Concepts and Techniques for the Analysis of Defect Reactions in Crystalline Silicon

Abstract

Crystalline silicon used for photovoltaic applications is one of the purest materials on earth used for mass production. Even small quantities of certain impurities or defects may seriously impair its electronic quality, in particular the excess charge carrier lifetime or the associated diffusion length being the most relevant figure-of-merit for solar cells made from crystalline silicon. And even though it is a solid, impurities may still move around and react with each other, eventually resulting in a delayed formation of harmful defects manifesting as degradation phenomena of which some are triggered under illumination. Within this work, a variety of concepts and techniques used for the analysis of defect reactions in silicon is presented. At first, a mathematical description of reaction dynamics is given and different scenarios are discussed. It is then shown how defect density, or at least a proxy for it, can be inferred from measurements of excess charge carrier lifetime, or from open circuit voltage in case of solar cells. Furthermore, the problems regarding the correct quantification of surface recombination arising from limited excess charge carrier lifetime are explained. Moreover, a method is presented that allows for an assessment of iron contamination in solar cells from open circuit voltages. Lastly, a method for the quantification of hydrogen in crystalline silicon based on its reactions with other impurities is presented. In addition, some side topics are covered as well, like the development of an LED-based illumination setup, a discussion on how (not) to report on illumination intensity, the problems encountered in intensity measurements, and problems in bifacial MIS-type samples used for the intentional manipulation of surface passivation quality.