This study investigates the effects of the thickness of CdS buffer layers on the photovoltaic properties of Cu(In,Ga)Se-2 solar cells. Continuous and dense CdS films were formed with a thickness of 103.7 nm. Increasing the film thickness caused cracks in the prepared films. The illuminated current-voltage characteristics of the cells revealed that V-oc and FF firstly increased and then decreased as CdS film thickness increased over 103.7 nm. The conversion efficiency of Cu(In,Ga)Se-2 solar cell increased with the thickness of CdS films. The proper CdS film thickness yielded a maximum conversion efficiency of 11.9 %. Complete coverage of the Cu(In,Ga)Se-2 layers by CdS films without cracking reduced the diode factor of the prepared Cu(In,Ga)Se-2 solar cells. The UV-Vis spectra and external quantum efficiency indicated that the absorption loss at short wavelengths decreased as the thickness of CdS films increased. This study found that a CdS film thickness was required to ensure the complete coverage by CdS films to optimize the photovoltaic properties of Cu(In,Ga)Se-2 solar cells.