Lead-free chalcogenide perovskites have garnered significant attention in recent years as promising candidates for sustainable, high-performance photovoltaics. These materials present an ideal combination of nontoxic elemental composition, exceptional phase stability, and outstanding optoelectronic properties. However, unlocking their full potential for solar cell applications requires advanced strategies to fine-tune their electronic and optical behavior. In this study, we take ${\mathrm{CaHfS}}_3$—a promising but underexplored candidate—and enhance its performance through strategic substitutions: Ti at the cation site and Se at the anion site. Using state-of-the-art computational techniques, including density functional theory, $GW$ calculations, and the Bethe-Salpeter equation, we reveal how these substitutions transform the material's properties. Our findings highlight that alloyed compounds such as ${\mathrm{CaHfS}}_{3-x}{\mathrm{Se}}_x$ and ${\mathrm{CaHf}}_{1-y}{\mathrm{Ti}}_y{X}_3$ ($X=\mathrm{S}$, Se) are not only phase stable but also feature adjustable direct $G_0{W}_0$@PBE band gaps ($1.29-2.67\mathrm{eV}$), reduced exciton binding energies, and significantly improved polaron mobility. These modifications enable better light absorption, reduced electron-hole recombination, longer exciton lifetimes, and enhanced quantum yield. The Ti-rich Se-based perovskites achieve a spectroscopic-limited maximum efficiency of up to 28.06%, outperforming traditional lead-based halide perovskites. Our results demonstrate that strategic alloying is a powerful tool to supercharge the optoelectronic properties of lead-free chalcogenide perovskites, positioning them as compelling candidates for next-generation photovoltaic technologies.

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• Dielectric properties
• Electronic structure
• Perovskites
• Photovoltaic absorbers
• Density functional calculations
• GW method