Room Temperature Recovery of Light Induced Changes in Amorphous Silicon Solar Cells
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ROOM TEMPERATURE RECOVERY OF LIGHT INDUCED CHANGES IN AMORPHOUS SILICON SOLAR CELLS G. Ganguly, D.E. Carlson, M.S. Bennett, F. Willing, R.R. Arya and P. Stradins* BP Solar, Toano, VA; National Renewable Energy Laboratory, Golden, CO ABSTRACT We have observed the recovery of the performance of amorphous silicon (a-Si:H) based solar cell (especially the fill factor) at temperatures between 25°C and 170°C after ~600 hours of light soaking under 1 sun illumination at ~40°C. We find that there is some recovery of the fill factor of the cells even at temperatures below the temperature of light soaking. The recovery is significantly greater in cells of poor quality. There is also some evidence of enhancement of the recovery rate due to an external electric field. Above the light soaking temperature, a sort of thermally activated recovery of the fill factor is observed. INTRODUCTION Light induced changes in amorphous silicon have been extensively studied since they were first observed in films and solar cells [1,2]. The changes are reversible through annealing at elevated temperatures ~90-200°C and the recovery is well documented as being thermally activated with a distribution of energies [3]. High light intensities, injection of carriers or electron irradiation also result is similar or more pronounced changes. The changes occurring under higher light intensities with the sample temperature increasing to 50-70°C have been observed to recover significantly at room temperature (25°C) [4] and the recovery is enhanced under electric fields [5]. Changes due to light soaking at temperatures significantly below room temperatures also result in some or complete recovery at room temperature [6]. Solar cell modules deployed in the field are exposed to intermittent periods of illumination and darkness as well as changes in temperature. Therefore, it becomes interesting to look for recovery effects at ambient temperatures and light soaking intensities that may be experienced under real operating conditions. Here we report on the observation of material quality dependent, fieldenhanced but temperature-independent recovery of the fill factor of a-Si:H, a-SiGe:H as well as a-Si:H/a-SiGe:H solar cell devices at temperatures below the temperature of the devices during the prolonged 1 sun light soaking. EXPERIMENT a-Si:H and a-SiGe:H single-junction and a-Si:H/a-SiGe:H tandem solar cells with the structure glass/SnO2/p/i/n/p/i/n /ZnO/Al were fabricated in one of several double load-lock research reactors using DC plasma decomposition of silane and/or germane mixed with the appropriate dopant gases and/or diluted with hydrogen. The i-layer was deposited under a variety of deposition conditions. The plasma was generated over areas of 0.1m2 or 0.8m2 and the cells were fabricated using 8cm x 8cm pieces of commercial SnO2 coated glass. The cells were light soaked using ~100mW/cm2 illumination from sodium vapor lamps at a temperature of ~40°C. In some cases the effect of increasing the temperature to ~60°C was investigated and the degrad
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