Gettering of metallic impurities in photovoltaic silicon |
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Authors: | S A McHugo H Hieslmair E R Weber |
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Institution: | (1) Department of Materials Science and Mineral Engineering, University of California at Berkeley, Berkeley, CA, USA 94720 (Fax: 001-510/643-1197, E-mail: sam@garnet.berkeley,edu; hhiesl@argon.eecs.berkeley.edu; weber@garnet.berkeley.edu), US |
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Abstract: | This work addresses the issue of structural defect-metallic impurity interactions in photovoltaic silicon and their effect
on minority carrier diffusion length values. Aluminium and phosphorus segregation gettering studies were performed on photovoltaic
silicon in order to gain insight into these interactions and quantify the effect of gettering on solar cell performance. Integrated
circuit grade silicon was also studied for comparative purposes. Additionally, a novel rapid thermal annealing technique,
designed to dissolve metallic impurity precipitates, and Deep Level Transient Spectroscopy were utilized to determine the
as-grown impurity concentration in both grades of materials. Significant differences in gettering responses between the two
grades of silicon are observed. Gettering treatments greatly improve I.C. grade silicon with a specific gettering temperature
providing the optimal response. Photovoltaic grade silicon does not respond as well to the gettering treatments and, in some
cases, the material degrades at higher gettering temperatures. The degradation is primarily observed in dislocated regions
of multicrystalline photovoltaic silicon. Additionally, these dislocated regions were found to possess the highest as-grown
metallic impurity concentration of all the materials studied. The dislocation-free photovoltaic silicon has a higher diffusion
length relative to dislocated silicon but could not be improved by the gettering methods employed in this study. A model is
presented to describe these phenomena where the high concentration of metallic impurities at dislocations produce relatively
low minority carrier diffusion lengths as well as the degrading response with higher gettering temperatures while microdefects
create an upper limit to the photovoltaic grade material’s diffusion length.
Received: 21 June 1996/Accepted: 2 September 1996 |
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Keywords: | PACS: 61 70 81 40 82 20 |
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