Surface sulfurization of Cu(In,Ca)Se-2 (CIGS) thin films was carried out using two alternative techniques that do not utilize toxic H2S gas; a sequential evaporation of In2S3 after CIGS deposition and the annealing of CIGS thin films in sulfur vapor. A Cu(In,Ga)(S,Se)(2) thin layer was grown on the surface of the CIGS thin film after sulfurization using In,S,, whereas this layer was not observed for CIGS thin films after sulfurization using sulfur vapor, although a trace quantity of S was confirmed by AES analysis. In spite of the difference in the surface modification techniques, the cell performance and process yield of the ZnO:Al/CdS/ CIGS/Mo/glass thin-film solar cells were remarkably improved by using both surface sulfurization techniques. (C) 2001 Elsevier Science B.V. All rights reserved.