It is promising that the efficiency of cuprous oxide (Cu2O) based heterojunction solar cell can be improved by inserting a buffer layer to mediate the energy alignment at the interface. In this work, a sulfur-doped ZnO (ZnOS) buffer layer with properly tuned band alignment was introduced to the Cu2O heterojunction solar cell with Al-doped ZnO (AZO) as the n-type transparent conductive oxide layer to form an interfacial energy barrier, which improved the open-circuit voltage extremely. The experimental parameters including the oxygen pressure, buffer layer thickness and the deposition temperature were optimized. The mechanism of buffer layer was investigated, which revealed the rise of conduction band minimum of ZnOS buffer layer, thereby mitigating the interfacial recombination. This work shed light on the optimization of the interfacial band alignment of heterojunction solar cell, improving the photoresponse property. (C) 2015 Elsevier B.V. All rights reserved.