Effects of low-temperature annealing on the initial and stabilized performance of amorphous silicon solar cells
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Effects of low-temperature annealing on the initial and stabilized performance of amorphous silicon solar cells D. Carlson, G. Ganguly and G. Lin BP Solar, Toano, VA 23168 ABSTRACT We have recently observed that the initial performance of amorphous silicon (a-Si) solar cells can be improved by up to several % by annealing the cells at successively lower temperatures for successively longer times. For some devices, we also observed an improvement in performance after light soaking that was statistically significant. We have also observed an improvement in the stabilized performance of cells that were subjected to reverse bias annealing after light soaking. This effect was clearly evident in single-junction p-i-n cells with amorphous silicon-carbon (a-SiC) i-layers where the density of metastable centers is very high (> 1018 cm-3). When we light-soaked a-SiC cells and then subjected them to low-temperature reverse bias annealing, they exhibited clear evidence of improved short-wavelength response after a second light soaking. These annealing effects are explained in terms of hydrogen motion within the a-Si network and within microvoids. INTRODUCTION It has been standard practice to anneal a-Si solar cells at moderate temperatures (~ 150 – 200°C) for 20 – 60 minutes after fabrication in order to optimize the performance. In the current study, we examined different annealing conditions and found that the performance can be improved even further by annealing the cells at successively lower temperatures for successively longer times. Swartz [1] showed in 1984 that the initial performance of a-Si solar cells could also be improved by the application of a strong reverse bias while annealing at temperatures in the range of 120 – 150°C. While this improvement in performance is largely wiped out by prolonged exposure to sunlight, some experiments showed that a small improvement was sometimes evident in reverse-biased cells after light soaking, but the effect was small and depended on fabrication conditions [2]. In more recent work, we have examined the effect of reverse bias annealing on the recovery of light-soaked a-Si cells and showed that the rate of recovery could be enhanced by the application of an electric field during annealing [3,4]. As part of the current study, we fabricated p-i-n cells with a-SiC i-layers under conditions that resulted in a very high density of metastable defects (> 1018 cm-3) and examined the effects of annealing these cells under various bias conditions. By increasing the density of metastable centers, we were able to observe statistically significant improvements in the stabilized performance by reverse bias annealing of degraded cells. EXPERIMENT The first set of annealing experiments were performed on both a-Si single-junction p-i-n cells [2] and on a-Si/a-SiGe tandem cells [5] that were grown on glass substrates coated with textured tin oxide. The samples consisted of 36 diodes (each 0.27 cm2) positioned in a 6 x 6 A25.4.1
array on a 3” x 3” substrate (see figure 1). The single-junction ce
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