We found a clear correlation between the solar conversion efficiency and the distribution of the ZnS secondary phase in Cu2ZnSnS4 (CZTS) solar cells. Five CZTS solar cells were prepared by the two-step process in which sputtered precursor multilayers were sulfurized at different temperatures. In higher efficiency solar cells, the ZnS secondary phase was found to accumulate near the interface between CZTS and MoS2 formed on top of the Mo layer. It was also found that a solar cell with a higher efficiency showed weaker ZnS Raman signal in the MoS2 layer. The highest conversion efficiency of 7.5% was obtained from a solar cell prepared at a sulfurization temperature Of 570 degrees C, in which no ZnS was detected in the MoS2 layer. We suggest that in order to obtain a high efficiency CZTS solar cell, it is critical to find a condition that keeps ZnS from the MoS2 layer. (C) 2016 Elsevier B.V. All rights reserved.