Lifetime Enhancement During Processing of Porous Silicon Cells

Abstract

Porous silicon (PS) induced lifetime enhancement was investigated in dendritic web and string ribbon silicon. As-grown lifetime in web was <1 µs, which improved by a factor of 3.27 due to 860 degrees C/2 min heat treatment. The same heat treatment after PS formation increased the lifetime by a factor of 8.33, indicating that PS alone is quite effective in raising the lifetime of web. Al gettering by itself raised the lifetime of as-grown web by a factor of 15, but the simultaneous annealing heat treatment increased the lifetime by a factor of 42, indicating a positive synergistic interaction between PS and Al. Finally, simultaneous phosphorus and Al diffusion, after PS formation, raised the as-grown web lifetime by a factor of 58. String ribbon Si, which has a low oxygen content and responds more favorably to hydrogenation, did not show much improvement due to PS+Al. Instead it showed greater response to SiN+Al treatment, suggesting that Al+PS induced lifetime enhancement is due to gettering and not hydrogenation. Screen-printed PS coated 100 µm web cells were formed by a single step firing and back diffusion with efficiency of 12 %. Cell analysis showed that process optimization can raise that efficiency by 2-3 % absolute.