Effect of excess selenium in the formation of $\mathrm{Cu}_2\mathrm{Zn}_{1.5}\mathrm{Sn}_{1.2}(\mathrm{S}_{0.9}+\mathrm{Se}_{0.1})_4$ alloys for solar cell applications

Copper zinc tin sulfide/selenide $\mathrm{Cu}_2\mathrm{Zn}\mathrm{Sn}(\mathrm{S},\mathrm{Se})_4$ (CZTSSe) is an alternative promising material for solar cell applications. It exhibits a high optical absorbance and tunable band gap. We have investigated the effect of excess selenium on the formation of CZTSSe phase which was prepared by the thermal melt method. The CZTSSe alloys were characterized by X-ray diffraction (XRD), Raman spectroscopy and UV-VIS spectroscopy. The crystallographic structure and phase were confirmed by X-ray diffraction and Raman spectroscopic techniques. In Raman spectroscopy, we found that the phase shifts from $327$ cm$^{-1}$ to $338$ cm$^{-1}$ when the selenium content excess is $5$ %. In optical studies, a band gap for the CZTSSe alloys of about $1.43$ eV to $1.44$ eV was observed.