Zinc oxide is a widely applied material in industry. It is produced in hundreds of thousands of tons for paints (chinese white), additive for rubber and plastics, catalysts, pharmaceuticals and cosmetics (sun creams), or as coating material for paper. In electronics industry ZnO is used in nickelor manganese-zinc ferrites, as ingredient of phosphors, in surface-acoustic wave filters and as a transparent electrode. Zinc oxide is a wide bandgap compound semiconductor (Eg - 3.2 eV) which has been investigated as an electronic material for many decades, starting in the 1930s. It belongs to the class of transparent conducting oxides (TCO). The other important oxides are indium and tin oxide. Recently, fundamental and applied research on zinc oxide experienced a renaissance due to the prospective use of zinc oxide as an optoelectronic material for blue and UV lasers. More over, thin ZnO films are important components in most thin film solar cells. The cost-effective large-scale production of these films on the one hand and the development of ZnO films with improved properties on the other hand are key challenges in the field of production and R&D in photovoltaics.
Renewable energies are one or even the only answer to provide the world energy demand on the long term in a sustainable way. Moreover, the world is facing climate changes due to green house gas emissions, and the limitation of these emissions is one of the key challenges of the global society. Solar energy bears the largest potential of all renewable energy sources, however, it is still rather expensive for many large-scale applications today. The direct conversion of sunlight into electricity by photovoltaic (PV) solar modules has grown by 20-40% per annum during the last decade and has emerged to a billion Euro market. Significant cost reductions have been achieved and there is a huge potential to bring the costs down further. Currently, PV module production is dominated by crystalline silicon solar cells, which are based on Si wafers with a typical thickness of around 150-300 μ m. However, the production of the comparatively thick silicon wafers involves high process temperatures and very pure silicon as an expensive feedstock material, partly limiting the potential for cost reductions. Thin-film solar cells require only a few micrometers of film thickness to absorb most of the sunlight and thus bear a great potential for significantly reducing the cost of photovoltaic energy conversion due to low material consumption, simple production techniques and high productivity by depositing on large areas. In addition, low-temperature processing and low material consumption save energy during production leading to short energy pay-back times.
Thin films of ZnO play an essential role in most thin film solar cells being produced today. ZnO films serve as transparent and conductive front contact, provide additional optical functions like light scattering and subsequent light trapping or enhance the reflection at the back contact. In heterojunction solar cells based on chalcopyrite absorber layers, ZnO is an inherent part of the p/njunction. In general, ZnO films with improved optical, electronic or structural properties promise higher conversion efficiencies while the development of process technologies providing an optimum film quality on large areas at high growth rates are an essential prerequisite to meet the cost targets in production. Examples of long-term scientific challenges are the development of adapted nano-structured ZnO films, like e.g. ZnO nanorods, or p-type ZnO films which may both open up new possibilities for designing future thin film solar cells.
This book is devoted to the properties, preparation and applications of zinc oxide (ZnO) as an transparent electrode material. It focuses on ZnO for thin film solar cell applications and hopefully inspires also readers from related fields. The book is structured into three parts to serve both as an overview as well as a data collection for students, engineers and scientists. The first part, Chaps. 1-4, provide an overview of the application and fundamental material properties of ZnO films and their surface and interfaces properties. Chaps. 5-7 review thin film deposition techniques applied for ZnO preparation on lab scale but also for large area production. Finally, Chaps. 8 and 9 are devoted to applications of ZnO in silicon and chalcopyrite-based thin film solar cells, respectively. One should note that the application of CVD grown ZnO in silicon thin film cells is discussed earlier in Chap. 6.
The idea to write this book evolved during a research project on zinc oxide in thin film solar cells, initiated by the German association "Forschungsverbund Sonnenenergie" and financed by the German Ministry of Education and Research which are gratefully acknowledged. Last, but not least, we thank all our colleagues who contributed with questions, discussions and data to this book.
Berlin, Darmstadt August 2007