Fundamental understanding and implementation of Al-enhanced PECVD SiNx hydrogenation in silicon ribbons

A low-cost, manufacturable defect gettering and passivation treatment, involving simultaneous anneal of a PECVD SiNx film and a screen-printed Al layer, improves the lifetime in Si ribbon materials from 1-10 ms to over 20 ms. Results indicate that the optimum anneal temperature for SiNx-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. Authors propose a three-step physical model, based on results, in which defect passivation is governed by the release of H from the SiNx film due to annealing, the generation of vacancies during Al-Si alloying, and the retention of H at defect sites due to rapid cooling. Controlled rapid cooling was implemented after the hydrogenation anneal to improve the retention of H at defect sites by incorporating an RTP contact firing scheme. RTP contact firing improved the performance of ribbon bsolar 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%. 9 refs.