In this work, we have studied the influence of sodium in CuZnSnS (CZTS) solar cells. The CZTS films were prepared on soda lime glass substrates with and without sodium barrier layers (the glass substrate served as a sodium source). Alternatively, a 40 nm NaF layer was deposited on the metallic precursor using glass substrates with a sodium barrier layer. The SEM showed a bi-layer structure of CZTS films. The bottom layer with small grains became thicker with sodium doping. Using the UV-Raman spectroscopy, ZnS phase was observed both in the surface and interface for the sample prepared with NaF. XPS indicated that sodium was rich on the film surface and at the CZTS/Mo interface. Simultaneously the surface and interface identified to be low concentrations of Cu as well as high concentrations of Zn. The I-V curve under light indicated that the performance could be improved by controlling the amount sodium. (C) 2016 Elsevier Ltd. All rights reserved.