First-principles investigation and photovoltaic assessment of Cs ₂ SnZ ₆ (Z = Cl, Br, I) lead-free perovskites for future solar technologies

This study presents a comprehensive first-principles and device-level investigation into the structural, electronic, optical, and photovoltaic properties of vacancy-ordered lead-free perovskites Cs₂SnZ₆ (Z = Cl, Br, I) for next-generation solar energy conversion. Using density functional theory (DFT), we examined the structural stability, band structure, density of states, and optical response of Cs₂SnCl₆, Cs₂SnBr₆, and Cs₂SnI₆. The calculated direct bandgaps are 2.652 eV for Cs₂SnCl₆, 1.358 eV for Cs₂SnBr₆, and 0.228 eV for Cs₂SnI₆, demonstrating significant tunability through halide substitution. Optical analyses reveal strong absorption in the visible spectrum, with a redshift in absorption onset from Cl to I, enhancing light-harvesting capabilities. To assess device performance, SCAPS-1D simulations were employed with four different electron transport layers (ETLs): CdS, IGZO, SnS₂, and ZnS. The Cs₂SnBr₆-based PSC with IGZO ETL achieved the highest power conversion efficiency (PCE) of 26.22%, driven by optimal band alignment and balanced charge transport. Meanwhile, Cs₂SnI₆, despite exhibiting ultrahigh short-circuit current densities (<i>J</i> _SC > 70 mA cm^-2) due to its narrow bandgap, showed poor performance with ZnS ETL due to mismatched energy levels. These results highlight the potential of Cs₂SnZ₆ perovskites as promising lead-free absorber materials and emphasize the critical role of ETL compatibility and absorber optimization in achieving high-efficiency solar cells.