The effects of annealing under various atmospheres on the electrical properties of Cu(In,Ga)Se-2 (CIGS) films and CdS/CIGS heterostructures were investigated. For CIGS films without CdS, the electrical properties of CIGS degraded under vacuum and O-2 annealing, although such degradations were not observed under N-2 annealing. For the CdS/CIGS heterostructures, the electrical properties of the junctions improved after annealing under all gas ambients. Therefore, CdS films prevent the chemical reactions at the CIGS surfaces and are necessary for effectively annealing the CIGS film. We observed a distinct correlation between the degradation of the electrical properties and increase in the defect density. Finally, we discussed the origin of the defect states. (c) 2007 Elsevier B.V. All rights reserved.