Improved Back Reflector for High Efficiency Hydrogenated Amorphous and Nanocrystalline Silicon Based Solar Cells Baojie Yan, Jessica M. Owens, Chun-Sheng Jiang 1 , Jeffrey Yang, and Subhendu Guha United Solar Ovonic Corporation, 1100 West Maple Road, Troy, Michigan 48084 1 National Renewable Energy Laboratory, Golden, Colorado 80401 ABSTRACT Ag/ZnO back reflectors (BR) on specular stainless steel substrates are optimized for hydrogenated amorphous silicon germanium alloy (a-SiGe:H) and nanocrystalline silicon (nc- Si:H) solar cells. The BRs are deposited using a sputtering method. The texture of the Ag and ZnO layers is controlled by deposition parameters as well as chemical etching with diluted HCl. The surface morphology is investigated by atomic force microscopy. The scattered light intensity from a He-Ne laser, which illuminates the sample surface perpendicularly, is measured at different angles. Finally, a-SiGe:H and nc-Si:H solar cells are deposited on the BR substrates prepared under various conditions. For a-SiGe:H bottom cells, the improved BR with large micro-features leads to an enhanced open-circuit voltage. For the nc-Si:H solar cells, large micro-features on the improved BR eliminate interference fringes otherwise observed in the quantum efficiency measurement and result in high short circuit current density. The result is consistent with an enhanced scattered light intensity. Hence, the cell performance was improved. We also deposited a-Si:H/a-SiGe:H/nc-Si:H triple-junction cells on the optimized BR and achieved a high initial active-area efficiency of 14.6%. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) and silicon germanium (a-SiGe:H) alloy based solar cell technology is one of the most attractive photovoltaic technologies, due to its low cost and large-scale manufacturing abilities. Among the techniques for improving cell performance, light trapping with a textured back reflector (BR) is an important one for enhancing short-circuit current density, J sc , [1]. A textured Ag/ZnO BR was used for achieving 14.6% initial and 13.0% stable efficiencies in an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure [2]. Recently, the hydrogenated nanocrystalline silicon (nc-Si:H) solar cell has attracted remarkable attention due to its superior long wavelength response and improved stability over a-SiGe:H [3]. Using our conventional Ag/ZnO BR, we have achieved high efficiencies with a-Si:H/nc-Si:H double- junction and a-Si:H/a-SiGe:H/nc-Si:H triple-junction structures [4,5]. However, because of the nature of the nc-Si:H structure, the optimized Ag/ZnO BR used for the a-Si:H and a-SiGe:H solar cells may not necessarily be the best choice for the nc-Si:H solar cells. Optimizing the Ag/ZnO BR has the potential of improving the nc-Si:H cell efficiency further, hence achieving even higher efficiencies in multi-junction cells with nc-Si:H bottom cells. In addition, an improved BR may result in an even higher efficiency for a-Si:H/a-SiGe:H/a-SiGe:H triple- junction cells. In this paper, we present our recent results on the optimization of Ag/ZnO BR for a-SiGe:H and nc-Si:H solar cells. A23.3.1 Mater. Res. Soc. Symp. Proc. Vol. 862 © 2005 Materials Research Society