Degradation Study of single a-Si:H Alloy Solar Cells
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DEGRADATION
STUDY OF SINGLE a-Si:H ALLOY SOLAR CELLS
ADAM H. PAWLIKIEWICZ, A. BANERJEE and S. GUHA United Solar Systems Corp. 1100 West Maple Road, Troy, MI 48084
ABSTRACT The paper presents the experimental data of single a-Si:H cells degraded under high intensity illumination and compares them to the numerically calculated J-V characteristics of degraded a-Si:H solar cells. The cells were deposited at two different substrate temperatures. The current voltage characteristics were calculated using our simulation program which allows for accurate determination of cell response under monochromatic or global AM1.5 spectrum. We have simulated the light-induced metastable defects (Staebler-Wronski degradation) by increasing the density of localized states. In order to test the validity of this approach we compared the calculated fill factors under AM 1.5, blue and red illumination with the measured fill factors for the same light. From the experimental data we observed that at near room temperature (-35'C) both cells reach saturation in degradation of fill factors under high intensity illumination. 17The saturation level corresponded to the minimum density of localized states (grin) of 1.2x10 cm- 3eV-1 and is independent of the light intensity used in experiment.
INTRODUCTION There is a lot of attention among the photovoltaic community directed towards understanding and minimizing the metastable degradation of amorphous silicon based solar cells. There are essentially two approaches to solving the negative effects of StaeblerWronski1 degradation. One is to come up with new amorphous silicon based alloys which are less prone to the light induced degradation and the other is to design solar cells such that they would perform adequately even in the degraded state. In this work we have investigated the SW degradation of single a-Si:H solar cells deposited at two different temperatures to see which one is better suited for fabrication of stable a-Si solar cells and also simulated these effects in order to use the results of modeling for the design of more stable devices. In the next sections we present details regarding the fabrication of the experimental devices subjected to the degradation tests and also a brief description of the solar cell model used in the calculation. Subsequently we will discuss -the results of the tests in light of the available theories on light induced degradation.
EXPERIMENTAL The devices used for accelerated tests were single junction a-Si:H cells. They were deposited on stainless steel substrates with a ZnO/Ag back reflector and ITO as a top contact. The cells were grown by GD method 2 and deposited at low (200'C - LT) and high temperature (300'C - HT). Both were of similar thickness of 3400A and 3700A. The optical bandgaps
Mat. Res. Soc. Symp. Proc. Vol. 219. @1991 Materials Research Society
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were 1.87eV and 1.81eV and the hydrogen content was 18% and 12% in the LT and HT samples, respectively. To measure photo-degradation, the cells were illuminated by low (5xAM1) and high (50xAMI)
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