Light trapping in hydrogenated amorphous and nano-crystalline silicon thin film solar cells

Abstract

AbstractLight trapping effect in hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) and nano-crystalline silicon (nc-Si:H) thin film solar cells deposited on stainless steel substrates with various back reflectors is reviewed. Structural and optical properties of the Ag/ZnO back reflectors are systematically characterized and correlated to solar cell performance, especially the enhancement in photocurrent. The light trapping method used in our current production lines employing an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure consists of a bi-layer of Al/ZnO back reflector with relatively thin Al and ZnO layers. Such Al/ZnO back reflectors enhance the short-circuit current density, Jsc, by ˜20% compared to bare stainless steel. In the laboratory, we use Ag/ZnO back reflector for higher Jsc and efficiency. The gain in Jsc is about ˜30% for an a-SiGe:H single-junction cell used in the bottom cell of a multi-junction structure. In recent years, we have also worked on the optimization of Ag/ZnO back reflectors for nano-crystalline silicon (nc-Si:H) solar cells. We have carried out a systematic study on the effect of texture for Ag and ZnO. We found that for a thin ZnO layer, a textured Ag layer is necessary to increase Jsc, even though the parasitic loss is higher at the Ag and ZnO interface due to the textured Ag. However, a flat Ag can be used for a thick ZnO to reduce the parasitic loss, while the light scattering is provided by the textured ZnO. The gain in Jsc for nc-Si:H solar cells on Ag/ZnO back reflectors is in the range of ˜60-75% compared to cells deposited on bare stainless steel, which is much larger than the enhancement observed for a-SiGe:H cells. The highest total current density achieved in an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure on Ag/ZnO back reflector is 28.6 mA/cm2, while it is 26.9 mA/cm2 for a high efficiency a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cell.