Defect recognition and impurity detection techniques in crystalline silicon for solar cells

The efficiency of multicrystalline solar cells is limited by defects and impurities, which include grain boundaries, dislocations, and transition metals. The density of these defects often varies from grain to grain. “Bad grains” with low minority carrier diffusion length generate low open circuit voltage and shunt the “good grains” with high minority carrier diffusion length, thus reducing the overall cell efficiency. It was found that it is more likely to find transition metal clusters in “bad grains” than in “good grains”, and that gettering is not efficient in improving the areas of low diffusion length. The primary objective of materials research in photovoltaics is identification of these lifetime-limiting defects. In this article we summarize the current state of understanding of lifetime-limiting defects in solar cells, summarize the advantages and limitations of traditional analytical tools and discuss novel emerging techniques, including X-ray fluorescence microprobe, X-ray absorption spectromicroscopy, and X-ray beam-induced current