TOWARDS HIGHER EFFICIENCY MICROMORPH SOLAR CELLS: THE 'INVERTED' (SUBSTRATE-N-I-P) CONFIGURATION

摘要

The aim of this study is the optimization of microcrystalline/amorphous silicon (i.e. "micromorph") tandem cells in the n-i-p configuration by focusing mainly on two issues : the reduction of light-induced degradation (Staebler-Wronski effect) in hydrogenated amorphous silicon (a-Si:H) films and the development of an efficient back reflector. Hydrogen dilution can improve both layer and solar cell stability against light-soaking. By increasing deposition temperature (T_(dep,)) the energy gap of hydrogen-diluted films can be reduced without any deterioration of the film quality. Thus, a combination of hydrogen dilution and moderately high values of T_(dep) (～325°C) leads to more stable a-Si:H layers with higher optical absorption, as compared to a-Si:H layers deposited at 275°C. However, this beneficial effect is not observed with entire solar cells incorporating such i-layer, certainly due to the low temperature discrepancy between 275°C and 325°C and the more severe diffusion of impurities occurring at higher temperature. The second optimization topic concerns the back reflector for micromorph solar cells. Several types of back reflectors were studied (ZnO doped with B or Al, textured, flat or etched ZnO:Al, flat and textured Ag). It was found that ZnO:Al (produced by magnetron sputtering) is more appropriate for a-Si:H n-i-p solar cells than ZnO:B (produced by LP CVD). Very efficient back reflectors were achieved by using as-grown textured Ag covered with ZnO:Al and promising results were obtained on μc-Si:H solar cells deposited one those latter reflectors.