Superstrate-type Cu(In,Ga)Se-2 (CIGS) thin film solar cells were fabricated using Zn1-xMgxO buffer layers. Due to the diffusion of Cd into CIGS during the growth of the CIGS layer, the conventional buffer material of CdS is not suitable. ZnO is a good candidate because of higher thermal tolerance but the conduction band offset (CBO) of ZnO/CIGS is not appropriate. In this study, the Zn1-xMgxO buffer layers were used to fulfill both the requirements. The superstrate-type solar cells with a soda-lime glass/In2O3:Sn/Zn1-xMgxO/CIGS/Au structure were fabricated with different band gap energies of the Zn1-xMgxO layer. The CIGS layers [Ga/(In + Ga)similar to 0.25] were deposited by co-evaporation method. The substrate temperature during the CIGS deposition of 450 degrees C did not cause the intermixing of the Zn1-xMgxO and CIGS layers. The conversion efficiency of the cell with Zn1-xMgxO was higher than that with ZnO due to the improvement of open-circuit voltage and shunt resistance. The results well corresponded to the behavior of the adjustment of CBO, demonstrating that the usefulness of the Zn1-xMgxO layer for the CBO control in the superstrate-type CIGS solar cells. (C) 2011 Elsevier B. V. All rights reserved.