Critical Assessment of Sub-Bandgap Primary Photocurrent in a-Si:H Based Solar Cells.
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CRITICAL ASSESSMENT OF SUB-BANDGAP PRIMARY PHOTOCURRENT IN a-Si:H BASED SOLAR CELLS. T.
X. Zhou',
S.S.
Hegedus,
and C.M.
Institute of Energy Conversion, 19716
Fortmann
University of Delaware,
Newark,
DE
ABSTRACT The sub-bandgap primary photocurrent and the solar cell performance of a-Si:H p-i-n devices have been studied before and after light induced degradation. The results indicate significant discrepancy between the two methods when used to estimate the degree of degradation and the defect density in the i-layers. A preliminary explanation is proposed. INTRODUCTION Deconvolution of the sub-bandgap optical spectrum is frequently used to estimate the density of states (DOS) in the mobility gap of a-Si:H and its alloys. The photothermal deflection spectroscopy (PDS) and the constant photocurrent method (CPM) are the most common methods to obtain the sub-bandgap absorption spectrum from homogeneous materials despite some persistent skepticism. It is desirable to be able to determine the sub-bandgap absorption and thus the DOS in the mobility gap of the intrinsic (i-) layer of a p-i-n device so that the cell performance and the material parameters can be characterized and analyzed on the same sample. Sub-bandgap primary (short circuit or reverse biased) photocurrent (PPC) has been considered (1-3] useful for this purpose. Detailed model analyses of sub-bandgap PPC have been provided by several authors. Okamoto et al. [1] applied non-equilibrium statistics for correlated defects to the DC and AC sub-bandgap PPC of a-Si:H p-i-n cells and found that the AC PPC originating from different optical transitions associated with correlated defects exhibited a unique phase shift. They applied vector analysis to determine the positions of charged and neutral dangling bond states from the AC PPC spectrum. However, in their analysis the recombination of photo-generated carriers was considered negligible. Recently A. Mittiga et al.[4] solved the full set of transport equations for a p-i-n device structure with sub-gap absorption and recombination taken into account. They concluded that the PPC under reverse bias (-2V) is in agreement with the absorption spectrum determined by CPM to within a factor of two. In this work we compare the cell performance and the magnitude of sub-bandgap PPC in p-i-n cells before and after light induced degradation in order to evaluate this method as a means to characterize the density of the Staebler-Wronski defects. EXPERIMENTAL Two series of p-i-n a-Si:H devices were prepared by photo-CVD technique on SnOx-coated glass [5]. The boron doped a-SiC:H players and phosphorus doped a-Si:H n-layers were the same for both Present Address: Microelectronics Research Center, Iowa State University, Ames, IA 50011 Mat. Res. Soc. Symp. Proc. Vol. 219. 01991 Materials Research Society
452
series. The back contacts are Ti/Ag layers. No a-SiC:H buffer layer was employed near the p-i interfaces in these devices. The i-layers were grown at 5 and 10 torr respectively and at the substrate temperature of 205
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