Influence of the US window layer on the PV performances of 2-mum thick CdS/CdTe solar cells has been studied as a function of the US thickness, d(CdS). With a reduction of d(CdS) from 114 to 95 nm, J(SC) increases due to an increase in blue response. While, at d(CdS) < 85 nm, the conversion efficiency largely decreases due to a decrease in V-OC and FF. The deterioration of the crystallinity of CdTe due to a decrease in the sulfur composition x of the CdTe1-xSx mixed-crystal layer is concluded to be the most possible mechanism for the large decreases in V-OC and FF. (C) 2002 Published by Elsevier Science B.V.