The direction of Se-Mo-Se sheets to the Mo film is crucial for the thickness of Mo(S,Se)(2) and the electrical conductivity, so as the back contact of Cu2ZnSn(S,Se)(4) solar cells. In this study, the preferred orientation of Mo (S,Se)(2) film changes from (100) peak to (103) peak, i.e. from perpendicular to the substrate to tilted to the substrate.for Se-Mo-Se sheets as the roughness decrease of Mo back contact layer. The Se vapor can easily diffuse through the channels between Se-Mo-Se sheets when the formed Mo(S,Se)(2) layer is (100)Deak preferred with SeMo-Se sheets perpendicular to the substrate, and thus excessively thick Mo(S,Se)(2) will be formed. Whereas, the Se-Mo-Se sheets are tilted to substrate when the preferred orientation of Mo(S,Se)(2) is (103) peak. The tilted SeMo-Se sheets can act as a natural Se diffusion barrier to suppress the Se vapor diffusion through the already formed Mo(S,Se)(2) layer to further selenize the remaining Mo film, and also can provide a good electrical conductivity. As a result, the thickness of Mo(S,Se)(2) sharply decreased from 1500 nest to 200 nm with the surface morphology change of Mo back contact, which resulting in the decrease of series resistance of Cu2ZnSn(S,Se)(4) solar cells from 2.94 Omega cm(2) to 0.49 Omega cm(2), and the increase of conversion efficiency of Cu2ZnSn(S,Se)(4) solar cells from 6.98% to 9.04%.