Effect of sulphur pressure on properties of ZnS thin film prepared by chemical bath deposition technique

Zinc sulfide (ZnS) thin films have been prepared by chemical bath deposition followed by thermal annealing. The influence of sulphur (S) pressure on the morphological, structural and optical properties of the ZnS thin film has been investigated. The results show that the annealed ZnS thin films have more homogeneous morphology and better crystallinity compared with the as-deposited film. It was found that the crystallinity and mean grain size of ZnS films increase with the S pressure. Meanwhile, the transmittance of the film was significantly enhanced in the visible spectrum region with the increase of S vapor pressure, which may be attributed to the lower defects density of the film annealed at higher S vapor pressure. However, the optical band gap of ZnS thin films decrease with the increase of S pressure, which is associated with the larger grain size and higher crystallinity. The decreased band gap can adjust the conduction band offset of the buffer/absorber heterojunction in copper based semiconductor material (such as Cu(In,Ga)Se-2, Cu(In,Ga)S-2 or Cu2ZnSnS4)solar cell to a lower energy barrier to enhance the electron injection from absorber to ZnS buffer layer. According to the structural and optical properties analysis, the obtained ZnS thin films annealed at higher S pressure can potentially be used as better buffer layer of thin film solar cells.