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ISBN 978-952-62-1442-9 (Paperback)ISBN 978-952-62-1443-6 (PDF)ISSN 0355-3213 (Print)ISSN 1796-2226 (Online)
U N I V E R S I TAT I S O U L U E N S I SACTAC
TECHNICA
U N I V E R S I TAT I S O U L U E N S I SACTAC
TECHNICA
OULU 2016
C 596
Bobins Augustine
EFFICIENCY AND STABILITY STUDIES FOR ORGANIC BULK HETEROJUNCTION SOLAR CELLS
UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF INFORMATION TECHNOLOGY AND ELECTRICAL ENGINEERING
C 596
ACTA
Bobins A
ugustine
C596etukansi.kesken.fm Page 1 Monday, October 31, 2016 1:08 PM
A C T A U N I V E R S I T A T I S O U L U E N S I SC Te c h n i c a 5 9 6
BOBINS AUGUSTINE
EFFICIENCY AND STABILITY STUDIES FOR ORGANIC BULK HETEROJUNCTION SOLAR CELLS
Academic dissertation to be presented, with the assent ofthe Doctoral Training Committee of Technology andNatural Sciences of the University of Oulu, for publicdefence in the Wetteri auditorium (IT115), Linnanmaa, on9 December 2016, at 12 noon
UNIVERSITY OF OULU, OULU 2016
Copyright © 2016Acta Univ. Oul. C 596, 2016
Supervised byProfessor Tapio FabritiusProfessor Risto Myllylä
Reviewed byProfessor Ellen MoonsProfessor Thomas Kirchartz
ISBN 978-952-62-1442-9 (Paperback)ISBN 978-952-62-1443-6 (PDF)
ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)
Cover DesignRaimo Ahonen
JUVENES PRINTTAMPERE 2016
OpponentProfessor Donald Way Lupo
Augustine, Bobins, Efficiency and stability studies for organic bulk heterojunctionsolar cells. University of Oulu Graduate School; University of Oulu, Faculty of Information Technologyand Electrical EngineeringActa Univ. Oul. C 596, 2016University of Oulu, P.O. Box 8000, FI-90014 University of Oulu, Finland
Abstract
The qualitative and quantitative characteristics of each component layer constituting the structureof organic bulk heterojunction solar cells (OSC-BHJ) contribute significantly towards its overallperformance. One of the prevalent issues resulting in reduced device efficiency is due to theconformational inhomogeneities in the active and buffer layers. The mechanical stress, extendedthermal exposure and presence of mutually reactive component layers etc., affects negatively onthe device stability. Effective methods to address these issues will be extensively benefited by theindustry since the current commercialisation of the technology is hindered owing to the lowerefficiency and stability of these devices.
This dissertation focuses on methods to coherently enhance the performance and longevity ofthe OSC-BHJ devices. The efficiency enhancements of the devices in this work were achievedthrough two main routes. The first route was through morphological improvement of the activelayer. The second route was through boosting the electrical characteristics of hole transportingconducting polymer layer (HTL) by controlled annealing conditions. The introduction of asuitable additive in the active layer was found to reduce unfavourable phase segregation thusresulting in enhanced morphology. Further, the annealing conditions in different atmospheres (air,nitrogen and vacuum) were found to have a clear influence on the optimum functioning of the HTLin the device. Regarding the stability improvement study done in this work, a method ofemploying suitable interlayer was developed to effectively abate the internal degradationoccurring in the device due to etching reaction on the indium tin oxide (ITO) anode by the HTL.Moreover, experimental investigations were carried out for drawing fundamental understandingof stability degenerating issues such as the influence of mechanical defects on transparentconducting metal oxide (ITO) anode on the performance of the device and heat induceddegradations in the low band gap polymer-fullerene active layer.
The highlight of this research is that the discovered methods are inexpensive, efficient, andeasy to adopt. The results of the study could help the technology to overcome some of itslimitations and accelerate its progress towards commercialisation.
Keywords: active layer, additive, annealing, cracks, excessive crystallisation, holetransporting layer, interlayer, power conversion efficiency, prolonged thermal exposure,transparent conducting metal oxide
Augustine, Bobins, Orgaanisten heteroliitosaurinkokennojen hyötysuhde- jastabiilisuustutkimuksia. Oulun yliopiston tutkijakoulu; Oulun yliopisto, Tieto- ja sähkötekniikan tiedekuntaActa Univ. Oul. C 596, 2016Oulun yliopisto, PL 8000, 90014 Oulun yliopisto
Tiivistelmä
Orgaanisten heteroliitosaurinkokennojen kerrosrakenteen ominaisuudet ja laatu vaikuttavat mer-kittävästi aurinkokennojen toiminnallisuuteen. Erityisesti rakenteelliset epähomogeenisuudetaktiivi- ja puskurikerroksissa heikentävät kennon hyötysuhdetta. Kennojen stabiilisuutta tarkas-teltaessa myös mekaanisella rasituksella, pitkittyneellä lämpöaltistuksella ja materiaalien rea-goinneilla keskenään kerrosten välillä, on selkeä negatiivinen vaikutus kennojen stabiilisuuteen.Orgaanisen aurinkokennoteknologian kaupallistamisen rajoitteina ovat kennojen heikko hyöty-suhde ja stabiilisuus, joten menetelmät jotka tarjoavat ratkaisuja edellä mainittuihin ongelmiin,ovat erittäin tärkeitä teknologiaa kaupallistavalle teollisuudelle.
Tämä väitöskirja keskittyy johdonmukaisesti selvittämään tapoja, joilla voidaan parantaaheteroliitosaurinkokennojen hyötysuhdetta ja elinikää. Hyötysuhteen tehostamiseksi valittiinkaksi eri lähestymistapaa, joista ensimmäisessä keskityttiin aktiivikerroksen morfologian paran-tamiseen ja toisessa aukkoja kuljettavan kerroksen sähköisten ominaisuuksien parantamiseenlämpökäsittelyprosessin avulla. Sopivan lisäaineen avulla aktiivikerroksen ei-toivottua kiteyty-mistä voidaan pienentää ja parantaa näin kerroksen morfologiaa. Lisäksi työssä todettiin, ettälämpökäsittelyn aikaisella ympäristöolosuhteella (ilma, typpi, tyhjiö) on merkittävä vaikutuspuskurikerroksen optimaaliseen toimintaan aurinkokennossa. Stabiilisuuden parantamiseksikehitettiin välikerroksen hyödyntämiseen perustuva menetelmä, jolla voidaan tehokkaasti vähen-tää kennojen sisäisessä rakenteessa tapahtuvaa toiminnallisuuden heikkenemistä, joka aiheutuuaukkoja kuljettavan kerroksen syövyttävästä vaikutuksesta indiumtinaoksidi (ITO) pohjaiseenanodiin. Tämän lisäksi työssä tutkittiin kokeellisesti stabiilisuuteen heikentävästi vaikuttaviatekijöitä, kuten mekaanisen rasituksen aiheuttamia vaurioita metallioksidi (ITO) anodissa ja läm-pöaltistuksesta aiheutuvia vikoja polymeeri-fullereeni rakenteeseen perustuvassa aktiivikerrok-sessa.
Tutkimuksen keskeisin tulos on, että esitellyt keinot aurinkokennojen hyötysuhteen ja stabii-lisuuden parantamiseen ovat edullisia, tehokkaita ja helppoja hyödyntää. Tulokset voivat merkit-tävästi edistää orgaanisten aurinkokennojen teknistä kehitystä ja kiihdyttää niiden tuloa kaupalli-siksi tuotteiksi.
Asiasanat: aktiivinen kerros, aukkoja kuljettava kerros, halkeama, hyötysuhde, liiallinenkiteytyminen, lisäaine, lämpökäsittely, läpinäkyvä metallioksidijohdin, pitkitettylämpöaltistus, välikerros
Dedicated to my lovely parents, elder brother, niece and sister in law
8
9
Acknowledgements
The work presented in this thesis was carried out in the Optoelectronics and
Measurement Techniques (OPEM) unit in the University of Oulu, during the period
of autumn 2011 – 2015.
I would like to express my sincere gratitude to my supervisor Prof. Tapio
Fabritius and co-supervisor Emeritus Prof. Risto Myllylä for their good guidance,
motivation and support. I especially thank my main supervisor Prof. Fabritius for
helping me and encouraging me to research independently and develop problem
solving skills. I highly appreciate the head of the OPEM, Prof. Igor Meglinski for
providing the necessary support in finalising my doctoral studies.
I express my acknowledgements to Prof. Ellen Moons (Karlstad University) and
Prof. Thomas Kirchartz (Forschungszentrum Jülich, Universität Duisburg-Essen)
for the pre-examination of this Thesis. I would also like to thank Dr. Keith A. Emery
(NREL) and Dr. Henry Tan (University of Aberdeen) for answering my research
questions through mails.
I like to thank my friends and colleagues who helped me to have a sound
working environment and also helped me to integrate easily in to the Finnish way
of life despite the cultural differences. I thank my co-authors Dr. Rafal Sliz, Dr.
Kimmo Leppänen, Prof. Mika Valden and Dr. Kimmo Lahtonen, for collaborating
with me and it was my pleasure vice versa. I express my thanks to my follow up
group members Docent Matti Kinnunen, Dr. Juha Saarela and Dr. Alexander Bykov,
for their support and helpful discussions. I highly appreciate the kind help from Dr.
Janne Lauri for the Finnish translation of the abstract.
I would like to thank the Tekes projects such as Autosys, Painettavan Paalutus
I and Hilla Diamond for providing the funding for my research. I also thank
Hindawi Publishing Corporation (I & II), Elsevier (III & IV) and EU PVSEC (V)
for granting the copyright permissions.
Finally, I express my thanks to God almighty and my family. My special
acknowledgment goes to my elder brother Prof. Robin Augustine who has been my
role model and motivator.
Oulu, January 2016 Bobins Augustine
10
11
List of terms, symbols and abbreviations
AFM Atomic Force microscopy
AM- 1.5G Air Mass 1.5 Global, reference solar spectrum
Al Aluminium
Ag Silver
AgOx Silver Oxide
BHJ Bulk heterojunction
CO2 Carbon dioxide
CAE Constant Analyser Energy
DCB 1,2-Dichlorobenzene
DIO 1,8- Diiodooctane
ETL Electron transport layer
ESCA Electron Spectroscopy for Chemical Analysis
eV Electron Volt
e Elementary charge
FF Fill Factor
HTL Hole transport layer
H2O Water
HOMO Highest occupied molecular orbital
ISOS International Summit on Organic photovoltaic stability
ITO Indium Tin Oxide
IPA Isopropyl alcohol
In 3d 3d orbital of the indium atom
I0 the intensity of light entering the sample at a particular wavelength
I the intensity of light exiting the sample at a particular wavelength
J Current density
JD Current density through diode
J0 Reverse saturation current density through diode
JPh Current density generated during illumination
JSC Short circuit current density
Jmax Maximum current density
KB Boltzmann’s constant
Kα Emission line resulting from electron transition from innermost K
shell (principal quantum number 1) to L shell (with principal
quantum number 2)
12
LiF Lithium Fluoride
LUMO Lowest unoccupied molecular orbital
Mg Magnesium
NREL National Renewable Energy Laboratory
N2 Nitrogen gas
n Ideality factor
OSC Organic solar cells
OLED Organic light emitting diode
OM Optical microscope
O2 Dioxygen
PV Photovoltaic
pH Measure of acidity or basity
P3HT Poly(3-hexylthiophene-2,5-diyl)
PC60BM [6,6]-Phenyl-C61-butyric acid methyl ester
PC70BM [6,6]-Phenyl-C71-butyric acid methyl ester
PEDOT:PSS Poly(3,4-thylenedioxythiophene)poly(styrenesulfonate)
PH500 Poly(3,4-thylenedioxythiophene)poly(styrenesulfonate) product
containing PEDOT to PSS ratio of 1:2.5 by weight.
PVP Al4083 Poly(3,4-thylenedioxythiophene)poly(styrenesulfonate) product
containing PEDOT to PSS ratio of 1:6 by weight.
PET Polyethylene terephthalate
PTB7 Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5- b′]dithiophene-
2,6-diyl)(3-fluoro-2[(2ethylhexyl)carbonyl]thieno[3,4-
b]thiophenediyl]]
PCE Power conversion efficiency
Pin Incident irradiance light power per unit area
Pmax Maximum power density
QE Quantum efficiency
IQE Internal quantum efficiency
EQE External quantum efficiency
RPM Rotations per minute
RS Series resistance
RSH Shunt resistance
S 2p 2 p orbital of the sulfur atom
T Temperature
UV Ultraviolet
UHV Ultra-high vacuum
13
V Voltage
VOC Open circuit voltage
Vmax Maximum voltage
vol% Percentage amount of additive present in total volume of the active
layer blend
XPS X-ray photoelectron spectroscopy
nm Nanometre
ºC Degree Celsius
Ω/ Ohm per square, Unit of sheet resistance
µm Micrometre
$/W Dollar/Watt
mm Millimetre
mA/cm2 Milliampere per square centimetre
mW/cm2 Milliwatt per square centimetre
mg/ml Milligram per millilitre
γ Mean free path of electrons
mN/m Millinewton per metre
θ Contact angle Theta
Interfacial tension between the solid and the vapour
Interfacial tension between the solid and the liquid
Interfacial tension between the liquid and the vapour
14
15
List of original articles
The thesis is based on the following articles (I-V). Copyrights for publications are
obtained from the respective journals.
I Augustine B & Fabritius T (2015) Synergetic enhancement of device efficiency in poly(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl C61 butyric acid methyl ester bulk heterojunction solar cells by glycerol addition in the active layer. International Journal of Photoenergy Article ID 414851, DOI: http://dx.doi.org/10.1155/2015/414851.
II Augustine B & Fabritius T (2015) How to control component ratio of conducting polymer blend for organic photovoltaic devices by annealing. International Journal of Photoenergy Article ID 532489, DOI: http://dx.doi.org/10.1155/2015/532489.
III Augustine B, Sliz R, Lahtonen K, Valden M, Myllylä R & Fabritius T (2014) Effect of plasma treated Ag/indium tin oxide modification on stability of polymer solar cells. Solar energy materials & Solar Cells 128: 330-334.
IV Leppänen K, Augustine B, Saarela J, Myllylä R & Fabritius T (2013) Breaking mechanism of indium tin oxide and its effect on organic photovoltaic cells. Solar energy materials & Solar Cells 117: 512-518.
V Augustine B & Fabritius T (2015) Performance of 1,8-diiodooctane (DIO) doped PTB7:PCBM based organic solar cell under simulated solar heating profile over 24 hours. Proceedings of 31st European photovoltaic solar energy conference and exhibition, 14-18 September, Hamburg, Germany: 1111-1113, DOI:10.4229/ EUPVSEC20152015-3BV.5.11.
Author’s contributions
I The author defined the research plan, carried out the experiments, fabricated
the devices and measured them. The AFM studies were done by Jarkko
Puustinen. The author did the literature review, analysed the results and wrote
the manuscript.
II The author defined the research plan and conducted the experiments. The XPS
analysis of the PEDOT:PSS layers were done by Santtu Heinilehto. The author
did the literature review, analysed the results and wrote the final manuscript.
III The author defined the research plan, fabricated the device and did the
photovoltaic measurements. The contact angle measurement was done by Rafal
Sliz. The XPS study of Indium diffusion was conducted by Kimmo Lahtonen
and Mika Valden. The author did the literature review, analysed the results and
wrote the final manuscript.
IV The author contributed in the article as the second author. The author´s
independent contribution was in the fabrication and measurement of OSC
16
devices made on top of mechanically treated ITO PET substrate produced by
the first author. Thus this thesis focuses only on the work which author of this
thesis has contributed.
V The author defined the research plan, fabricated the OSC devices and
conducted the experiment. The author did the literature survey, analysed the
results obtained from the experiments and wrote the final manuscript.
17
Contents
Abstract
Tiivstelmä
Acknowledgements 9
List of terms, symbols and abbreviations 11
List of original articles 15
Contents 17
1 Introduction 19
1.1 Solar cell classifications .......................................................................... 20
1.2 Organic solar cells (OSC) and working principle ................................... 21
1.3 Main junction types ................................................................................. 23
1.3.1 Planar heterojunction .................................................................... 23
1.3.2 Bulk heterojunction (BHJ) ........................................................... 24
1.4 Solar cell characteristics .......................................................................... 26
1.5 Life time .................................................................................................. 28
1.6 OSC - current status, challenges and prospect ........................................ 32
2 Aim and structure of study 37
3 Materials and methods 39
3.1 OSC device fabrication and experimental methods for (I – V)
articles ..................................................................................................... 39
3.2 Characterisation methods ........................................................................ 42
3.2.1 Optical microscopy (OM) ............................................................. 42
3.2.2 Atomic force microscopy measurements (AFM) ......................... 43
3.2.3 X-ray photoelectron spectroscopy measurements (XPS) ............. 44
3.2.4 Contact angle measurement .......................................................... 45
3.2.5 Transmittance measurement ......................................................... 46
3.2.6 Photovoltaic characteristics measurement .................................... 46
4 Results 49
4.1 Efficiency studies .................................................................................... 49
4.1.1 Performance of P3HT:PC60BM devices ....................................... 49
4.1.2 Effect of annealing on P3HT:PC60BM ......................................... 50
4.1.3 Effect of glycerol additive ............................................................ 50
4.1.4 Controlling component ratio of conducting polymer blend
by annealing ................................................................................. 53
4.2 Stability studies ....................................................................................... 56
18
4.2.1 Effect of acidic nature of PEDOT:PSS on ITO anode and
performance of OSC device ......................................................... 57
4.2.2 Method to counter ITO degradation by PEDOT:PSS ................... 58
4.2.3 Effect of cracked ITO anode on the device performance ............. 60
4.2.4 Effect of prolonged thermal exposure over 1,8-
diiodooctane (DIO) doped PTB7:PC70BM based organic
solar cells ...................................................................................... 61
5 Discussion 63
5.1 An inexpensive route for performance improvements ............................ 63
5.2 Stability enhancement and experimental analysis ................................... 65
5.3 Future outlook ......................................................................................... 68
6 Summary 71
References 73
Original articles 85
19
1 Introduction
Early humans started to harness energy in the form of fire for their various needs
such as light, heat, cooking and safety at least 1.9 million years ago [1]. The current
world of the 21st century depends on fossil fuels on a large scale for meeting its
various energy demands. Since electricity is a flexible and convenient form of
energy in modern society, a portion of energy obtained from fossil fuels is also
converted to it. The use of non-renewable sources of energy such as petroleum, coal
and wood would emit considerable levels of pollutants in to the atmosphere [2].
The greenhouse gas like CO2 and many other industrial pollutants are causing
global warming, environmental and ecological problems. The CO2 concentration in
the atmosphere has risen from 280 ppm since the industrial revolution and
surpassed 400 ppm level in March 2015 [3]. It is expected that the levels will be
further elevated and reach around 570 ppm at the culmination of this century [4].
Thus there is a huge demand for finding clean and renewable energy sources. The
renewable energy sources include hydroelectricity, wind energy, biomass,
geothermal and solar radiation. Among the renewable energy sources, solar energy
is the most abundantly available form of energy on earth. It has been estimated [5]
that the solar radiation received by the earth’s surface in 90 minutes is enough to
feed the global energy requirement for an entire year, and this signifies the huge
potential of solar energy. The use of photovoltaic devices or solar cells is the easiest
way to convert solar radiation directly into electricity and that is why remarkable
efforts have been invested in the development of photovoltaic technology during
past decades. The field of photovoltaics is currently dominated by inorganic and
silicon based solar cells owing to its appreciable efficiency and stability.
Although solar cells have significant potential in providing clean energy, the
biggest challenge is that traditional solar cells are still not competitive enough with
fossil fuel based energy technologies in terms of cost per power generated.
Renewable energy sources such as wind and solar are currently more expensive
than non-renewables because they need intensive capital and costlier storage grids.
Photovoltaic technologies need improvements for achieving higher efficiency, low
cost, larger scale production and better storage of energy produced to realise the
technology as a real solution for the world´s clean energy demand. In a recent report
[6], UK climate advisors estimate that wind and solar energy prices would likely
match fossil fuel based energy in 2020, if carbon prices are imposed. Although the
price of oil is now down due to uncertainty in the global economy, the costs of non-
renewable sources of energy are expected to rise further in the future owing to its
20
limited reserves on the earth, thus making the clean technology like photovoltaics
more competitive in terms of cost. If we consider the scenario in 1977, the price of
energy production from crystalline silicon solar panels was around 76 US $/W, and
the price has plummeted over the years and reached around 0.3 US $/W in 2015
[7]. The photovoltaic technology which can ensure cheaper, efficient and more
mobile energy production has significant potential to impact positively on the
commercial market as well as the environment.
1.1 Solar cell classifications
Nowadays, there are many solar cell technologies existing with different degrees
of development. The first photovoltaic effect was observed by Alexandre-Edmond
Becquerel in an electrolyte solution in the year 1839 [8], and the initial
development of modern day solid-state solar cells was initiated by the research at
Bell Labs in 1954 [9]. Present solar cell technologies can be classified as first,
second and third generation cells.
The first generation of solar cells include the predominant silicon solar cells.
These types of solar cells include monocrystalline and polycrystalline silicon solar
cells [10]. These devices can deliver efficiencies of up to 20% and last more than
20 years. The technology is very mature because of its development during the past
50 years. However, the cost is relatively high, owing to the need for high purity
silicon and rigid structures.
The Second generation solar cells are also called thin film solar cells, because
these are made from layers of semiconductor materials, only a few micrometres
thick. This class of solar cells includes copper indium gallium selenide (CIGS) [11-
14], cadmium telluride (CdTe) [15-22], amorphous silicon [23-26] and gallium
arsenide (GaAs) [27-31] based solar cells. Second generation solar cells have lower
associated costs compared to first generation solar cells. However, some of the
materials used in the second generation devices such as cadmium are toxic, and the
technology predominantly depends on scarce elements causing some limitations.
The devices are flexible to some degree, which is an advantage but production costs
are still high due to the need for vacuum processing and very high temperature
curing.
Third generation solar cells are referred to as emerging solar cell technology,
since it does not yet have a realised, large scale commercial application. This
includes quantum dots [32-35], dye sensitised solar cells (DSSC) [36], perovskite
solar cells [37-47], organic solar cells (OSC) [48-62] and hybrid solar cells [63-74].
21
In third generation solar cells, higher efficiencies can also be achieved through
multi-junction architecture, which is the stacking of thin layers of material with
varying bandgaps on top of each other. It was reported recently that a multi-junction
solar cell based on GaAs achieved efficiency beyond 44% [75]. The perovskite
devices recently achieved efficiencies of around 21%, which is closer to silicon
solar cells [76]. The flexibility, ease of transport, freedom in material design and
fabrication makes OSC devices attractive to the electronic market [77-80]. This
thesis focuses on issues relating to organic solar cells, specifically organic bulk
heterojunction OSC devices.
1.2 Organic solar cells (OSC) and working principle
The term “organic solar cells” refers to the device in which an organic layer based
on conjugated polymers or small molecules is an essential part of the photovoltaic
process for generating electricity. The OSCs may also be referred as the organic
photovoltaic devices (OPV) or polymer solar cells. The typical active layer of the
OSC device consists of two components, one which donates electron (donor) and
one which accepts electron (acceptor). These are then brought in to intimate contact
to form a donor-acceptor heterostructure. In OSC devices, a combined electron-
hole pair called an exciton which is bound by coulombic force is created in the
donor material when the incident photons are absorbed by the active layer.
The energy state diagram of the heterojunction OSC device is depicted in Fig.
1(a). The basic charge generation and its transfer process during illumination in
OSC are depicted in Fig. 1(b).
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Fig. 1. a) Energy state diagram in heterojunction OSC ; WA- work function of the anode,
WC - work function of the cathode, ∆EA- difference in electron affinity between donor
and acceptor, ∆Ip- difference in ionization potential of donor and acceptor. b) charge
transfer process during illumination.
The electron affinity (EA) and the ionization potential (Ip) are the most important
factors while choosing the donor and acceptor materials. The ionization potential
and electron affinity of the donor or acceptor can be approximated as their highest
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) level energy respectively. The ionization potential of the donor should be
lower than that of the acceptor, because the donor is the species that most easily
gives away an electron. Conversely, the electron affinity of the acceptor should be
larger than the electron affinity of the donor. When the photons are incident on the
photoactive layer, they are absorbed and a photovoltaic effect is built in a sequence
of processes as in Fig.1 (b) : (1) excitation of the donor upon absorption of light
and formation of exciton, (2) Diffusion of the generated exciton to the
donor/acceptor interface, (3) If the off-sets of the energy levels of the donor of the
acceptor materials are higher than the exciton binding energy, photo generated
excitons in the donor side will dissociate by transferring electrons to the LUMO
levels of the acceptor, while those created at the donor side will transfer the holes
to the HOMO of the donor, (4) The separated electrons and holes are collected at
the cathode and anode electrodes respectively. These steps constitute the basic
working principle of the device.
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1.3 Main junction types
The donor and acceptor materials can be deposited as two distinct layers (planar
heterojunction) or blended in a homogeneous mixture throughout an
interpenetrating network known as bulk heterojunction (BHJ) which presents a
larger interfacial area. With the aim of covering the solar spectrum more effectively,
another useful approach consists of stacking multiple photoactive layers with
complementary absorption spectra in series to reach a tandem device. This thesis
focuses mainly on the conventional BHJ-OSC device architecture.
1.3.1 Planar heterojunction
The first reported organic solar cell was based on a single layer of poly(acetylene),
which demonstrated very low device efficiency [81]. Later, the modern solid-state
solar cell based on organic materials with bi- active layer was developed by C. W.
Tang [82]. In his work, the device was made by stacking the donor and acceptor.
This type structure is referred to as planar heterojunction or bi-layer. The device
studied by Tang had (3,4,9,10)-perylenetetracarboxylic-bis-benzimidazole (PTCBI)
as the donor material and copper phthalocyanine (CuPc) as the acceptor material.
The structure is depicted in Fig. 2.
Fig. 2. Planar Heterojunction.
Later, fullerene was introduced as the acceptor, which further increased efficiencies
[83, 84]. Although a thick layer of the organic photoactive materials may be
providing for higher absorption, in the planar heterojunction, the excitons will be
24
recombined excessively because of the short exciton diffusion length. The exciton
diffusion length is the average distance that an exciton travels before it recombines.
Thus it restricts the thickness of the active layers and should be thin enough within
the exciton diffusion length for efficient charge transfer.
1.3.2 Bulk heterojunction (BHJ)
The major limitation of the planar heterojunction OSC due to the short exciton
diffusion length can be solved by blending donor and acceptor materials together
and thus minimising the travelling distance to donor-acceptor interface. Since the
heterojunction is dispersed in the bulk of the layer by blending donor and acceptor
materials, it is called bulk heterojunction. Bulk heterojunctions solar cells (BHJ)
may consist of a large variety of materials. Generally, the mixtures of conjugated
donor polymer with a fullerene derivative have been used as the active layer [85-
101]. The first bulk-heterojunction structure was demonstrated by Friend [102] and
Heeger [103] almost simultaneously in 1995 and the devices were showing superior
performance than earlier planar heterojunction architectures. The typical
conventional bulk heterojunction OSC is depicted in Fig. 3.
Fig. 3. Conventional BHJ-OSC.
In the conventional structure, the bottom anode must be a transparent conducting
electrode which is generally indium tin oxide (ITO). The substrate can be either
glass, polyethylene terephthalate (PET) or even transparent paper based on
cellulose fibers. Normally, ITO is coated with a hole transporting layer (HTL) layer
of poly(3,4-thylenedioxythiophene)-poly(styrenesulfonate) for effective transfer of
25
holes to the anode. A low work function electrode like Aluminum is deposited as
cathode. A thin layer of lithium fluoride (LiF) is used as intermediate layer between
the active layer and cathode to improve device performance by functioning as an
electron transport layer (ETL). The BHJ architecture is widely opted for in OSC
devices, since the advantage of having larger interface between the donor and
acceptor materials and hence enhancing the amount of exciton dissociation at the
interface and the collection of charges at their respective electrodes. The exciton
diffusion lengths of various conjugated polymers in OSC are around 10 nm. The
non-radiative recombination occurring when the photo generated excitons are
unable to reach the donor-acceptor interface will significantly reduce the device
efficiency [104,105].
The performance of BHJ OSC devices significantly depends on the
morphology of the active layer, since it facilitates efficient percolation pathways
for the generated holes and electrons to be transported through the active layer to
their respective electrodes. As a result of the intimate mixing, the interface
increases enormously where the charge transfer can occur. The exciton, created
after the absorption of light, has to diffuse towards this charge-transfer interface for
charge generation to occur. For efficient charge generation after absorption of light,
each exciton has to find a donor acceptor interface within a few nanometres,
otherwise it will be lost without the charge generation. An intimate bicontinuous
network of donor- acceptor materials in the nanometres range should suppress
exciton loss prior to charge generation. Control of morphology is essential for not
only the creation of large charge generating interfaces, but also in the creation of
efficient percolation pathways for the transfer of electrons and holes. The holes
travel to the higher work function electrode and the electrons travel to the lower
work function electrode. Annealing can enhance the performance in some polymer-
fullerene active layers with the improvement in crystallinity, phase segregation and
donor-acceptor network etc., which also helps in the efficient transport of charge
carriers [106]. The BHJ structure also facilitates the solution processing of the
organic layers. This is helpful especially in large-scale, high-speed roll to roll
manufacturing process. The solution processing of both acceptor and donor layers
for a planar heterojunction is challenging because the deposition of the second layer
should not dissolve and remove the first layers beneath. Utilisation of bulk
heterojunctions device structure simplifies solution processing because only single
layer of donor and acceptor blend is needed to deposit as the active layer. The main
challenge would be the proper percolation pathway creation for the holes and
electrons. The solution processability of OSC is a great advantage in industrial
26
point of view, as these devices can be fabricated by a number of high throughput
printing methods [107-111]. Process condition optimisation of each layer in the
OSC is very essential since it affects crucially to the final performance of the device.
1.4 Solar cell characteristics
The electrical characterisation such as the current density vs voltage (J-V)
characteristics of the inorganic and organic solar cells under dark condition
resembles exponential response of the diode in forward bias. The illumination of
the device generates the current in the cell in addition to the diode behaviour.
Therefore, the solar cell can be considered as a current generator in parallel with a
diode, as shown in Fig. 4. The Jph is the current density generated during
illumination.
Fig. 4. Solar cell equivalent circuit.
The contributions from the series resistance (RS) and shunt resistances (RSH) in the
devices are also taken in to account in the model. The current density through the
diode is given by the Shockley equation (1),
exp V /nk T 1 (1)
Where JD is the current density through the diode, VD is the voltage across the diode,
Jo is the reverse saturation current density of the diode, e is the elementary charge,
n is the ideality factor, kB is the Boltzmann constant, and T is the temperature. The
output current density (J) from the solar cell under illumination as a function of the
voltage applied (V) is described in equation (2),
27
∙ exp
1
(2)
The performance of the solar cell is analysed from the measured J-V characteristic
curve. The Fig. 5 depicts the J-V plot for an idealised solar cell under illumination.
The most important performance parameters that can be obtained from the J-V
curve measured for the device under illumination are open-circuit voltage (VOC),
short-circuit current density (JSC), fill factor (FF), and power conversion efficiency
(PCE).
Fig. 5. Typical J-V curve of solar cell under illumination.
The VOC is the voltage across the solar cell when J = 0 or when the device being
open-circuited. The open-circuit voltage can be considered as the point at which
the photocurrent generation and dark current process compensate each other. The
short-circuit current density JSC is the current density when V = 0, or when the
electrodes are short circuited. The VOC and JSC mark the boundaries of power
production in a solar cell, the maximum power density Pmax occurs at the voltage
Vmax and current density Jmax where the product of J and V is at the maximum. Due
to the factors such as diode behaviour, additional resistance and recombination
losses, the values of Jmax and Vmax in practice are always less than JSC and VOC. The
Fill Factor (FF) is the parameter that determines maximum power output from the
device and is defined as in equation (3),
28
(3)
Graphically FF can also be described as the degree of squareness of the J-V curve.
The power conversion efficiency (PCE) is defined as the percentage of incident
irradiance light power per unit area (Pin) that is converted to output power.
The PCE is represented as in equation (4),
PCE
(4)
The PCE determines how effectively the active area of the solar cell is utilised in
producing electricity. PCE is very dependent on the power and spectrum of the light
source, since all the solar cells do not convert all the incident radiation which has
different wavelengths.
Solar panels do not generally operate under exactly one atmosphere's thickness
since the sun is at an angle to the Earth's surface and thus the solar irradiation is
influenced by the varied effective thickness. An Air Mass (AM) number is
representing the spectrum at mid-latitudes. AM 1.5 G corresponds to a solar zenith
angle of z = 48.2° and it is used as the standard spectrum which matches the
spectrum of solar irradiation on earth’s surface.
The quantity that defines the quality of energy conversion in the device is
called quantum efficiency (QE). It is basically the number of charge carriers
collected at the electrode per number of incident photons on the area of the solar
cell at a given wavelength. It can be defined in two ways, as external and internal
quantum efficiencies.
The external quantum efficiency (EQE) is the fraction of incident photons
converted in to current by the device. It can be defined as the ratio of the number
of electrons generated by the device to the incident number of photons at the given
wavelength.
The internal quantum efficiency (IQE) is the ratio between the number of
charge carriers collected and the number of all photons absorbed by the active layer
at a given wavelength. It does not take into account the photons that transmit
through or reflect from the cell or absorbed by other layers since it does not take
part in the process of photo conversion.
1.5 Life time
The stability of OSC is an important factor which must be controlled to some extent
in order to make OSC technology commercially successful. The quality of a device
29
or material to retain a major portion of its original state of performance over a
prolonged time period is referred to as stability. Currently, the OSC stability is
relatively low, lasting only few years whereas the inorganic silicon solar cells have
a lifetime exceeding 20 years [112].
In order to improve lifetime, it is necessary to investigate the factors that are
causing the degradation OSC devices. Life time tests carried out by various
research groups [113-115] confirm the fact that the degradation in OSC does not
take place as the result of a single cause. There might be many parallel degrading
mechanisms taking place and the dominance of each degrading factor may differ
according to the device structure, used materials and surrounding conditions.
Among the component layers in OSC, some layers and layer interfaces are more
prone to degradation than the other. In case of common OSC device structures, the
interface between the PEDOT:PSS and ITO is problematic due to the acidic nature
of PEDOT:PSS which etches the ITO anode. The PEDOT:PSS and active layers
are sensitive to heat, light, oxygen and moisture. To understand the trend of decay
in performance in OSC devices, the best way is to plot the variation of its
photovoltaic parameters such as VOC, FF, JSC and PCE with aging time. The crucial
parameters in lifetime measurement are JSC and PCE, since it directly reflects the
ability of the device to convert the incident light to the electricity. Degeneration of
the devices occurs even in dark conditions, but it decays faster when the device is
illuminated. The speed of degradation even varies with the intensity of the light. It
is clear that the lifetime of OSC can be improved with suitable shielding. For
instance, the LiF layer in OSC structure acts as such a shielding layer preventing
the diffusion of cathode electrode in to the polymer layer [116].
The main degradation of OSCs can be divided into intrinsic as well as extrinsic
degradations. The intrinsic factors include the degradation occurring inside the
device such as the degradation of the active layer, instability between component
layers etc. The donor polymer and the acceptor material present inside the active
layer can also degrade upon prolonged illumination [113,115,117-120]. The
extrinsic factors include instances such as the degradation of the metal cathode, due
to delamination, oxygen and moisture ingression. The electrode–organic interface
is the spot where the major degradation dominantly occur [121].
In simple lifetime measurement, the parameters JSC, VOC, FF and PCE are
measured as the function of time. The JSC is the crucial factor that gets degrades
over the period of time and its rate of degradation depends on the type of solar cell.
The VOC also degrades with time, but it is slower in most cases. The FF is also a
measure of the internal losses influencing on the charges generated in the active
30
layer. The sensitivity to degradation may vary according to the active material and
the structure of the device. The typical reasons for degradations in OSC devices are
due to mechanical stress, temperature, oxygen, moisture and UV radiation. The
performance variation of the OSC devices varies according to the testing conditions.
In order to make traceable lifetime testing, ISOS standards have been published
since 2008 [121]. The standards are defined for basic, intermediate and advanced
levels. The specifications also include if the devices are encapsulated, measured
outdoor or indoor and surrounding conditions. The standards for stability studies
are defined as ISOS D, ISOS L and ISOS T. ISOS D corresponds to the shelf life
time study in the ambient conditions. ISOS L corresponds to the life time testing
under laboratory weathering. ISOS T corresponds to the thermal cycling testing for
life time. In the advanced level of thermal cycling test, the samples are stored in a
thermal chamber and cycles of temperature ranging from – 40 °C to +85 °C are
applied whereas for lower levels of ISOS T, the range is between room temperature
and 65/85 °C. Those laboratories which are not able to fulfil these standards are
recommended to report the measuring conditions together with the life time data.
The ISOS protocols can be adopted for evaluating the effect of air, moisture, light
and temperature on OSC device performance.
The typically observed decay pattern for OSC is a “burn in” period
characterised by an exponential loss in efficiency followed by a linear decay period.
The decay pattern followed by most of the OSC devices as mentioned by ISOS
[121] is depicted in Fig. 6.
31
Fig. 6. Typical decay curve of OSC device.
The parameters in Fig. 6 correspond as follows,
E0- Initial PCE, T0- Time when t = 0,
E80- PCE after E0 has decayed by 20%,
T80- Time when the PCE has decayed 20% of E0,
ES- PCE at the point when there is change in the slope of the curve,
TS- Time at ES,
ES80 - PCE after ES has decayed 20%, TS80- Time at ES80
In the case of extensive analysis of the decay mechanism, ISOS recommends
measuring these above mentioned parameters. However, there is no straight
forward method for choosing TS and ES points and thus often marked arbitrarily. It
can be difficult in some cases to identify these points since the decay curve can
vary depending on the type of measurement and the type of degradation under study.
The device undergoing complete degradation will give significant information
regarding the behaviour of the materials as well as the stability.
32
An overview of the degradation pattern can be analysed by allowing the device
to age in the specific condition over a considerable span of time and comparing the
trend of change in the photovoltaic parameters over the course till the final
measurement. The internal degradation can occur simultaneously inside the device
in parallel with other degradations occurring due to the influence of light,
temperature, air and moisture etc., [113, 114]. By excluding some of the agents of
degradation, certain other degradation factors can be specifically studied.
The internal degradation process like chemical reaction between the ITO and
PEDOT:PSS starts already during the so called ‘burn in period’, so in our study the
initial PCE to the final PCE value over the appreciable amount of time was
important. The ITO is sensitive to acidic materials, releasing indium atoms causing
its diffusion in to subsequent layers [122]. The most widely used hole transporting
layer PEDOT:PSS has hygroscopic and acidic nature [123] and thus it can etch the
bottom ITO anode. Thus analysing the degradation pattern over a broad time period
accounting the initial and final device values helps to explain better the stability
state of the device.
There are still quite few stability studies conducted in the field of OSC when
compared with efficiency enhancement research carried out in the same technology.
Thus more research efforts on enhancing the device stability are needed to make
the OSC technology more competitive among other renewable technologies.
1.6 OSC - current status, challenges and prospect
OSC was hailed since a decade owing to its various merits [124]. The OSC
technology has a major strength due to the fact that the photoactive materials of the
device can be synthesised and tailored in number of ways to match the performance
needs. The organic photovoltaic devices have also the advantage of having wide
range of colours which is increasing its aesthetic values in products. OSC devices
can be made even semi-transparent or even close to fully transparent. The time line
of progress in OSC technology among various PV technologies according to NREL
is depicted in Fig. 7.
33
Fig. 7. Certified best power conversion efficiencies over time for a variety of
photovoltaic technologies. This plot is courtesy of the National Renewable Energy
Laboratory, Golden, CO [125].
The highest reported efficiency by NREL for OSC devices recently was 11.5%,
achieved by researchers in Hong Kong UST [126]. Although not confirmed by
NREL, the Heliatek reported 12% efficiency for OSC devices couple of years ago.
Thus the state of art OSC devices has the capability to deliver efficiencies in the
range of 10 to 12%. The successful preliminary stage implementations of OSC
devices are achieved by leading PV companies like BELECTRIC, Nano Flex and
Heliatek. However, the price of silicon and other thin film based photovoltaics had
plummeted currently which makes them more competitive with OSC technology.
Although such situations arise at the moment, the OSC devices still has a place in
the market and a role to play in building integrated photovoltaics and flexible
electronics. The OSC technology is currently facing some major challenges for its
large-scale adoption and these issues are discussed as below.
34
1. Manufacturing costs: The major cost contributor in traditional OSC is the use
of ITO and its patterning on the substrate. The maximum percentage price
contribution for this can account to almost 51.2% of total material cost for the
OSC module, having an area of 1 m2 [127]. The economical alternative for ITO
can cause significant reduction in the manufacturing cost. Fullerenes based
acceptor materials are widely used till date despite their relatively high costs.
An inexpensive alternative for fullerene would greatly reduce the overall cost
of the photovoltaic devices as well. The primary advantage of OSC technology
over inorganic counterparts is its ability to facilitate high throughput
production techniques which helps in cost reduction. The substrate and
electrode reuse can also account to cost reduction. The other cost is related to
the expense regarding the encapsulation of the OSC devices. In some cases,
there might be better encapsulation which is expensive, but the incorporation
of such barrier layers would raise production costs. Thus, techniques are
necessary to develop inexpensive and effective protective barriers for organic
PV technology.
2. Initial PCE/ Efficiency: The current efficiency ranges are not competitive in
the commercial market. The OSC devices that are manufactured in high
throughput techniques still holds lower performances. The efficiency of these
devices can be increased to a certain extent by sandwiching active layers of
complementary absorption ranges in the device forming a tandem device. The
morphology of the active layer is a critical factor to be optimised for enhancing
the BHJ device performance [128-132]. For instance, large domains in the
phase-separated structure will prevent efficient charge separation, whereas
small domains will result in a poorly percolated domain structure and thereby
increase the possibility of charge carrier recombination [132]. Recently, the
Israeli research group led by Prof. Nir Tessler reported [133] that they have
developed a patented technique for improving the efficiency of converting
solar energy into electric current inside the cell from 10% to 15%, and adding
0.2 volts to the cell's voltage. The development is based on increasing the
energy gap between the electrodes by changing their fixed position in the
system. By doing so, the researchers were able to increase the voltage, leading
to an increase in system power. There are also some design rules for active
layer identified in view of enhancing the efficiency. It was understood based
on previous research [134] that the gap between HOMO of the electron
donating polymer and the LUMO of the electron acceptor should be maximised
35
in order to increase the VOC, at the same time the band gap of the polymer
should be minimised to increase photon absorption and thus short circuit
current. The LUMO of the donor polymer should be positioned above the
LUMO of the acceptor fullerene derivative at least 0.2 to 0.3 eV, to ensure
efficient electron transfer [134].
3. Life time/ stability: The organic materials used in the devices are prone to
degradation from moisture, air, light and heat, and degradation from the
organic and metallic layer interfaces inside the device. Polymer materials have
dynamic character, and are prone to degrade by a range of factors. When the
organic active layer is illuminated, the materials react through photolytic and
photochemical processes [135-138]. The materials are also not heat stable and
can suffer from heat and solar radiation-induced morphology changes or
interfacial degradation over their lifetime. The bulk heterojunction with a
specific morphology of interconnected domains do not necessarily represent
the most thermodynamically stable configuration and the alteration in optimal
morphology under prolonged thermal exposure has been observed [139-142].
Polymer solar cells can also sustain damage to the top electrode, often made
from a low work-function metal that is reactive and easily oxidised in ambient
air [143, 144]. The device operation could be prolonged to few years with
suitable encapsulation [145]. Other predominant degradation happens at the
organic- metallic interface of the devices and the electrodes. The presence of
hot spots and considerable increase in overall temperature of the device under
prolonged illumination is a common problem in solar cells. OSC devices are
sensitive towards the temperature changes due to the fact that organic materials
can degrade and even change morphology under prolonged thermal exposure
and illumination. Energy of UV photons is in the order of bonding energies of
some organic materials present in the active layer of OSC devices and result in
accelerated degradation [138]. Thus, it is necessary to have modified
encapsulation techniques which provide protection in addition to oxygen and
moisture, also from thermal exposure and unfavourable wavelengths that
account to photodegradation.
Prospects of OSC technology: The OSC devices hold currently a small portion in
the market and its applications are predominantly in mobile/disposable electronics.
Unless the OSC technology surpasses the previously mentioned challenges, the
36
introduction of the devices to the commercial market would be prolonged. The
photovoltaic technologies in general are becoming cheaper and their efficiencies
are getting higher. Most probably, the difference between the solar cell technologies
will be decreased in terms of cost per power generated. In such cases, the next edge
on which different PV technologies will compete would be based on factors such
as stability, low weight, form factors, aesthetic appeal, transparency, recyclability
and eco-friendliness. OSC devices already have the advantage of aesthetic appeal,
form factors, transparency and low weight per device etc. If the OSC technology
evolves by intensive research in coming years to highly efficient and stable devices,
it has the great potential of ruling the flexible and portable electronics market.
Significant improvements in the efficiency can be achieved in future by forming
hybrid with inorganic structures such as perovskites. That may provide key
advantages to these devices like cost efficiency, material abundance, ease of
preparation, near perfect crystallinity at low temperatures and large carrier
diffusion length. According to Swanson’s law [146], the price of solar cells will
drop 20% for every doubling of the industrial capacity. But the interesting aspect is
that the Swanson´s law is applicable to all kinds of PV technology exist today. Thus
the large industrial scale ups for various PV technologies can be expected in the
future. The price reduction is the prime motive for major share of various PV
technologies. The OSC technology although was very promising in the initial years
of the launch, the large scale market realisation as product has not yet realised
owing to the fact that many of the initial challenges are not yet tackled. At last, the
race in various PV technologies is about harnessing solar energy in the most
efficient, cheap and eco-friendly way.
37
2 Aim and structure of study
The aim of this doctoral thesis was to develop methods to analyse and reduce
various problems affecting the efficiency as well as the stability of the OSC devices.
The studies can be divided in to two main categories: Efficiency studies and
Stability studies. The work conducted in articles I & II comprises the efficiency
studies. Methods to enhance efficiency of the polymer-fullerene BHJ-OSC devices
were developed with the emphasis on the OSC active layer morphology
enhancement with an additive and optimising the component ratio in PEDOT:PSS
hole transporting layer (HTL). The articles III-V comprise the stability studies
addressing various internal and external degradation issues such as instability at the
ITO/PEDOT:PSS interface, the effect on OSC performance due to ITO breakage
during bending and the effect of prolonged thermal exposure on the active layer of
low band gap polymer-fullerene based OSC device. In these studies, solutions are
developed for the interface instability problems and the extents and influence of
these degradation phenomena are analysed. The structure of the study is depicted
in Fig. 8.
Fig. 8. Structure of the study.
38
39
3 Materials and methods
This section describes the materials and methods used to fabricate and characterise
the OSC devices during the studies. The materials and fabrication parameters used
in the articles within the scope of this thesis are depicted in the following Table 1.
Table 1. Materials used in the work.
Articles Substrates Dimension / Specifications Producer
I,II,III,V Glass with patterned
ITO
6.25cm2 square glass substrate with
1.3 cm wide ITO stripe in the
middle, 20 Ω/ sheet resistance,
ITO thickness -150 nm
Thin Film Devices Inc.
IV PET with patterned ITO Dimension of substrate for OSC device
was same as above, Nominal resistance
were 40-60 Ω/, ITO thickness -125 nm
Dupont
Articles HTL (PEDOT:PSS) Spin coating parameter Producer
I PH500 6700 RPM / 1 minute H.C.Starck
II,V PH500 6700 RPM/ 1 minute Heraeus
III,IV PVP AI4083 PEDOT:PSS (III) 5200 RPM/1minute & (IV)
6200 RPM/1minute
H.C.Starck
Articles Donor Acceptor solvent Active layer blend ratio Annealing Temp.(oC)
I,II P3HT PC60BM DCB 1:0.8 (30 mg/ml) 150 oC / 30 minute
III,IV P3HT PC70BM DCB 1:0.8 (30 mg/ml) (III) 160 oC/ 30 minute &
(IV) 150 oC/ 15 minute
V PTB7 PC70BM DCB + 3 vol% DIO 1:1.5 (25 mg/ml) no annealing
Articles Donor Producer Acceptor Producer Cathode for devices in (I-V)
I- IV Sigma-Aldrich Nano C LiF and Al
V 1-Material Inc. Nano C
3.1 OSC device fabrication and experimental methods for (I – V)
articles
The fabrication of organic solar cell consists of sequential deposition process. The
basic structure of the conventional bulk heterojunction organic solar cell is
illustrated in Fig. 3 and this structure was employed for the OSC devices in this
work. It consists of a transparent anode which is usually a glass substrate coated
with ITO. Since each organic layer in the device is deposited by spin coating, it is
40
necessary that the surface of the substrate is devoid of any contaminants. To ensure
this, the substrates are first cleaned by washing with acetone, isopropyl alcohol
(IPA) and methanol respectively. Then it is plasma treated for one to five minutes
inside a Plasma PREEN II-862 asher, which helps in enhancing the surface energy
of the substrate so that the hole transporting layer has superior wettability over the
surface. Then a hole transporting layer (HTL), usually PEDOT:PSS is used. After
the deposition of the HTL, it is then subjected to 1 hour annealing (at 120 o C inside
the vacuum for articles I-III & V and at 110 0C in article IV), to ensure the removal
of the moisture content. The active layer blends in article I- IV were stirred at 55 o
C and at 70 0C for article V. Then the active layer material which was stirred
overnight is deposited by spin coating. Subsequently, it is dried at room temperature
for 1hour inside the glovebox and then annealed at optimum temperature. Finally,
the electrodes are deposited. A 0.6 nm thick LiF and 120 nm thick aluminum
respectively are deposited on the sample as cathode by thermal evaporation
technique. The materials are used inside the nitrogen filled glovebox which ensured
inert atmosphere. The photovoltaic polymers used are very sensitive to moisture
and oxygen. Thus every processing was handled inside the glovebox, except for the
spin coating of the HTL.
In the article I, the experiment performed was to identify the inhomogeneities
in the morphology of the active layer based on the P3HT:PC60BM blend during the
annealing process. The active layer was spin coated at 450 RPM for 2 minutes. As
a method to reduce the non-uniformity in the morphology, the effect of glycerol
addition in varying concentration to the active layer is analysed.
In article II, the effect hole transporting layer PH500 annealed in different
atmospheres was studied. The cleaned and plasma treated substrate was spin coated
with PH500 in three batches. Then it is annealed in air, nitrogen and vacuum for 1
hour. The temperature ranged from 70 oC to 270 oC with 50 oC interval variation
for each annealing step. After the annealing the active layer of P3HT:PC60BM was
spin coated at 450 RPM as before annealed at 150 o C for 30 minutes.
In article III, the study was focused on understanding the degradation caused
by the chemical reactions between the hole transporting layer and the ITO anode
layer of the device and also develop solutions to reduce such degradation. The hole
transport layer (HTL) used in the device was PVP AI4083 PEDOT:PSS, which is
poly(3,4-ethylenediioxythiophene)-poly(styrenesulfonate). The active layer was
spin coated at 450 RPM for 2 minutes. After drying the sample inside the glove box
for 1 hour in room temperature, it was annealed at 160 0C for 30 minutes. For
studying the effect of the presence of the plasma treated Ag interlayer on reducing
41
chemical instability caused by PEDOT:PSS on ITO, a different set of devices were
made in which the Ag layers were deposited by thermal evaporation on top of ITO
patterned substrates before the deposition of PEDOT:PSS. The two thicknesses that
tried for Ag interlayers in OSC in the study were 1 nm and 5 nm. After the Ag layer
deposition, plasma treatment was done for 5 minutes to enhance the wettability of
subsequent deposition of PEDOT:PSS. By conducting contact angle measurement,
it was found that the plasma treatment of the sample with thin Ag layer increased
its surface energy significantly from 36.6 mN/m to 75.2 mN/m.
The article IV was a collaborative work. The contribution of the author was the
experiment conducted to understand the effect of the bending of ITO substrate on
the performance of the OSC of smaller area. This thesis focuses only on the author’s
independent contribution. The substrates were made from PET-ITO sheets. The
brittle nature of ITO and the bending stress motivates the substrate to undergo
certain defects such as cracks. The analysis of ITO surface defects on OSC device
performance was carried out by fabricating OSC device on top of the cracked ITO
by bending stress. The bending procedures were carried out by placing the substrate
strip on the surface of cylinder 10 mm in diameter and hanging a weight of 1 kg.
The diameter of the cylinder was chosen as mentioned to ensure definite cracking
of the PET ITO substrate. The samples were initially UV treated for 1 minute to
enhance the wetting properties. Later the successive component organic layers and
electrodes are deposited over it.
In article V, the motivation of the experiment was to understand the effect of
varying ranges of thermal exposure on the performance of the low band gap
polymer: fullerene active layer blend. The blend of PTB7:PC70BM with a 3 vol%
of 1,8 diiodooctane (DIO) additive, was utilised as the active layer. It was reported
that the presence of additives like DIO in the active layer enhances the initial device
efficiency [147]. Firstly the optimised spin coating speed of active layer is
determined by analysing the parameter providing highest PCE value for the device.
The active layer was spin coated at the optimised spin speed of 700 RPM. To
understand specifically the thermal degradation happening to the active layer, the
thermal exposure was done solely to the active layer coated substrates before the
deposition of the electrode. The varying temperature profile was selected for pre-
annealing to mimic the outdoor exposure of such solar cells in intense thermal
conditions. The temperature was varied starting from room temperature by
increasing 20 0C every 3 hours till it reached 107 0C and then decreased by the same
amount for every other 3 hours under dark condition till it reached the initial
temperature of the experiment.
42
In our specific degradation studies in Article III and Article V, we have studied
unencapsulated devices inside the inert glovebox which eliminated the influence of
moisture and air. In article III, the initial and final efficiencies were very important,
since it makes possible the complete analysis of the performance changes due to
the presence of the interlayers. The unencapsulated OSC device when exposed to
light and air, there can be various degrading phenomena occurring at the same time
such as oxygen and moisture ingression, reaction of oxygen and water with
electrodes and active layer. To study specifically certain degrading factors, the
procedure adopted was to isolate the circumstances causing the degradation other
than the interested phenomena. However, the ISOS related standards were not
strictly followed in our experiments. Analysing the degradation pattern over a broad
time period accounting the initial and final PCE values help in understanding better
the stability state of the device. Shelf lifetime provides significant information
regarding the inherent capability of the organic materials to retain its initial
efficiency over a period of time. The shelf life time measurements are conducted
inside an inert atmosphere like the nitrogen filled glovebox. The devices are stored
in the dark and photovoltaic performance was monitored at specific intervals of
time by illuminating the device with solar simulator.
3.2 Characterisation methods
This section mentions all the characterisation techniques employed in this work for
understanding topographical and photovoltaics properties of the samples and
devices. Although these surface characterisation methods are well known and
commonly used in this type of research, the principles behind some of these are
explained briefly.
3.2.1 Optical microscopy (OM)
The excessive crystallisation of the acceptor components in the active layer of the
OSC devices in the experiments of article I were identified using a Nikon universal
design optical microscope (UDM) ECLIPSE LV 100DA-U. For this measurement,
the samples with Pristine PC60BM and PC60BM with glycerol additive, pristine
P3HT, P3HT with glycerol additive were initially analysed for annealed and not
annealed samples. The glycerol additive concentration used was 30 vol% and
annealing conditions applied in the investigation were the same as during the
fabrication process. The split analysis was also conducted where a section of the
43
sample was coated with pristine PC60BM and another section coated with PC60BM
with glycerol additive to understand clearly the distinction in phase segregation due
to the excessive crystallisation of the fullerenes. The effect in real active layer blend
of P3HT:PC60BM with and without the glycerol additive on the ITO\PH500\active
layer structure is analysed finally with annealed and unannealed conditions.
3.2.2 Atomic force microscopy measurements (AFM)
The changes in surface roughness due to the presence of additive in active layer of
OSC devices in the experiments of article I were studied using the AFM
measurement technique. The AFM employed in our experiment was AFM Veeco
Dimension 3100. The projected image area was 25µm2. The device set up utilised
in the work is depicted in Fig. 9.
Fig. 9. AFM Veeco Dimension 3100 tapping mode set up.
In AFM measurement system, the information regarding the surface of the sample
is obtained by scanning a small probe across the sample. The AFM can be used as
a tool to understand the physical topography as well as material properties of the
sample such as its magnetic properties, stiffness, surface chemistry etc., [148-151].
The three dimensional images of solid surfaces including non-conducting samples
such as polymers and ceramics could be generated by AFM at a very high resolution.
However, in case of topographical investigation on soft samples such as polymers,
44
the lateral forces exerted by the tip of the probe can lead to image artefacts due to
rupture of the surface. The tapping mode is typically used to solve this problem. In
tapping mode, a cantilever oscillating at or near its resonant frequency with high
amplitude is employed. The oscillating tip is then scanned at a height where it
barely touches or taps the sample surface. Thus, the shear forces under which the
tip and the sample interact are greatly reduced by this method. The vibration is set
such that the tip contacts the sample surface once in every vibration period. Because
of the tip-induced damage, the AFM measurement, the studies done on the sample
surfaces in this work were therefore taken in the tapping mode.
3.2.3 X-ray photoelectron spectroscopy measurements (XPS)
The information regarding the composition and the chemical state of the material
surfaces could be obtained using the analysis technique called X-ray Photoelectron
Spectroscopy (XPS) [152,153] and was utilised for articles II & III. The XPS is
also known as Electron Spectroscopy for chemical Analysis (ESCA). In this
measurement system, the specimens are placed in an ultra-high vacuum (UHV) to
prevent contamination of the surfaces and then exposed to x-ray source. The
ejection of core-level electrons from the sample atoms occurs when x-rays are
incident. The emitted electrons, which have mean free path lengths of the order of
1 nm, are detected over the energy range 0 to 1000 eV. The mean free path γ is
determined by the thickness of matter through which 63% of the traversing
electrons will lose energy. As a consequence of this energy loss, only electrons out
of the first ten nanometers may leave the surface and be detected as photoelectrons.
By knowing the binding energy of a particular shell of an atom, the element can be
identified. The photoelectron peaks are associated with a particular core level of an
element. All elements, except hydrogen and helium can be detected. The
concentrations of the elements are determined by integrating the area under a
characteristic peak for each element taking sensitivity factors in to account.
In article II, component variation of PEDOT and PSS in the hole transporting
layer PH500 under different annealing condition was analysed by using X-ray
photoelectron spectroscopy (XPS). The XPS experiments were performed by the
system Thermo Fisher Scientific ESCALAB- 250Xi. Avantage software was used
for the analysis of the data and background subtraction. Monochromatic Al Kα X-
rays (1486.68 eV) were utilised for excitation and detection area was set to 900 µm
in diameter. The detector mode was in Constant Analyser Energy (CAE). Survey
scan pass energy was 150 eV with step size 1 eV. The chemical states of compounds
45
were identified and quantified by analysing the photoelectron transitions of S 2p.
In article III, the XPS technique was utilised for analysing the indium diffusion and
etching issue in the OSC device. The XPS experiments were performed in a UHV
system equipped with VG Microtech Multilab ESCA 3000 electron spectrometer.
Non-monochromatised Mg Kα X-rays (1253.6 eV) were utilised for the excitation,
and the detection area was set to 600 µm in diameter. The chemical states of
compounds were identified and relative atomic concentrations of indium were
quantified by analysing the photoelectron transition of In 3d.
3.2.4 Contact angle measurement
The changes in the wettability due to plasma treatment of the thin Ag coated ITO
layer (article III) were identified with the contact angle measurement. Contact angle
measurement is a surface analytical method for investigating the wetting and
surface energy properties. The surface energy of a solid material can be understood
from the known surface tension values of the probing liquids and the contact angle
θ, made by them with the surface of the solid material. The contact angle can be
measured by the angle between the tangent plane to the surface of the liquid and
the tangent plane to the surface of the solid substrate, at any point along their line
of contact. The contact angle formed between a liquid and a solid in equilibrium
with vapour phase, and the surface energy difference between these were
determined by Young [154]. The inferences made by Young are graphically
depicted in Fig. 10 and its simplified mathematical form is represented in equation
(5).
= (5)
Fig. 10. Force balance according to Young.
46
Where θ is the contact angle, is the interfacial tension between the solid and the
vapour, is the interfacial tension between the solid and the liquid and is the
interfacial tension between the liquid and the vapour. The Owens-Wendt-Rabel-
Kaelble [155] method is used to calculate the surface energy. In this method, the
surface energy of a solid consists of two components, a dispersive and a polar
component. In our experiment, the static sessile drop contact angle measurement
method was applied by using Kruss DSA100 system to measure the surface energy.
In order to calculate the surface energy, two fluids such as deionised water and
ethylene glycol were used. For each fluid, five contact angle measurements were
performed and the results were averaged. Importantly, the measurement was done
separately for plasma – treated and untreated samples.
3.2.5 Transmittance measurement
The normal transmittance measurement of the samples in article III were done to
understand the effectivity of photons to pass through the substrate with the presence
of additional interlayer. The samples undergone the specified measurement were
the ITO coated glass substrate and the substrate modified with plasma treated Ag
layers (1 nm and 5 nm thick layers). For the normal transmittance measurement,
the spectrophotometer system (Optoelectronics Laboratory, USA) in the range 300
nm – 900 nm was unutilised. The samples were placed in the line of the illumination
from the spectrophotometer and the automated system detected the percentage
transmittance for each sample in the specified wavelength range. The transmittance
is defined mathematically in equation (6).
Transmittance = (6)
The transmittance is the ratio of the intensity of light entering the sample (I0) to that
exiting sample (It) at a particular wavelength.
3.2.6 Photovoltaic characteristics measurement
The photovoltaic measurement for all the devices fabricated in the experiments
were evaluated under AM 1.5G, class AAA solar simulator (Oriel Inc.), as the light
source. A solar simulator (also artificial sun) is a device that provides illumination
approximating natural sunlight. An ideal spectral match for a solar simulator is
based on the percentage of the integrated light intensity in the six spectral ranges
47
having 100 nm interval ranges from 400 nm to 1100 nm. The class AAA solar
simulator meets the class A requirement for all the three performance criteria such
as spectral performance, uniformity of irradiance and temporal stability. The
purpose of the solar simulator is to provide a controllable indoor test facility under
laboratory conditions, used for the testing of solar cells, sun screen, plastics, and
other materials and devices. The light intensity was calibrated to be 100 mW/cm2
by using NREL calibrated crystalline silicon reference cell. The parameters such as
VOC, JSC, FF and PCE are measured from corresponding J-V curve obtained from
the device under illumination. The information of the photovoltaic characteristics
of the device is derived by Botest LIV organic electronics tester system connected
to it.
Each and every solar cells fabricated during individual experiments in the study
had an active cell area of 15 mm2. For each experiment, the average performances
were measured for at least 15 cells (5 individual cells per 3 solar cell modules). The
O2 and H2O levels were normally 0.1 ppm each inside the nitrogen filled glovebox.
For our stability studies, the unencapsulated devices were stored inside the
glovebox in dark condition. The photovoltaic measurements were carried out under
the solar simulator at specific intervals inside the glove box, thus reducing the
parallel degrading mechanisms such as influence of oxygen and moisture.
48
49
4 Results
The results are divided in to two major sections. The first section describes about
the efficiency studies of bulk heterojunction solar cells. Various factors affecting
the efficiency of the device are analysed in this section and methods are
implemented to enhance the efficiency of the devices. The second section deals
with the stability studies of the device. Some of the internal and external
degradation factors causing reduction in stability of the device are investigated and
techniques are developed to reduce those degrading factors.
4.1 Efficiency studies
The efficiency enhancement studies were carried out for the P3HT:PC60BM bulk
heterojunction solar cell. The component layers such as active layer and the hole
transporting layer contributes significantly towards the performance of the device.
The irregularities in these layers can influence the final device efficiency to a
greater extent. In this section, some of such issues responsible for the lower
efficiency of the device were identified and suitable methods to enhance the
performance of these devices have been found out.
4.1.1 Performance of P3HT:PC60BM devices
In bulk heterojunction solar cells based on P3HT:PC60BM active layers, the
annealing process helps to facilitate better interlinking of the donor and acceptor
materials in the active layer. The P3HT acts as the donor and PC60BM acts as the
acceptor material respectively in the layer. During the annealing process, the extent
of donor material crystallisation may affect the morphology crucially since it is
necessary to have nanoscale bicontinuous network between acceptor and donor
materials to have excellent route for charge transfer [156]. It is known that some of
the unfavourable mechanisms taking place during the processing of active layer can
reduce the efficiency of the device [157]. The good morphology of the active layer
will result in higher efficiency for the device. The annealing of the active layer is
essential for controlling the morphology, but since some unfavourable phase
segregation can easily arise due to excessive crystallisation, the better
understanding of the phenomena is needed.
50
4.1.2 Effect of annealing on P3HT:PC60BM
The optical microscope images in Fig. 11 reveals the excessive crystallisation of
PC60BM components present inside the active layer after annealing. The excessive
crystallisation issue was investigated using the optical microscope since the
micrometer scale crystals are formed during annealing.
Fig. 11. (a) ITO\PH500\P3HT:PC60BM (pristine) unannealed (b) ITO\PH500\P3HT:PC60BM
(pristine) after annealing, scale 100 µm. (Article I, published with the permission of ©
Hindawi Publishing Corporation 2015).
The morphology changed crucially with excessive crystallisation of the acceptor
material. This happens due to the fact that the solvent present inside the layer
evaporates suddenly with the thermal exposure leaving the components to
crystallise under annealing. This type of morphology change causes unfavourable
condition for the ease of charge transfer. The optimal condition is to have the
morphology to be in nanoscale bi continuous network [156].
4.1.3 Effect of glycerol additive
The problem of fast evaporation of solvents from the layer can be reduced by
adding some amount of additive solvents with higher boiling point. Thus a very
inexpensive and common material, glycerol was added to the blend to investigate,
if it can help to reduce the excessive crystallisation problem. The glycerol
concentrations from 0 to 70 vol% have been investigated. Fig. 12 shows the PCE
51
variation of the device with active layer having glycerol doping in various
concentrations.
Fig. 12. Variation of PCE with glycerol additive in the P3HT:PC60BM active layer.(Article
I, published with the permission of © Hindawi Publishing Corporation 2015).
It can be seen from the plot that average efficiency of the device from 2% at 0 vol%
concentration is increased almost near to 3% at 30 vol% glycerol concentration in
the active layer. Further presence of higher concentration of glycerol was found to
reduce the efficiency. The morphology changes by the annealing were investigated
by the optical microscope, in Fig. 13.
52
Fig. 13. ITO\PH500\P3HT:PC60BM with 30 vol% glycerol additive without annealing (b)
ITO\PH500\ P3HT:PC60BM with 30 vol% glycerol additive after annealing, scale 100 µm.
(Article I, published with the permission of © Hindawi Publishing Corporation 2015).
As comparing with the morphology of the blend in Fig. 11 (b) and Fig. 13 (b), it
can be clearly seen in the latter that the amount and extent of excessive
crystallisation of PC60BM due to annealing is lesser with the presence of glycerol
additive. This mechanism greatly contributes to the enhanced performance of the
device as in OSC devices since the morphology acts as the significant factor in
determining the efficiency of the device. It was reported that [156] when the contact
surface area between donor and acceptor components in the active layer are higher,
better would be the transport of charges at the interfaces and hence resulting in
better working of the device.
Further, the AFM measurements revealed the fact that the active layer with glycerol
additive was having a smoother surface. It was reported [130] that the smoother
surface lead to better adhesion of the successive component layers and enhances
device efficiency. On the contrary, there were other reports [158] which stated that
the rough surface of blend film is a signature of higher efficiency. It was argued
that the rougher surface increased charge collection by increasing the contact area
between blend and metal cathode at the interface and also increased light absorption
by improving internal reflection. However, it was explained by Dutta et.al [159] in
his report that an increase in efficiency was not directly attributed to the changes in
roughness, but to the improved ordering of P3HT chains and optimal donor-
acceptor phase distribution in both lateral and vertical directions. Thus it can be
53
expected that the efficiency improvement observed in this study may
predominantly depend on the morphological improvement obtained in the active
layer.
4.1.4 Controlling component ratio of conducting polymer blend by
annealing
The electrical performance and also surface energy properties of conducting
polymers used as hole transporting layer depends on its surface compositions.
Appropriate annealing of the HTL polymer layer could enhance its morphology and
hence the device performance. In this study, we investigated how the different
atmospheres such as vacuum, N2 and air with various temperatures affect to the
performance of PEDOT:PSS in OSC. The temperature was varied from 70 0C to
270 0C. The performance variation of PH500 in OSC with temperature and different
atmosphere is represented in Fig. 14.
Fig. 14. PCE variation with temperature and atmospheres. (Article II, published with the
permission of © Hindawi Publishing Corporation 2015).
At temperature 220 0C, it can be seen from Fig. 14 that the performance was higher
for devices with PH500 annealed in each of the three atmospheres. Annealing in
air was demonstrating lower PCE value and the efficiency values were higher for
54
the devices annealed in N2 and vacuum. Annealing in N2 has also found to cause an
increase in JSC values than for devices annealed in vacuum. It was found that the
optimum temperature was 220 0C in all three atmospheres. The Nitrogen
atmosphere was demonstrating better condition for the PEDOT:PSS layer to
achieve higher PCE in OSC devices. The average performances in the three
atmospheres are depicted in Table 2.
Table 2. Average photovoltaic parameters at optimum annealing temperature. (Article II,
published with the permission of © Hindawi Publishing Corporation 2015).
Annealing atmosphere VOC (V) FF (%) JSC (mA/cm2) PCE (%)
Vacuum 0.625 61.85 6.70 2.58
N2 0.635 56.57 7.43 2.67
Air 0.631 61.20 5.62 2.17
In order to understand better the efficiency variation caused by annealing, the
behaviour of PEDOT and PSS component variation in PH500 was analysed by XPS
spectroscopy. The sulfur atoms in both PEDOT and PSS can be easily distinguished
because they have different binding energies. The sulfur peaks at S 2p having lower
binding energy, 164.6 eV and 163.4 eV correspond to the PEDOT component and
the higher binding energy 169 eV and 167.8 eV correspond to PSS [160]. The
variation S2p peaks of PH500 annealed in different atmosphere at the optimum
temperature are represented in Fig. 15.
55
Fig. 15. S 2p XPS peaks of PH500 samples annealed at optimum temperature in different
atmospheres. (Article II, published with the permission of © Hindawi Publishing
Corporation 2015).
In PEDOT:PSS, the excess PSS present in the blend acts slightly insulating and
PEDOT is relatively more conductive [161,162]. Analysing the component content
of PEDOT and PSS in the Fig. 15, it was found that the PEDOT content in PH500
layer annealed in N2 and vacuum was higher and PSS content lower compared to
the PH500 layer annealed in air. The PH500 layer annealed in N2 had comparatively
the highest PEDOT content and lower PSS content. The reason for such kind of
component variation might be due to the reported phenomena that at higher
annealing temperature the PSS shell around the conducting PEDOT grain gets
shrunk and thus exposing more conductive PEDOT to the surface [163, 164]. At
temperature greater than 220 0C, the performance of PH500 deteriorates and might
be due to the fact that as larger component variation is taking place. The variation
of ratio content of PSS/PEDOT with power conversion efficiency and current
density is plotted in Fig. 16.
56
Fig. 16. Plot showing PCE and JSC variation with PSS/PEDOT ratio at 220 0C. (Article II,
published with the permission of © Hindawi Publishing Corporation 2015).
The strong behavioural dependence of PSS/PEDOT ratio content with PCE and JSC
is evident in Fig. 16. It was found that when the component ratio of PSS/PEDOT
is closer to 1.1 the efficiency and current density is the highest in any atmosphere
of N2, vacuum and air. The results achieved are very relevant due to the fact that it
helps the manufacturer to understand how the specific component ratio formulation
of the conducting polymer blend performs in the respective atmospheres. In our
further experiments, we have also investigated the effect in various other types of
PEDOT:PSS such as PVP AI4083, neutralised PEDOT:PSS, HTL solar
PEDOT:PSS and similar kind of behaviour was observed.
4.2 Stability studies
Organic solar cell (OSC) devices currently face many stability issues. Additional
focus on studying degradation happening inside the device is needed. This is due
57
to the fact that a straightforward solution does not exist for addressing internal
degradation issues as compared with external degrading factors where the choice
of suitable encapsulation of the device can be adopted to restrict the degradation to
a certain degree. We had invested our effort to study some of such factors causing
internal degradation and explored a suitable modus operandi to tackle etching of
ITO by PEDOT:PSS. Further the investigations were done for understanding the
factors such as influence of ITO surface defects due to bending stress on the device
performance and effect of extended thermal exposure on the active layer of high
efficiency OSC devices.
4.2.1 Effect of acidic nature of PEDOT:PSS on ITO anode and
performance of OSC device
The poly(3,4-ethylenedioxythiophene) (PEDOT) normally does not dissolve in
usual solvents and it has to be attached with PSS in order to make it more soluble.
PEDOT:PSS has acidic pH value between 1 and 2 at 20 oC [165]. The PEDOT:PSS
is hygroscopic and with the presence of moisture during PEDOT:PSS deposition,
it can react with ITO layer beneath. The ITO is highly soluble in acidic medium.
The interfacial degradation of ITO anode in the device takes place due to the
etching effect of the acidic PEDOT:PSS and the indium and tin components will
diffuse through PEDOT:PSS to the active layer. The degradation of the ITO anode
due to etching and the migration of etched products through the subsequent organic
layers will together contribute in reducing the performance of the device and the
performance gets reduced as it ages.
58
Fig. 17. PCE degradation of OSC within 41 days. (Article III).
From the Fig. 17, it can be seen that the efficiency falls rapidly within first five days
and continues the degradation pattern and at the last day of measurement the
degradation was about 66% from the initial efficiency of 2.65%. In order to
understand the stability degradation of polymer solar cell predominantly due to ITO
etching, the studies and measurement were conducted so that other parallel
degrading factors such as photo bleaching, thermal exposure, oxidation and
moisture were restricted to the minimum.
4.2.2 Method to counter ITO degradation by PEDOT:PSS
The presence of interlayer between the ITO and the conducting polymer layer
might lessen the direct acidic influence of PEDOT:PSS. The interlayer cannot be
too thick because it will affect to the transmittance and the performance of the
layers. The properties of plasma treated silver as the interlayer was investigated.
The interlayer thicknesses of 1 nm and 5 nm were used to demonstrate the influence.
59
Fig. 18. Influence of plasma treated Ag interlayers on stability of OSC (Article III,
published with the permission of © Elsevier 2014).
From Fig. 18, it can be seen that the initial PCE value is changed to 2.32% with the
insertion of 1 nm thick interlayer and to 2.21% with 5 nm thick plasma treated Ag
layer. The thinner interlayer of 1 nm resulted in degradation up to 56% from its
initial value, whereas with 5 nm thick plasma treated Ag layer contributed to
enhancing the stability by restricting the degradation to only 30% from its initial
PCE. Additional transmittance measurement showed that the influence of light
transmission was reduced slightly with the presence of the interlayer which was
reflected in the initial PCE values of devices with the interlayers.
The interfacial degradation mechanism with indium diffusion due to etching
by PEDOT:PSS was later analysed with XPS measurements. It was found that the
indium atoms etched out from ITO starting from the first day and at the end of 30
days, there was significant percentage of indium content diffused through
PEDOT:PSS. It was found for the samples with 5 nm interfacial layer that the
diffusion of indium was restricted by the presence of the plasma treated Ag layer.
This was thus reflected in the enhanced stability with 5 nm thick interlayer. The use
of interlayer of 1 nm was found to be not very effective due to the fact the layer
60
may be very thin or may have non uniform distribution on the surface to contribute
shielding against acidic PEDOT:PSS.
4.2.3 Effect of cracked ITO anode on the device performance
The solution processability of the OSC technology enable large scale flexible
devices to be made on PET substrates. Since ITO is brittle [166-170], large bending
of the substrate can cause cracks on the ITO surface and it will reduce the
conductivity of the surface which can reflect in the device performance. In this
section, we tried to understand how the device would perform if the OSC is
fabricated on the already cracked ITO substrates. The mechanism behind ITO
breaking is studied and its effect on OSC is analysed. Table 3 depicts the average
performance of the functioning devices.
Table 3. OSC- PCE for functional cells based on cracked ITO (Samples) and uncracked
ITO (References). (Article IV, published with the permission of © Elsevier 2013).
Total number of solar cells Functioning cells Average PCE (%) Standard deviation
Samples 40 36 0.78 0.25
References 16 15 0.52 0.21
It was surprising that the statistical difference in the broken and reference samples
are not very significant. During the initial stages of cracking, the decrease in
conductivity may be compensated with increased effective surface. These results
showed that even with samples cracked with 10 mm cylinder did not collapse the
OSC device performance. Although the substrates were broken in a direction that
expected to decrease the performance drastically, it was found that the direction of
the crack does not have major influence on the performance of the OSC devices
made on top of it. The efficiency of the device did not collapse although the ITO
substrate was undergone bending stress under the critical 10 mm cylinder which
created higher density of cracks. This might be insightful by revealing the fact that
the thin film non uniformities in ITO is not very critical in smaller area. The sheet
resistance of the ITO surface has greater influence when the surface area is higher
[171], but the effect of cracked ITO requires more investigation to be understood
better.
61
4.2.4 Effect of prolonged thermal exposure over 1,8-diiodooctane
(DIO) doped PTB7:PC70BM based organic solar cells
The active layer is both prone to degradation due to continual illumination as well
as continuous thermal exposure. In this study, other degrading factors are restricted
and specifically studied how an extended and varying temperature profile would
affect the performance of the low band gap polymer over an entire day. This study
is very crucial in developing thermally stable and efficient OSC devices. The
research on thermal stability of polymer solar cells is important since the devices
are exposed to the sun for a prolonged time and the degradation of the polymer can
take place due to prolonged heating effect associated with illumination [172,173].
The prolonged thermal exposure affects adversely the efficiency of the low band
gap active layer polymers in OSC devices. We have investigated the effect of
varying temperature on the performance of DIO doped PTB7:PC70BM organic
solar cells. The whole study was conducted inside the nitrogen filled glove box and
hence the inert atmosphere restricted other parallel degrading mechanisms [115]
and helped to study the effect specifically coming from thermal exposure of the
active layer. The plot in the Fig. 19 represents the changes in power conversion
efficiency (PCE) of the devices under varying thermal exposure over the whole day.
Fig. 19. Average variation of PCE with varying pre-annealing condition. (Article V,
published with the permission of © EUPVSEC 2015).
62
From the measurement plots in Fig. 19, the initial average efficiency was found to
be 7.15%. In the case of device under thermal exposure, the efficiency had steep
degradation slope in the first 6 hours and the next six hours the efficiency was
almost constant. The efficiency was slightly increased at 18th hour which
characterised a short recovery period and sheer decline in efficiency thereafter. This
may be associated with the critical changes in morphology due to prolonged
temperature variation. The efficiency reached to 5.47% at the end of the day with
heating. Almost 23% degradation took place from the initial value. The pristine
device which had no thermal exposure had shown some degradation but was not
significant as compared to the active layer under thermal exposure. The unheated
devices degraded only about 2.15% from its initial value. Although the devices
showed clear decrease in power conversion efficiencies from thermal exposure, it
was observed that the VOC of the devices increased during the period whereas the
JSC has fallen. The FF values were also found to drop with the thermal exposures
after 6th hour. This might suggest that the morphology of the active layer has
undergone a process of dynamic change where the generated excitons are limited
somehow from dissociation and charge transfer. However, the accurate knowledge
of the observed performance variations can be obtained only after further detailed
morphological investigations.
63
5 Discussion
It is interesting to draw implications of the outcome generated from the different
methods employed and experiments conducted in this work regarding the device
performance and stability enhancement. These issues are discussed below.
5.1 An inexpensive route for performance improvements
Achieving the highest possible power conversion efficiency for OSC is a necessity
for ensuring its competence among other thin film technologies. The cost effectivity
of the applied methods for achieving significant efficiency improvements was an
important consideration during our research. Moreover, we have utilised a targeted
approach for performance improvement, i.e. targeting and mitigating the
anomalous characteristics of the constituent layer of the device or the components
inside the layer. The morphology improvements of the bulk heterojunction active
layer and the optimisation of conducting polymer HTL characteristics through
regulated annealing conditions were found to be the straightforward methods to
enhance the device efficiency in this work. The morphology enhancement achieved
in this study was demonstrated by reduction in unfavourable phase segregation with
addition of glycerol additive at optimised concentrations. The enhanced interfacial
contact area between acceptor and donor phases due to reduction in excessive
fullerene crystallisation [156] may have resulted in effective exciton dissociation.
The probability of enhanced charge transport and reduction in series resistance is
also suggested by the observation of significantly improved JSC. In addition to
reduction in interfacial contact area, it was reported that the irregularity occurring
due to excessive crystallisation of fullerene components can also result in reducing
light-harvesting capability of the active layer [156]. These factors reinforce the
gravity of our achieved results regarding morphology improvement. The
motivations for choosing glycerol as an additive in the active layer were based on
the priorities such as low cost, eco-safe, hydrophilic nature and having higher
boiling point. We expected that this type of additive could effectively target the
PC60BM components in the active layer which was reported to have higher surface
energy and hydrophilicity [174]. We also assumed that the rapid evolution of the
solvent could also be prevented by elevating the boiling point with the introduction
of suitable additive. The additive was found to be very effective in enhancing the
performance of the device by 42% from its initial PCE. Apart from the BHJ lateral
morphology, the vertical composition distribution is also an important issue, since
64
charges have to be vertically transported to their respective electrodes after they are
generated at the donor-acceptor interface of the polymer-fullerene active layer. The
other similar reported methods where additives were used to address the
unfavourable phase segregation in the active layer include the use of 1,2,3,4-
tetrahydronaphthalene (THN) [175] and bisadduct of PC60BM [156]. But our
method outweighs these reported methods in terms of cost and rate of improvement
from the initial PCE values.
Considering the scenario in large scale manufacturing process, a variety of
annealing requirements are necessary for buffer layers constituting the device
structure. For instance, annealing of the conducting polymer HTL such as
PEDOT:PSS is essential for moisture removal. The cost effective methods for
annealing in large scale processing is definitely not cleanroom and vacuum based
conditions. We thus need to understand effect of other non-optimal processing and
annealing conditions which are cost effective for making the technology profitable.
Understanding the factors causing the improvement or deterioration in the layer
during annealing in various atmospheres, would provide information enabling us
to even design the specific conducting polymer buffer layer material to perform
superior in non-optimal but cost effective atmosphere such as air. Thus it motivated
us to investigate OSC performance with device structure consisting of water based
and atmosphere susceptible conducting polymer such as PEDOT:PSS and
annealing it under different pressures and atmospheres, for instance in vacuum, N2
filled glovebox and ambient air atmosphere. Previous studies had given very little
focus on the effect of atmosphere conditions on the performance conducting
polymer PEDOT:PSS layer during annealing. We have thus enhanced the efficiency
of the OSC-BHJ device by obtaining optimised annealing condition for
PEDOT:PSS, which was actually a distinct route employed to enhance device
efficiency apart from morphological improvement of active layer. It was reported
[176] that the contribution of electrical performance of the conducting polymer
layer depends predominantly on the composition ratio between PSS and PEDOT in
the layer. The interesting observation made from our experiments is the fact that
the samples coated with the conducting polymer buffer layer at the same parameters,
subjected to same annealing temperatures but distinct atmospheres, behaved rather
differently. Moreover, the proportion content of PSS to PEDOT on the surface
varied depending on the atmosphere and temperature in which the sample was
annealed. There are other consequences with composition ratio differences and one
of the major influences is on the subsequent wetting of the active layer. When the
PSS content is getting reduced, the layer may become less hydrophilic and the
65
active layer consisting of non-polar solvent might have better wettability.
Analysing only the conductivity changes in the sole layer would not guarantee its
optimum functionality in the overall performance of the device. Thus the
conducting polymer layer functionality on the whole device performance was
important for us owing to the wettability variations after annealing. The
conformational structure of PEDOT:PSS consists of random coil entanglement of
PSS chains with attached PEDOT oligomers, and the components are distributed
such that the PEDOT-rich core is covered by PSS-rich shell [177]. The shrinkage
and expansion of these shells depends on the water content and evidently the
annealing affect significantly on these variations, morphology of the layer and how
it may expose more conductive PEDOT grains to the surface etc., which is directly
reflected in enhanced JSC and PCE values. It was reported that the degradation of
the conducting polymer initiates at higher temperatures above 260 °C [163]. This
effect is consistent with our results where the devices exhibited the lowest
performance at its maximum annealing peak in the experiment. The implication of
the achieved results in the study is the fact that proper annealing condition can be
used as a tool to control the composition of the HTL/PEDOT:PSS layer to obtain
enhanced device performance. Moreover the optimal compositional ratios for
PEDOT:PSS in various annealing conditions were also found out. This may help
the industry to design the conducting buffer layers in future to deliver cost effective
and superior performance irrespective of atmosphere annealing constraints.
5.2 Stability enhancement and experimental analysis
Once the highest possible efficiency of the OSC-BHJ device is achieved, the next
challenge is to sustain at least 80% of its initial efficiency as long as possible. This
is a general problem for the devices in the field of organic electronics. In this section
of work regarding the stability studies of the devices, we have utilised an empirical
approach which identified and limited some of the stability degenerating
aberrations occurring in various strata and component layer interfaces within the
device structure. Precisely the reactions [122] occurring at ITO anode and
PEDOT:PSS conducting polymer buffer layer interface contribute significantly
towards the stability degeneration. In the conventional OSC-BHJ structure, the
same interface occurs predominantly. Moreover, the previously mentioned etching
reaction on ITO could result in decreasing the conductivity of the transparent
conducting anode. This might in turn act responsible for decreasing the device
efficiency in the process owing to increased series resistance. It can be assumed
66
that a suitable interlayer would reduce or screen the direct acidic effect of the
conducting polymer buffer layer on the anode. It was reported by W.J.Yoon [178],
that the plasma treated AgOx interlayer between the ITO and PEDOT:PSS enhanced
the efficiency of the device by providing the interface energy step which was
reflected by 2 mA/cm2 increase in JSC. Thus, it inspired us to conduct the study
utilising plasma treated Ag as an interlayer in view of obtaining both increased
efficiency and stability. From our experiment, the stability of the device was
significantly improved with a thicker interlayer, although there was no significant
increase in the efficiency. The situation is ideal if the interlayer could increase the
efficiency of the device significantly and contribute to increase stability by
reducing the internal degradation of the device. The other methods that can be
employed in mitigating such interface degradation inside the device are the choice
of ITO compatible HTL layers such as molybdenum trioxide, adoption of inverted
architecture, use of neutralised conducting polymer buffer layer and use of ITO
alternatives. However, all these methods are not perfect and have many
shortcomings. For instance, the processability of the neutralised PEDOT:PSS is
difficult due to its low oxidation potential [179,180] and presence of unfavourable
additives. Our method is found to be very relevant and efficient among them, since
we had addressed the problem directly at the interface and not avoided either of the
mutually reactive component layers which were some of the previously existing
solutions.
During the usage cycle of OSC-BHJ devices, there are two major stability
degenerating stress issues occurring on the device. These are issues relating to
bending stress and thermal exposure due to prolonged illumination. The devices
undergo bending or flex stresses during fabrication (roll to roll production),
deployment, transport and critical angular stress for portable devices in the hands
of the end user etc. Especially when ITO is utilised as their transparent conducting
anode, these bending stresses can seriously influence the device performance
owing to the brittle nature of the anode. Thus, we have done experimental
investigation to obtain an understanding of the influence of already cracked ITO on
the device performance. But our experiments done on the substrates revealed that
if the ITO is broken within certain limiting diameter of bending, the influence is
not significant on smaller scale OSC. Previous reports [181] suggest that solar cell
device efficiency should drop with the conductivity drop due to mechanical defects
on ITO. There are many possible explanations for the results we obtained. One of
the possible explanations why the performance of the device did not collapse with
the presence of cracks in ITO may be due to the fact that the conductivity losses
67
due to cracks may be compensated with the infiltration of conducting polymer
buffer layer. It might also have influenced on enhancing light scattering in to the
active layer. In addition to these, the dimensions of the devices were rather small.
It is possible therefore to expect that the mechanical defects on ITO might not yet
decrease the efficiency of the device in such smaller areas. There is also a high
probability that the performance depends on the extent of congruency in the surface
defects on ITO. Further investigations are necessary to understand the real
mechanism behind these observed effects and analysis on larger dimensions might
provide better insights regarding the phenomenon.
In the case of outdoor application of OSC, these devices are exposed to
prolonged solar radiation. The significant elevation in module temperature and hot
spots of over hundred degrees are some of its ramifications. The investigations on
these thermal variations generated and subsequent effect on the device performance
in OSC devices was rarely focussed. The organic active layers of the OSC-BHJ
devices which are known to be thermally sensitive [141] is a core area to
concentrate in view of enhancing stability of the device. Unlike some of the semi-
crystalline polymers like P3HT, the active layer based on low band gap polymer
PTB7 and fullerene does not have any positive effect on thermal influence. Thus
the experimental investigation analysing the impact of varied thermal exposure on
the high efficiency DIO doped PTB7:PC70BM active layer on device performance
is interesting. Most of the previous studies were based on analysing the effect at
constant temperature. But we explored the issue closer to real life situation where
the temperature changes are dynamic. The efficiency of the device was found to
drop with the prolonged thermal exposure. Degradations due to fullerene
aggregation and its excessive crystallisation under prolonged thermal exposure can
also contribute to the observed decline in PCE values. Although the additive DIO
enhances the initial device efficiency in the PTB7:PC70BM based active layers by
enhancing fine scale morphological distribution [147], it is reported to have many
deteriorating properties [182] in the long run, specifically under extended thermal
influence. Moreover it was reported [183] that the presence of DIO makes the
active layer more prone to photo degradation when exposed to sunlight, thus
contributing adversely in the outdoor application of the device. The temperature
variations undergone by the active layer are well below the boiling point of the
additive, thus the presence residual additive might have continued its negative
contributions towards stability throughout the process. The morphological changes
of the active layer blend during prolonged thermal exposure can affect the charge
mobility and the generation of charge transfer complex which is reflected in
68
reduced current density of the devices under prolonged thermal exposure. The
probability of donor-acceptor phase segregation and changes in defect states which
control the recombination is reflected in the VOC of the device which was found to
rise during the heating process. The fill factors of the devices were also found to
drop which suggest an increase in series resistance and limitation in the charge
transport. The existence of some period of duration where the device recovered its
performance to a certain extent was also observed. The morphological
transformation of the active layer under study has undergone a fluctuated
temperature conditions characterised by high and low values. Thus the real
characteristic transformations and mechanisms behind this could be expected to be
much more complex. The deeper analysis on the phenomenon could be gathered
by analysing carefully the morphological transformations at each temperature and
correlating it with the photovoltaic characteristics. Nevertheless, the strategies to
enhance the thermal stability allow OSC technology to ensure the continuous,
prolonged and efficient operation of the devices under extensive outdoor
applications.
5.3 Future outlook
In our future work, the focus would be placed more on obtaining very high
efficiency devices by conducting deeper investigations on low band gap polymer
based active layers and also the organic-inorganic hybrid perovskite devices. Some
of the methods developed in this study are equally applicable in inverted
perovskite-organic photovoltaic devices. The effectivity of the techniques
developed during this study would be also explored in those devices as well.
Although the most efficient acceptor material till date is fullerene, it is one of
the expensive parts in OSC. Thus a low-cost and superior alternative for fullerenes
would be investigated in our further studies. There is also as added reason which
motivates us for the search of fullerene alternatives and it is because of the fact that
the fullerenes, especially [6,6]-Phenyl-C61-butyric acid methyl ester contributes
towards the stiffness of the active layer which in turn reduces the mechanical
freedom of the flexible OSC devices.
Another issue that will be handled in our future work is the reuse and recycling
of the device substrate. The thorough focus on fully recyclable organic
photovoltaics will give boost to the environmental friendliness of the technology.
Strategies to reuse the substrate would greatly help to reduce the overall
manufacturing cost which helps the industry to produce devices at higher profit
69
margin. The reduced manufacturing cost will also reflect in the final product price
which makes the devices more affordable and attractive for the consumers in the
commercial market. Especially the re-use of ITO patterned PET substrates can help
in reducing the production cost due to limited indium supply and expenses related
to patterning requirements. This will also enable the technology to attain full green
label since the plastic flexible substrates also get recycled or reused in the process.
The ITO is still is the leader in transparent electrode market owing to its excellent
optical transparency and conductivity. There are a few alternative exists today
although their performance is little less than ITO. Few of these include graphene
layers, highly conductive poly (3,4-ethylene- dioxythiophene)-poly(styrenesulfo-
nate) and transparent conducting layer with silver nanowires embedded in it.
However, most of these alternatives are still in its research phase and not widely
available with a reasonable price.
Our future research will be taking in to account all the above mentioned aspects
in a view of obtaining efficient, inexpensive, stable and eco-friendly organic
photovoltaic devices.
70
71
6 Summary
OSC is currently facing a variety of challenges and the main bottleneck is its lower
efficiency and stability when compared with other thin film solar cell technologies.
The motivation of this thesis was based on the fact that simple and robust
techniques are necessary to synergistically enhance the efficiency and stability of
the device. This thesis identified various factors causing low efficiency and stability
in BHJ-OSC, some of its mechanisms were understood and methods to reduce
those degradations were also explored. Fig. 20 illustrates problems studied in the
scope of this thesis.
Fig. 20. Issues analysed in the thesis.
The studies conducted and methods developed in this thesis were regarding mostly
the materials commonly used among many other organic devices apart from OSCs.
The organic photovoltaic materials are also useful in photodetector applications
such as photoelectric transducers for optical communications or optical imaging
systems. Thus the results of the study on component layers have broader scope of
utility. Especially the results regarding ITO and PEDOT:PSS may find its
applicability also among Organic light emitting diodes (OLEDs) and inverted
perovskite-organic PV devices where some of the mentioned component layers are
also used. The efficiency and stability issues described in the thesis occur usually
in the device and the organic solar cell industry faces these problems very often.
We recognise that our conducted studies addressing these issues might therefore
impact the OSC technology by providing choices in problem mitigating techniques
and pin pointing the core areas to focus.
72
73
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Original articles
I Augustine B & Fabritius T (2015) Synergetic enhancement of device efficiency in poly(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl C61 butyric acid methyl ester bulk heterojunction solar cells by glycerol addition in the active layer. International Journal of Photoenergy Article ID 414851, DOI: http://dx.doi.org/10.1155/2015/414851.
II Augustine B & Fabritius T (2015) How to control component ratio of conducting polymer blend for organic photovoltaic devices by annealing. International Journal of Photoenergy Article ID 532489, DOI: http://dx.doi.org/10.1155/2015/532489.
III Augustine B, Sliz R, Lahtonen K, Valden M, Myllylä R & Fabritius T (2014) Effect of plasma treated Ag/indium tin oxide modification on stability of polymer solar cells. Solar energy materials & Solar Cells 128: 330-334.
IV Leppänen K, Augustine B, Saarela J, Myllylä R & Fabritius T (2013) Breaking mechanism of indium tin oxide and its effect on organic photovoltaic cells. Solar energy materials & Solar Cells 117: 512-518.
V Augustine B & Fabritius T (2015) Performance of 1,8-diiodooctane (DIO) doped PTB7:PCBM based organic solar cell under simulated solar heating profile over 24 hours. Proceedings of 31st European photovoltaic solar energy conference and exhibition, 14-18 September, Hamburg, Germany: 1111-1113, DOI: 10.4229/ EUPVSEC20152015-3BV.5.11.
Reprinted with permissions from Hindawi Publishing Corporation (I & II), Elsevier
(III & IV) and EU PVSEC (V).
Original publications are not included in the electronic version of the dissertation.
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