We compare the performance of two back reflector designs on the optoelectrical properties of microcrystalline silicon solar cells. The first one consists of a 5-mu m-thick low-pressure chemical vapor deposition (LPCVD)-ZnO electrode combined with a white sheet; the second one incorporates an Ag reflector deposited on a thin LPCVD-ZnO layer (with thickness below 200nm). For this latter design, the optical loss in the nano-rough Ag reflector can be strongly reduced by smoothing the surface of the thin underlying ZnO layer, by means of an Ar-plasma treatment. Because of its superior lateral conductivity, the thin-ZnO/Ag back reflector design provides a higher fill factor than the dielectric back reflector design. When decreasing the roughness of the front electrode with respect to our standard front LPCVD-ZnO layer, the electrical cell performance is improved; in addition, the implementation of the thin-ZnO/Ag back reflector leads to a significant relative gain in light trapping. Applying this newly optimized combination of front and back electrodes, the conversion efficiency is improved from 8.9% up to 9.4%, for cells with an active-layer thickness of only 1.1 mu m. We thereby highlight the necessity to optimize simultaneously the front and back electrodes. Copyright (c) 2014 John Wiley & Sons, Ltd.