In the solution-based preparation of CZTS (Cu2ZnSnS4) thin films followed by a sulfurization process, a layer of MoS2 is formed at the CZTS-Mo interface. Formation of this MoS2 layer is mainly governed by the sulfurization process in H2S ambient gas rather than diffusion of a sulfur source in the CZTS film. Growth of CZTS grain and grain boundaries facilitates the formation of a MoS2 layer in any sulfurization process. A decrease in the series resistance and an increase in the current density and solar cell efficiency were achieved through an increase in the temperature of the second sulfurization sequence in a two-step sulfurization sequence. The formation of a CZTS grain dominates the performance of CZTS thin film solar cells with a relatively thin MoS2 layer, but the performance is degraded by an increase in recombination rate and the hole barrier effect between CZTS and Mo when the MoS2 is sufficiently thick. (C) 2015 Elsevier B.V. All rights reserved.