Optical optimization of planar junction amorphous and microcrystalline silicon-based thin-film solar cells offers improved conversion efficiency. Planar junction solar cells with diffuse and selective angle rear reflectors were examined in detail. The refractive index of the diffuser material needs to be as close to that of silicon as possible. Insulating diffuse rear reflectors must be used in conjunction with conductive rear-window layers of the appropriate band gap, conductivity and refractive index. The refractive index of the rear window layer should be at least as large as that of the diffuser material. Rear windows comprised of ZnS, CdS and ZnSe (as well as other II-VI semiconductors and semiconductor alloys) used with a TiO2 diffuse reflector could increase the current of optically thin silicon solar cells beyond that possible using rough TCO/glass substrates. Selective angle reflectors must also be comprised of either the solar cell semiconductor itself (not possible in many cases) or comprised of a material having properties similar to those of the conducting rear windows used with the diffuse reflectors.