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Rohatgi, Ajeet

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Author: Hanoka, J. I. × Author: Jeong, Ji-Weon × Author: Yelundur, Vijay × End date: 2009 × Start date: 2000 ×

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Fundamental Understanding and Implementation of Al-enhanced PECVD SiN(x) Hydrogenation in Silicon Ribbons

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%.

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PECVD SiN(x) Induced Hydrogen Passivation in String Ribbon Silicon

2000-09 , Yelundur, Vijay , Rohatgi, Ajeet , Jeong, Ji-Weon , Gabor, A. M. , Hanoka, J. I. , Wallace, R. L.

To improve the bulk minority carrier lifetime in String Ribbon silicon, SiN(x) induced defect passivation during a post deposition anneal is investigated. Our results indicate that SiN(x) induced hydrogen passivation is very effective when the SiN(x) film is annealed in conjunction with a screen-printed AI layer on the back. In addition, it is found that controlled rapid cooling can be used to enhance the defect passivation process. A model is proposed which relates the high temperature passivation to the release of hydrogen from the SiN(x) film, the injection of vacancies from backside AI alloying, and the retention of hydrogen at defect sites. High efficiency screen-printed String Ribbon solar cells (>14.5%) are fabricated utilizing the simultaneous SiN(x)/AI anneal in a belt furnace for hydrogenation and AI-BSF formation, followed by RTP firing of screen-printed contacts to improve the retention of hydrogen at defects.

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