The structural, electrical, chemical, and optical properties of ternary Zn(O,S) thin films formed by atomic layer deposition (ALD) were investigated. It was revealed that the film's characteristics were highly influenced by the O/(O+S) ratio. The n-type Zn(O,S) layer was applied to both S-rich and Se-rich Cu2ZnSn (S,Se)(4) (CZTSSe) absorbers as an alternative buffer layer to conventional CdS. The device performance relationship to the O/(O+S) ratio was examined. The highest power-conversion efficiency (PCE) of 2.75% and 3.30% was achieved using an actual O/(O+S) ratio of similar to 0.67 in the buffer layer for S-rich and Se-rich CZTSSe solar cells, and these PCE5 correspond to 77% and 67% of the standard CdS-based solar cells, respectively. Further improvement in Se-rich CZTSSe was demonstrated by using NH4OH solution instead of pure H2O as oxygen source during ALD process. The dependence of the solar cell performance on the O/(O+S) ratio was investigated using dark current density-voltage (J-V), external quantum efficiency (EQE), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX). (C) 2016 Elsevier B.V. All rights reserved.