Polymer Photovoltaics: A Practical Approach (SPIE Press Monograph Vol. PM175)

Polymer Photovoltaics A Practical Approach

Polymer Photovoltaics A Practical Approach

Frederik C. Krebs, Editor

Bellingham, Washington USA

Library of Congress Cataloging-in-Publication Data Polymer photovoltaics : a practical approach / Frederik Krebs, editor. p. cm. Includes bibliographical references and index. ISBN 978-0-8194-6781-2 1. Photovoltaic cells--Materials. 2. Conducting polymers. I. Krebs, Frederik. TK8322.P69 2008 621.31'244--dc22

2007050208

Published by SPIE P.O. Box 10 Bellingham, Washington 98227-0010 USA Phone: +1 360 676 3290 Fax: +1 360 647 1445 Email: [email protected] Web: http://spie.org Copyright © 2008 Society of Photo-Optical Instrumentation Engineers All rights reserved. No part of this publication may be reproduced or distributed in any form or by any means without written permission of the publisher. The content of this book reflects the work and thought of the author(s). Every effort has been made to publish reliable and accurate information herein, but the publisher is not responsible for the validity of the information or for any outcomes resulting from reliance thereon. Printed in the United States of America.

Contents List of Contributors

ix

Preface

xi

List of Abbreviations 1 Introduction 1.1 Human Energy Consumption Now and in the Future 1.2 Renewable Energy Sources 1.3 Important Facts About Energy, Energy Conversion, the Earth, and the Sun 1.4 Solar Energy 1.5 The Storage and Relocation Problem 1.6 Types of Solar Cells 1.7 Current Challenges References 2 The Polymer Solar Cell 2.1 Introduction 2.2 Materials 2.2.1 Polymers 2.2.2 Molecules and oligomers 2.3 Fast and Simple Guide to a Polymer Solar Cell from Scratch 2.3.1 Equipment 2.3.2 The substrate 2.3.3 The PEDOT:PSS layer 2.3.4 The active layer 2.3.5 Evaporating the electrode 2.3.6 Applying electrodes and measuring the electrical properties of the devices 2.3.7 Device preparation and performance References

xiii 1 1 2 4 4 5 6 8 9 11 11 12 12 34 42 42 43 46 47 64 65 68 79

vi

Contents

3 Characterization of Organic Solar Cells 3.1 Taking the Sun Inside 3.1.1 Air mass 3.1.2 The ASTM E 927-05 standard and the IEC 904-9 standard 3.1.3 Types of simulators 3.1.4 Halogen lamps 3.1.5 Recording the spectrum 3.1.6 Applying filters to improve the spectrum 3.1.7 Spectral, temporal, and spatial homogeneity of the light field 3.1.8 Calibration of the sun simulator 3.2 IV-Curves and Efficiencies 3.2.1 The source meter 3.2.2 Where the electrons are and how to connect your cell to the outside world 3.2.3 Speed of IV-curve measurement, dielectric relaxation, and capacitive loading 3.2.4 Action spectra using a high-power spectrometer 3.2.5 IPCE measurements using a simple high-power spectrometer 3.2.6 Environmental effects 3.3 Outdoor Measurements 3.3.1 Why outdoor photovoltaic characterization is necessary for organic solar cells 3.3.2 Experimental procedure 3.3.3 Temperature dependence of the photovoltaic parameters of BHJ solar cells 3.3.4 Example of long-term outdoor testing of stability of organic solar cells 3.3.5 Some new experimental possibilities and suggestions for future studies 3.4 Methods for Preparation and Characterization of Thin Films 3.4.1 Controlling morphological properties 3.4.2 Techniques for monitoring morphology References

91 91 92

128 131 133 144 148

4 Lifetime and Stability Studies 4.1 Overview 4.2 Studies of Degradation Mechanisms Using TOF-SIMS 4.2.1 Principle of TOF-SIMS 4.2.2 Isotopic labeling

155 155 156 156 159

94 97 97 99 100 101 103 105 105 105 108 109 110 112 112 112 113 116 123

Contents

4.2.3 TOF-SIMS depth profiling 4.2.4 Gaining access to the various layers in the photovoltaic device 4.2.5 TOF-SIMS imaging 4.2.6 Chemical structure elucidation based on mass spectral information 4.2.7 Monitoring photooxidation in time—mapping the “history” of degradation 4.3 Studies of Degradation Mechanisms Using XPS 4.3.1 The principle of XPS 4.3.2 Chemical shifts 4.3.3 Angle-dependent studies 4.3.4 Experimental details 4.3.5 Device aging and IV measurement 4.3.6 XPS overall observations 4.3.7 Li and F distribution 4.4 Studies of Degradation Mechanisms Using RBS 4.4.1 Principles and quantitative depth profile of the composition 4.4.2 Studies of cathode degradation using RBS 4.5 Studies of Degradation Mechanisms Using Physical and/or Spectroscopic Techniques 4.5.1 Interference microscopy 4.5.2 Atomic force microscopy (AFM) 4.5.3 Scanning electron microscopy (SEM) 4.5.4 Fluorescence microscopy 4.6 Accelerated Lifetime Measurements for Extended Periods of Time 4.7 Apparatus for Lifetime Measurements and for Isotope Labeling References 5 Processing and Production of Large Modules 5.1 Printing and Coating Methods 5.1.1 R2R coating 5.1.2 Screen printing 5.1.3 Pad printing 5.1.4 Doctor blading 5.1.5 Other printing methods 5.2 Printing the Active Layer 5.2.1 Screen printing 5.3 Carrier Substrates

vii

159 167 170 178 185 187 188 189 189 189 191 192 193 197 198 201 210 211 212 213 216 217 220 223 229 232 232 234 237 237 239 239 239 266

Contents

viii

5.4 Anodes and Cathodes 5.5 Processing of the Transparent Front-Side Contact 5.5.1 PEDOT as transparent contact 5.5.2 Introduction of a conductive grid into photovoltaic devices 5.6 Processing of the Opaque Back-Side Contact 5.6.1 Ag-based pastes as back-side contacts 5.7 Encapsulation and Permeability 5.7.1 Measurement of permeability 5.7.2 Measurement of the diffusion coefficient D 5.7.3 Units 5.7.4 Apparatus 5.7.5 An example of a commercial instrument 5.7.6 The calcium test 5.7.7 Mass spectrometry 5.7.8 Tritiated water 5.7.9 Oxygen permeation in PEDOT 5.8 Practical Encapsulation Techniques 5.8.1 Rigid encasement at IMEC (Belgium) 5.8.2 Small rigid encasement at Risø National Laboratory (Denmark) 5.8.3 Large rigid encasement at Risø National Laboratory (Denmark) 5.8.4 Flexible encasement 5.9 Production and Companies 2007 5.9.1 Intellectual property rights in Europe, the United States, and Asia 2007 5.9.2 A road map for setting up a company producing OPVs in Europe 5.9.3 What production equipment is available in 2007 References 6 Outlook 6.1 Where Is the Technology Now? 6.2 Where Is It Suitable? 6.3 Where Could It Be in the Next Decades? References Index

266 267 269 272 274 275 279 279 281 282 282 283 283 285 285 285 286 287 287 289 290 291 293 294 295 295 301 301 302 303 306 307

List of Contributors Tom Aernouts IMEC Kapeldreef 75 B-3001 Leuven Belgium Rémi de Bettignies Commissariat à l’Ènergie Atomique 17, rue des Martyrs 38054 Grenoble cedex 9 France Eva Bundgaard Risø National Laboratory Technical University of Denmark Frederiksborgvej 399 DK-4000 Roskilde Denmark Stéphane Cros Commissariat à l’Ènergie Atomique 17, rue des Martyrs 38054 Grenoble cedex 9 France Muriel Firon Commissariat à l’Ènergie Atomique 17, rue des Martyrs

38054 Grenoble cedex 9 France Mikkel Jørgensen Risø National Laboratory Technical University of Denmark Frederiksborgvej 399 DK-4000 Roskilde Denmark Eugene A. Katz Ben-Gurian University of Negev P.O.B. 653 Beer-Sheva 84105 Israel Frederik C. Krebs Risø National Laboratory Technical University of Denmark Frederiksborgvej 399 DK-4000 Roskilde Denmark Kion Norrman Risø National Laboratory Technical University of Denmark Frederiksborgvej 399 DK-4000 Roskilde Denmark

Preface Polymer photovoltaics is a discovery that potentially houses the solutions to many of the problems currently encountered with traditional photovoltaic technologies. Most notably, the technology offers the possibility for ultrafast processing, low cost, light weight, flexibility, and a very low thermal budget. The technology rests on a moderately solid base of scientific literature spanning from the first prototypical literature reports. Among the most prominent contributors are the groups of C.W. Tang, R. Friend and A.J. Heeger through an impressive number of original research papers documenting a steady increase in the performance at the level of very small devices with power-conversion efficiencies of up to around 5% for single junctions, which today represent the state of the art. This base of research reports, conference proceedings, reviews, and even many books makes the topic highly accessible to the newcomer and as such there is no need for a new book on the topic from a theoretical or explanatory point of view. One of the problems when entering the field of organic photovoltaics is getting a good idea of how to actually make devices, how to study them, and how to characterize them. The ambition of this book is that it should be a practical guide in the laboratory for the experimental solar cell scientist whether he or she is involved with synthesis, device preparation, processing, or device characterization. Our feeling is that such an experimental guide will be useful to all scientists working practically in the field. This book presents the process of creating a polymer solar cell device starting with a description of materials including how they are made and characterized, followed by how the materials are processed into devices and films and how these are characterized. Following on from this, the status of two emerging fields of polymer solar cells are described, namely, degradation and stability, and large-scale processing. Frederik C. Krebs December, 2007

Acknowledgements Technical assistance by Jan Alstrup is greatly acknowledged. Rolf H. Berg is acknowledged for carrying out a detailed freedom-to-operate analysis within organic photovoltaics. Suren Gevorgyan is acknowledged for preparing small encapsulated modules for the purpose of this book.

List of Abbreviations [60]PCBM or PCBM [70]PCBM AA AFM Ag paste Al Alq3 AM ATRP BCP BE BHJ BIF Ca CAFM CdTe CEA CHA Cu(In,Ga)Se2 CuPc CV CVD DA dppp DSC DTR DUT Ea EBPVD ECN EDG EDX EQE ERDA

[6,6]-phenyl C61 -butyric acid methyl ester [6,6]-phenyl C71 -butyric acid methyl ester Atmospheric air Atomic force microscopy Silver-(epoxy) paste Aluminum Tris(8-hydroxyquinolinato)aluminum Air mass, amount of atmosphere light passes through Atom transfer radical polymerization Bathocuproin Binding energy Bulk heterojunction Barrier improvement factor Calcium Conductive atomic force microscopy Cadmium telluride Commissariat à l’Énergie Atomique Concentric hemispherical analyser Copper indium-gallium diselenide Copper phthalocyanine Cyclic voltammetry Chemical vapor deposition Dry air 1,3-bis(diphenylphosphino)propane Differential scanning calorimetry Diffusion transfer reversal Device under test Activation energy Electron beam physical vapor deposition Energy Research Centre of the Netherlands Electron donating group Energy-dispersive x-ray analysis External quantum efficiency Elastic recoil detection analysis

xiv

ESCA eV EVA EWG F8BT FF FWHM GaAs GW HC-PEDOT HOMO HPLC HTO HWE Impp IPCE IPR Isc ISE ITN ITO IV IV-curve I-V-L K kB kdeg KE KHS575 KOH LCD LED LiF LIP LUMO MALDI-TOF MDMO-PPV MEH-PPV MeOH MeV mfp

List of Abbreviations

Electron spectroscopy for chemical analysis Electron volt ethyl vinyl acetate Electron withdrawing group Poly(9,9-dioctylfluorene-co-benzothiadiazole) Fill factor full width at half maximum Gallium arsenide Gigawatt = 109 W Highly conductive PEDOT Highest occupied molecular orbital High-perfomance liquid chromatography tritium-containing water Horner-Wadsworth-Emmons Current at maximum power point Incident photon to current efficiency Intellectual property rights Short-circuit current Institut für Solare Energisysteme Isothianaphthene Indium tin oxide Current-voltage Current-voltage diode characteristics Current-voltage-luminance Acceleration factor Boltmann constant Degradation constant Kinetic energy Sun simulator from Steuernagel Lichttechnik Potassium hydroxide Liquid crystalline display Light-emitting diode Lithium fluoride Localized irradiation probe Lowest unoccupied molecular orbital Matrix-assisted laser desorption/ionization–time-of-flight Poly(2-methoxy-5-(3,7-dimethyloctyloxy)1,4-phenylenevinylene) Poly[2-methoxy-5-(2-ethylhexyloxy)1,4-phenylenevinylene] Methanol Megaelectron volt Mean free path

xv

Mg MgSO4 Mol wt MW NBS NIR NMR NRA NREL ODCB OLED OPV OTR P3CT PCE PCT PD PE PEDOT PEN PEOPT PET PFB PFO Pin PITN PLD PLED Pmax POMeOPT Pout PPV PSS PTCA PTCBI PTOPT PTV PV PVD R2R

Magnesium Magnesium sulfate Molecular weight Megawatt N-bromosuccinimide Near infrared Nuclear magnetic resonance Nuclear reaction analysis National Renewable Energy Laboratory Orto-dichlorobenzene or 1,2-dichlorobenzene Organic light-emitting device or organic light emitting diode Organic photovoltaic Oxygen transmission rate Poly(3-carboxythiophene-co-thiophene) Power conversion efficiency Patent cooperation treaty Polydispersity Poly(ethylene) Poly(ethylendioxythiophene) Poly(ethylenenaphthalate) Poly[3-(4 -(1 ,4 ),7 -trioxaoctyl)phenylthiophene] Poly(ethylene terephthalat) Poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bisN,N-phenyl-1,4-phenylenediamine) Poly(9,9-dioctyl-fluorene) Incoming solar power Poly(isothianaphthene) Pulsed laser deposition Polymer light-emitting device or polymer light emitting diode Maximum power Poly[3-(2 -methoxy-5 -octylphenyl)-thiophene] Output electrical power Poly(phenylenevinylene) Poly(styrene sulfonic acid) Perylene tetracerboxylic acid Perylene tetracarboxylic acid bisimide Poly[3-(4-octylphenyl)-2,2 -bithiophene] Poly(thienylenevinylene) Photovoltaic Physical vapor deposition Roll-to-roll

xvi

RBS RR Rs SD SEC SEM SIMS SMU STC Ta TCO TE TEM TEMPO Tg THF TOF TOF-SIMS TW UHV UPS UV UV-vis VD VE Vmpp Voc VTE Vth W Wp WVTR XPS XRD XRD ZJ η ηe ηn Φ Ø

List of Abbreviations

Rutherford backscattering Rectification ratio Series resistance Sputter deposition Size exclusion chromatography Scanning electron microscope Secondary-ion-mass spectrometry Source measure unit Standard test conditions Annealing temperature Transparent conductive oxide Thermal evaporation Transmission electron microscopy 2,2,6,6,-tetramethylpiperidin-1-oxyl Glass transition temperature Tetrahydrofurane Time-of-flight Time-of-flight secondary-ion-mass-spectrometry Terawatt = 1012 W Ultrahigh vacuum Ultraviolet photoelectron spectroscopy Ultraviolet Ultraviolet-visible Vapor deposition Vacuum evaporation Voltage at maximum power Open-circuit voltage Vacuum thermal evaporation Theoretical paste volume Watt Watt peak Water vapor transmission rate X-ray photoelectron spectroscopy Grazing-incidence x-ray diffraction X-ray diffraction Zettajoule = 1021 J Viscosity Energy conversion efficiency Viscosity in a spin-coating process Work function Diameter

Chapter 1

Introduction Frederik C. Krebs Photovoltaic devices convert light energy directly into electrical energy, and the primary objective of their use is the harvesting of light energy from the sun. The photovoltaic devices are silent when they operate and have no mechanical movement associated with their function. They operate under illumination and produce an electrical current that can be directly consumed or stored by chemical (batteries, hydrogen, etc.) or mechanical means (flywheels). The current conversion of power from sunlight into electrical power is not highly efficient; however, improvements are being made all the time. Aside from the very occasional solar eclipse, the sun is a reliable source of energy that the human race can depend on for the next five billion years.

1.1 Human Energy Consumption Now and in the Future The level of energy consumption by the humans on the planet in 2004 was approximately 15 terawatts (TW), with everything included. Most of this energy (87%) was derived from fossil-fuel sources.1, 2 Continuous industrialization of developing countries, growth in human population, and a general increase in human welfare is projected to increase the demands for energy in the future by a large proportion. By the year 2050, the anticipated level of energy consumption by humans is 28– 35 TW, which is a challenge we currently cannot meet with the sources of energy available. The global economy today is based on fossil fuel (coal, oil, and, gas) and, as a resource, is generally accepted to follow the Hubbert peak theory that, in brief, claims the rate of petroleum production follows a bell-shaped curve. The point at which the global demand for petroleum exceeds the rate of production is termed “peak oil.” This point in time is when the global economy is predicted to collapse with more or less disastrous consequences. Depending on who you ask, peak-oil production may already have been reached or is imminent. The Hubbert peak theory is based on the fact that the resources are finite and that the rate of production to a rough approximation follows the rate of discovery with a time lag. For any resource, the rate of discovery is small to begin with followed by an increase to a certain maximum and then finally a decrease.

Chapter 1

2

Table 1.1 Fossil fuel use (in 2004) compared with the reserves.1, 2 The estimated time left is under the assumption that the annual consumption does not increase (1 TW = 1 terawatt = 1012 W, 1 ZJ = 1 zettajoule = 1021 J).

Form

Consumption (TW)

Coal Solid 3.8 Oil Liquid 5.6 Natural gas Gaseous 3.5

Annual consumption (ZJ year−1 ) 0.12 0.18 0.11

Energy reserve (ZJ2 )

Time left (years)

290 57 30

2400 316 272

Note: There is an estimated reserve of 2500 ZJ if using the uranium from the earth’s crust with currently available extraction techniques. This assumes the use of breeder reactors that generate more fissile material than they consume. Fusion is currently not in practical use and is not currently considered as a serious alternative to energy production.

The generally accepted view is that there are still plenty of fossil fuels left (see Table 1.1) and that it may be an overreaction to change our energy consumption pattern toward renewable energy sources, nuclear fission reactors, and nuclear fusion. Nuclear energies will not be discussed in this introduction simply because they are, as it will become apparent, not necessary seen in the light of the wealth of renewable energy sources that are available. However, energy derived from fossil fuels does produce carbon dioxide and the disaster that looms may not be a shortage of fossil fuels but rather global warming as a result of our excessive abuse of the energy derived from fossil sources. In reality, there is no argument; we should urgently change horses and aim for renewable and non-CO2 -emissive sources of energy.

1 2 Renewable Energy Sources The most ironic outcome of the analysis of renewable energy sources3, 4 is that there is a multitude of sources to choose from and the energy available from them is plentiful; from some of the sources unfathomable amounts of energy are available. The renewable energy sources available are largely covered by hydropower, biomass energy, solar energy, wind energy, geothermal energy, and ocean energy. The current use of the various renewable energy sources are presented in Table 1.2. It is interesting to note that about three-quarters of the renewable energy produced is in the form of electrical energy. This is due to the extensive use of hydropower. Some of the renewable energy sources are inherently inferior, such as ocean energy that can be divided into tidal energy and wave energy. The former is due to the gravitational pull of mainly the moon but also the sun, while the latter is derived from wind that is derived from the sun and thus has a low potential due to losses with each conversion. When comparing all these technologies according to the theoretical potential (how much energy there is available), the technical potential (how much we can extract), and the current use, it quickly becomes apparent

Introduction

3

Table 1.2 The current use of renewable energy sources as electricity and heating where applicable in figures of continuous energy consumption (1 GW = 109 J s−1 ).4

Hydropower Biomass energy Solar energy Wind energy Geothermal energy Ocean energy Total

Electricity (GW) 816 44 5.4 59 9.3 0.3 934

Heating (GW) − 220 88 − 28 − 336

Total (GW) 816 264 93.4 59 37.3 0.3 1270

Table 1.3 The renewable energy resources in terawatt (1 TW = 1012 W). Global use in 2004 = 15 TW.

Hydropower Biomass energy Solar energy Wind energy Geothermal energy Ocean energy Total

Current use (TW) 0.816 0.264 0.0934 0.059 0.0373 0.0003 0.934

Technical potential (TW)a 1.6 >7.9 >51 19 158 6c >238

Theoretical potential (TW)a 4.8 92 124000 190(370b ) 4440000 235 >4560000

a

Esti–mates taken from Ref. [4]. Upper estimate. c Tidal power = 3 TW, wave power = 3 TW. b

that solar energy is used little, while there is a massive theoretical and technical potential for its use.3, 4 By comparison, hydropower and biomass are the only two well-exploited renewable energy sources. Wind energy is established and is widely used; however, the technical potential is much larger than its current use. In Table 1.3, the continuously available energy is compared in terms of the current use and the technical and theoretical potential. Table 1.3 shows that solar energy and geothermal energy outweigh all other renewable energy sources by orders of magnitude when comparing the theoretical potentials. In terms of the technical potential that comprises an estimate of our ability to make use of the theoretical potential, solar energy and geothermal energy are still the two renewable sources with the largest potential, but by a smaller margin. After this brief examination of the currently accepted renewable energy sources, it should be clear that solar energy is a renewable energy with a large potential. This is the motivation for many scientists working in this field.

4

Chapter 1

1.3 Important Facts About Energy, Energy Conversion, the Earth, and the Sun The sun is at the center of our solar system and is a type G2 star. It consists of 70% hydrogen, 28% helium, and 2% of all the other elements. While essentially being a cloud of gasses, it is a massive object held together by gravitational forces. The sun has a diameter of 1.39 × 107 m, and with a mass of 1.989 × 1030 kg it accounts for 99.8% of the total mass in our solar system. The temperature and pressure in the center of the sun reach very high levels, with temperatures of 15.6×105 K and pressures of more than 25 × 1010 atmospheres that allow for nuclear fusion processes to take place, thus producing energy. The fusion processes mainly involve hydrogen, which gives helium as a product. At the core of the sun, 7 × 1011 kg of hydrogen is converted into 695 × 109 kg of helium every second, releasing 5 × 109 kg of energy in the form of gamma rays, which is equivalent to 3.86 × 1026 J. The gamma rays make their way to the surface of the sun by absorption and reemission at lower and lower temperatures until they reach the surface mainly as visible light. The temperature at the surface that is what we can observe directly on Earth amounts to 5800 K (while the temperature of sunspots can descend to 3800 K). The distance from the sun to the earth is 149.6 × 108 m and the earth has, with its much smaller mass of 5.98 × 1024 kg, a diameter of 12.756 × 103 m. The energy from the sun is emitted in all directions meaning that the energy density at the distance from the sun where the earth is situated is lower. The energy in space just outside the earth’s atmosphere is 1366 W m−2 , and when the light energy passes the atmosphere some of the visible light energy is lost by absorption in specific regions of the spectrum. This is termed the “air mass” and is abbreviated AM followed by a number (see Fig. 1.1 and Chapter 3 for further details). The absorption loss amounts to 28%, giving about 1000 W m−2 at the surface of the earth under ideal conditions throughout the year. A year has 8760 hours and at any given location on earth half of these are nighttime, giving 4380 hours of daylight averaged over a year. In practical terms, there are many dependencies, such as latitude, earth rotation, and cloud cover, that on the average give 1800 hours of sunlight available for power production every year. In a country such as Denmark, this implies that a solar panel of 1 kWpeak will generate 850 kW-hours of electricity on an annual basis.

1.4 Solar Energy The earlier discussion of our fossil-fuel energy past gives a daunting projection of our energy future. The good news, however, is that we receive plenty of energy from the sun and the challenge is simply to make use of it. The side of the earth that is exposed to the sun receives approximately 1.2 × 105 TW from the sun continuously, which is approximately 10,000 times the energy we consumed in the year 2004. Even with the projected doubling of our energy consumption over the

Introduction

5

Figure 1.1 An illustration of the solar spectrum in space just outside the atmosphere (air mass is negligible; AM 0) and the solar spectrum at the surface of the earth at the northern latitudes of Europe (AM 1.5). The distances in the drawing of the sun, earth, and atmosphere is not to scale.

next 50 years, there is plenty of energy available from the sun alone. The plentitude is such that we can easily accommodate for energy conversion factors well below 100% and we still have to cover only a fraction of the earth’s surface with energy extraction devices such as solar cells. There are obvious logistical problems associated with solar energy because the energy we receive and convert into electricity is lost if it is not used as it is generated. Even for a large energy-consumption system such as the earth, it is useful to buffer or store the energy if there is no immediate need for it when generated. Wind, wave, and photovoltaic solar energy all suffer from the problem of not being energy technologies that inherently allow for storage. Some good examples of energy technologies that intrinsically achieve this are nuclear power, hydropower, biomass, and geothermal power. In the latter case, energy is extracted from the subsurface of the earth and is in principle wasted if not extracted; thus, it does not fully comply with the concept of storage. Hydropower is a very good example of a renewable energy source, where water is collected in a reservoir and emptied from the reservoir through a turbine when needed. Biomass is another good example, where wood can be stored until needed. By today’s standards, biomass energy also accounts for biofuels such as bioethanol and biodiesel.

1.5 The Storage and Relocation Problem Our society has evolved over centuries, and our habit of consuming energy is deeply anchored in our way of thinking, working, and behaving. Most of our methods for extracting energy are naturally centered on the most efficient energy storage materials available such as coal, oil, and natural gas. As seen from the above, this is a fantastic solution since we have access to stored energy in solid, liquid, and gaseous

6

Chapter 1

form. It is easy to relocate the energy material to the consumer by transport through a carrier or a pipeline, and we are thus used to treating energy as something that is available at any given location at any given time. The main reason for this is that the energy is efficiently stored and there are few good alternatives to coal, oil, and gas. Renewable energy sources all suffer from the weakness that they are not all reliably available or storable. With photovoltaic systems, we only have reliable operation when the sun is shining, and while this happens at regular intervals (day and night), the energy output from a photovoltaic is highly dependent on the weather (cloud cover, time of the year in some locations, etc.). Wind energy has similar problems and while it is not limited in operation to hours of sunlight, it does depend heavily on the weather. It is paramount to view renewable energy sources in a large systems perspective. They all have advantages and disadvantages and must be coupled to some means of storage. This could be conversion to a storable chemical such as hydrogen, formic acid, methanol, or methane. As an example, wind energy is widely exploited in a country such as Denmark where 20% of the electricity production is covered by wind energy. This is a large proportion of the national electricity consumption and the nature of the energy source is highly dependent on the weather, meaning that there are periods when the energy that can be produced is not used because the demand is not there. Fortunately, there is a natural gas network in Denmark and if the technology for converting excess electrical energy into methane was available, it would be possible to produce methane from excess wind energy for storage and transmission through the existing gas distribution network.

1.6 Types of Solar Cells The archetypical photovoltaic device5 is a silicon-based PN-junction. Since its first incarnation, the silicon solar cell has taken many forms and is by far the most exploited photovoltaic technology. The monocrystalline silicon solar cells are generally better than the polycrystalline silicon cells with module efficiencies of up to 20% in the case of float zone–grown monocrystalline silicon. In terms of the best reported efficiencies for small-scale laboratory devices, crystalline silicon (24.7% efficiency) is only rivaled by gallium arsenide (GaAs, 25.1% efficiency) for singlejunction devices. Generally, photovoltaics based on the monocrystalline materials are costly and various thin film-technologies exist that offer lower cost both in terms of stock material and thermal budget. Amorphous silicon is perhaps the most well-known technology, and while it suffers from degradation under illumination, the technology offers stable efficiencies of around 5–10%. Other thin-film technologies are cadmium telluride (CdTe) and copper indium-gallium diselenide [Cu(In,Ga)Se2 ], and they both offer very attractive efficiencies that rival those of silicon with best efficiencies of, respectively, 16.5% and 18.9%. Finally, solar cells falling broadly under the heading of third-generation photovoltaics can be divided into dye-sensitized, organic, and polymer solar cells. The dye-sensitized

Introduction

7

Figure 1.2 Exploded view of the organic solar cell described by Tang.7

solar cells are best exemplified by the Grätzel cell, with best reported efficiencies of ∼11%.6 One of the earliest examples of organic solar cells was described by Tang7 in 1986 and consisted of two different small molecule organic compounds sandwiched between a glass substrate with indium oxide and a silver electrode as shown in Fig. 1.2. The organic components copper phthalocyanine (CuPc) and a bis imide derivative of perylene tetracarboxylic acid (PTCA) were robust enough to be evaporated at high temperature and under high vacuum onto the substrate. This device achieved a remarkably high power-conversion efficiency of ∼1% in the conversion of light to electricity. The light comes in through the transparent substrate, continues through the indium-oxide electrode, and is then absorbed in the organic layers. The excited state (exciton) can then give rise to a pair of charge carriers, namely, an electron and a hole. This is facilitated at the interface between the two types of organic materials: a donor (CuPc) and an acceptor (PTCA). The electrodes on each side of the cell then collect the charges. The small molecule organic solar cells today have best reported efficiencies of >5%.8 The solar cells based on small organic molecules are prepared by evaporation of the active material and this is viewed by some as being impractical in a large-scale production due to the requirement for a vacuum step. The preparation of solar cells entirely via solution processing techniques such as coating and printing are highly compatible with soluble conjugated polymers, and this is the most recent type of solar cell. The polymer solar cell has been subject to many advances and today almost all efficient polymer solar cells are based on the concept of a so-called bulk heterojunction (BHJ),9 whereby an acceptor and a donor component are mixed and processed simultaneously into the active film. Today, single-layer BHJ cells give efficiencies of ∼5% (see Ref. [10]), and efficiencies of 6.5% (see Ref. [11]) have been reported for tandem cells whereby polymer solar cells, as a technology, have become competitive with amorphous silicon.

8

Chapter 1

1.7 Current Challenges The use of photovoltaics is increasing dramatically, but in the context of the global energy consumption photovoltaics only account for an insignificant part of the amount of energy that is produced (∼0.036%). Most of the established photovoltaic technologies are stable and exhibit a relatively high power-conversion efficiency in the range of 10–20%, qualifying for electrical energy production for on-grid or local use provided that the land mass is available. However, they all suffer from a very high cost, giving energy prices of a few euros per watt, in 2006 terms. This is believed to be one of the reasons that the use is mainly limited to niche products and applications at remote locations. The general view is that a photovoltaic technology has to fulfill three criteria to reach a large market and provide on-grid electricity production.12, 13 The photovoltaic technology has to be stable, efficient, and of low cost (see Fig. 1.3). Crystalline silicon photovoltaics are very stable, with estimated operational lifetimes in excess of 25 years and module power conversion efficiencies as high as 20%. The cost, however, is too high and this is seen as the main reason for the scarcity of photovoltaic technologies on the energy scene. The special focus area of this book, polymer photovoltaics, holds some promise in this respect because it is a technology that in many ways is complementary to the prototypical silicon-based solar cells. The technology offers intrinsic flexibility, low cost, a low thermal budget, solution processing, and very fast methods for fabrication. The technology succeeds where the inorganic photovoltaic technologies have failed, namely, the cost issue. Conversely, the organic photovoltaics have until recently exhibited low stability and low efficiency, while this is improving and power conversion efficiencies of up to 6.5% and estimated operational lifetimes of many years have been reported.14 The main reasons for pursuing a technology that,

Figure 1.3 The Brabec triangle6 contrasting organic PV and silicon PV. The technologies are in many ways complementary; organic PV succeeds where silicon fails and vice versa. Silicon technologies are becoming available at a lower cost and organic PV is improving in terms of stability and efficiency.

Introduction

9

Figure 1.4 The unification challenge of combining efficiency, stability, and large-scale processing for the same material. The properties have been demonstrated individually and the combination of some stability and some efficiency has been achieved with some success.15

in principle, is inferior to the silicon photovoltaic technology is that it offers low production costs and fast processing, while there are other soft advantages, such as flexibility, light weight, and environmental benefits. Seen in light of the recent advances within the field of organic photovoltaics with respect to stability and power-conversion efficiency, the current challenge is the industrial demonstration of a low-cost organic photovoltaic module with moderate stability and efficiency. The secondary challenges are a higher stability of more than 10 years and efficiencies above 10%. Most notably, the demonstrations of moderate efficiency,11 high stability,14, 15 and large-scale processing16 have not been demonstrated for the same material. However, the fact that isolated studies can reach any of the three goals does hold promise for the possibility of combining all three goals in the same material, and this is the overall current challenge known as the unification challenge as illustrated in Fig. 1.4.

References 1. http://www.eia.doe.gov/pub/international/iealf/table18.xls. 2. http://pubs.usgs.gov/dds/dds-060/ESpt4.html#Table. 3. Simms, A., “It’s time to plug into renewable power,” New Scientist, 183(2454), pp. 18–19 (2004). 4. http://www.ren21.net/globalstatusreport/download/RE_GSR_2006_ Update.pdf

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5. Markvart, T., and Castañer, L., Solar Cells: Materials, Manufacture and Operation, Elsevier, Oxford (2005). 6. O’Regan, B., and Grätzel, M., “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films,” Nature, 353, pp. 737–740 (1991). 7. Tang, C.W., “2-Layer organic photovoltaic,” Appl. Phys. Lett., 48, pp. 183– 185 (1986). 8. Xue, J., Rand, B.P., Uchida, S., and Forrest, S.R., “A hybrid planar-mixed molecular heterojunction photovoltaic cell,” Adv. Mater., 17, pp. 66–70 (2005); Xue, J., Uchida, S., Rand, B.P., and Forrest, S.R., “Asymmetric tandem organic photovoltaic cells with hybrid planar-mixed molecular heterojunctions,” Appl. Phys. Lett., 85, pp. 5757–5759 (2004). 9. Yu, G., Gao, J., Hummelen, J.C., Wudl, F., and Heeger, A.J., “Polymer photovoltaic cells – enhanced efficiencies via a network of internal donor-acceptor heterojunctions,” Science, 270, pp. 1789–1791 (1995). 10. Li, G., Shrotriya, V., Huang, J., Yao, Y., Moriarty, T., Emery, K., and Yang, Y., “High-efficiency solution processable polymer photovoltaic cells by selforganization of polymer blends,” Nature Mater., 4, pp. 864–868 (2005); Ma, W., Yang, C., Gong, X., Lee, K., and Heeger, A.J., “Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology,” Adv. Funct. Mater., 15, pp. 1617–1622 (2005). 11. Kim, J.Y., Lee, K., Coates, N.E., Moses, D., Nguyen, T.Q., Dante, M., and Heeger, A.J., “Efficient tandem polymer solar cells fabricated by all-solution processing,” Science, 317, pp. 222–225 (2007). 12. Brabec, C.J., “Organic photovoltaics: technology and market,” Sol. Energy Mater. Sol. Cells, 83, pp. 273–292 (2004). 13. Krebs, F.C., “Alternative PV,” Refocus, 6, pp. 38–39 (2005). 14. Krebs, F.C., and Spanggaard, H., “Significant improvement of polymer solar cell stability,” Chem. Mater., 17, pp. 5235–5237 (2005). 15. Yang, X., Loos, J., Veenstra, S.C., Verhees, W.J.H., Wienk, M.M., Kroon, J.M., Michels, M.A.J., and Janssen, R.A.J., “Nanoscale morphology of highperformance polymer solar cells,” Nano Lett., 5, pp. 579–583 (2005). 16. Krebs, F.C., Spanggaard, H., Kjær, T., Biancardo, M., and Alstrup, J., “Large area plastic solar cell modules,” Mater. Sci. Eng. B, 138, pp. 106–111 (2007).

Chapter 2

The Polymer Solar Cell Mikkel Jørgensen, Eva Bundgaard, Rémi de Bettignies and Frederik C. Krebs 2.1 Introduction The first half of this chapter briefly describes the conceptual buildup of a polymer solar cell followed by an overview of the organic materials that have been used. The second half is devoted to a practical guide to making solar cells complete with synthesis, device fabrication, and testing. Today, most polymer solar cells are based on the bulk heterojunction concept described in 1995 by Yu et al.1 In this type of solar cell, the donor material (typically a polymer) is mixed with an acceptor (a soluble fullerene) in an organic solvent and then spin coated or cast on a substrate of indium-tin oxide (ITO) on glass. During evaporation of the solvent and latter treatments, a microphase separation takes place with the formation of an interpenetrating network. A hole-blocking layer of, e.g., lithium fluoride may be added, and in a last step a metal electrode (aluminum) is evaporated on top (Fig. 2.1). The bulk heterojunction is important because a large interfacial area between the donor and acceptor materials is created where charge separation can take place.

Figure 2.1 Exploded view of a bulk heterojunction type of polymer solar cell.

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Exciton generation on excitation of the mixture is generally short-lived, with a diffusion length on the order of 10 nm. The size of the individual domains is therefore critical. It is also important that the domains are interconnected so that continuous paths exist for both electrons and holes for transportation to the external electrodes. The advantages of the BHJ-type cell have increased the efficiency to about 5–6%. Unfortunately, the optimal structure for generating the most efficient device may not be the most thermodynamically stable configuration. Much effort has therefore been devoted to creating technical recipes for optimizing the best devices. A large number of different polymers and small molecules have been tested as active materials in organic solar cells, as described in the overview below. Only very few of these new materials have been tested by more than one research group and most of them failed to produce devices with efficiencies of even 1%. As a consequence, nearly all physical studies of polymer solar cells have been conducted with only a few popular polymers. Experience shows that the efficiency can be increased by adjusting fabrication parameters. This requires, however, that sufficient quantities be available. Development of new materials is still very important and among the recent trends are low bandgap polymers that extend the harvesting of photons above 600 nm. Another is the advent of thermocleavable polymers that combine processability with high stability of the finished device.

2.2 Materials 2.2.1 Polymers Most of the common conjugated polymers have been tested as active materials for solar cells, but two classes have attained a special status. The poly(phenylenevinylene)s (PPVs) exemplified by poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4phenylenevinylene) (MDMO-PPV) have a conjugated PPV core, as shown in Fig. 2.2. Early examples of polymer-based solar cells include polyacetylene with an efficiency of 0.3%.2 Karg et al. were the first to investigate PPV in a photovoltaic

Figure 2.2 Chemical structures of the most commonly used conjugated organic polymers, namely, MEH-PPV, MDMO-PPV, and poly(3-hexylthiophene) (P3HT).

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Figure 2.3 Chemical structures of PEDOT and PSS.

device in 1993.3 PPV in itself is a very intractable material due to its insolubility. Adding alkyl or alkoxy chains on the phenylene rings, as in MEH- and MDMOPPV, makes these materials processable and soluble in some organic solvents such as chloroform, chlorobenzene, or 1,2-dichlorobenzene (ODCB). Yu et al. reported the use of MEH-PPV in a dual function light-emitting diode (LED) photovoltaic device also in 1994.4 Higher efficiency devices were prepared from MDMO-PPV/ [60]PCBM blends reaching 2.5% in the so-called bulk heterojunction geometry.5 More recently, highly regioregular P3HT has come to be a material of choice. By tuning several parameters such as annealing, solvent, and film formation, efficiencies of approximately 5% have been achieved recently.6, 7 Using a tandem geometry with different polymers has extended the efficiency even to 6.5%.8 Another polythiophene material that is used extensively in organic solar cells is PEDOT:PSS or poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (Fig. 2.3). A thin film of PEDOT:PSS is usually applied as a hole conducting material directly on top of the ITO electrode by spin coating the commercially available dispersion in water followed by removal of the water by heating. 2.2.1.1 Poly(phenylenevinylenes)

PPVs have the general structure of phenylene rings (aromatic rings) joined by ethylene bridges (Fig. 2.4). PPV and substituted versions have been prepared in numerous ways, as reviewed earlier.9 One of these methods is the sulfonium precursor route,10 where α, α -dichloro-p-xylene is treated with tetrahydrothiophene to give a bis-sulfonium salt (Scheme 2.1). Treatment of this salt with sodium hydroxide generates a quinoid intermediate that polymerizes. The remaining sulfonium groups are eliminated in a later heat treatment generating the PPV structure. This procedure has the advantage that the intermediate polymer salt is soluble and can be processed into films, while PPV itself is intractable. Karg et al. prepared photovoltaic cells from PPV with an ITO/PPV/Al structure and measured maximum monochromatic light conversion efficiencies of about 0.1% (at 514 nm).3 Marks et al. also described the use of PPVs in solar cells with different metal top layers (Al, Mg, Ca) and reported open-circuit voltages of 1.2, 1.2, and 1.7, respectively.11 Soon after, mixtures of PPV with the electron accepting C60 fullerene were tested in solar cells.

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Figure 2.4 General chemical structure of poly(phenylenevinyles) (PPVs). When R = H, it is the archetypal PPV structure. More commonly, one or more of the R groups represent flexible alkyl or alkoxy groups that aid solubility.

Scheme 2.1 Synthesis of PPV using the sulfonium precursor (Wessling) method.

Alkyl group- or alkoxy group-substituted PPVs allowed solution processing and made these materials more suitable for device preparation, such as for organic light-emitting diodes (OLEDs). The main focus has been on finding materials with emission in different regions of the optical spectrum. PPV itself emits green light, while alkoxy-substituted versions emit in the orange part of the spectrum. The major absorption band of PPVs is in the range of 400–500 nm with a bandgap of 2.1–2.7 eV (electron volts). MEH- and MDMO-PPV can be prepared using several types of reactions that all seem to be based on creating a reactive intermediate, a quinodimethane. A common starting material is hydroquinone monomethyl ether, which can be alkylated with a branched alkane such as 2-ethyl-hexyl-1-bromide or 2,7-dimethyl-octyl-1bromide. Chloromethylation can be carried out in both the 2- and 5-positions to obtain the bis-chloromethyl-dialkoxy monomer that can be utilized directly in the Gilch reaction.12 There is still some discussion about the actual mechanism of the reaction13, 14 shown in Scheme 2.2. While these polymers perform well in certain applications such as PLEDs (polymer light-emitting diodes), a number of defects (up to several mol %) are present due to incomplete reactions and side reactions. Becker et al. made a detailed investigation of the defects using nuclear magnetic resonance (NMR) to characterize them and found the presence of both fully saturated ethylene bridges and

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Scheme 2.2 Gilch polymerization producing MEH-PPV and MDMO-PPV.

triple bonds to a level of about 2%. The conjugation, of course, breaks at many of these defects, influencing the electronic properties.15 Yu et al. were among the first to utilize MEH-PPV to prepare photovoltaic devices with the ITO/MEH-PPV/Ca geometry with an open-circuit voltage (Voc ) of 1.6 V and short-circuit current (Isc ) of 6 μA/cm2 at 20 mW/cm2 light intensity.4 A year later, in 1995, Yu et al. reported a new and more efficient cell with a blend of MEH-PPV and [6,6]-phenyl C 61-butyric acid methyl ester, or [60]PCBM, in a so-called bulk heterojunction geometry.1 Compared with previous bilayer devices, heterojunction devices have a microphase separated interpenetrating network of polymer and acceptor domains. The internal distance to a phase boundary is on the order of a few nanometers and the chance of successful charge carrier generation was improved greatly, leading to a better energy conversion efficiency (ηe ) of 2.9%. Wienk et al. have increased the efficiency even further by optimizing the devices and exchanging [60]PCBM with [70]PCBM (phenyl-C71 -butyric acid methyl ester).16 Alkyl substituted PPVs have been synthesized using a number of routes that generate the vinylene groups. These are usually condensation reactions, such as the Horner-Wadsworth-Emmons (HWE) reaction between an aromatic aldehyde and an aryl methyl phosphonate ester (Scheme 2.3). Alternatively, a Heck-type coupling reaction between an aryl halide and a styrene type monomer may be utilized. Such alkyl substituted PPVs have been shown to exhibit very high carrier mobility.17 The structure of the alkyl substituted PPVs were also investigated by x-ray powder diffraction and it was found that the relatively high carrier mobility could be explained by the good overlap between polymer chains (Fig. 2.5). Although many other variants of PPV structures have been made, few have been applied to photovoltaic devices. One instance is the use of a cyano substituted MEH-PPV (Fig. 2.6) by Granström et al.18 in a laminated bilayer structure with a polythiophene, obtaining an overall power conversion efficiency of 1.9%. CN-

Chapter 2

16

Scheme 2.3 Two examples of PPV syntheses that involve two different monomers resulting in the same polymer. The reaction on top is a classical phosphonate-ester condensation, while the bottom one is a palladium catalyzed Heck reaction.

Figure 2.5 Stereo drawing of an x-ray structure of a poly[2,5-dioctyl-(1,4-phenylene)vinylene-1,4-phenylene-vinylene] type PPV. (Reprinted with permission from Ref. [17]. Copyright 2003, American Chemical Society.)

Figure 2.6 Chemical structure of CN-MEH-PPV.

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MEH-PPV had previously been prepared by Greenham et al. for use in LEDs.19 The electron attracting cyano groups change the material to be an acceptor compared to MEH-PPV. Organic materials with a high electron mobility are rare and the fullerenes must still be considered unrivaled in this respect. 2.2.1.2 Poly(thiophenes)

Polythiophene is, like PPV, an intractable material, although it can easily be produced by chemical or electrochemical oxidation of thiophene. Side groups of various types are therefore necessary for obtaining a usable polymer that can be cast or spin coated. The 2- and 5-positions of thiophene are the most reactive and are preferred for polymerization. 3-alkyl thiophenes can be polymerized by oxidation with, for example, ferric chloride. The initial cation radical reacts with other thiophene molecules and protons are eliminated to regain conjugation. This material is a regiorandom polymer with thiophene units joined at the 2- or 5-positions (head to head, head to tail, and tail to tail) (Fig. 2.7). In the head-to-head type dimer, the alkyl groups force the thiophenes to adopt an out-of-plane geometry, reducing the electronic overlap and limiting the intermolecular interactions. Regioregular P3HT with almost exclusive head-to-tail geometry can be prepared via two related routes. In the McCullough route,20 3-bromothiophene is alkylated with hexyl magnesium bromide in a Kumada coupling, followed by bromination in acetic acid with elemental bromine at the 2-position. The pure 2-bromo-3-hexylthiophene can then be polymerized in one pot by lithiation at the 5-position using LDA conversion to the magnesium derivative with MgBr2 · Et2 O, which is then treated with a Ni(dppp)Cl2 catalyst, where dppp is 1,3-bis(diphenylphosphino)propane (Scheme 2.4). Another procedure for the synthesis of regioregular P3HT is based on the activation of 2,5-dibromo-3-hexylthiophene with so-called Rieke-zinc. As in the McCullough route, the metal is inserted in the 5-position creating a nucleophilic species. At a low temperature (−78◦ C) and for the 3-hexyl substituted thiophene, the selectivity in this reaction was found to be 97:3. The intermediary is then polymerized with the Ni(dppp)Cl2 catalyst to create regioregular material with a molecular weight (mol wt) of 15.000.21 The UV-visible (UV-vis) spectra of the regioregular material has a maximum of 450 nm, which is red shifted some 25 nm relative to the random material. Also, the

Figure 2.7 Head-to-head, head-to-tail, and tail-to tail geometries in part of a random 3-alkyl polythiophene chain.

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Chapter 2

Scheme 2.4 Regiorandom P3HT is produced by ferric chloride polymerization (top). The McCollough route (middle) and the Rieke route to highly regioregular P3HT (bottom).

emission spectra differ with a maximum of 570 nm for the regular versus 550 nm for the random P3HT (see later in this chapter). The electronic spectra in the solid state show a pronounced red shift of the lowest absorption from about 440 nm to more than 500 nm due to close intermolecular interactions. This is only possible for the regioregular P3HT, which forms a well-ordered structure with interdigitating side chains (Fig. 2.8). Several studies have shown that the structure of heat-treated (annealed) regioregular P3HT film is semicrystalline. Diffraction techniques, such as grazingincidence x-ray diffraction (XRD), show peaks that are interpreted as a lamellae structure with an alternating orientation of the thiophene moieties, where the alkyl groups of neighboring polymer chains interdigitate.22, 23 A regular surface structure can be observed with atomic force microscopy (AFM), showing needlelike structures in an amorphous matrix. P3HT, like all the other condensation polymers, have a broad distribution of molecular masses where the smaller masses might act like glue between crystalline domains. Other types of polythiophenes (see Fig. 2.9) that have been used in solar cells include poly[3-(4-octylphenyl)-2,2 -bithiophene] (PTOPT),24 poly[3-(2 -methoxy5 -octylphenyl)-thiophene] (POMeOPT),25 and poly[3-(4 -(1 ,4 ),7 -trioxaoctyl) phenyl)thiophene] (PEOPT).26

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Figure 2.8 Schematic representation of the crystaline part of P3HT.

Figure 2.9 Some other polythiophenes used in organic solar cells.

2.2.1.3 Low bandgap polymers

Low bandgap polymers are defined as polymers absorbing light with wavelengths above 600 nm. The traditional polymers used in organic photovoltaics, such as MEH-PPV, have an absorption that extend to wavelengths of 550 nm. Commonly employed P3HT has an absorption that extends out to 650 nm and comparing the absorption spectra of this polymer with the solar spectrum (Fig. 2.10), a strong mismatch between the absorption spectrum of P3HT and the emission spectrum of the sun is seen.27

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Chapter 2

Figure 2.10 Absorption spectra of P3HT and a low bandgap polymer and the solar spectrum (AM 1.5D).

This mismatch could be alleviated if a low bandgap polymer was used, that is, the absorption spectrum of the low bandgap polymer has a better overlap with the sun emission spectra. Therefore, in the past decade the focus on low bandgap polymers have increased tremendously. But why are low bandgap polymers important? And furthermore, how can we obtain the low bandgap? Before answering these questions, we take a look at the definitions of a bandgap. The bandgap of a polymer is defined as the difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The bandgap can be determined from an optical absorption spectrum (UV-vis), which gives the optical bandgap, or by cyclic voltammetry (CV), which gives the electrical bandgap. The bandgap is in principle used for the determination of the energy that can be extracted from the cell. However, neither the optical nor the electrical bandgaps give this information directly. In the optical bandgap, the binding energy of the exciton is not accounted for and in the CV experiment the energy of solvation of the electrochemical species is unknown. The optical bandgap gives a somewhat larger value. To answer the question on why we need low bandgap polymers, we take a closer look at the solar spectrum. In Fig. 2.11, the AM 1.5G spectrum of the sun is shown together with the number of photons as a function of wavelength. The spectrum clearly shows that the maximum number of photons is at wavelengths up to 900 nm. However, when we look at the integrated number of photons and integrated current as a function of the wavelength (Fig. 2.12) some important results are found. We compare two examples of polymers, that is, one absorbing light with wavelengths up to 500 nm (A) and one absorbing light with wavelengths up to 1000 nm (B). Figure 2.12 shows that polymer A will absorb 9.4% of the photons in the solar spectrum. Polymer B, however, will absorb 55.1% of the photons. By converting to the maximum theoretical current, this corresponds to Jsc = 5.1 mA cm−2 for A and Jsc = 33.9 mA cm−2 for polymer B, that is, lowering

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Figure 2.11 Sun irradiance spectrum AM 1.5G (black) and the number of photons (red) as a function of wavelength.

Figure 2.12 Number of photons for AM 1.5G (black), integrated photons (in percent from 280 to 4000 nm), and integrated current (in mA cm−2 ) as a function of wavelength (red).

the bandgap of the polymer results in a higher theoretical current. More values are given in Table 2.1. The values given for the maximum theoretical current is calculated from the assumption that the polymer absorbs all the photons from 280 to the wavelength given. The current measured depends on the absorption of the device, which includes absorption in the material, but also reflection losses from the window and interfaces. As an example, we have plotted the absorption spectrum of a thin film of a low bandgap polymer with the photon flux and performed the same calculations (Fig. 2.13). As can be seen from Fig. 2.13, the absorption spectrum of the polymer does not correspond completely with the photon flux, and hence the maximum theoretical current is around 14 mA cm−2 and not around 20 mA cm−2 , which is indicated in

Chapter 2

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Table 2.1 for a polymer absorbing light with wavelengths up to 750 nm. The actual current measured for an OPV depends on several factors, such as morphology, and hence, to obtain a more precise calculation the incident photon to current efficiency (IPCE) should be considered, since this is a device measurement and factors like morphology, thickness, carrier mobility, carrier lifetime, and reflection losses have thus been taken into account.

Table 2.1 The integrated photon flux and maximal theoretical current for an OPV with a polymer absorbing all wavelengths from 280 nm to the wavelength given assuming every photon is converted to an electron in the external circuit. The maximum current may increase in bulk heterojunctions due to the absorption of PCBM (the values are for direct and circumsolar AM 1.5D and AM 1.5G in brackets).

Wavelength 500 600 650 700 750 800 900 1000 1250 1500

Max. % harvested (280 nm →) 8.0 (9.4) 17.3 (19.0) 22.4 (24.3) 27.6 (29.6) 32.6 (34.7) 37.3 (39.5) 46.7 (48.8) 53.0 (55.0) 68.7 (70.4) 75.0 (76.4)

Current density in mA cm−2 (AM1.5G) 5.1 (6.47) 11.1 (13.15) 14.3 (16.77) 17.6 (20.42) 20.8 (23.9) 23.8 (27.23) 29.8 (33.67) 33.9 (38.0) 43.9 (48.57) 47.9 (52.75)

Figure 2.13 Number of photons (AM 1.5G, black), integrated photon flux (blue), current (blue) as a function of wavelength, and an absorption spectrum of a low bandgap polymer (red).

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Above we have shown that low bandgap polymers are important to increase the short-circuit current and hence the efficiency of the OPV due to a better overlap with the solar spectrum. Another important factor that influences the efficiency of the device is the open-circuit voltage. In Fig. 2.14 it can be seen that maximum voltage obtainable decreases as a function of wavelength (as calculated from the optical bandgap). The optimum bandgap is in the region of 0.9–1.2 eV. From this crude approximation, the maximum power conversion efficiency is 44% (in reality, thermodynamical factors have to be taken into account). The maximum Voc that can be obtained from a device based on donor and acceptor components in a bulk heterojunction geometry is roughly the difference between the HOMO of the donor and the LUMO of the acceptor. The energy level alignment between the polymer, the electron acceptor, and the electrodes becomes very crucial when low bandgap polymers are used in OPV devices. In Fig. 2.15, the energy level alignment in a bulk heterojunction between a low bandgap polymer and PCBM is shown. The energies β and ΔE have to be high enough to ensure a high Voc . A decrease in the LUMO of the polymer and an increase in the LUMO of the acceptor, that is, a decrease in α, will cause an increase in the maximum Voc that can be obtained and a decrease in the bandgap assuming a better overlap with the solar spectrum; hence, a higher Isc as described above. However, α has to be high enough to ensure an efficient charge transfer from the electron donor (polymer) to the electron acceptor (most commonly PCBM). These factors are of great importance when a low bandgap polymer is applied and often it becomes difficult to align the energy levels when the LUMO of the acceptor is fixed. Therefore, when low

Figure 2.14 Maximum power obtainable in OPVs based on the AM 1.5G emission spectrum. The power is taken as the product of the integrated current assuming an IPCE of 100% and the voltage of the device as the value for the bandgap. Thus, the power is the maximum theoretical value, neglecting thermodynamic effects and losses. The pink box is the range of bandgaps where the most efficient devices can be found.

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Figure 2.15 Energy level alignment in a bulk heterojunction based on a low bandgap polymer and PCBM. The sum of ΔE and β represents the maximum voltage that can be obtained (Voc ).

Figure 2.16 Resonance forms of a fused ring system, here PITN.35

bandgap polymers are applied in OPV devices, the choice of acceptor may have to be reconsidered to ensure a good energy level alignment.27, 28 We will now focus on how to design these important low bandgap polymer materials. There are several factors that influence the bandgap of the polymer, for example, conjugation length, bond length alternation, intrachain charge transfer, intermolecular interactions, aromaticity, and substituents. The effect of an increased conjugation length, for example, is seen for poly(3alkylthiophenes), where the regioregularity of the alkyl groups are of great importance. In the randomly coupled poly(3-alkylthiophene), the torsion between the side groups causes the backbone to twist, reducing the effective conjugation length and thus increases the band.29 Planarity along the aromatic polymer backbone increases delocalization of the π-electrons, reducing the bandgap.30 This can be further enhanced by intermolecular interactions observed for P3HT and other poly(3-alkylthiophenes) when comparing the absorption spectra of solid and liquid phases of the polymer.31–34 The low bandgap polymers described in the literature are based on fused ring systems or are copolymers with alternating donor and acceptor groups. Fused ring systems, such as those shown in Fig. 2.16,35 enhance the quinoid resonance structure, which in turn reduces the bond alternation.36–41 Finally, examples in the literature demonstrate the effect of additional electron donating groups (EDGs) and electron withdrawing groups (EWGs) to the polymer backbone.42–55 For example, in a bithiophene repeating unit, one thiophene has

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Figure 2.17 Examples of low bandgap polymers. Fused ring systems (PITN), and copolymers based on benzothiadiazole, pyrrole, and thiophene (PBPT); benzothiadiazole and thiophene (PBT); benzo-bis(thiadiazole) and thiophene (PBBT); thienopyrazine and thiophene (PTP); thienopyrazine, thiophene, and fluorene (PTTF); and benzothiadiazole, thiophene, and fluorene (PBBTF) R = alkyl, phenyl or chlorine.

an EDG and one has a EWG. This results in a donor/acceptor-based copolymer.56 In Fig. 2.17, some examples of low bandgap polymers that are described in the literature are shown. These polymers can be divided into two categories, namely, (1) a fused ring HOMO polymer, for example, poly(isothianaphthene), PITN, and (2) copolymers based on donor and acceptor majorities, for example, the polymer based on benzothiadiazole and thiophene, PBT. Below is a short summery of these low bandgap polymers and the results obtained from OPV devices prepared with some of these low bandgap polymers. First, a general summary of the polymerization methods used to prepare these polymers. The low bandgap polymers shown in Fig. 2.17 have been synthesized by different methods as described below. The most important ones are (1) Stille cross-coupling polymerization, (2) Suzuki cross-coupling polymerization, (3) Yamamoto condensation polymerization, (4) oxidative ferric chloride polymerization,

Synthesis

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Chapter 2

Figure 2.18 Synthesis methods for low bandgap polymers: (1) Stille, (2) Suzuki, (3) Yamamoto, (4) oxidative ferric, and (5) electrochemical polymerizations.

and (5) electrochemical polymerization. These reactions are shown in a general form in Fig. 2.18. In Stille cross-coupling polymerization, a dibromo derivate and a distannyl derivate are coupled using a palladium catalyst. In Suzuki cross-coupling polymerization, a bis-boronic acid monomer and dibromo monomer are coupled, also using a palladium catalyst. In both Stille and Suzuki coupling, it is possible to synthesize polymers with two different monomers forming a donor/acceptor copolymer. Unfortunately, in these polymerization methods there are a number of side reactions such as HOMO coupling of the stannyl compounds, dehalogenation, destannylation, etc. This may result in low molecular weights of the polymer products.57 Yamamoto condensation polymerization is a reaction between dibromo monomers using a nickel catalyst. In oxidative ferric chloride polymerization, a monomer is polymerized using FeCl3 as a reagent and results in high molecular weights. Finally, in electrochemical polymerization, a potential is applied on a solution containing the monomer and a buffer. This results in formation of the polymer product at the anode. PITN Poly(isothianaphthene) (PITN) is prepared from a monomer with a benzene fused to a thiophene ring. The fused ring system causes the bandgap to decrease

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Figure 2.19 Derivates of ITN-based polymers. (a) Poly(naphthothiophene) and (b) copolymer of thiophene and isothianaphthene p(T-ITN-T), R = alkyl or chlorine.

due to the quinoid resonance structure (Fig. 2.16).30 The bandgap of PITN was found to be 1.0 eV.58 Addition of another fused benzene ring (Fig. 2.19) caused an increase in the electron delocalization over the aromatic system, and hence a suppression of the quinoid structure and an increase in the bandgap was observed.59 The isothianaphthene (ITN) unit has also been applied in a copolymer based on donor/acceptor systems where the ITN unit is the electron acceptor and a thiophene unit is the electron donor (Fig. 2.19).60, 61 The bandgap of this copolymer was found to be 1.5 eV.60 The synthesis of PITN was first described in 1984 and was carried out by a electrochemical polymerization. Since then, the synthesis has been carried out using P4 S10 (see Refs. [62] and [63]) and by other methods.64–66 PBPT PBPT is a copolymer based on benzothiadiazole, pyrrole, and thiophene, where thiophene and pyrrole are the electron-donating units and benzothiadiazole is the electron-accepting unit. Benzothiadiazole is a fused ring system where a thiadiazole ring is fused to a benzene ring. The polymer has been synthesized by a Stille cross-coupling reaction; however, this results in low molecular weight products and the Suzuki cross-coupling polymerization was therefore used, which resulted in higher molecular weight products.37, 57, 67 The bandgap of PBPT was found to be 1.6 eV for the polymer with the highest molecular weight.67, 68 PBT PBT is also a copolymer where thiophene corresponds to the electron-donating unit and benzothiadiazole corresponds to the electron-accepting unit. The synthesis of these types of polymers have been carried out by Stille cross-coupling, by oxidative ferric chloride, or by Yamamoto coupling polymerization.69–72 Stille cross-coupling polymerization resulted in four different polymer products with a variation in the number of thiophene units between the benzothiadiazole unit (n = 1–4). The building blocks for Stille cross-coupling polymerization are shown in Fig. 2.20.69 The bandgap was shown to decrease with an increase in number of thiophenes in the repeating unit from 2.1 to 1.65 eV for the polymer with one and four thio-

28

Chapter 2

Figure 2.20 Building blocks for Stille cross-coupling polymerization of PBT. R = alkyl.

phenes in the repeating unit, respectively.69 This is ascribed to the optimization of the donor unit with respect to the acceptor unit (benzothiadiazole). It also means that there is an optimal length of the thiophene segment for one benzothiadiazole acceptor unit, and thus increasing the number of thiophenes beyond the optimum may cause an increase in the bandgap, which eventually will approach the bandgap of polythiophene.69 Studies of side groups showed that the solubility and film forming ability of the polymer was highest when 3,7,11-trimethyldodecyl was applied as a side chain.69 The position of the side chain also showed an effect on the bandgap, that is, steric hindrance of the side chain in the 3-position on thiophene, which results in an increase of the bandgap.37 PBBT PBBT is similar to PBT, having the thiophene unit as the donor and a benzo-bis(thiadiazole) unit as the acceptor. Benzo-bis(thiadiazole) has two thiadiazole rings that are fused to a benzene ring. The synthesis of PBBT was also carried out by Stille cross-coupling using the three building blocks shown in Fig. 2.21, creating two different polymer products, one with three and one with four thiophenes in the repeating unit.69 The bandgap of the two polymers was found to be 0.65 and 0.67 eV, respectiv69 ely. The low bandgap of this type of copolymer is ascribed to the quinoid form of the polymer, which is stabilized by the 1,2,5-thiazole unit (Fig. 2.22).73 It has been found that the benzo-bis(thiadiazole) has the highest electron-accepting ability among other acceptor units, for example, thienopyrazine and benzothiadiazole; hence, polymers based on benzo-bis(thiadiazole) also show the lowest bandgap.36 PTP PTP is another example of a copolymer where thiophene is the electron donor unit, and here thienopyrazine is the electron acceptor. The thienopyrazine is also a fused ring system where a pyrazine ring is fused to a thiophene ring. The synthesis of PTP was carried out by electrochemical, oxidative ferric chloride, or Yamamoto coupling polymerization.36, 74–76 The bandgap of PTP was found to be around 1.3 eV; however, a bandgap down to 1.20 eV was found for a more coplanar structure without bulky side chains on the backbone.74, 75

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Figure 2.21 Building blocks for Stille cross-coupling polymerization of PBBT. R = alkyl.

Figure 2.22 Resonance forms in the PBBT. Table 2.2 Bandgaps of the copolymers based on fluorene shown in Fig. 2.23.

Polymer 1 2 3 4 5 6

Band gap (eV) 2.01 ∼1.9 1.27 2 1.4 1.3

λmax (nm) 530 780 780 740

Ref. [77,78] [79] [30,80,81] [82] [83] [57]

PF Several copolymers based on fluorene have been described in the literature (Fig. 2.23). In the copolymers, the fluorene and thiophene unit functions as the electron-donating unit and these are coupled with varying electron accepting groups such as benzothiadiazole (1 and 2), thienopyrazine (4), and thiadiazolequinoxaline (3, 5, and 6). The side chains on the polymer backbone was varied to ensure solubility and to vary the bandgap of the polymers. The bandgap of the polymers is summarized in Table 2.2. The polymers were synthesized by Suzuki cross-coupling reaction.36, 56, 77, 78 Photovoltaic devices Some of the low bandgap polymers described above have been applied in solar cell devices and the best reported results are summarized in Table 2.3. As can be seen from this table, the efficiency of the devices with the low bandgap polymers is small compared with the efficiency of P3HT.7 The measured Isc is several times lower than the maximum theoretical current (Table 2.1). The choice of PCBM as an electron acceptor in these devices might ex-

Chapter 2

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Figure 2.23 Copolymers based on fluorene. R and R1 = alkyl.

plain the low Voc caused by a poor overlap between the HOMO of the polymer and the LUMO of the acceptor. For reviews on low bandgap polymers see Refs. [29] and [89]. 2.2.1.4 Other polymers

A number of less-used polymers have been studied as active materials for solar cells. They may not perform particularly efficiently in photovoltaic devices, but are included to show the scope and because some interesting and alternative principles have been used. Henckens et al. exchanged phenyl groups with thiophene in the PPV type polymer to obtain PTVs or mixed structures.84 Smith et al. prepared regioregular poly(3-dodecyl-2,5-thienylene-vinylene) by a Stille type cross-coupling between 3-dodecyl-2,5-dibromothiophene and (E)-1,2-bis(tributylstannyl)ethylene, as shown in Scheme 2.5.90

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Table 2.3 Photovoltaic responses reported for OPV devices with low band gap shown in Fig. 2.17. The device structure was ITO/PEDOT:PSS/polymer:PCBM/Al.

Polymer PITN P(T-ITN-T) PBPT PBT PTT 1 3a

Isc (mA cm−2 ) 0.045 1.13 3.1 3.59 3.5 4.66 3.4

Voc (V) 0.55 0.88 0.72 0.61 0.56 1.04 0.58

FF (%) 30 25 37 46 58 46 35

ηe (%) 0.008 0.31 1.0 1.0 1.1 2.2 0.7

4 5a

3.0 2.4

0.78 0.61

– 40

0.9 0.59

a

IPCE (%) – 20/480 20/550 18/600 10/800 40/550 8.8/850 7/900 10/650 8/800

Ref. [84] [61] [85,86] [87] [74] [77] [88] [80,82] [80,82]

The devices was prepared with BPF70 in stead of PCBM.

Scheme 2.5 Synthesis of a poly(thienylene-vinylene).

This PTV blended with PCBM in a bulk heterojunction type solar cell had an efficiency of 0.24%, a short-circuit current of 0.8 A cm−2 , and an open-circuit voltage of 0.54 V at AM 1.5G conditions. Branched PTV type polymers have been prepared by Li et al., who found similar values for the efficiencies of the solar cell devices.49 A special type of organometallic polymer with alternating platinum alkyne bridges and bithiazole units has been prepared by Wong et al.91 (Fig. 2.24). This polymer had an absorption maximum at 460 nm and a bandgap of 2.35 eV in the solid state. A very low photo current was generated and an efficiency of ∼10−5 %. Diblock copolymers such as polystyrene-polydimethylsiloxane form a number of microphase-separated structures depending on the size of each of the blocks. This was utilized by de Boer et al. to prepare interesting examples of a selfassembling PPV C60 conjugates.92 These conjugates were prepared by first synthesizing a PPV domain using the Siegrist reaction on (4-methyl-2,5-bis-octyloxybenzylidene)-phenyl-amine. NMR indicated an approximate length of 10 units. The aldehyde group at the end of this oligomer was then used to transform the PPV into a macroinitiator by reaction with a 2,2,6,6,-tetramethylpiperidin-1-oxyl (TEMPO) derivative. Atom-transfer radical polymerization (ATRP) with a mix-

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Figure 2.24 A platinum alkyne bithiadiazole polymer.

Figure 2.25 A diblock copolymer of PPV and polystyrene/C60 (PPV-b-P(S-stat -C60 MS).

ture of styrene and 4-chloromethyl-styrene then gave a diblock copolymer with the PPV block and a polystyrene block. Some of the styrene units were functionalized with chloromethyl groups that were used to react with C60 forming the PPV-b-P(S-stat-C60 MS) diblock (Fig. 2.25). A similar diblock copolymer without C60 was investigated with SEM and showed a honeycomb structured film indicating the expected microphase separation. The photovoltaic response was measured for films with and without C60 at a single wavelength of 458 nm to give short-circuit currents of 5.8 μA cm−2 and 0.15 μA cm−2 , respectively. This is rather low, but it is encouraging that the diblock copolymer with C60 performs best. Krebs et al.93 also tried to use the diblock copolymer approach to make materials with separated channels of hole and electron transporting domains (Fig. 2.26). A dialkyl substituted PPV domain was used as the hole conducting block, while a cyanovinylene terphenylene was presumed to be electron conducting. These two domains were coupled to a porphyrin system. As in the other studies of microphase segregated systems, rather low efficiencies were obtained for devices based on this material. Furthermore, a negative photovoltaic response was seen in the part of the spectrum where the porphyrin moiety was absorbed through its Soret band. Krebs94 also tested another self-assembling system consisting of a polar head group chromophore attached to a poly(cyanovinylene-terphenylene). This mater-

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Figure 2.26 Diblock copolymer of a central porphyrin unit joined to a dialkyl PPV and a poly(cyanovinylene terphenylene) at each end.

Figure 2.27 The self-assembling polar molecule with a poly(cyanovinylene-terphenylene) domain coupled to an aza-triangulenium dye group.

ial had previously been found to self-assemble using the Langmuir–Blodgett technique, giving films with a rectifying behavior95 (Fig. 2.27). Donor/acceptor copolymers based on a PPV, triphenylamine, and a perylene bisimide have been prepared by Hua et al.96 Lu et al. used MEH-PPV blended with (PTCA) and investigated the effect of annealing on the morphology and photovoltaic efficiency.97 Barber et al. prepared devices from a block copolymer [MEHPPV]-co-[biphenylene vinylene] blended with PCBM having a maximum power conversion efficiency of 2.4%.98 2.2.1.5 Thermocleavable materials

Alkyl and alkoxy polymers such as P3HT, MEH-PPV, and MDMO-PPV have been very successful materials in solar cells with desirable properties such as solubility in organic solvents, processability, and semicrystallinity. These properties are mainly due to the judicious choice of flexible side chains. On the other hand, they take no part in the function of the solar cell, they do not absorb light, and they do not transport carriers. The film thickness is a compromise between the wish for maximum absorbance and carrier (exciton) lifetime/mobility. Another factor is the semiliquid nature of the film that is also imparted by the side chains. This flexibility allows changes in the structure to occur either with time or with heat. The latter property is explored in the annealing procedure where the PCBM component

Chapter 2

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Scheme 2.6 Transforming the soluble prepolymer to PPV according to the Wessling reaction.

Scheme 2.7 Thermocleavable polythiophene carboxylic acid ester. R = alkyl.

of the film is typically allowed to form microcrystallites of a certain domain. It is, however, important that this structural development is not carried too far. A possible solution would be to remove the solubilizing groups after film formation in a similar way that PPV itself is prepared from a prepolymer. This principle has been realized for a special type of polythiophene with carboxylic acid groups developed by Liu et al.99 The acid groups were esterified with a tertiary alcohol and polymerized. The alkyl ester groups make the polymer soluble in organic solvents and allow film formation through spin coating and drop casting. The ester groups can then be removed simply by heating it to 190–210◦ C for a short time. Tertiary esters are susceptible to this thermocleavage reaction, forming the free acid and an alkene as the only products (Scheme 2.7). This type of polythiophene carboxylic acid was later found by Krebs and Spanggaard to be exceptionally stable in an organic solar cell.100 2.2.2 Molecules and oligomers 2.2.2.1 Small molecules

The first organic solar cell reported by Tang in 1986 had a bilayer structure of CuPc and a bis-benzimide derivative of perylene tetracarboxylic acid bisimide (PTCBI), shown in Fig. 2.28.101 Both of these materials were vacuum deposited. The phtalocyanine acted as the electron donor part while the perylene imide was the acceptor to a given cell that had a respectable ∼1% efficiency. Vapor deposition techniques continue to be an efficient method for creating layered solar cells from smaller molecular weight organic compounds. One of the recent examples is an asymmetric tandem photovoltaic cell with a front subcell com-

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Figure 2.28 CuPc and PTCBI used in the first organic solar cell by Tang.

Figure 2.29 Chemical structures of SubPC and BCP.

prising CuPc/CuPc:C60 /C60 /PTCBI and a back subcell similar to the front, but having a diphenyl-dimethyl-phenanthroline, bathocuproin (BCP), instead of PTCBI. The front and back subcells were sandwiched between ITO and silver metal electrodes. This type of organic solar cell was reported to give a very high efficiency of 5.7%.102 Another type of phthalocyanine with only three arms (SubPc) has been shown to enhance the open-circuit voltage with up to 0.5 V (Fig. 2.29).103 Self-organizing molecular materials have been used as an alternative method to create structure in the active layer of an organic solar cell. A substituted hexabenzocoronene donor together with N,N -bis(1-ethylpropyl)-3,4,9,10-perylenetetracarboxylic acid diimide acceptor in a 40:60 blend was used to obtain device efficiencies of about 2% (Fig. 2.30).104 The disk-shaped benzocoronene formed columnar structures with the perylene derivative in between to give a material with a very large interfacial area. Another disk-shaped molecule that has been employed in self-organized solar cells is DL-CuPC in combination C60 bathocuproine105 (Fig. 2.31). 2.2.2.2 Fullerenes

The PPVs and P3HT organic polymers are electron rich materials that can be oxidized fairly readily, having high-energy HOMO levels, and are typically hole conducting materials. Organic materials with high electron affinity are much rarer. One of the few reasonably electron conducting materials are C60 and derivatives

Chapter 2

36

Figure 2.30 Hexakis(4-dodecylphenyl) benzocoronene (left) and N,N -bis(1-ethylpropyl)3,4,9,10-perylenetetracarboxylic acid diimide (right).

Figure 2.31 Chemical structure of the mesogenic molecule DL-CuPC.

thereof. C60 is in itself a rather insoluble material, but a soluble PCBM derivative is commercially available. The active organic layer in organic solar cells is typically constructed from a mixture of polymer and PCBM that forms an interpenetrating network—the so-called bulk heterojunction.1 PCBM was first described by Hummelen et al.106 The synthesis is shown in Scheme 2.8, where the methyl ester of 4-benzoylbutyric acid tosylhydrazone is treated with sodium methoxide, creating a reactive diazo intermediate. This compound then reacts with C60 in a 1,3-dipolar addition to one of the double bonds in C60 , followed by elimination of nitrogen. Two adducts, [5, 6] and [6, 6], are possi-

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Scheme 2.8 Synthesis of PCBM, a soluble derivative of C60 .

Figure 2.32 Variant types of soluble C60 derivatives used in solar cells instead of [60]PCBM.

ble with the [5, 6] isomer dominating the reaction product. The numbers indicate between which type of rings the former double bond was situated. Heating the mixture in 1,2-dichlorobenzene convert it to the [6, 6] isomer, leading to color change of the solution from burgundy to more brownish. It is remarkable that this particular derivative of C60 with the phenyl ring was chosen “just for synthetic convenience” and is now the material of choice with bulk heterojunction solar cells. [6, 6]PCBMs have a series of characteristic UV-vis absorption at 210, 258, 328, 430, 492, and 696 nm in hexane solution. These optical transitions have low extinction coefficients and PCBM usually does not harvest the main part of photons in a solar cell. Symmetrical C60 can be replaced by the oval egg-shaped C70 , where lowenergy transitions become allowed, increasing light absorption. Cells with blends of [70]PCBM with MDMO-PPV have obtained an efficiency of 3%.16 Variation in the substitution pattern of PCBM with electron withdrawing and electron donating groups have been shown to affect the reduction potential of these materials and, hence, also the level of the LUMO.107 PCBM forms crystallites in the active layer, and control over size and morphology has a great influence on the efficiency of the devices. It has been shown that crystallite size increases on heat treatment of a PCBM/P3HT device improves the efficiency dramatically.108 Ultimately, the PCBM crystallites may grow too large to function well in the solar cell devices and an optimum heat treatment (annealing) can be found. Diffusion

Chapter 2

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Scheme 2.9 The Prato reaction between C60 , N-methyl-glycine, and benzaldehyde.

of PCBM is controlled both by temperature and the glass transition temperature (Tg ) of the polymer. C60 can also be coupled to aromatic aldehydes using the Prato reaction. The aldehyde is reacted with N-methyl-glycin, forming an iminium salt that can act as a 1,3-dipole toward a double bond in C60 . The product is an aryl fulleropyrrolidine, as shown below in Scheme 2.9. Nierengarten et al.109 utilized the Prato reaction to make a C60 moiety bound to a trimeric oligophenylenevinylene with two alkoxy groups, reported to give a solar cell device with a monochromatic efficiency of 1% (Isc = 10 μA cm−2 , Voc = 0.2 V) at 400 nm. Peeters et al.110 prepared a more extended version with four phenylenevinylene units, each having alkoxy groups as shown in Fig. 2.33. UV-vis spectra show that the electronic transitions of the individual oligophenylenevinylene and C60 parts are retained in the combined molecule. Other photophysical studies indicated that a photogenerated charge-separated state is formed for the larger oligophenylenevinylenes, and solar cells were prepared from the tetrameric oligophenylenevinylene -C60 adduct. A short-circuit current of 235 μA and an open-circuit voltage of 650 mV were obtained under white light illumination (65 mW cm−2 ). Other structures incorporating thiophene and naphthalene units in the oligophenylenevinylene structure reported by Guldi et al.111 gave similar results. The fluorescence of the oligophenylenevinylene fragment is effectively quenched by the C60 moiety, presumably leading to a charge-separated state. Solar cell devices made from this material gave a rather low overall white light efficiency of