Light trapping in hydrogenated amorphous and nano-crystalline silicon thin film solar cells
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1153-A13-02
Light trapping in hydrogenated amorphous and nano-crystalline silicon thin film solar cells Jeffrey Yang, Baojie Yan, Guozhen Yue, and Subhendu Guha United Solar Ovonic LLC, 1100 West Maple Road, Troy, Michigan 48084 ABSTRACT Light 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 nanocrystalline 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/aSiGe:H triple-junction cell. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H), silicon-germanium alloy (a-SiGe:H), and nano-crystalline silicon (nc-Si:H) are the three intrinsic materials commonly used in multijunction solar cells [1,2]. Because of the nature of the disorder, the amorphous materials normally have a low mobility-lifetime product, which determines the fundamental limit of the solar cell thickness. For a-Si:H and a-SiGe:H cells, the intrinsic layers are in the range of a few thousand angstroms. For nc-Si:H cells, the intrinsic layer can only extend to one to two micrometers for maintaining a reasonable fill factor (FF). However, a thin intrinsic layer cannot produce enough short-circuit current density (Jsc). To resolve this issue, light trapping with textured substrates becomes an important technique for improving the Jsc in a-Si:H based solar cells without significant losses in the FF [3-7]. Except for the
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