In this research, we present thermal evaporated MoO3 layer as a primary back contact for pure sulphide Cu2ZnSnS4 solar cells. The effects of incorporating MoO3 in primary back contact and its thickness on the Cu2ZnSnS4 device performance are investigated. The MoO3 layer acts as enhancing sodium diffusion site where sodium is more effectively diffused into CZTS absorber, facilitating the formation of larger CZTS grains during sulfurization process compared to the reference absorber. The power conversion efficiency is improved from 5.2% to 6.2% on average, yielding the highest efficiency of 6.52% by the insertion of 10 nm MoO3 layer between Cu2ZnSnS4 absorber and molybdenum contact. The major efficiency enhancement arises from open circuit voltage which is improved from 530 to 615 mV.