Thermal Annealing Recovery and Saturation of Light-Induced Degradation of Amorphous Silicon Alloy Solar Cells with Diffe
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THERMAL ANNEALING RECOVERY AND SATURATION OF LIGHT-INDUCED DEGRADATION OF AMORPHOUS SILICON ALLOY SOLAR CELLS WITH DIFFERENT MICROVOID DENSITY X. XU, J. YANG, AND S. GUHA United Solar Systems Corp., 1100 W. Maple Rd., Troy, MI 48084 ABSTRACT We have studied the light-induced degradation and thermal annealing recovery of amorphous silicon alloy solar cells with different microvoid density in the intrinsic layer. The microvoid density was changed by altering the deposition rate. The experiments show that cells with higher microvoid density need longer annealing time to recover after prolonged light-soaking. As a consequence, cells with high density of microvoids do not seem to saturate even after long duration of light exposure. The cells with high microvoid density also show much lower degraded efficiency. A careful comparison between degradations caused by accelerated and one-sun light soaking and subsequent annealing recovery indicates that the defects created in the two cases have different nature. INTRODUCTION A good correlation between microstructure of amorphous silicon (a-Si) alloy films and solar cell performance has been recently observed [1]. With increasing microvoid density and microstructure fraction in the intrinsic layer, both the initial and light-degraded solar cell performance are found to deteriorate. It is interesting to see if the microstructure affects the saturation and annealing behavior of lightinduced defects of a-Si alloy solar cells. In this paper, we report the thermal annealing recovery and saturation behavior of a-Si alloy cells with different microvoid density in the intrinsic (i) layer produced by different deposition rates. The comparison of the cells degraded under one-sun and accelerated light-soaking will also be discussed. EXPERIMENTAL The a-Si alloy pin solar cells used in this study were grown by the rf glowdischarge technique on stainless steel substrates kept at 300 'C. Details of deposition parameters are given elsewhere [2]. The intrinsic layer was grown using disilanehydrogen mixture, and the dilution of the mixture and rf power density were changed to obtain deposition rates between 0.14 and 1.35 nm/s. The thicknesses of the i layers were kept constant at -420 nm. The deposition conditions for the doped layers were kept the same for all the samples. The top contact was made using thermally evaporated indium tin oxide. Cell performance was measured under global AMI.5, red, and blue illumination. The cells were degraded both under one-sun and accelerated light soaking. For one sun degradation, the cell temperature was controlled at 50 'C, while for accelerated degradation the light illumination intensity was 30 suns and the temperature was kept at 48 'C by continuously blowing N2 to the sample, and water cooling the sample holder plate. Thermal annealing was done in a vacuum system with a cryogenic sorption pump, and the temperature fluctuation during the annealing was within +/-3 "C.
Mat. Res. Soc. Symp. Proc. Vol. 297. (D1993 Materials Research Society
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