By the end of 2014, almost 50% of the world's photovoltaic (PV) capacity of 178 GWp was installed in Europe, with Germany alone counting for over 38 GWp. The exponential growth of PV has continued in spite of the world-wide economic recession. China is now catching up rapidly and even the UK, which has traditionally taken a conservative view of PV, has seen an astonishing burst of activity, with the installed capacity exceeding 8GWp by the second quarter of 2015. The vast majority of PV installations is based on highly developed first generation silicon technology. With efficiencies of over 20%, the best modern commercial silicon panels achieve around two thirds of the theoretical limit, setting the target that must be matched by any alternative PV technology. However, silicon has the drawback that its production is highly energy demanding, which also impacts on the carbon footprint of the technology. An important factor in this respect is the thickness of the silicon layer (and hence the amount of silicon per unit area of solar cell) needed to absorb sunlight efficiently. Because silicon is an indirect bandgap semiconductor, it absorbs red light rather weakly, and therefore, the typical thickness of the silicon absorber layer is of the order of 200 microns. The search for alternatives to silicon has generated second generation technologies based on thin film solar cells that utilise semiconductors such as cadmium telluride (CdTe) or copper indium gallium selenide (CIGS). These are direct band gap semiconductors that absorb light much more efficiently than silicon, so that the absorber layers only need to be a few microns thick. All of these technologies could be called ‘conventional’ in the sense that they have all progressed from lab bench to manufacture. By contrast, this book is concerned with newer types of photovoltaic cells that utilise what may loosely be called ‘soft’ materials, either organic compounds such as polymers and larger single molecules or the interesting class of hybrid perovskite materials such as methylammonium lead tri-iodide (MAPbI3). As the title of this book indicates, these solar cells can be considered to be unconventional in so far as they are based on emerging types of materials that differ from conventional inorganic semiconductors. The field of unconventional PV is developing very rapidly, and the editors are to be congratulated for having managed to bring together some of the best known specialists in the fields of organic and hybrid perovskite photovoltaics to give an account of the current state of the art, both from an experimental and from a theoretical standpoint. These areas of research continue to attract some of our brightest minds, and the breadth of activity illustrated by this book underlines the huge importance of PV research in the context of the challenges that mankind faces as a consequence of its thirst for energy.
The chapters in this book are intended to present a balanced combination of in-depth explorations of the experimental and theoretical aspects of the subject, allowing the reader to gain a deeper understanding of the two main types of unconventional photovoltaic cells: hybrid organolead halide perovskite cells and organic photovoltaics (OPV). The list of authors may appear rather ‘eurocentric’, but this reflects in part the fact that recent research in this area has been particularly intense in Europe, although major contributions have also been made by groups in Asia and USA. In particular, in the case of hybrid perovskites, pioneering studies on their use as sensitizers in mesoporous solar cells have been carried out in Tokyo and Seoul (Tsutomu Miyasaka and Nam-Gyu Park, respectively). The subsequent development of solid state hybrid perovskite solar cells (both mesoporous and planar) then took place essentially simultaneously in labs in Lausanne (Michael Grätzel) and Oxford (Henry Snaith).
The first part of the book provides an excellent overview of current activity in the area of hybrid perovskites. Many aspects of the behaviour of these fascinating materials are still controversial, and this is reflected in the variety of different explanations suggested for the hysteretic behaviour and apparent ferroelectric properties, to give just two examples. The jury is still out on the validity of these explanations, but the ideas presented in this volume certainly represent a useful step forward. The importance of solution processing methods and nanostructuring for achieving high performance in hybrid perovskite cells will be evident from chapter 1 (Park) and chapter 2 (DoCampo et al.), whereas chapter 3 (Bisquert et al.) and chapter 4 (Petrozza et al.) focus on characterization methods and on the photophysical processes that determine cell performance. Chapters 6 (Yan et al.), 7 (Even et al.), 8 (De Angelis et al.) and 9 (Walker and Richardson) illustrate the wide range as well as the extraordinary sophistication of current theoretical work on hybrid perovskites. The theme of nanostructuring contacts, in this case for OPV devices, is addressed in chapter 5 (Schmidt-Mende and Dorman), and the OPV theme is the focus of the chapters in the latter part of the book, which deal with different kinds of theoretical approaches to understand the physics of OPV. Leo et al. give an authoritative review of small molecule OPV in chapter 10 and, in chapter 11, Beljonne et al. provide an excellent status assessment and identify the key issues facing OPV. In chapter 12, Zannoni and Roscioni describe molecular dynamics simulations and show how they can be applied to thin films, whereas Koster et al. address the problem of 3-D simulations of organic systems in chapter 13. The final chapter by Michels and Schaefer shows how modelling can be used to unravel the complex processes that take place during solution processing of OPV devices based on bulk heterojunctions, where the energetics and kinetics of phase separation are key issues.
I would like to thank the authors who have contributed to this book, and I am also grateful to the four editors who have guided the process from the outset. All have taken time from their busy schedules to put together a book that I hope will be an important landmark that will help propel unconventional PV from the laboratory to the market place.