Degradation and Regeneration in mc-Si after different gettering steps

Abstract

Light and elevated temperature induced degradation (LeTID) affects significantly the performance of multicrystalline (mc) Si PERC solar cells, and underlying mechanisms of LeTID are still unknown. In this work LeTID and following regeneration of an industrial mc-Si PERC solar cell is compared to differently processed minority charge carrier lifetime samples under illumination (1 sun) and elevated temperature (75°C). LeTID on cell level reveals the same kinetics compared to lifetime samples. Varying the processing sequence has a significant effect on LeTID of lifetime samples. Ungettered samples with fired SiNx:H surface passivation show a very strong LeTID and regeneration effect, with degradation kinetics being similar for all wafer areas irrespective of initial material quality. In contrast, regeneration sets in earlier in good quality areas. Differently gettered samples are less sensitive to LeTID, while overall degradation and regeneration behaviour is strongly influenced by applied gettering sequences. Al-gettered samples show a more pronounced degradation effect than P-gettered samples, leading to the assumption that P-gettering is more effective in the reduction of LeTID sensitive defects. A model is presented, describing LeTID in boron as well as gallium doped mc-Si being based on impurities that can be gettered and redistributed during high temperature steps.