The optoelectronic properties of oxygen, copper and chlorine doped CdTe thin films are being investigated. It is found that Cu co-dopant increases the p-type resistivity of CdTe:Cl films up to some orders of magnitude whereas, oxygen co-dopant increases the resitivity of CdTe:Cl:Cu films up to some orders of magnitude, and increases the quantity of liquid phase at thermal treatments. It is thought that Cu complexes, with oxygen and the isoelectronic complex defects V(Cl)2Cl(Te.) can limit the efficiency of CdS/CdTe solar cells.