Advanced Characterization Techniques for Thin Film Solar Cells

Inorganic thin-film photovoltaics is a very old research topic with a scientific record of more than 30 years and tens of thousands of published papers. At the same time, thin-film photovoltaics is an emerging research field due to technological progress and the subsequent tremendous growth of the photovoltaic industry during recent years. As a consequence, many young scientists and engineers enter the field not only because of the growing demand for skilled scientific personal but also because of the many interesting scientific and technological questions that are still to be solved. As a consequence, there is a growing demand for skilled scientific staff entering the field who will face a multitude of challenging scientific and technolo­gical questions. Thin-film photovoltaics aims for the highest conversion efficiencies and at the same time for the lowest possible cost. Therefore, a profound under­standing ofcorresponding solar-cell devices and the photovoltaic materials applied is a major prerequisite for any further progress in this challenging field.

In recent years, a wide and continuously increasing variety of sophisticated and rather specialized analysis techniques originating from very different directions of physics, chemistry, or materials science has been applied in order to extend the scientific base ofthin-film photovoltaics. This increasing specialization is a relatively new phenomenon in the field of photovoltaics where during the ‘‘ old days’’ everyone was (and had to be) able to handle virtually every scientific method personally. Consequently, it becomes nowadays more and more challenging for the individual scientist to keep track with the results obtained by specialized analysis methods, the physics behind these methods, and on their implications for the devices.

The need for more communication and exchange especially among scientists and Ph. D. students working in the same field but using very different techniques was more and more rationalized during recent years. As notable consequences, very well attended ‘‘Young Scientist Tutorials on Characterization Techniques for Thin-Film Solar Cells’’ were established at Spring Meetings of the Materials Research Society and the European Materials Research Society. These Tutorials were especially dedicated to mutual teaching and open discussions.

The present handbook aims to follow the line defined by these Tutorials: providing concise and comprehensive lecture-like chapters on specific research methods,

written by researchers who use these methods as the core of their scientific work and who at the same time have a precise idea of what is relevant for photovoltaic devices. The chapters are intended to focus on the specific methods more than on significant results. This is because these results, especially in innovative research areas, are subject to rapid change and are better dealt with by review articles. The basic message of the chapters in the present handbook focuses more on how to use the specific methods, on their physical background and especially on their implications for the final purpose of the research, that is, improving the quality of photovoltaic materials and devices.

Therefore, the present handbook is not thought as a textbook on established standard (canonical) methods. Rather, we focus on emerging, specialized methods that are relatively new in the field but have a given relevance. This is why the title of the book addresses ‘‘advanced’’ techniques. However, also new methods need to be judged by their implication for photovoltaic devices. For this reason, an introductory chapter (Chapter 1) will describe the basic physics of thin-film solar cells and modules and also guide to the specific advantages that are provided by the individual methods. In addition, we have made sure that the selected authors are not only established specialists concerning a specific method but also have long-time experi­ence dealing with solar cells. This ensures that in each chapter, the aim of the analysis work is kept on the purpose of improving solar cells.

The choice of characterization techniques is not intended for completeness but should be a representative cross section through the scientific methods that have a high level of visibility in the recent scientific literature. Electrical device characterization (Chapter 2), electroluminescence (Chapter 3), photoluminescence (Chapter 7), and capacitance spectroscopy (Chapter 4) are standard optoelectronic analysis techniques for solid-state materials and devices but are also well-established and of common use in their specific photovoltaic context. In contrast, characteriza­tion of light trapping (Chapter 5) is an emerging research topic very specific to the photovoltaic field. Chapters 6, 8 and 9 deal with ellipsometry, the steady-state photocarrier grating method, and time-of-flight analysis, which are dedicated thin – film characterization methods. Steady-state photocarrier grating (Chapter 8) and time-of flight measurements (Chapter 9) specifically target the carrier transport properties of disordered thin-film semiconductors. Electron spin resonance (Chapter 10) is a traditional method in solid-state and molecule physics, which is of particular use for analyzing dangling bonds in disordered semiconductors.

The disordered nature of thin-film photovoltaic materials requires analysis of electronic, structural, and compositional properties at the nanometer scale. This is why methods such as scanning probe techniques (Chapter 11) as well as electron microscopy and its related techniques (Chapter 12) gain increasing importance in the field. X-ray and neutron diffraction (Chapter 13) as well as Raman spectroscopy (Chapter 14) contribute to the analysis of structural properties of photovoltaic materials. Since thin-film solar cells consist of layer stacks with interfaces and surfaces, important issues are addressed by understanding their chemical and electronic properties, which may be studied by means of soft X-ray and electron spectroscopy (Chapter 15). Important information for thin-film solar cell research

and development are the elemental distributions in the layer stacks, analyzed by various techniques presented in Chapter 16. Specifically for silicon thin-film solar cells, knowledge about hydrogen incorporation and stability is obtained from hydro­gen effusion experiments (Chapter 17).

For designing photovoltaic materials with specific electrical and optoelectronic properties, it is important to predict these properties for a given compound. Combining experimental results from materials analysis with those from ab-initio calculations based on density-functional theory provides the means to study point defects in photovoltaic materials (Chapter 18). Finally, in order to come full circle regarding the solar-cell devices treated in the first chapters of the handbook, the information gained from the various materials analyses and calculations may now be introduced into one-dimensional (Chapter 19) or multidimensional device simu­lations (Chapter 20). By means of carefully designed optical and electronic simula­tions, photovoltaic performances of specific devices may be studied even before their manufacture.

We believe that the overview of these various characterization techniques is not only useful for colleagues engaged in the research and development of inorganic thin-film solar cells, from which the examples in the present handbook are given, but also to those working with other types of solar cells as well as with other optoelec­tronic, thin-film devices.

The editors would like to thank all authors ofthis handbook for their excellent and (almost) punctual contributions. We are especially grateful to Ulrike Fuchs and Anja Tschortner, WILEY-VCH, for helping in realizing this book project.

Подпись: August 2010Daniel Abou-Ras, Berlin Thomas Kirchartz, London and Uwe Rau, Julich

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