Buffer layers for high-efficiency Cu(In,Ga)Se-2 (CIGS) solar cells are commonly prepared by chemical bath deposition. The highest efficiencies are obtained using a CdS buffer layer. In view of the detrimental environmental impact of Cd and to improve in-line processing for mass production, Cd-free buffer layers fabricated by a dry process are attractive. In this study, ZnO1-xSx thin films were prepared by co-sputtering of ZnO and ZnS targets and they were used as the buffer layer in CIGS solar cells. The conduction band offset (CBO) between ZnO1-xSx/CIGS layers was controlled by varying the sulfur content x in ZnO1-xSx. CIGS solar cells with an In2O3:Sn/ZnO1-xSx/CIGS/Mo/soda-lime glass structure were fabricated with different x. The variations in the solar cell parameters with the sulfur content were consistent with that of the CBO. The maximum efficiency was obtained at x = 0.18 and it was >90% that of CdS/CIGS solar cells. (C) 2010 Elsevier B.V. All rights reserved.