Abstract. We develop a detailed formalism to photoconversion efficiency η of hydrogenated amorphous silicon ( a-Si:H ) based solar cells with a contact grid. This efficient three-dimensional model allows firstly optimization of the p i n sandwich in terms of carrier mobilities, thickness of the layers, doping levels and others. Secondly, geometry of the grid fingers that conduct the photocurrent to the bus bars and ITO/SiO 2 layers has been optimized, and the effect of non-zero sun beam incidence angles has been included as well. The model allows optimization of the amorphous Si based solar cells in a wide range of key parameters.