In this study, pure and 1%, 2% and %3 In-doped CdS thin films were produced by spray pyrolysis method. CdS is an n-type (II-VI group) semiconductor material and used as a buffer layer in solar cells. By doping In into CdS thin film, it was investigated how optical and crystalline behavior of thin film are changed. Using Moss and Herve&Vandamme and Ravindra relations, refractive indices and dielectric coefficients were investigated depending on the band gap of the obtained CdS sample. It has been observed that In element decreases the band gap of CdS thin film, improved its crystal structure and reduced its roughness. Therefore, 3% In doped CdS has gained a more ideal feature for use as an n-type semiconductor in solar cells. CIGS/In doped CdS solar cell was modelled and analysed by SCAPS-1D simulation program by using the physical parameters of the semiconductor layers that make up solar cells as imputs of program. Photovoltaic parameters of solar cell based on donor defect density, the neutral interface defect density and Auger electron/hole capture coefficient which were calculated by using In %3 doped CdS thin film, which has the most ideal n-type semiconductor properties.