Abstract 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 CdS layer and the adjustment of a CBO value by a Zn 1− x Mg x O (ZMO) layer. In ZnO/CdS/CIGS solar cells, open-circuit voltage ( V oc) and shunt resistance ( R sh) decreased with reducing the CdS thickness. In constant, significant reductions of V oc and R sh were not observed in ZMO/CdS/CIGS solar cells. With decreasing the CdS 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.