Light-Induced Degradation in Upgraded Metallurgical-Grade Silicon Solar Cells

We report on light-induced degradation (LID) of multicrystalline solar cells made of upgraded metallurgical-grade (UMG) silicon. Cells made of wafers from various locations in an UMG ingot were subjected to LID study. We investigated the kinetics of the LID by measuring changes in the solar cell performance parameters with illumination time and under various illumination intensities; the LID occurred in a logarithmic scale with time. The amount of LID changes with the wafer location, depending on boron and oxygen concentration profiles in the ingot. Further, by mapping the diffusion length using light beam-induced current, we found that the degradation occurred in the bulk of grains in the multicrystalline cells but not at extended defects, such as grain boundaries and dislocations. The LID of UMG-Si is essentially consistent with the literature-reported LID of electronic-grade (EG) Si when it contains high levels of boron and oxygen concentration. Despite the significantly higher levels of many impurities in UMG-Si than in EG-Si, the LID of UMG-Si is dominated by the formation of the boron and oxygen defect complex, which is the mechanism of LID for EG-Si.