This article investigates the role of the interfaces of US and In(OHx,S-y) buffer layers in thin film ZnO/buffer/Cu(In,Ga)Se-2 heterojunction solar cells. The presence of acceptor defects at the ZnO/In(OHx,S-y) interface explains the poor performance of solar cells using the In(OHx,S-y) buffers. The standard US buffer has better quality at both interfaces to the ZnO window layer and to the Cu(In,Ga)Se-2 absorber. By using a combination of buffer layers of thin US and In(OHx,S-y), the short circuit current density is improved by 2 mA cm(-2) and thus, the efficiency is 1% higher than that of devices with the standard US buffer. (C) 2003 Elsevier Science B.V. All rights reserved.