Secondary Crystalline Phases Influence on Optical Properties in Off-Stoichiometric Cu₂S-ZnS-SnS₂ Thin Films

Cu₂ZnSnS₄ (CZTS) is an economically and environmentally friendly alternative to other toxic and expensive materials used for photovoltaics, however, the variation in the composition during synthesis is often followed by the occurrence of the secondary binary and ternary crystalline phases. These phases produce changes in the optical absorption edge important in cell efficiency. We explore here the secondary phases that emerge in a combinatorial Cu₂S-ZnS-SnS₂ thin films library. Thin films with a composition gradient were prepared by simultaneous magnetron sputtering from three binary chalcogenide targets (Cu₂S, SnS₂ and ZnS). Then, the samples were crystallized by sulfurization annealing at 450 °C under argon flow. Their composition was measured by energy dispersive X-ray spectroscopy (EDX), whereas the structural and optical properties were investigated by grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy and optical transmission measurements. As already known, we found that annealing in a sulfur environment is beneficial, increasing the crystallinity of the samples. Raman spectroscopy revealed the presence of CZTS in all the samples from the library. Secondary crystalline phases such as SnS₂, ZnS and Cu-S are also formed in the samples depending on their proximity to the binary chalcogenide targets. The formation of ZnS or Cu-S strongly correlates with the Zn/Sn and Cu/Zn ratio of the total sample composition. The presence of these phases produces a variation in the bandgap between 1.41 eV and 1.68 eV. This study reveals that as we go further away from CZTS in the composition space, in the quasi-ternary Cu₂S-ZnS-SnS₂ diagram, secondary crystalline phases arise and increase in number, whereas the bandgap takes values outside the optimum range for photovoltaic applications.