The conventional CdS layer in Cu(In1-x,Ga-x)Se-2 based solar cells replaced by (Zn1-y,Sn-y)O as a buffer layer and the solar performance analyzed. An initial model based on an experimental device has been established. The dependence of solar cells performance on the change of Ga concentrations in CIGS absorber and Sn concentrations ZnSnO (band gap of ZnSnO) buffer layer was investigated. The optimum values of Ga and Sn concentrations were found at x = 1 and y = 0.2, leading to a conversion efficiency of 25.36%.