Band gap engineering in Cu2FeSnS4 nanostructures via single (Ag) and co-substitution (Ag–Mn): Correlating structure, morphology, and optical properties

In this study, a quaternary Cu₂FeSnS₄ (CFTS) nano-compound was prepared and characterized using the solvothermal method, with partial substitution between copper and silver ions to form the Cu_1.8Ag_0.2FeSnS₄ compound, in addition to a partial co-substitution between iron and manganese ions to form the Cu_1.8Ag_0.2Fe_0.5Mn_0.5SnS₄ compound. The aim of the study is to compare single doping and co-doping and their effect on structural, optical, and morphological properties. The X-ray diffraction and Raman spectroscopy results showed that all the prepared compounds have a kesterite crystal structure with the absence of secondary phases, while maintaining the crystalline phase during the substitution process, accompanied by a slight change in the crystal lattice constants. Field emission scanning electron microscopy analysis revealed a clear evaluation in nanoparticle morphology with doping and co-doping. Brunauer–Emmett–Teller analysis indicated a decrease in surface area upon doping. It was also observed that there is a gradual decrease in the energy gap accompanied by strong absorption in the visible region when the doping and co-doping process is carried out. According to these results, the authors confirm that co-doping with silver and manganese is an effective strategy for controlling optoelectronic properties. It also has a clear impact on surface properties, which must be considered to balance maintaining an effective surface area with minimizing the energy gap. Therefore, these co-doped compounds are promising candidates for photocatalysis and thin-film solar cell applications.