Reactions of H with C in Multicrystalline Si Solar-cell Materials
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1268-EE02-02
Reactions of H with C in multicrystalline Si solar-cell materials Michael Stavola1, Chao Peng1, Haoxiang Zhang1, Vijay Yelundur2, Ajeet Rohatgi2 Lode Carnel3, Mike Seacrist4 and Juris Kalejs5 1 Department of Physics, Lehigh University, Bethlehem, Pennsylvania 18015 USA 2 School of Electrical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 USA 3 REC Wafer AS, NO-3908 Porsgrunn, Norway 4 MEMC Electronic Materials, St Peters, Missouri 63376 USA 5 American Capital Energy, N. Chelmsford, MA 01863 USA
ABSTRACT Hydrogen is commonly introduced into silicon solar cells to reduce the deleterious effects of defects and to increase cell efficiency. When hydrogen is introduced into multicrystalline Si that is often used for the fabrication of solar cells, the H atoms become trapped by carbon impurities to produce defect structures known at H2*(C). These defects act as both a source and a sink for hydrogen in H-related defect reactions. IR spectroscopy has been used to determine what H- and C-related defects are formed in multicrystalline Si when the carbon concentration is varied. A process that is used by industry to introduce hydrogen into Si solar cells is the post-deposition annealing of a hydrogen-rich SiNx layer. The H2*(C) defects provide a strategy for estimating the concentration and penetration depth of the hydrogen that is introduced by this method.
INTRODUCTION Crystalline-Si, solar-cell materials are fabricated by a variety of methods (Czochralski, cast, ribbon …), with an important goal being to reduce cost [1-3]. These Si materials typically have higher concentrations of structural defects and impurities (carbon, nitrogen, transition metals …) that can be tolerated for microelectronics applications. Carbon is often the impurity with the highest concentration which can approach near 1018 cm-3. Hydrogen is commonly added to Si solar cells to passivate bulk defects [4-7]. Hydrogen improves the minority carrier lifetime and increases solar-cell efficiency. However, what hydrogen exactly does in the Si containing a high concentration of carbon that is typically used to fabricate solar cells remains unknown. It is not known what hydrogen-defect complexes are formed or what harmful defects are passivated. The present paper reports a study of the hydrogen-related defect reactions that can occur in multicrystalline (mc) Si solar cells. Low temperature vibrational spectroscopy [8] has been used to investigate the reactions of carbon and hydrogen in mc Si that contains a high concentration of carbon. The dependence of these reactions on the concentration of carbon has also been explored. The C-H defect complexes that are seen also provide a strategy for characterizing the hydrogenation process by helping to estimate the concentration of H and its penetration depth when H is introduced into mc Si by the post-deposition annealing of a SiNx antireflection coating.
EXPERIMENT Multicrystalline (mc) Si samples with grain sizes from a few millimeters to one centimeter were used for our experim
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