Cation Substitution of Copper by Silver in the Earth-Abundant Compound Cu₂ZnSnS₄: Comparative Study of Structural, Morphological, and Optical Properties

Quaternary compound Cu₂ZnSnS₄ (CZTS) is widely recognized as a promising candidate for use as an absorber layer for photovoltaic applications. However, its efficiency is limited by the high density of antisite defects that shorten the performance of CZTS-based solar cells. Cation substitution of copper by other elements has been proposed as a potential solution in order to control and eliminate the cation disorder within the kesterite structure. In this work, Ag₂ZnSnS₄ (AZTS) and Cu₂ZnSnS₄ (CZTS) powders were successfully synthesized by solid state reaction. XRD and Raman measurements were performed to study the crystallographic structures and their lattice vibration spectrum. The results confirmed the presence of the pure phase of CZTS and the stannite phase of AZTS with an occurrence of the secondary phase Ag₈SnS₆. Morphological properties and the composition analysis of synthesized powders were analyzed by scanning electron microscopy and energy dispersive x-ray analysis. A significant change is noticed in the band gap energy value established by the diffuse reflectance spectroscopy from 1.44 eV for CZTS to 2.55 eV for AZTS. Lastly, measurements with the hot probe method indicate the p-type conductivity of CZTS and the n-type conductivity for AZTS as predicted by theoretical studies. These results highlight that the substitution of silver for copper in CZTS leads to significant changes in the properties of CZTS.