(Georgia Institute of Technology, 2001-06)
Rohatgi, Ajeet; Yelundur, Vijay; Jeong, Ji-Weon; Ebong, Abasifreke; Rosenblum, M. D.; Hanoka, J. I.
A low-cost, manufacturable defect gettering and passivation treatment, involving simultaneous anneal of a PECVD SiN(x) film and a screen-printed Al layer, is found to improve the lifetime in Si ribbon materials
from 1-10 μs to over 20 μs. Our results indicate that the optimum anneal temperature for SiN(x)-induced hydrogenation is 700°C for EFG and increases to 825°C when Al is present on the back of the sample.
This not only improves the degree of hydrogenation, but also forms an effective back surface field. We propose a three-step physical model, based our results, in which defect passivation is governed by the release of hydrogen from the SiN(x) film due to annealing, the generation of vacancies during Al-Si alloying, and the retention of hydrogen at defect sites due to rapid cooling. Controlled rapid cooling was
implemented after the hydrogenation anneal to
improve the retention of hydrogen at defect sites by incorporating an RTP contact firing scheme. RTP contact firing improved the performance of ribbon solar cells by 1.3-1.5% absolute when compared to slow, belt furnace contact firing. This enhancement
was due to improved back surface recombination velocity, fill factor, and bulk lifetime. Enhanced hydrogenation and rapid heating and cooling resulted
in screen-printed Si ribbon cell efficiencies
approaching 15%.