One-stage co-evaporation of Cu(In,Ga)Se2 (CIGS) thin films has strong potential for wide adoption in industry because of its simplicity compared with multiple-stage processes. This study investigated the effects of substrate temperature during CIGS film growth on the efficiency of the resulting ITO/ZnO/CdS/CIGS/Mo structures. CIGS thin films were grown by using one-stage co-evaporation at substrate temperatures ranging from 525 to 550 °C. The device with a CIGS film grown at 535 °C was found to have the highest cell efficiency. In the XRD patterns, films grown at this substrate temperature had the largest texture coefficient for the (220) plane and the smallest full width at half maximum for both the (112) and the (220) planes. Other cell electrical characteristics were also largest for this substrate temperature. Thus, we conclude that the optimal morphological characteristics of CIGS thin films grown at a temperature of 535 °C are responsible for the high efficiency.