The surface treatment of Cu2ZnSnS4 thin films was carried out by utilizing ultrasonically generated water vapor. Suppression of the unintentional fluctuation of chemical composition near the heterointerface and enhancement in the homogeneity of CdS deposition on the Cu2ZnSnS4 thin films were observed in the samples with water vapor treatment at the temperatures between 250 degrees C and 350 degrees C. The effect on the variation of conduction band and valence band offsets at the Cu2ZnSnS4/CdS heterojunction, which was observed to be in a staggered alignment, was less than +/- 50 meV due to the treatment. Reproducibility of the cell characteristics was improved by the H2O spray treatment of the Cu2ZnSnS4 absorbers before the deposition of CdS buffer layers. At treatment temperature of 300 degrees C, the improvement of efficiency by about 25% compared to that of the untreated samples was achieved. (C) 2016 Elsevier B.V. All rights reserved.