Light Trapping Effect from Randomized Textures of Ag/ZnO Back Reflector on Hydrogenated Amorphous and Nanocrystalline Silicon Based Solar Cells

We report our progress in the optimization of Ag/ZnO back reflectors (BR) for a-Si:H and nc-Si:H solar cells. Theoretically, a BR with a smooth metal surface and a textured dielectric surface would be more desirable. A smooth metal/dielectric interface reduces the plasmonic resonance loss and parasitic losses due to light trapped in sharp angles; a textured dielectric/semiconductor interface provides scattering for light trapping. In order to obtain sufficient light scattering at the ZnO/silicon interface, a highly textured ZnO layer is normally used. However, a highly textured ZnO surface causes deterioration of nc-Si:H material quality. In addition, to make a highly textured ZnO surface, a thick ZnO layer is needed, which could introduce additional absorption in the bulk ZnO layer and reduce the photocurrent density. Therefore, Ag/ZnO BR structures for nc-Si:H solar cells needs to be optimized experimentally. In this study, we found that an optimized Ag/ZnO BR for nc-Si:H solar cells is constructed with textured Ag and thin ZnO layers. Although a textured Ag layer might cause certain losses resulting from plasmonic absorption, the enhanced light scattering by a moderately textured Ag layer makes it possible to use a thin ZnO layer, where the absorption in the ZnO layer is low. With such a BR, we achieved a short-circuit current density of over 29 mA/cm² from a nc-Si:H single-junction solar cell. Using the high performance nc-Si:H cell in an a-Si:H/nc- Si:H/nc-Si:H triple-junction structure, we achieved an initial active-area efficiency of 14.5% with a total current density exceeding 30 mA/cm².