Enhancing crystalline/optical quality, and photoluminescence properties of the Na and Sn substituted ZnS thin films for optoelectronic and solar cell applications; a comparative study

Zinc sulphide (ZnS) is one of the most important semiconductor for the optoelectronic devices. It is generally used as a buffer layer in highly efficient Cu(In,Ga)Se-2 solar cells due to its environmentally friendly characteristics. Here, we studied the effects of Na+/Sn2+ substituent using a solution based method for improving characteristics and functions of Zn1-xNaxS/Zn1-xSnxS (x = 0-15%) thin films for such applications. The formation of Zn2+-S2-, Zn2+-Sn2+-S2-, and Zn2+-Na+-S2- bonding as well as the existence of Zn2+/Zn, Sn2+/Sn, S2-/S, Na+/Na, ions/elements have been examined by the FT-IR and XPS/EDX analysis. The produced films have mixed cubic and hexagonal polycrystalline structure with highly preferred orientation along (111) plane of predominant cubic phase. Comparatively, XRD, SEM, and AFM analysis clearly showed that the crystalline quality of the Zn1-xSnxS thin films is higher than those of Zn1-xNaxS thin films. The surface morphology of both type films is dense and homogeneous. The surface roughness of Zn1-xSnxS films has higher value as compared to the Zn1-xNaxS thin films. The optical quality of Zn1-xSnxS films was better than those Zn1-xNaxS films confirmed by UV-Vis analysis. Amongst of all the synthesized materials, 1% Sn substituted ZnS film sample has the best film crystallization and optical quality. The observed results suggested that it has a promising potential for the optoelectronic and solar cell applications.