This report describes the long-term research and development issues related to polycrystalline thin-film solar cells. Our general research approach is based on combining activities aimed at improvement of cell performance and stability with activities aimed at increasing our fundamental understanding of the properties of materials making up the cells: CdTe, CdS, multi-layer back-contact, and transparent conducting oxide (TCO) front-contact. We emphasize the relation between structural and electronic material properties and various processing procedures, as well as the microscopic mechanisms responsible for the cell performance and its degradation. There is a lack of knowledge and understanding of basic issues behind the CdTe/CdS cell performance and stability, such as the nature and electronic properties of impurities and defects that control the majority-carrier concentration, mechanisms of the dopant compensation, recombination centers, their nature and properties, diffusion, electro migration and transformation of defects under various processing, stress, and operating conditions. We believe that better basic understanding of the specific influence of polycrystallinity, especially for fine-grain materials characteristic of CdTe-based cells, is now one of the most important issues we must address. We need to clarify the role of grain boundaries in forming the film electronic properties, as well as those of the p-n junction. It is important to study and understand the influence of the grain boundaries on the spatial distribution and migration of impurities and electrically active defects. To fulfill these tasks, one needs to develop new methods and techniques (or adjust existing ones) for material characterization, as well as develop more sophisticated approaches to data analysis and modeling.
