Stability and Degradation of Organic and Polymer Solar Cells E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 617

Veröffentlichungsjahr: 2012

Beschreibung

Organic photovoltaics (OPV) are a new generation of solar cells with the potential to offer very short energy pay back times, mechanical flexibility and significantly lower production costs compared to traditional crystalline photovoltaic systems. A weakness of OPV is their comparative instability during operation and this is a critical area of research towards the successful development and commercialization of these 3rd generation solar cells. Covering both small molecule and polymer solar cells, Stability and Degradation of Organic and Polymer Solar Cells summarizes the state of the art understanding of stability and provides a detailed analysis of the mechanisms by which degradation occurs. Following an introductory chapter which compares different photovoltaic technologies, the book focuses on OPV degradation, discussing the origin and characterization of the instability and describing measures for extending the duration of operation. Topics covered include: * Chemical and physical probes for studying degradation * Imaging techniques * Photochemical stability of OPV materials * Degradation mechanisms * Testing methods * Barrier technology and applications Stability and Degradation of Organic and Polymer Solar Cells is an essential reference source for researchers in academia and industry, engineers and manufacturers working on OPV design, development and implementation.

Leseprobe

Contents

Cover

Cover

Title Page

Copyright

Preface

Acknowledgements

List of Contributors

1: The Different PV Technologies and How They Degrade

1.1 The Photovoltaic Effect and the Overview

1.2 The Photovoltaic Technologies

1.3 Intrinsic Versus Extrinsic Stability

1.4 Degradation – The Culprits, the What, the Why and the How

1.5 Some Representative Technologies and How They Degrade

References

2: Chemical and Physical Probes for Studying Degradation

2.1 Introduction

2.2 Physical Probes

2.3 Chemical Probes

2.4 Summary and Outlook

References

3: Imaging Techniques for Studying OPV Stability and Degradation

3.1 Introduction to Imaging Techniques

3.2 Reports

3.3 Discussion: Comparison of Imaging Techniques

3.4 Summary

Acknowledgement

References

4: Photochemical Stability of Materials for OPV

4.1 Introduction

4.2 Methods

4.3 State-of-the-Art

References

5: Degradation of Small-Molecule-Based OPV

5.1 Comparison to Small-Molecule OLEDs

5.2 Comparison to Polymer Solar Cells

5.3 Small-Molecule Organic Materials

5.4 Degradation Conditions

5.5 State-of-the-Art in Lifetime Studies

5.6 Summary and Outlook

References

6: Degradation of Polymer-Based OPV

6.1 Focus on the Degradation and Stability of Polymer Solar Cells

6.2 A Chart of Degradation and Stability of Polymer Solar Cells

6.3 A Short Account of the OPV Stability/Degradation History

6.4 Modus Operandi for Evolving OPV

6.5 The Recent Developments

6.6 Interlaboratory Studies and Round Robins

6.7 Outside Studies

6.8 How Far Can OPV Be Taken in Terms of Stability?

References

7: Test Equipment for OPV Stability

7.1 Introduction

7.2 Reliability and Durability Testing of PV Products

7.3 Laboratory Weathering Testing

7.4 Durability Testing Techniques

7.5 Conclusion

References

8: Characterization and Reporting of OPV Device Lifetime

8.1 Introduction

8.2 Photoelectric Characterization of OPV Devices

8.3 Interlaboratory Studies of OPVs

8.4 Lifetime Testing and Reporting: Standardized Approach

8.5 Conclusions

List of Abbreviations

References

9: Concentrated Light for Organic Photovoltaics

9.1 Introduction

9.2 Light-Concentration Setups

9.3 Experimental Work Performed with Concentrated Light

9.4 Physical Characterization by Concentrated Sunlight

9.5 Conclusion

References

10: Barrier Technology and Applications

10.1 Encapsulation Requirements

10.2 Thin-Film Permeation Physics

10.3 Measurement of Barrier Properties

10.4 Barrier Technologies

10.5 Barrier Application in OPV

10.6 Conclusion

References

11: Summary and Outlook

Color Plate

Index

This edition first published 2012 © 2012 John Wiley & Sons, Ltd

Registered office John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom

For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com.

The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.

Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought.

The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of fitness for a particular purpose. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for every situation. In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read. No warranty may be created or extended by any promotional statements for this work. Neither the publisher nor the author shall be liable for any damages arising herefrom.

Library of Congress Cataloging-in-Publication Data

Stability and degradation of organic and polymer solar cells / editor, Frederik C. Krebs. p. cm. Includes bibliographical references and index. ISBN 978-1-119-95251-0 (cloth) 1. Polymers–Deterioration. 2. Photovoltaic cells. 3. Organic compounds–Biodegradation. I. Krebs, Frederik C. QD381.9.D47S73 2012 621.3815′42–dc23 2011051140

A catalogue record for this book is available from the British Library.

Print ISBN: 9781119952510

Preface

Photovoltaic cells or solar cells made out of organic materials have become the subject of intense study by many research groups and a significant industrial activity is emerging within several areas spanning from suppliers of materials, substrates and barriers, through ink development to full manufacture and integration of organic photovoltaics (OPV). The traditional photovoltaic industry is well established and the photovoltaic as a useful energy-producing unit has been around for nearly 60 years. The interest in OPV technology is that it potentially provides an efficient solution to many of the struggles that the traditional PV technologies have been fighting for decades. OPV also has its share of problems but they are different and the interest emerges in the belief that these new problems for a new and different technology might be easier to tackle than the ones we have failed to tackle efficiently for the traditional PV technologies. Traditional PV in the form of crystalline PV was born as an environmentally and inherently stable technology and many of the later developments never knew different. However, they all suffer from the same problem of a massive energy input being required in their manufacture, and sometimes also from scarcity of the elements employed or significant toxicity of the components that constitute them or in the processes leading to them. OPV in its intended form does not share either of these problems and potentially allows for very short energy pay back times as they are very thin, common elements can be employed and very little energy is required for their manufacture. They also have a weakness and that is their instability during operation and sensitivity to some of the atmospheric components that are evidently present on earth. The early OPVs were not very stable and had a duration of operation in air measured in seconds or some minutes at most. Today, many thousands of hours is common place, however, it is still lagging behind the stability presented by for instance crystalline silicon. This book is dedicated to a description of what this instability is, where it has its roots, how it is measured and characterized, the physical means available to investigate it and also how it can be countermeasured through removal or elimination of the source of the problem or through design of the materials and device. This book is dedicated to the topic of degradation of OPV and should serve as a source of reference for the student, the expert, the experimentalist, the interested and the generalist. I wish you a pleasant read and hope that you will find most of the questions on the topic answered or find a path towards further development.

Frederik C. Krebs

Acknowledgements

During the writing of this book the authors have been supported by several funding organisations and would like to express thanks to:

The Danish Strategic Research Council (2104-07-0022) and EUDP (j.no. 64009-0050).The Dresdner Innovationszentrum Energieeffizienz, funded by the European Union, the State Saxony and the Fraunhofer Gesellschaft for funding encapsulation research at IAPP and IPMS.The Thuringian Ministry of Culture in the framework of FIPV II.The European Commission, the German Ministry for Education and Research (BMBF), Deutsche Forschungsgemeinschaft (DFG), Free State of Saxony, and further funding agencies. K. Leo thanks the DFG for the support via the Leibniz Prize.Public-funded research projects supported by the German Federal Ministry for Education and Research under reference number 13N8858 and 01RI0628 and by the Saxony State Ministry for Economic Affairs, Labor and Transport under reference number 3957/623

Several people have supported the authors and we would like to express sincere gratitude to:

Toni Müller and Bert Männig from Heliatek GmbH for supporting this work with data and knowledge. Furthermore, we want to thank the Fraunhofer IPMS and the companies Creaphys GmbH, Heliatek GmbH, Novaled AG, and Sim4Tec GmbH for continued good collaboration. This work would not have been possible without the help of the technical and scientific staff of the organic solar cell group at the IAPP, especially Sven Kunze, Carsten Wolf, Tobias Günther, Ines Rabelo de Moraes and Sebastian Scholz.Sylvio Schubert, Hannes Klumbies and Claudia Keibler for their contributions to encapsulation at IAPP and IPMS, Karl Leo, Olaf Hild and Christian May for their support.Colleagues from Fraunhofer FEP, especially, Nicolas Schiller, Steffen Straach, Steffen Günther, Matthias Fahland, Olaf Zywitzki and Thomas Modes for their work on single and multilayer barriers as well as for providing some of the figures being used for this book chapter, Allessandro Patelli from CIVEN, Italy, for allowing reproduction of SEM images of the multilayer barrier presented by CIVEN.Markus Hösel is thanked for taking photographs for the cover art to this book.

List of Contributors

Birgitta Andreasen Department of Energy Conversion and Storage, Technical University of Denmark, Roskilde, Denmark

John Fahlteich Fraunhofer-Institut für Elektronenstrahl und Plasmatechnik, Dresden, Germany

Jean-Luc Gardette Laboratoire de Photochimie Moléculaire et Macromoléculaire, Université Blaise Pascal, Ensemble Universitaire des Cézeaux, Aubiere, France

Suren A. Gevorgyan Department of Energy Conversion and Storage, Technical University of Denmark, Roskilde, Denmark

Olivier Haillant Atlas MTT GmbH, Vogelsbergstrasse 22, Linsengericht-Altenhasslau, Germany

Martin Hermenau Technische Universität Dresden, Institut für Angewandte Photophysik, Dresden, Germany

Harald Hoppe Institute of Physics, Ilmenau University of Technology, Ilmenau, Germany

Mikkel Jørgensen Department of Energy Conversion and Storage, Technical University of Denmark, Roskilde, Denmark

Frederik C. Krebs Department of Energy Conversion and Storage, Technical University of Denmark, Roskilde, Denmark

Karl Leo Technische Universität Dresden, Institut für Angewandte Photophysik, Dresden, Germany

Matthieu Manceau CEA-INES RDI, Savoie Technolac, Le Bourget Du Lac, France

Lars Müller-Meskamp Institut für Angewandte Photophysik, Technische Universität Dresden, Dresden, Germany

Kion Norrman Department of Energy Conversion and Storage, Technical University of Denmark, Roskilde, Denmark

Moritz Riede Technische Universität Dresden, Institut für Angewandte Photophysik, Dresden, Germany

Agnès Rivaton Laboratoire de Photochimie Moléculaire et Macromoléculaire, Université Blaise Pascal, Ensemble Universitaire des Cézeaux, Aubiere, France

Roland Rösch Institute of Physics, Ilmenau University of Technology, Ilmenau, Germany

Marco Seeland Institute of Physics, Ilmenau University of Technology, Ilmenau, Germany

Thomas Tromholt Department of Energy Conversion and Storage, Technical University of Denmark, Roskilde, Denmark

1

The Different PV Technologies and How They Degrade

Frederik C. Krebs

Department of Energy Conversion and Storage, Technical University of Denmark, Roskilde, Denmark

1.1 The Photovoltaic Effect and the Overview

Shining electromagnetic radiation on matter has been employed by scientists to make observations and study the fundament of nature for centuries and the list of experiments that has been carried out is almost endless. Some of the experiments have led to deep understanding of our world and others have led to discoveries that have been reduced to practical applications that serve our society today. One particular effect is where light with wavelengths from the ultraviolet (UV) to the infrared (IR) interact with matter to create an electrical current in an external circuit. This effect is called the photovoltaic effect and there have been many experiments that documented the phenomenon very early on. One of the best known examples is that of Becquerel [1] and even if this is considered by many the first proof of principle it is difficult to extrapolate this early description to any useful application.

It was not until the bulk semiconductors arrived in the early 1950s that the photovoltaics developed into the more useful form of a solar cell as we know them today. Chapin, Fuller and Pearson [2] made the first solar cell and many applications were envisaged very shortly thereafter. Progress has been massive and today solar cells represent a large multifaceted industry, even if solar cells still contribute little to the overall production of electrical energy when viewed globally. This is likely to change radically in years to come as the annual production capacity in terms of Wpeak (Wp) increases. In the year 2010 the annual production grew by more than 15 GWp alone [3]. The early solar cell has over the past 60 years developed into several different technologies that are fundamentally different in their manufacture, use, operation, mechanism and stability, to a degree that the only common point is that they convert light (sunlight) into electricity.

In this introductory chapter the evolution of the photovoltaic technologies is briefly outlined and exemplified with some of the most important examples. The overview is meant to provide you with enough knowledge on the different technologies and to understand how they differ in the context of degradation and stability. This implies that not all solar cell technologies and disciplines will be mentioned and the literature covered is exemplary rather than exhaustive. Several books have been dedicated to general aspects of solar cells and the reader is referred to those [4–6].

1.2 The Photovoltaic Technologies

Broadly speaking the development of photovoltaic technologies has been driven with the aim of providing a stable and low-cost source of electrical energy from light (sunlight). The first solar cells that fulfilled this are often called the 1st-generation solar cells and the monocrystalline silicon solar cells should be considered prototypical for this type comprising a semiconductor p-n junction. In terms of stability the 1st generation had few problems and emerged as an intrinsically very stable technology. The energy requirements in its making and the relatively large amounts of bulk material needed resulted in the desire to develop new technologies with lower energy and material requirements. This next generation (naturally named the 2nd generation) solar cells generally encompass all the thin-film solar cells. Generally speaking the 2nd generation of solar cells solved the problems but very early on new problems of stability emerged, at least when compared to the 1st generation. The 2nd generation became much more diverse, while still being exclusively based on inorganic materials and in terms of speed of development and performance they quickly rivaled the 1st generation. The 2nd generation, however, proved remarkably slow in being upscaled reliably and this made room for the 3rd generation of solar cells that is broadly different in the sense that they encompass multijunction tandem cells and a diverse set of materials such as for instance organic polymers. The polymer or small-molecule organic solar cell is thus similar to the 2nd generation of solar cells and in essence qualify as a thin-film solar cell except that its constitution comprise organic materials. The second most preponderant organic solar cell is the dye-sensitized solar cell that is a thicker solar cell relying on the interplay between an organic and an inorganic material. Hybrid cells that are a mix of organic and inorganic material are also a 3rd-generation type of solar cell. The 3rd generation of solar cells elegantly addressed the problem of manufacturing complexity and can in essence be prepared with reasonable efficiency and very modest equipment. They also possess the potential for inherently low-cost and fast manufacture using only abundant elements. Few of the 1st- and 2nd-generation solar cells share this latter point (essentially only silicon). The 3rd-generation solar cells, however, had several weaknesses in their generally low performance and also a significantly more pronounced tendency for degradation. The diversity of the 3rd generation of solar cells is even larger than the preceding generations, but this diversity is not only linked to the constitution but also to the manners in which they degrade and this is what serves as the basis of this book. There has been discussion of whether a 4th generation of solar cells can be identified but in essence these recent types of solar cells (quantum dots, plasmonics, etc.) either fit under the hat of the 3rd generation or have a degree of esotericism that makes it difficult to pull a classification together. Another rough distinction between the generations is that the 1st generation is processed from a solid block of semiconductor by sawing it into thin slices, whereas the 2nd generation is prepared by depositions of the materials from the gas phase, and the 3rd generation is processed from solution by coating and printing. This processing evolution has transcended back and forth and today there are examples of 2nd-generation solar cells (i.e. CIGS) that can be processed from solution and 3rd generation (i.e. small molecule) prepared by evaporation.

1.3 Intrinsic Versus Extrinsic Stability

When considering the stability of any photovoltaic the question of where the stability (or instability) comes from arises. There are several examples of solar cells that prove unstable in operation while their constituents are stable. On the other hand, there are no examples of solar cells where stable operation is achieved while their constituents are unstable. One may then ask why raise the question at all and the answer is that during development of a solar cell technology one of course strives to achieve stable operation, but when, for one reason or another, this is not reached and the cause to degradation is established it is useful to know whether the source of degradation is something you can solve or whether the degradation is fundamentally linked to the materials and the approach. A poor intrinsic stability can for instance be linked to an interface inside the working device, whereas a poor extrinsic stability can be caused by corrosion or crack formation causing failure of an otherwise well-operating solar cell.

1.3.1 Intrinsic Stability

A good example of intrinsic stability for a solar cell is the pn-heterojunction in monocrystalline silicon solar cells. Being a single-crystalline material that is passivated at the surfaces with stable materials and interfaces yields a solar cell where the part that converts sunlight into an electrical current is intrinsically stable during operation.

1.3.2 Extrinsic Stability

Taking a monocrystalline silicon solar cell module as an example it was sometimes observed for the early versions of the technology that the module performance failed quickly due to corrosion of the interconnections or dropped significantly due to yellowing of the encapsulation material upon exposure to sunlight without proper UV-blocking using, e.g., cerium ions in the front window.

1.4 Degradation – The Culprits, the What, the Why and the How

When approaching the stability of solar cells, it is most useful to examine degradation as this, from a scientific point of view, is more easily studied and characterized. A very stable solar cell is of course of great technological relevance but does not leave a lot to be studied as there ideally is no change in performance or appearance over time, regardless of the conditions the solar cell or module is subjected to. For this reason failure modes or sources of degradation are often deliberately sought to enable observations to be made. For the more novel technologies that do present significant instabilities this is straightforward. For the more stable solar cells special conditions are employed to accelerate the occurrence of failure modes or degradation. Typical stress conditions are high temperatures, high/low humidity, salt-spray, electrical stress, mechanical stress, intense light, ionizing radiation or strong UV-light. Very often combinations of those stress conditions are employed to provoke the preponderance of a particular failure type or a cycling of parameters between for instance light/dark, dry/wet, hot/cold, etc. When employing these conditions (that can be viewed as environmental or surrounding conditions) to deliberately observe changes in the performance (or even catastrophic failure) it often becomes possible to identify “what causes degradation”. This is the first important step but to find a remedy for the problem it is necessary to establish answers to the two more elaborate questions; “why it degrades” and “how it degrades”. With those three answers at hand one is left in a powerful situation where decisions on a technology can be made, further research can be planned or the technology abandoned. The stress factors described above combined with careful analysis of the results obtained when using them can be used to gain insight into intrinsic instabilities even though the conditions are external to the device. It is thus possible to gain knowledge on instabilities rooted in the materials, interfaces and instabilities linked to processing and preparation of the solar cell or module. In this book you should find the methodologies that cover most of it (if not all) in the context of polymer and organic solar cells. To round off this chapter some examples of some of the most distinct solar cell technologies are given with special focus on some of their most well-known degradation paths and failure mechanisms.

1.5 Some Representative Technologies and How They Degrade

Even though this book deals only with the degradative behavior of a branch of the 3rd-generation solar cells (polymer and organic solar cells excluding dye-sensitized solar cells) it is considered instructive to present well-known technologies and their most preponderant failure modes and degradation paths. Also, reflection is given on why a given solar cell type may be particularly stable under a given set of conditions, whereas another technology might be very sensitive under that set of conditions. One very obvious overall observation that is also summarized in Figure 1.1 is that when the operating temperature for a solar cell is much lower than the processing temperature the stability generally seems to be larger. Also, the more degrees of freedom that there are in the constitution the more sources of degradation are found. While this corroborates well with the observation that the newer the generation the more challenges there are with ensuring operative stability, it does not mean that it is fundamentally impossible to make a polymer solar cell that will work well for 25 years. We just do not know how to get there yet. Since stability studies on polymer solar cells started 10 years ago the stability under ambient conditions was measured in minutes or even within the time span of recording one or a few I–V curves and until today where we have continuous operation outdoor on the order of years, it is not unreasonable to expect that we can improve it by the remaining factor of 10–20 when considering that we have already improved it by a factor of 10 000–1 000 000 under ambient conditions.

Figure 1.1 An arbitrary scale illustration of how the different solar cell generations have evolved in time with respect to stability, processing temperature, cost, simplicity and materials use.

1.5.1 Mono- and Polycrystalline Silicon Solar Cells

Monocrystalline silicon solar cells can be viewed as the first ‘real’ solar cell. It was developed in 1954 by Chapin et al. [2] and came out with a power-conversion efficiency of 6% with fast development to 10% and in excess of 20% today even for multicrystalline silicon [7]. Already at birth operation was exceptionally stable with little concern being raised over operation for extended periods of time. A schematic of a typical crystalline silicon solar cell is shown in Figure 1.2 where a few of the degradation paths are shown. Possibly the first real surprise regarding the stability of crystalline silicon came with space exploration where solar panels were employed to power electronics in satellites. Here, radiation damage (from ionizing radiation) was found to be a significant source of gradual degradation in performance with increasing dose [8,9]. On earth operation proved to be very stable and radiation damage cannot be viewed as a significant cause of performance degradation for crystalline silicon. The largest problem with outdoor deployment and operation of silicon solar cells were external to the device and linked to mechanical cracking [10] of the silicon wafer or corrosion of electrodes and interconnections [11]. The encapsulation techniques quickly developed into a form where a thermoplastic polymer such as ethylvinylacetate (EVA) was employed to embed the wafer and wiring. The use of EVA has been extensively studied with respect to use stability, yellowing, water ingress, etc. [12–16].

Figure 1.2 A schematic view of a typical crystalline silicon solar cell with an indication of where the most preponderant failure modes are observed. The processing starts with a silicon wafer that is processed on both sides. The completed wafers with front electrodes are stringed together and hot-melt laminated between two sheets of EVA (0.5 mm thick) onto an antireflective glass plate and a TedlarTM or glass back plate.

Typically, a glass plate has been employed and the mechanical problems were mostly solved and are a guarantee of being durable to weathering outside for many years. The use of a printed silver grid on the front of the wafer along with stringing a tape wire across the front even solved problems related to complete failure when the devices crack. Early on some yellowing of EVA was observed outside but once cerium-doped glass was employed, thus eliminating UV-B admission to the EVA, operation for 25 years or more would seem to be the general expectancy for silicon solar cells. In terms of reports dedicated to degradation of crystalline silicon solar cell modules studies on EVA degradation have been accounting for most of the recent literature [13–16] simply because the crystalline silicon solar cells are so stable that anything you develop has to comply with that stability.

1.5.2 Amorphous, Micro- and Nanocrystalline Silicon Solar Cells

Amorphous silicon was developed with the aim of reducing the large materials usage and thermal budget of crystalline silicon solar cells and this was achieved by Carlson and Wronski in 1976 [17].

Even though the performance was lower the excitement was enormous and expectations were high. It did not, however, take long before the first surprise of degradation came and this was reported by Staebler and Wronski in the following year [18] and became known as the Staebler–Wronski effect. The Staebler–Wronski effect is what happens when an amorphous silicon solar cell degrades in performance when subjected to illumination. The typical amorphous silicon solar cell is shown in Figure 1.3. The studies of the Staebler–Wronski effect led to the relatively quick development of doped versions of a-Si:H and more importantly by controlling the ratio between SiH4 and H2 in the feed gas during PECVD deposition of the a-Si:H layer it was found that nano- and microcrystallinity could be induced and that this was an efficient remedy for the Staebler–Wronski effect [19–21] whereas chemical means have also been attempted [22]. This has also led to the further development of stacked junctions giving higher efficiencies that today rival that of polycrystalline silicon wafers. Similarly to crystalline silicon a:Si:H is also sensitive to ionizing radiation to a certain degree. It must have been a fact that was emotionally difficult to handle as everyone was accustomed to the stability of the silicon solar cells and the means available to stabilize crystalline silicon (extrinsically), while handy, would not solve this intrinsic stability problem that amorphous silicon presented. This naturally opened a whole new research discipline on what the underlying physical reasons were [23–29] and also a large technological development on how to eliminate it [30–32]. Thus, amorphous silicon became the first example of a solar cell technology that was not simply dismissed because it presented degradation to certain conditions but where scientists invested a lot of time in fixing the problem. The reason for this was of course (as always) the potential financial gain but also because amorphous silicon (potentially at least) efficiently addressed so many challenges that one would be able to sacrifice some performance to get something else.

Figure 1.3 A schematic view of the amorphous silicon solar cell with an indication of where the iconic Staebler–Wronski failure mode is observed.

1.5.3 CIS/CIGS Solar Cells

Just like amorphous silicon solar cells are prepared by a low-temperature (relative to crystalline Si) gas phase deposition technique so is the copper indium diselenide (CIS) and copper indium gallium diselenide (CIGS) solar cells, as illustrated for the CIGS solar cell in Figure 1.4. The CIGS solar cell relies on the codeposition of all the components onto a molybdenum electrode and thus provides a solution similar to the a-Si:H solar cells and even presents a very significant power-conversion efficiency of around 20%. It has some drawbacks in that very thick active layers are required and also that some of the elements are not the most abundant. The largest degradation path for CIGS is their sensitivity to humidity where especially the front electrode suffers. This is viewed as an extrinsic stability problem and has been addressed by encapsulation techniques well known from the crystalline silicon solar cells such as EVA and a glass front with special edge sealing [33]. The CIGS solar cells presented great difficulty in passing a damp heat test for 1000 h and it is likely that further development of the front electrode and encapsulation will enable an improvement in the technology [34–42]. One particular advantage of CIGS solar cells are their resistance to radiation [43] which makes them particularly suited for space application, granted their high efficiency and the possibility of a thin lightweight outline. In space, the humidity is not a problem and the CIGS cell is well suited for space application from that point of view. Several reports have suggested that CIGS solar cells present self-healing properties towards defects in the bulk [44,45].

Figure 1.4 A schematic view of the typical CIGS solar cell illustrating the most dominant failure mode associated with humidity is highlighted.

1.5.4 CdS/CdTe Solar Cells

The cadmium sulphide–cadmium telluride solar cell is also prepared by deposition from the gas phase and is typically constituted as shown in Figure 1.5. It employs thin films and was possibly the first of the thin-film solar cell technologies that very quickly reached a very large production capacity (First Solar claims in excess of 1 GWp/year). Most importantly, a cost of < 0.7 USD/Wp is what has warranted such intense technological development and investment in increased production capacity [46]. There have been claims of uncharted long-term toxicity issues with cadmium emission from decommissioned solar panels and also tellurium is one of the rarer elements. The debate is ongoing and has so far been warded off and has at least satisfied investors in the technology well enough to consider large-scale production (First Solar). In terms of stability CdTe solar cells present a significant stability towards ionizing radiation but has been deemed unsuited for space application [47]. The most pertinent degradation path for CdTe solar cells is linked to the back electrode that traditionally comprised a copper–carbon electrode that had a tendency to induce diffusion of copper into the bulk and creation of a transport barrier and degradation in performance, seen as the appearance of an inflection point in the I–V curve [48]. The CdTe solar cells also present a significant sensitivity towards humidity at the back electrode and this has resulted in a concentrated effort towards development of alternative back electrodes that are more stable towards humidity and prevents diffusion into the CdTe bulk [49–53], In particular, Sb/Mo back electrodes have shown that this intrinsic problem is one where a solution exists or at least significant reduction of the degradation mode can be produced [54].

Figure 1.5 A schematic view of the typical CdTe solar cell with an indication of the two commonly observed failure modes are indicated.

1.5.5 Dye-Sensitized Solar Cells (DSSC)

The dye-sensitized solar cell appeared in 1991 as reported by O’Regan and Grätzel and came with a reported efficiency that competed with polycrystalline silicon and for sure surpassed amorphous silicon (∼11%) [55]. It presents an elegant solution to principally all of the disadvantages that one can think of when it comes to preparing solar cells. It is simple to make and has proven to be a fantastic education tool and has been employed by many teachers of school children. One would have thought that the DSSC would have industrialized more quickly. The intrinsic stability of electrolytes dye and the construct have been convincingly reported [56–72].

The Achilles heel of the DSSC, as shown if Figure 1.6, is the fact that a liquid electrolyte is required for most efficient operation and while solid-state electrolytes have been reported as an efficient solution this has not made it beyond a scientific curiosity and being a great teaching tool. The absence of successful commercialization of the DSSC must at this point be considered as an abysmal failure.

Figure 1.6 A schematic view of a DSSC that is the only solar cell technology to comprise a liquid layer (an electrolyte). Some of the identified sources of degradation have been labeled.

1.5.6 Organic and Polymer Solar Cells (OPV)

Finally, we turn towards the solar cell type that is the topic of this book, namely the organic and polymer solar cells that do present significant degradation when operated. The degradative behavior is in fact so extensive that an entire book (this one) can be dedicated towards it. The OPV has had a slow start and the evolution this far could justify the view that OPV is the ugly duckling of solar cells and perhaps also of organic electronics taken as a whole. No matter how OPV has been examined it always came out inferior, mediocre or downright poor in performance. The only reason it has not been dismissed as a possibility is that it so elegantly promises low cost, use of only abundant materials and facile and fast manufacture to an extent that would make all other PV technologies seem like a necessary step on the path towards development of the ultimate energy technology. Of course, provided that a few challenges could be solved first, i.e. stability and power-conversion efficiency. OPV has consistently improved and now approaches performance at all levels that justifies its consideration as an industrially relevant technology, even if it is still in the low end in terms of performance (at all levels). Considering the consistent improvement that OPV has undergone there is no reason to believe that the development will stop.

OPV is undoubtedly the most diverse solar cell type with a myriad of different materials and reported device geometries Figure 1.7. It is also characterized by being the solar cell where almost all materials and every interface present intrinsic stability. Most often this is due to a poor choice of combining device structure, processing and materials. Fortunately, the diversity is very large and most often a particular choice made in the aim of solving one problem, causes another one [73]. This, however, can gradually be rectified in an iterative process where modification followed by analysis is applied until all failure modes have been weeded out.

Figure 1.7 A schematic view of one type of organic or polymer solar cell that comprises a multilayer structure with identified degradation problems at all interfaces and in the bulk of all layers. Some failure modes have been highlighted.

References

[1] Becquerel, A. E. (1839) Mémoire sur les effets électriques produits sous l’influence des rayons solaires. Comptes Rendus. 9:561–567.

[2] Chapin, D. M., Fuller, C. S., Pearson, G. L. (1954) A new silicon p-n junction photocell for converting solar radiation into electrical power J. Appl. Phys. 25:676–677.

[3] a) EPIA European Photovoltaic Industry Association: “GLOBAL MARKET OUTLOOK FOR PHOTOVOLTAICS UNTIL 2015” b) http://www.solarbuzz.com/our-research/reports/marketbuzz

[4] Markvart, T., Castañer, L. (2003) Practical Handbook of Photovoltaics: Fundamentals and Applications, Elsevier Ltd., Oxford, UK.

[5] Markvart, T., Castañer, L. (2005) Solar Cells: Materials, Manufacture and Operation, Elsevier Ltd., Oxford, UK.

[6] Goetzberger, A., Hebling, C., Schock, H.-W. (2003) Photovoltaic materials, history, status and outlook. Mater. Sci. Eng. R 40:1–46.

[7] Schultz, O., Glunz, S. W., Willeke, G. P. (2004) Multicrystalline silicon solar cells exceeding 20% efficiency. Prog. Photovoltaics: Res. Appl. 12:553–558.

[8] Mandelkorn, J., Schwartz, L., Broder, J., Kautz, H., Ulman, R. (1964) Effects of impurities on radiation damage of silicon solar cells. J. Appl. Phys. 35:2258–2260.

[9] Yamaguchi, M. (2001) Radiation-resistant solar cells for space use. Sol. Energy Mater. Sol. Cells 68:31–53.

[10] Köntges, M., Kunzen, I., Kajari-Schröder, S., Breitenmoser, X., Bjørneklett, B. (2011) The risk of power loss in crystalline silicon based photovoltaic modules due to micro-cracks. Sol. Energy Mater. Sol. Cells 95:1131–1137.

[11] Sastry, O. S., Saurabh, S., Shil, S. K., Pant, P. C., Kumar, R., Kumar, A., Bandopadhyay, B. (2010) Performance analysis of field exposed single crystalline silicon modules. Sol. Energy Mater. Sol. Cells. 94:1463–1468.

[12] Czanderna, A. W., Pern, F. J. (1996) Encapsulation of PV modules using ethylene vinyl acetate copolymer as pottant: a critical review. Sol. Energy Mater. Sol. Cells 43:101–181.

[13] Jorgensen, G. J., Terwilliger, K. M., DelCueto, J. A., Glick S. H., Kempe, M. D., Pankow, J. W., Pern, F. J., McMahon, T. J. (2006) Sol. Energy Mater. Sol. Cells 90:2739–2775.

[14] Kempe, M. D. (2006) Modelling the moisture ingress into photovoltaic modules. Sol. Energy Mater. Sol. Cells 90:2720–2738.

[15] Kempe, M. D., Jorgensen, G. J., Terwilliger, G. J., McMahon T. J., Kennedy, C. E., Borek, T. T. (2007) Acetic acid production and glass transition concerns with ethylene-vinyl acetate used in photovoltaic devices. Sol. Energy Mater. Sol. Cells 91:315–329.

[16] Kempe, M. D. (2010) Ultraviolet test and evaluation methods for encapsulants of photovoltaic modules. Sol. Energy Mater. Sol. Cells 94:246–253.

[17] Carlson, D. E., Wronski, C. R. (1976) Amorphous silicon solar cell. Appl. Phys. Lett. 28:671–673.

[18] Staebler, D. L., Wronski, C. R. (1977) Reversible conductivity changes in discharge-produced amorphous Si. Appl. Phys. Lett. 31:292–294.

[19] Meier, J., Dubail, S., Cuperus, J., Kroll, U., Platz, R., Torres, P., Anna Selvan, J. A., Pernet, P., Beck, N., Pellaton Vaucher, N., Hof. Ch., Fischer, D., Keppner, H., Shah, A. (1998) Recent progress in micromorph solar cells. J. Non-Cryst. Solids. 227–230:1250–1256.

[20] Keppner, H., Meier, J., Torres, P., Fischer, D., Shah, A. (1999) Microcrystalline silicon and micromorph tandem solar cells. Appl. Phys. A. 69:169–177.

[21] Shah, A. V., Meier, J., Vallat-Sauvain, E., Wyrsch, N., Kroll, U., Droz, C., Graf, U. (2003) Material and solar cell research in microcrystalline silicon. Sol. Energy Mater. Sol. Cells 78:469–491.

[22] Kobayashi, H., Kasama, Y., Fujinaga, T., Takahashi, M., Koinuma, H. (2002) Chemical prevention of light-induced degradation in amorphous silicon films. Solid State Commun. 123:151–154.

[23] Branz, H. M. (1999) Hydrogen collision model: Quantitative description of metastability in amorphous silicon. Phys. Rev. B. 59:5498–5512.

[24] Yan, B., Yue, G., Owens, J. M., Yang, J., Guha, S. (2004) Light-induced metastability in hydrogenated nanocrystalline silicon solar cells. Appl. Phys. Lett. 85:1925–1927.

[25] Meier, J., Flückiger, R., Keppner, H., Shah, A. (1994) Complete microcrystalline p-i-n solar cell-crystalline or amorphous cell bahavior? Appl. Phys. Lett. 65:860–862.

[26] Yang, L., Chen, L., Catalano, A. (1991) Intensity and temperature dependence of photodegradation of amorphous silicon solar cells under intense illumination. Appl. Phys. Lett. 59:840–842.

[27] Crandall, R. S. (1991) Defect relaxation in amorphous silicon: Stretched exponentials, the Meyer-Neldel rule, and the Staebler-Wronski effect. Phys. Rev. B. 43:4057–4070.

[28] Crandall, R. S., Carlson, D. E., Catalano, A., Weakliem, H. A. (1984) Role of carbon in hydrogenated amorphous silicon solar cell degradation. Appl. Phys. Lett. 44:200–201.

[29] Zanzucchi, P. J., Wronski, C. R., Carlson, D. E. (1977) Optical and photoconductive properties of discharge-produced amorphous silicon. J. Appl. Phys. 48:5227–5236.

[30] Rech, B., Wagner, H. (1999) Potential of amorphous silicon for solar cells. Appl. Phys. A. 69:155–167.

[31] Beneking, C., Rech, B., Fölsch, J., Wagner, H. (1996) Recent developments in amorphous silicon-based solar cells. Phys. Status Solidi 194:41–53.

[32] Fritzsche, H. (2001) Development in understanding and controlling the Staebler-Wronski effect in a-Si:H. Annu. Rev. Mater. Res. 31:47–79.

[33] Yanagisawa, T., Kojima, T., Koyanagi, T. (2004) Behavior of Cu(In,Ga)Se2 solar cells under light/damp heat over time. Microelectron. Reliab. 44:229–235.

[34] Wennerberg, J., Kessler, J., Stolt, L. (2003) Cu(In,Ga)Se2-based thin-film photovoltaic modules optimized for long term performance. Sol. Energy Mater. Sol. Cells. 75:47–55.

[35] Lam, K. H., Close, J., Durisch, W. (2004) Modelling and degradation study on copper indium diselenide module. Solar Energy. 77:121–127.

[36] Kessler, F., Rudmann, D. (2004) Technological aspects of flexible CIGS solar cells and modules. Solar Energy. 77:685–695.

[37] Medvedkin, G. A., Terukov, E. I., Hasegawa, Y., Hirose, K., Sato, K. (2003) Microdefects and point defects optically detected in Cu(In,Ga)Se2 thin film solar cells exposed to the damp and heating. Sol. Energy Mater. Sol. Cells. 75:127–133.

[38] Guillemoles, J. F. (2002) The puzzle of Cu(In,Ga)Se2 (CIGS) solar cells stability. Thin Solid Films. 403–404:405–409.

[39] Powalla, M., Dimmler, B. (2001) Process development of high performance CIGS modules for mass production. Thin Solid Films. 387:251–256.

[40] Radue, C., van Dyk, E. E. (2009) Degradation analysis of thin film photovoltaic modules. Physica B. 404:4449–4451.

[41] Durisch, W., Lam, K.-H., Close, J. (2006) Efficiency and degradation of a copper indium diselenide photovoltaic module and yearly output at a sunny site in Jordan. Appl. Energy. 83:1339–1350.

[42] Kijima, S., Nakada, T. (2008) High-temperature degradation mechanism of Cu(In,Ga)Se2-based thin film solar cells. Appl. Phys. Exp. 1:075002-1-3.

[43] Yamaguchi, M. (1995) Radiation resistance of compound semiconductor solar cells. J. Appl. Phys. 78:1476–1480.

[44] Guillemoles, J. F., Kronik, L., Cahen, D., Rau, U., Jasanek, A., Schock, H.-W. (2000) Stability issues of Cu(In,Ga)Se2-based solar cells. J. Phys. Chem. B. 104:4849–4862.

[45] Guillesmoles, J. F., Rau, U., Kronik, L., Schock, H.-W., Cahen, D. (1999) Cu(In,Ga)Se2 solar cells: Device stability based on chemical flexibility. Adv. Mater. 11:957–961.

[46] Dhere, N. G. (2010) Considerations for low cost, reliable and durable packaging for flexible PV modules. Proc. SPIE. 7773:777302-1–4.

[47] Woodyard, J. R., Landis, G. A. (1991) Radiation resistance of thin-film solar cells for space photovoltaic power. Solar Cells. 31:297–329.

[48] Visoly-Fisher, I., Dobson, K. D., Nair, J., Bezaldel, E., Hodes, G., Cahen, D. (2003) Factors affecting the stability of CdTe/CdS solar cells deduced from stress tests at elevated temperature. Adv. Funct. Mater. 13:289–299.

[49] Mendoza-Pérez, R., Sastre-Hernandez, J., Contreras-Puente, G., Vigil-Galán, O. (2009) CdTe solar cell degradation studies with the use of CdS as the window material. Sol. Energy. Mater. Sol. Cells. 93:79–84.

[50] Hegedus, S. S., McCandless, B. E. (2005) CdTe contacts for CdTe/CdS solar cells: effect of Cu thickness, surface preparation and recontacting on device performance and stability. Sol. Energy Mater. Sol. Cells. 88:75–95.

[51] Grecu, D., Compaan, A. D., Young, D., Jayamaha, U., Rose, D. H. (2000) Photoluminescence of Cu-doped CdTe and related stability issues in CdS/CdTe solar cells. J. Appl. Phys. 88:2490–2496.

[52] Dobson, K. D., Visoly-Fisher, I., Hodes, G., Cahen, D. (2000) Stability of CdTe/CdS thin-film solar cells. Sol. Energy Mater. Sol. Cells. 62:295–325.

[53] Singh, V. P. Erickson, O. M., Chao, J. H. (1995) Analysis of contact degradation at the CdTe-electrode interface in thin film CdTe-CdS solar cells. J. Appl. Phys. 78:4538–4542.

[54] Bätzner, D. L., Romeo, A., Terheggen, M., Döbeli, M., Zogg, H., Tiwari, A. N. (2004) Stability aspects in CdTe/CdS solar cells. Thin Solid Films. 451–452:536–543.

[55] O’Regan, B., Grätzel, M. (1991) A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature. 353:737–740.

[56] Kato, N., Higuchi, K., Tanaka, H., Nakajima, J., Sano, T., Toyoda, T. (2011) Improvement in long-term stability of dye-sensitized solar cell for outdoor use. Sol. Energy Mater. Sol. Cells. 95:301–305.

[57] Harikisun, R., Desilvestro, H. (2011) Long-term stability of dye solar cells. Solar Energy. 85:1179–1188.

[58] Nguyen, P. T., Andersen, A. R., Skou, E. M., Lund, T. (2010) Dye stability and performances of dye-sensitized solar cells with different nitrogen additives at elevated temperatures – Can sterically hindered pyridines prevent dye degradation? Sol. Energy Mater. Sol. Cells. 94:1582–1590.

[59] Grätzel, M. (2009) Recent advances in sensitized mesoscopic solar cells. Acc. Chem. Res. 42:1788–1798.

[60] Tributsch, H. (2004) Dye sensitization solar cells: a critical assessment of the learning curve. Coord. Chem. Rev. 248:1511–1530.

[61] Tanaka, H., Takeichi, A., Higuchi, K., Motohiro, T., Takata, M., Hirota, N., Nakajima, J., Toyoda, T. (2009) Long-term durability and degradation mechanism of dye-sensitized solar cells sensitized with indoline dyes. Sol. Energy Mater. Sol. Cells. 93:1143–1148.

[62] Kato, N., Takeda, Y., Higuchi, K., Takeichi, A., Sudo, E., Tanaka, H., Motohiro, T., Sano, T., Toyoda, T. (2009) Degradation analysis of dye-sensitized solar cell module after long-term stability test under outdoor working condition. Sol. Energy Mater. Sol. Cells. 93:893–897.

[63] Fredin, K., Anderson, K. F., Duffy, N. W., Wilson, G. J., Fell, C. J., Hagberg, D. P., Sun, L., Bach, U., Lindquist, S.-E. (2009) Effect on cell efficiency following thermal degradation of dye-sensitized mesoporous electrodes using N719 and D5 sensitizers. J. Phys. Chem. 113:18902–18906.

[64] Li, Y., Xie, W., Hu, X., Shen, G., Zhou, X., Xiang, Y., Zhao, X., Fang, P. (2010) Comparison of dye photodegradation and its coupling with light-to-electricity conversion over TiO2 and ZnO. Langmuir. 26(1):591–597.

[65] Kroon, J.M., Bakker, N.J., Smit, H.J.P., Liska, P., Thampi, K.R., Wang, P., Zakeeruddin, S.M., Grätzel, M., Hinsch, A., Hore, S., Würfel, U., Sastrawan, R., Durrant, J.R., Palomares, E., Pettersson, H., Gruszecki, T., Walter, J., Skupien, K., Tulloch, G.E. (2006) Nanocrystalline dye-sensitized solar cells having maximum performance. Prog. Photovolt: Res. Appl. 15:1–18.

[66] Nour-Mohammadi, F., Nguyen, H.T., Boschloo, G., Lund, T. (2007) An investigation of the photosubstitution reaction between N719-dyed nanocrystalline TiO2 particles and 4-tert-butylpyridine. J. Photochem. Photobiol. A: Chem. 187:348–355.

[67] Nogueira, V.C., Longo, C., Nogueira, A.F., Soto-Oviedo, M.A., De Paoli, M.-A. (2006) Solid-state dye-sensitized solar cell: improved performance and stability using a plasticized polymer electrolyte. J. Photochem. Photobiol. A: Chem. 181:226–232.

[68] Junghänel, M., Tributsch, H. (2005) Role of nanochemical environments in porous TiO2 in photocurrent efficiency and degradation in dye sensitized solar cells. J. Phys. Chem. B. 109:22876–22883.

[69] Nakade, S., Kanzaki., T., Kambe, S., Wada, Y., Yanagida, S. (2005) Investigation of cation-induced degradation of dye-sensitized solar cells for a new strategy to long-term stability. Langmuir. 21:11414–11417.

[70] Sommeling, P.M., Späth, M., Smit, H.J.P., Bakker, N.J., Kroon, J.M. (2004) Long-term stability testing of dye-sensitized solar cells. J. Photochem. Photobiol. A: Chem. 164:137–144.

[71] Agrell, H.G., Lindgren, J., Hagfeldt, A. (2003) Degradation mechanisms in a dye-sensitized solar cell studied by UV-VIS and IR spectroscopy. Sol. Energy. 75:169–180.

[72] Pettersson, H., Gruszecki, T., Johansson, L.-H., Johander, P. (2003) Manufacturing method for monolithic dye-sensitised solar cells permitting long-term stable low-power modules. Sol. Energy Mater. Sol. Cells. 77:405–413.

[73] Jørgensen, M., Norrman, K., Krebs, F. C. (2008) Stability/degradation of polymer solar cells. Sol. Energy Mater. Sol. Cells. 92:686–714.

2

Chemical and Physical Probes for Studying Degradation

Birgitta Andreasen and Kion Norrman

Department of Energy Conversion and Storage, Technical University of Denmark, Roskilde, Denmark

2.1 Introduction

Degradation has a detrimental effect on efficiency and lifetime for organic solar cells [1]. It is therefore highly desirable to prevent (or delay) degradation processes to an extent that will make the technology attractive from a commercialization point of view so that the large-scale vision can be carried out. In order to prevent degradation one needs to understand the degradation mechanisms that are in play. The most common approach to study OPV degradation is based on trial and error. Photovoltaic properties (JSC, VOC, FF, and PCE) are monitored as a function of lifetime (described in Chapter 8) and modifications to prevent degradation is based on guessing what the problem is, typically by exposing the OPV devices to various experimental conditions. This approach is empirical and indirect. However, it is nevertheless the approach that is responsible for the majority of progress within OPV degradation research.

Physical and chemical analytical techniques are more direct methods used to obtain a greater insight into degradation processes in OPV devices and materials from a physical and chemical point of view. There are two overall approaches to study degradation of organic solar cells using physical and chemical probes: (i) individual materials can be analyzed prior to being incorporated into an OPV device, and (ii) the complete OPV device (or a partial device) can be analyzed. Analyzing the individual materials systematically provide information on relative material stability, which is useful, especially when synthesizing/designing new OPV materials. In addition, by studying individual materials a broader range of analytical probes are available, i.e. materials are easier to analyze than complex three-dimensional multilayer structures. However, studying the individual materials excludes a wide range of degradation phenomena related to interlayer and interface processes, which could contribute significantly to the overall degradation of the organic solar cell. Furthermore, when studying individual materials the study does not take into account the barrier effect [2] that each layer represents, which is relevant with respect to diffusion of water and molecular oxygen into the OPV device.

There are an overwhelming number of degradation processes taking place simultaneously during operation (and storing) of organic solar cells, which suggests a systematic approach involving both individual materials and complete devices that should be analyzed using a broad suite of analytical techniques in order to gain as much complementary information as possible [3]. This chapter describes physical and chemical probes that have been used to study degradation processes in organic solar cells. Each technique is discussed in terms of applicability and supported by illustrative examples.

2.2 Physical Probes

Physical probes provide structural information and/or information on physical properties of materials. As will be evident from the examples presented in this chapter, some physical probes provide additional indirect chemical information, which is useful when studying chemical degradation of organic solar cells.

2.2.1 UV-vis Spectroscopy

UV-vis spectroscopy is a widely used technique and because it measures the absorption spectrum it is particularly relevant for OPV research. Besides being used for monitoring the absorption spectrum for newly designed OPV materials for the purpose of increasing efficiency the technique has proven useful in mapping relative material stabilities by monitoring loss of absorption caused by degradation [4]. Photo-oxidative degradation of active OPV materials is also called photobleaching because the process leads to loss in conjugation destroying the chromophores responsible for the color.

Manceau et al. [4] exposed various active OPV materials to simulated sunlight in air and periodically removed them in order to record the UV-vis absorbance, which produced plots like those shown in Figure 2.1. The plots show that the active materials P3HT and JC1 have equivalent stabilities and that MEH-PPV has a relative much lower stability. The authors were able to map the photochemical stability for a wide range of p-conjugated polymers used in OPV devices. The study revealed various points critical for stability, which enabled them to rationalize how variations in the chemical structure affect the photochemical stability. This subject is discussed in greater detail in Chapter 4.

Figure 2.1 Normalized amount of absorbed photons extracted from UV-vis spectra. The samples were illuminated (AM1.5G, 1000 W m−2) in ambient air and periodically removed in order to record the UV-vis absorbance. MEH-PPV is poly(2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene), P3HT is poly(3-hexylthiophene), and JC1 is poly(2,2’-(2,5-bis(2-hexyldecyloxy)-1,4-phenylene)-dithiophene). Reproduced from [4] with permission of The Royal Society of Chemistry ©2011.

2.2.2 Atomic Force Microscopy (AFM)

Atomic force microscopy (AFM) is a well-established imaging technique most commonly used to visualize surface topography. However, the development of different AFM acquisition modes allows for simultaneous collection of different types of physical and chemical information, e.g. conductive-AFM, Kelvin probe force microscopy (KPFM), noncontact scanning force microscopy (NC-SFM), etc. AFM provides Ångstrøm range resolution in both the lateral and vertical planes under ideal conditions. As well as obtaining topographic images AFM is cable of acquiring so-called phase images that visualize contrast in physical properties on the surface, and since most different materials have different physical properties, it is often possible to visualize chemical contrast. A clear disadvantage is the limited analysis area and a very slow acquisition time. In OPV research the main use of AFM to date has been focused on optimizing the nanostructure in the bulk heterojunction active layer (solvent choice, annealing method, blend ratios, etc.) with the aim of optimizing OPV efficiency [5–8]. The contribution of AFM to stability and degradation research has typically been limited to support conclusions obtained using other methods [9–14].

Motaung et al. [12] used AFM phase imaging (Figure 2.2) and surface roughness measurements along with other techniques to investigate the thermal degradation of P3HT:C60 blended films during prolonged annealing. The authors observed that the morphology changed with annealing time, manifested by an increase in surface roughness induced by overgrown large-scale C60 domains, which resulted in decreased efficiency of the P3HT:C60 solar cells.

Figure 2.2 AFM phase images (2 × 2 μm2) of P3HT:C60 films. (a) As-prepared. (b) Annealed for 5 h. (c) Annealed for 30 h. The images (a–c) show increasing surface roughness and the formation of large-scale C60 domains. Reproduced from [12] with kind permission from Springer Science + Business Media ©2011. (See colored version on color plate).

Noncontact scanning force microscopy (NC-SFM) has been used to study the effect of UV irradiation on morphology and electric properties of the OPV material P3OT [14]. Nanoscale topographic and electrostatic characterizations were performed on the same surface location of the sample for every cycle of UV radiation. Selected topographic NC-SFM images are shown in Figure 2.3.

Figure 2.3 NC-SFM topographic images (20 × 20 μm2) of a poly(3-octylthiophene) (P3OT) thin film showing morphology at various UV exposure times. (a) Pristine sample. (b) After 7 min of UV irradiation. (c) After 15 min of UV irradiation. The outlined area in (a) was used for roughness analysis. Reprinted from [14] with permission from Elsevier ©2011. (See colored version on color plate).

The authors came to the conclusion that a two-stage degradation process occurred during the UV irradiation. The first 10–15 minutes of exposure presented a chemical modification of the polymer (discoloration of the sample) and a significant change in the electrostatic properties and conductivity. After 10–15 minutes the film entered a second degradation stage, which was characterized by a strong structural modification, thickness reduction, oxygen doping, and further mobility reduction.

2.2.3 Interference Microscopy

Interference microscopy [15] is an old technique that has been successfully adapted to the field of biology [16] but has yet to establish a foothold in the field of OPV. It is one of the numerous techniques that maps surface topography. The lateral resolution is inferior to AFM, but its strength lies in the large scan range and the fast acquisition time. The transition from laboratory-scale to large-scale solar cells (e.g. via a R2R process) changes the requirements for analytical techniques accordingly. The ability to perform fast analyses on large areas will become important in the future. Interference microscopy has been used to screen aluminum electrode surfaces on OPV devices with respect to protrusions and associated pinholes (Figure 2.4) [3]. The protrusions are formed after molecular oxygen has diffused through the pinholes and reacted with the organic material that consequently expands in all directions (due to the oxygen uptake) forcing a protrusion to be formed. The density and size of protrusions can be mapped fast and easily on any nonencapsulated OPV electrode surface, which is an efficient way to monitor a specific degradation mechanism.

Figure 2.4 Interference microscopy image (134 × 179 μm2) of an aluminum electrode from an illuminated (AM1.5G, 1000 W m−2) organic solar cell with the composition ITO/PEDOT:PSS/C12-PPV/C60/Al. The electrode surface is covered with protrusions and associated pinholes. The horizontal white line shows from where the line profile was obtained. (See colored version on color plate).

2.2.4 Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) has a superior spatial resolution (subnano to ∼20 nm) and offers, in contrast to AFM, a relatively large analysis area ranging from nanometers to millimeters. A higher resolution can be achieved by employing transmission electron microscopy (TEM) that utilizes a shorter wavelength (i.e. higher energy). SEM is a vacuum technique and thus a rather slow technique, but the most serious disadvantage is the lack of depth scale, which would have been extremely useful. A 3D effect can be obtained by utilizing inverse reconstruction using electron-material interactive models, i.e. 3D surface reconstruction [17].

In OPV research SEM has most commonly been used to visualize and monitor material morphology in order to optimize OPV efficiency [18,19], which is very similar to the use of AFM in the same context. SEM also allows cross-sectional analysis of OPV devices. If the SEM apparatus is equipped with a focused ion beam (FIB) source then it is possible to obtain more well-defined cross sections, which improves quality. Furthermore, if the SEM apparatus is equipped with an energy-dispersive X-ray spectroscopy (EDX) detector it is possible to obtain some degree of chemical information. However, there are certain limitations when characterizing OPV devices using SEM combined with FIB and/or EDX. The probe depth of EDX is far greater than the thickness of the device, so it makes little sense to perform an EDX analysis perpendicular to the electrodes. EDX is thus more advantageous with respect to a cross-sectional analysis. A FIB source could be a problem in regard to OPV degradation studies. The ion beam is highly energetic and could thus degrade the organic components (e.g. induce morphological changes) in the OPV device, which would complicate the interpretation of subsequent analysis data. However, the effect of FIB on OPV devices has not been studied, so it is uncertain to what extent the process affects the OPV device. A cross-sectional analysis is useful to monitor morphological changes in the layer stack such as interlayer mixing and increasing/decreasing of layer thicknesses for experimental conditions relevant to OPV operation.

One of the few studies on OPV degradation involving SEM deals with, among other things, a cross-sectional analysis involving a partial OPV device with the configuration ITO/PEDOT:PSS/P3HT:PCBM (no cathode) [20]. The partial OPV device was exposed to long-term UV-vis light irradiation under conditions of accelerated artificial aging at 60°C in the absence of oxygen. SEM and various other techniques were employed to monitor the modification/degradation during aging. The resulting SEM images at various UV-vis exposure times are shown in Figure 2.5.

Figure 2.5 Cross-sectional SEM images (1.1 × 0.7 μm2) of a partial OPV device with the composition glass/ITO/PEDOT:PSS/P3HT:PCBM before UV-vis exposure (a), after 3000 h (b), and after 6000 h (c). Reprinted from [20] with permission from Elsevier ©2011.

Based on the combined result from UV-vis spectroscopy, SEM, and TEM analyses, the authors found that the UV-vis exposure induced three types of modification/degradation (besides a significant decrease in photovoltaic performance): (i) a decrease of UV-vis absorbance (i.e. photoinduced chemical degradation of organic components), (ii) formation of PCBM-rich domains in the active layer, and (iii) probable diffusion of PSS into the active layer. The cross-sectional SEM images revealed that after 4500 h of UV-vis exposure the limit between PEDOT:PSS and the active layer could no longer be distinguished. The SEM study confirmed the observations from the TEM analysis, i.e. that UV-vis exposure at 60°C provoked phase segregation between P3HT and PCBM at the nanometer scale. This is a good example of the benefit in using a suite of analysis techniques to gain complementary and supportive information in order to better understand the complex degradation mechanisms that are in play during OPV operation.

2.2.5 Fluorescence Microscopy