Full recovery of red zone in p-type high-performance multicrystalline silicon

Abstract

Between 10% and 30% of commercial cast silicon ingots is discarded due to contamination caused by the casting process. A significant contaminant in the scrap volume is metal precipitates, which are difficult to getter effectively and degrade minority charge carrier lifetime, hence limiting solar cell efficiency potential. We show here that the unusable red zone can be restored in high-performance multicrystalline silicon wafers. Adding a high temperature dissolution anneal prior to phosphorus diffusion dissolves the metal precipitates within the wafer bulk, and leaves the metal point defects in a mobile state to be readily gettered by phosphorus diffusion gettering during the solar cell process. The efficiency of the dissolution gettering treatment increases with increasing temperature, with a temperature of 1150°C eliminating the very low lifetime region of the wafers completely. Additionally, we find that the red zone does not re-emerge after a 60min oxidation anneal at 900°C, confirming that the achieved benefit is tolerant to any high temperature processing following the phosphorus diffusion gettering process.