The MoSe2 layers, as a necessary interface layer in Cu(In,Ga)Se-2/Mo for high-efficiency Cu(In,Ga)Se-2 solar cells, were observed to form in the Mo and CIGS/Mo films during the selenization process. In order to reveal the determining factor of MoSe2, the correlation of MoSe2 thickness with the Mo sputtering pressure was investigated. The MoSe2 thickness increased with decreasing the sputtering pressure. Furthermore, the dependence of MoSe2 thickness on the deposition time of SiO2 layer and the characterizations of X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy demonstrate that the formation of Na2Sex compounds plays a determining role for the formation of MoSe2 layer. The illuminated I-V characterization showed that the high series resistance of the CIGS solar cell was induced by too thick MoSe2 layer. The significant deterioration of efficiency was observed because of the peeling-off of the absorber layer from Mo layer when the MoSe2 layer was too thin. (C) 2012 Elsevier B.V. All rights reserved.