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

- A high temperature anneal before PDG eliminates the heavily contaminated red zone.
- At best, the lifetime in the red zone increases from below 5 µs to over 80 µs.
- Higher anneal temperature is more efficient in recovering the red zone.
- The benefit is tolerant to post-PDG high temperature processing.

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 60 min oxidation anneal at 900 °C, confirming that the achieved benefit is tolerant to any high temperature processing following the phosphorus diffusion gettering process.