Sputtered (Zn,Mg)O buffer layers are one of the few promising options for completely dry and cadmium-free manufacturing of chalcopyritebased solar cells. The performance of a heterojunction solar cell depends critically on the electrostatic charge contained in the interface. In chalcopyrite-based cells this charge can be influenced by redox treatments. In this work we have added hydrogen to the working gas when sputtering, the buffer layer in an effort to optimize the interface charge. We report on the unexpectedly complex effects even at low hydrogen flow. They include a reduced deposition rate, significantly altered diffractograms and higher Mg/Zn-ratio in the film. X-ray photoelectron spectroscopy reveals the deposition of a thin layer of metallic zinc in the initial deposition stage which is not observed in the absence of hydrogen. Small hydrogen concentrations appear to be beneficial in terms of cell performance and reproducibility. However, higher concentrations typically cause a loss in blue response which indicates the formation of a homojunction buried within the chalcopyrite absorber. (C) 2006 Elsevier B.V All rights reserved.