An adjustment of a conduction band offset (CBO) of a window/absorber heterointerface is important for high efficiency Cu(In,Ga)Se-2 (CIGS) solar cells. In this study, the heterointerface recombination was characterized by the reduction of the thickness of a Us layer and the adjustment of a CBO value by a Zn1-xMgxO (ZMO) layer. In ZnO/CdS/CIGS solar cells, open-circuit voltage (V-oc) and shunt resistance (R-sh) decreased with reducing the US thickness. In constant, significant reductions of V-oc and R-sh were not observed in ZMO/CdS/CIGS solar cells. With decreasing the Us thickness, the CBO of (ZnO or ZMO)/CIGS become dominant for recombination. Also, the dominant mechanisms of recombination of the CIGS solar cells are discussed by the estimation of an activation energy obtained from temperature-dependent current-voltage measurements. (C) 2008 Elsevier Ltd. All rights reserved.