Effect of White and Red Light Illumination on the Degradation of a-Si:H Solar Cells
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ABSTRACT
Structures were carried out on the degradation of heterojunction and homojunction a-Si:H solar cells, using Xenon arc and tungsten halogen light sources. The degradations were carried out with both white and red light with the intensities being calibrated using the saturated reverse biased photocurrents. Because of the different light intensities, I, involved, comparisons between the different illumination were made after exposure times, t, where I1' t = constant. The degraded states of the cells were characterized with light I-V, dark I-V, and quantum efficiency measurements. The same degraded states, as indicated by the all three characteristics, are obtained with either white and red light providing the intensities are calibrated using the actual p-i-n cells. It is also found that the empirical relation for obtaining the same degradation found by L. Yang holds for these illuminations being about I and 10 suns. Analysis of the dark I-V's using AMPS with a charged defect distributions of defects indicates that with the exposures used, the light induced defects have a close to uniform distribution throughout the .-layer.
INTRODUCTION There is considerable interest in quantifying the degradation in solar cells in terms of the material parameters associated with the intrinsic region of the cell. Despite the well-established dependence of degradation on i-layer thickness in p-i-n solar cells [I], there are still unanswered questions about the exact role of the p-i and n-i interfaces as well as on the densities and spatial distributions of the light induced defects. This is in part due to a lack of systematic studies on the cell structures thick enough for the contribution of the i region to be clearly identified and related to the properties of the corresponding thin film materials. It is also a consequence of the different types of light sources used to degrade the materials and solar cells. These sources include: Xenon arc, tungsten white light (GE ENH, GE ELH); sodium vapor light sources as well as a variety of lasers. In addition to the different spectra present in these light sources, a wide range of intensities are used in order to accelerate the degradation. As a consequence, it is very difficult to correlate the results obtained by the different methods. The work reported here addresses two issues related to the degradation of p-i-n solar cells using different light sources. The first is on how equivalent degradation can be obtained using Xenon arc and tungsten halogen white light sources. The other is on how similar are the light induced defects created using white and red light for these two sources. The studies were carried out on p-i-n solar cell structures, 0.6 to 1.0 pm thick, with different i-layer thickness and p-layers. In order to reliably quantify the degrees of degradation, in addition to the light I-V's, quantum 627 Mat. Res. Soc. Symp. Proc. Vol. 3770 1995 Materials Research Society
efficiencies and dark I-V's were also measured. The dark I-V characteristics were analyzed using AMPS,
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