AbstractTechnology variations involving Cu and Cl impurities are among the major performance influencing factors for CdS/CdTe thin film solar cells. CuCl and CdCl2 influence on the energetic diagram of impurity levels with respect to variation of deposition parameters has been investigated. A comparative analysis has been carried out by using low temperature photoluminescence (PL) studies (17-98K) of CdTe thin films in the device configuration (from CdS/CdTe inteface and CdTe sides). To study the effect of CuCl influence, as-deposited, annealed heterojunctions, with CuCl treatment of CdS have been investigated.