Investigation of pre-deposition temperature effect on n+-BSF properties in co-diffusion process

Abstract

In this study our aim was to investigate the variation effect of the phosphorus pre-deposition (P-D) temperature on n+-BSF properties. A co-diffusion process of n type solar cells was used as a drive-in step. This step concerned two types of as-pre-deposited n-type silicon test-wafers, with PSG and PSG etched. When increasing the phosphorus P-D temperature from 800 °C to 950 °C, we found that the sheet resistance after drive-in decreases exponentially and then saturates at the equal value beyond 900 °C. In addition, the BSF depths and phosphorus sheet concentration increased in this range of temperature. It was also found that the passivation quality depends greatly on the P-D temperature and on whether or not the processed wafer surface was capped with the PSG layer. Indeed, the effective lifetime increased significantly to a maximum value greater than 300 μs at a temperature range of 900 °C–920 °C and then decreases. BSFs performed at the P-D temperature of 900 °C produced the best passivation quality but were not well suited for contact formation. Those performed at 920 °C produced slightly lower passivation quality and were well suited for a metal contact formation.