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University Center of Excellence for Photovoltaics
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ItemImplementation of Rapid Thermal Processing to Achieve Greater Than 15% Efficient Screen-Printed Ribbon Silicon Solar Cells(Georgia Institute of Technology, 2003-05) Rohatgi, Ajeet ; Yelundur, Vijay ; Jeong, Ji-Weon ; Kim, Dong Seop ; Gabor, A. M.This paper summarizes our progress in fabricating record-high efficiency ribbon Si solar cells with screen-printed and photolithography defined contacts. We have developed and optimized rapid thermal processing enhanced SiN(x)-induced hydrogenation to achieve record-high efficiency screen-printed EFG (15.9%) and String Ribbon (15.6%) cells and a high-efficiency String Ribbon cell (17.8%) with photolithography defined contacts. A low-frequency SiN(x) film and a two-step RTP firing process were critical in achieving high-efficiency screenprinted cells. Step 1 provides SiN(x) induced hydrogenation and forms an aluminum doped back surface field. Step 2 is designed for Ag grid firing and includes rapid cooling to retain hydrogen introduced in Step 1.
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ItemLifetime Enhancement and Low-Cost Technology Development for High-Efficiency Manufacturable Silicon Solar Cells(Georgia Institute of Technology, 2001-08) Rohatgi, Ajeet ; Yelundur, Vijay ; Jeong, Ji-WeonA 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. Controlled rapid cooling was implemented after the hydrogenation anneal and contact firing to improve the retention of hydrogen at defect sites using RTP. RTP contact firing improved the performance of ribbon solar cells by 1.3-1.5% absolute when compared to slow, belt furnace contact firing. Enhanced hydrogenation and rapid heating and cooling resulted in screen-printed Si ribbon cell efficiencies approaching 15%. A combination of screen-printed Al and a two minute RTP anneal in an oxygen ambient produced simultaneously a high quality rapid thermal oxide (RTO) and an aluminum back surface field (Al-BSF) with a back surface recombination (BSRV) of 200 cm/s 2-3 Ohm-cm single and multicrystalline silicon solar cells. In addition, RTO/SiN(x) stack passivation was found to be superior to SiN(x) surface passivation. RTO/SiN(x) passivation reduces the BSRV to ~10 cm/s on 1-2 Ohm-cm p-type single crystal Si and also lowers the Joe of 40 and 90 Ohm/sq emitters by a factor of three and ten, respectively. Integration of RTP emitters, screen-printed RTP Al-BSF and RTO produced 19% and 17% efficient monocrystalline cells with photolithography and screen-printed contacts, respectively.
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ItemFundamental Understanding and Implementation of Al-enhanced PECVD SiN(x) Hydrogenation in Silicon Ribbons(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%.
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ItemPECVD SiN(x) Induced Hydrogen Passivation in String Ribbon Silicon(Georgia Institute of Technology, 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.