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Metastability in Hydrogenated Nanocrystalline Silicon Solar Cells

Published online by Cambridge University Press:  01 February 2011

Guozhen Yue ,
Baojie Yan ,
Gautam Ganguly ,
Jeffrey Yang  and
Subhendu Guha
Guozhen Yue
Affiliation:
[email protected], United Solar Ovonic Corporation, Research and Development, 1100 West Maple Road, Troy, MI, 48084, United States, (248)519-5317, (248)362-4442
Baojie Yan
Affiliation:
[email protected], United Solar Ovonic Corporation, 1100 West Maple Road, Troy, MI, 48084, United States
Gautam Ganguly
Affiliation:
[email protected], United Solar Ovonic Corporation, 1100 West Maple Road, Troy, MI, 48084, United States
Jeffrey Yang
Affiliation:
[email protected], United Solar Ovonic Corporation, 1100 West Maple Road, Troy, MI, 48084, United States
Subhendu Guha
Affiliation:
[email protected], United Solar Ovonic Corporation, 1100 West Maple Road, Troy, MI, 48084, United States
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Abstract

We have recently reported that hydrogenated nanocrystalline silicon (nc-Si:H) solar cells exhibit metastability behavior that is significantly different from hydrogenated amorphous silicon (a-Si:H) solar cells. First, we studied the light-induced change of nc-Si:H cells under different light spectra, and observed that no light-induced degradation occurs when the photon energy is lower than the bandgap of a-Si:H, while light-induced degradation indeed occurs when the photon energy is higher than the bandgap of a-Si:H. Based on this observation, we conclude that the light-induced defect generation occurs mainly in the amorphous and/or grain boundary regions. Secondly, we found that forward-bias current injection in the dark does not cause any degradation in the performance of nc-Si:H cells, in contrast to observations in a-Si:H cells. This phenomenon can be explained by assuming transport percolation through crystallites, where excess carrier recombination does not cause degradation. Third, we found that a reverse electrical bias does not reduce, but rather enhances the light-induced degradation in the nc-Si:H cell performance. This enhancement increases with the magnitude of the reverse bias. We carried out a systematic study including measurements of quantum efficiency losses and fill factors under different color filters. In view of the heterogeneity of the material structure, we proposed a “back-to-back” diode model which explains most of the experimental results. Finally, we present results of the improvement of stability of nc-Si:H cells made using hydrogen dilution profiling. We find that the nc-Si:H cells with an optimized hydrogen dilution profile show very little light-induced degradation though these cells have a high amorphous volume fraction. This indicates that the amorphous volume fraction is not the only factor determining the degradation. Other factors, such as the structure and distribution of the amorphous phase, as well as the properties of the grain boundaries, apparently play important roles in the overall stability of nc-Si:H cells.

Keywords

amorphousSiphotovoltaic
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 910: Symposium A – Amorphous and Polycrystalline Thin-Film Silicon Science and Technology – 2006 , 2006 , 0910-A02-01
Copyright
Copyright © Materials Research Society 2006

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