waardoor de e�ciëntie nog niet hoog ge-noeg is om aan bovenstaand potentieel te voldoen. In dit proefschrift word onderzocht ofdeze verliezen vermindert kunnen worden door het gebruik van goedkope en eenvoudigeoplossingen.
Als eerste is het gedrag van de FGG onderzocht als functie van de hoeveelheid geab-sorbeerd licht, door de concentratie van de �uorescerende kleurstof te veranderen. Zoweltheoretische benaderingen als uitgevoerde experimenten toonden aan dat een toename inkleurstofconcentratie boven een bepaalde kritische waarde slechts leid tot een verwaar-loosbare toename in de hoeveelheid energie die de zijkant van de FGG verlaat. Boven-dien neemt boven deze kritische concentratie de verhouding tussen geëmitteerde en geab-sorbeerde fotonen af. Deze daling in e�ciëntie kan verklaard worden doordat een deelvan het uitgezonden licht weer opnieuw geabsorbeerd wordt door de kleurstof moleculen.Hierdoor neemt ook het verlies van licht door de oppervlakken van de lichtgeleider toe.Een ander verlies dat zorgt voor een lage e�ciëntie van de FGG is het feit dat slechts eendeel van het inkomende zonlicht geabsorbeerd kan worden door de kleurstof moleculen.Om dit deel te vergroten zijn twee golfgeleiders met verschillende kleursto�en op elkaargelegd. De hoeveelheid licht die de zijkant van deze twee golfgeleiders verlaat is meer dantwee keer zo groot in vergelijking met de enkele golfgeleiders.
Om de kans op herabsorptie van het uitgezonden licht te verminderen is de �uo-
149
150 Samenvatting
rescerende kleurstof in een coating aangebracht op de golfgeleider en is deze coating vervol-gens gepatroneerd. Door het patroneren van de kleurstofcoating ontstaan er transparantedelen in de golfgeleider waar geen herabsorptie kan plaats vinden. Zowel een theoretischebenadering als experimenten tonen aan dat de verhouding tussen het aantal geabsorbeerdefotonen en het aantal fotonen dat de zijkant van de golfgeleider verlaat, toeneemt als hetdeel van het oppervlak dat kleurstof moleculen bevat verkleind wordt. Echter, door hetontstaan van de transparante delen in de FGG wordt de absolute hoeveelheid licht diegeabsorbeerd wordt kleiner. Hierdoor zal de totale hoeveelheid licht dat de zijkant van degolfgeleider verlaat ook afnemen.
Om de hoeveelheid geabsorbeerd licht in de gepatroneerde FGGs te verhogen zijn mi-crolenzen toegepast. Deze lenzen focusseren licht dat normaal op een transparant deelvan de golfgeleider valt, op een deel dat wel kleurstofmoleculen bevat. Om de totale e�-ciëntie van de FGG zo hoog mogelijk te maken, moet de acceptatiehoek van de lenzen zogroot mogelijk gemaakt worden, zodat zonlicht uit zo veel mogelijk hoeken gefocusseerdkan worden. Om hieraan te voldoen zonder dat de focusspot te groot te maken, is eenasferische lens met een acceptatiehoek van 30◦ is ontworpen. Dergelijke microlenzen zijngecombineerd met gepatroneerde FGGs en uit experimenten blijkt dat de totale hoeveel-heid licht dat de zijkant van de golfgeleider verlaat is 20% hoger dan bij een standaardniet-gepatroneerde FGG.
Door de introductie van de microlenzen en het patroneren is de hoeveelheid licht die dezijkant van de golfgeleider verlaat niet meer hetzelfde voor de vier verschillende zijkantenvan de FGG. Uit twee tegenoverliggende zijkanten die parallel zijn aan de lenzen verlaateen grotere hoeveelheid licht de golfgeleider dan uit de twee andere zijkanten. Dit e�ectkan vergroot worden door de kleurstofmoleculen uit te lijnen. Als de kleurstofmoleculenplanair en parallel aan de lenzen worden uitgelijnd, neemt het verschil tussen de hoeveel-heid licht die de twee parallelle zijkanten verlaat en de hoeveelheid die de twee loodrechtezijkanten verlaat toe. Hierdoor kan het aantal zijkanten dat bedenkt moet worden metzonnecellen worden gereduceerd van vier naar twee.
Ook gaat een deel van het licht dat uitgezonden wordt door de kleurstof moleculenverloren door de twee oppervlakken van de golfgeleider. Om deze verliezen te verminderenis een gol�engte selectieve spiegel, gemaakt van chiraal nematische vloeibaar kristallijnepolymeren, op de golfgeleider geplaatst. Deze laat het licht dat geabsorbeerd wordt doorde kleursto�en door, maar re�ecteert het uitgezonden licht, dat de golfgeleider verlaat viahet oppervlak, terug in de golfgeleider. De totale hoeveelheid licht die de golfgeleider aande zijkanten verlaat wordt verhoogd als deze selectieve spiegels op een gepatroneerde FGGgeplaats worden, zowel in het geval dat microlenzen gebruikt worden als in het geval datdeze niet gebruikt worden. De relatieve verhoging van de hoeveelheid licht die de zijkantenvan de golfgeleider verlaat is hoger bij systemen met een lage kleurstof concentratie omdatde kans op herabsorptie verliezen lager is.
De FGGs die beschreven zijn in dit proefschrift bestaan uit relatief goedkope materi-alen en kunnen eenvoudig geproduceerd worden. De verhoogde e�ciëntie van de FGGszorgt ervoor dat deze toegepast kunnen worden op daken om ter plekke energie op tewekken, zowel in steden als in afgelegen gebieden. Doordat de energie die opgewektword door FGGs relatief goedkoop is in vergelijking met standaard zonnepanelen, kun-nen FGGs ook als alternatief gezien worden voor grootschalige installaties, bijvoorbeeld
Samenvatting 151
op daken van grote industriële gebouwen. De �exibiliteit in ontwerp van FGGs, zoals�exibiliteit in kleur en vorm, opent de deur naar andere toepassingen, bijvoorbeeld in deglastuinbouw.
152 Samenvatting
Acknowledgements
Being Canadian, the question I have been asked the most since I arrived here is �Whydid you come to the Netherlands for your Ph.D.?� The answer to this question lies in themeeting of my promotor, an inspiring professor, Dick Broer during my Master studies inCanada. I, who did not consider doing a PhD for various personal reasons, least of allin the chemistry department as I was an electrical engineer in training, must thank Dickfor suggesting the idea of doing a PhD in The Netherlands. His suggestion initiated aseries of events, including the meeting of my co-promotors, Kees Bastiaansen and MichaelDebije, which resulted in the beginning of my PhD studies in the Netherlands. As thisPhD study is coming to an end �lled with anticipation, stress, and uncertainty, I realizethat one cannot complete such a journey without the wisdom, inspiration, help, time andpatience of many people within and outside the chemistry department. The �rst fewpeople that come to mind are Dick, Kees, Michael and BJ, whom I must thank for givingme the opportunity to do research on an interesting and challenging topic. Dick, thankyou for your inspiring ideas and support, particularly when I am immobilized in a di�cultproblem or situation. Kees, your insight and constructive comments were invaluable tome as they imposed me to re-evaluate my project using di�erent methods and openedmy eyes to di�erent �elds of studies. BJ, thank you for your advices, and you've taughtme much regarding business presentations. Mike, as one of my old o�ce mates, you werealways there for me when I was lost in the project, and you always had the patienceto listen when I bombard you with questions. So, thank you for your time, patience,support, `out-of-the-box' ideas, and most of all, I truly enjoyed all our discussions duringand outside of work (and the boardgames were a lot of fun).
My gratitude also goes to another person deeply involved in my project, Dick de Boerfrom Philips Research, who always had great insight in the optical modeling problemsI faced and often came up with solutions or di�erent approaches to the problem. TheSenterNovem meetings every two or three month in collaboration with people from PhilipsResearch helped to focus my research and the discussions often bought about new ideas.
It was a pleasure to be involved in the KWR project, a collaboration between our groupand Anteryon. The meetings I had with Koen Demeyer, Ruben Tibben, Edwin Wolterink,Klaas Pauluse and various others were enlightening and provided insight to the di�erentaspects of my project. I especially would like to thank Ivar Boere�jn for his help withcharacterization of lens arrays and Jules Kierkels for arranging everything. I have also hadthe pleasure of working with a number of students, Sara Peeters and Martijn van Gerwen,though their work is not used in this thesis, it helped me to gain some fundamental insightinto my research. Special thanks also goes out to Albert Schenning and Stefan Meskerswho had the patience to teach me �uorescence time-resolve measurements and how to
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154 Acknowledgements
analysis the data presented in chapter 2.For experimental setups and experimental procedures, Bob Fi�eld and Pit Teunissen
were great help and they often came up with practical and clever solutions to my problems.Bob, thanks for all the chattering and jokes as we work through problems. It made theprocedures less tedious and that much more bearable. I would also like to thank PaulineSchmit for helping and training me on optical and SEM imaging systems.
I have been lucky to be working in a group with such a broad range of technical andcultural backgrounds as the old PICT and the current SFD group. We have shared manyideas, frustrations, laughs, and fun moments in the four years and here, I would like tothank all my colleagues for their help: Amol, An, Antonio, Blanca, Chi-Wen, Danqing,Irén, Ivelina, Johan, Jelle, Katherine, Ko, Laurens, Maud, Mian, My, Natalia, Robert,Shabnam, Ties, Thijs, Tom, Youseli, Xiaoran, and anyone else I might have forgotten.Special thanks goes out to Casper, Helena, Nick, Joost, and Irina whom o�ered theirkindness, friendship and help to someone arriving in a foreign city of Eindhoven with noprior knowledge of The Netherlands, as well as my o�ce mates: Nicole, thank you for yoursmiles and small chitchats; Debarshi, thank you for your help in one of our collaboratedprojects; and Paul, without you, chapter 6 would not have been possible. Many thanksfor your help and I always enjoyed your jokes and discussions, rather it be work, music,books or boardgames.
It has been an interesting experience being involved in the Chinese community inEindhoven, and it allowed me to improve my Chinese and gain a better understanding ofthe customs of di�erent regions of China. Here, I would like to acknowledge a few peoplewithin this community whom are both colleagues and friends to me: Lu Kanbo, SongLiguo, Tang Donglin, Wang Ting, Xiao Yan, and Zhang Yi, thank you for the wonderfuldinners and parties. Piming and Xiaoxia, thank you for all your help, kindness, and theChinese dumplings you two made were delicious. Lijing Xue (shijie), thank you for yourhelp both at work and in life. You were my �rst o�ce mate at the university, and yourfriendliness and kindness were invaluable to me. Dancing is one of my hobbies, and it wasa lot of fun performing with the girls in the Chinese Dance Group. I would like to thankall the girls in the dance group for the gossips, laughs, and fun. A special thanks to LiJing, the founder of the dance group, who not only accepted me into the dance group,but was also a good friend who supported and helped me on many occasions despite hercrazy schedule.
Of course there were many di�cult times throughout a PhD study, but the moralsupport of three friends and colleagues in particular has made things seem a bit easier.Chunxia, thank you for always being there for me. I admire your everlasting energy andpositive attitude, and it was your attitude that motivated me to try harder. Weizhen,your sensibility, kindness, and innocence have given me much comfort and support inthese four years. Thank you for all the good times we had together (and I am not sorryfor pranking you, as I still laugh remembering your reaction). Mian, your practical andsensible perception of things was invaluable to me. Thank you for your advice and helpboth in the lab and outside of work.
I would also like to thank Duy's family for their support. The days I've spent at hisparents' house provided a moment of relaxation and pleasure among the stressful andfrustrating times in a PhD. In particular, I would like to thank Duy's mother for cooking
Acknowledgements 155
all the great food, which always managed to make me feel better.The last few months have been exceptionally hectic, trying to �nish a PhD, �nd a job,
and dealing with sudden arising of family issues. I am not sure how well I would havesurvived if not for Duy's help with my thesis and everything else. Duy, I must apologizefor making both of our lives quite chaotic in this past year, but having you beside memade all the di�erence, even if we were both working on separate things. Thank you fordoing all the little things around the house, taking care of me and making my life moreenjoyable, but most of all, thank you for just being there. You have become an essentialpart of my life. I look forward to having a vacation and sharing the rest of my life withyou.
Lastly, I would like to thank my parents. Mom and dad, thank you for always trustingme and allowing me to choose my own path, even when you disagreed. Thank you forsupporting me and being with me every step of the way. Your understanding, wisdomand love have been essential to the completion of this thesis, and I just want to tell youthat I love you both.
156 Acknowledgements
Curriculum Vitae
Shufen Tsoi was born on the 24th of January 1980 in Guangzhou, China. After com-pleting secondary school diploma in 1999, at Glenmary Highschool in Peace River, Al-berta, Canada, she studied Electrical Engineering at University of Alberta in Edmonton,Canada. In 2006, she received her Master of Science degree under the supervision of dr.Jeremy Sit within the glancing angle thin �lm deposition group on the topic of `ChemicalTunability of Glancing Angle Deposition Thin Films'. After 2006, she continued workingas a research associate in the same group on the topic of volatile organic sensors using 3Dnanostructures. In November 2007, she started a PhD project at Eindhoven University ofTechnology at Eindhoven, The Netherlands, with prof.dr. Dick Broer, prof.dr.Ing. CeesBastiaansen, and dr. Michael Debije, resulting in the publication of this dissertation.Since December 2011, she is employed at Philips Lighting BV as an optical designer.
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158 Curriculum Vitae
Publications and Patents
S. Tsoi, C. W. M. Bastiaansen, and M. G. Debije, and D. J. Broer, Aspherical-shapedlens arrays on structured organic solar concentrators, in preparation (2012).
P.P.C. Verbunt, S. Tsoi, D. J. Broer, C. W. M. Bastiaansen, and M. G. Debije, Organicwavelength selective mirrors for structured luminescent solar concentrators, Adv. Mater.in preparation (2012).
S. Tsoi, P.P.C. Verbunt, D. J. Broer, C. W. M. Bastiaansen, and M. G. Debije, Perfor-mance comparison of thin-�lm and dye-embedded luminescent solar concentrators, Sol.Energ. Mat. Sol. C. submitted (2011).
S. Tsoi, D. J. Broer, C. W. M. Bastiaansen, and M. G. Deije, Patterned dye structures limitreabsorption in luminescent solar concentrators, Opt. Express 18, A536-A543 (2010).
S. Tsoi, H. Plasschaert, C. W. M. Bastiaansen, and B. J. Lommerts, Photoreactor, PatentNo. EP2194117 A1 (2010).
M. G. Debije, C.W. M. Bastiaansen, S. Tsoi, and C. L. van Oosten, Luminescent Opticaldevice and solar cell system with such luminescent optical device, Patent No. WO/2011/012545A1 (2011).
Conference contributions
P.P.C. Verbunt, S.Tsoi, C. W. M. Bastiaansen, M. G. Debije, D.J. Broer,Building in-tegrated light harvesting polymer, oral presentation at 11th Dutch Polymer Days, TheNetherlands (2011).
S.Tsoi, C. W. M. Bastiaansen, M. G. Debije, and D. J. Broer, Limiting reabsorption in�uorescent waveguides via patterning and aligned dye, poster presentation at 23rd Inter-national Liquid Crystal Conference, Poland (2010).
S.Tsoi, C. W. M. Bastiaansen, M. G. Debije, and D. J. Broer, Using microlens system toimprove the e�ciency of luminescent solar concentrator, oral presentation at 10th DutchPolymer Days, The Netherlands (2010).
S.Tsoi, C. W. M. Bastiaansen, D. J. Broer, and M. G. Debije, Enhancing light output of
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160 publications
�uorescent waveguides with a microlens system, 24th European Photovoltaic Solar EnergyConference proceedings, 377-380 (2009).
Publications and Patents not related to this work
J. G. C. Veinot, D. Rollings, S. Tsoi, and J. C. Sit. Substrates coated with organosiloxanenano�beres, methods for their preparation, uses and reactions thereof, Patent No. US2008/0311337 A1, (2008).
S. Tsoi, B. Szeto, M. D. Fleischauer, J. G. C. Veinot, and M. J. Brett, Control of Alq3wetting layer thickness via substrate surface functionalization, Langmuir, 23, 6498-6500(2007).
S. Tsoi, E. Fok, J. C. Sit, and J. G. C. Veinot, �Surface functionalisation of porous nanos-tructured metal oxide thin �lms fabricated by glancing angle deposition�, Chem. Mater.,18, 5260-5266 (2006).
S. Tsoi, E. Fok, J. C. Sit, and J. G. C. Veinot, Super-hydrophobic, high surface area,3-D SiO2 nanostructures through siloxane-based surface functionalization, Langmuir, 20,10771-10774 (2004).
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