Bandgap engineering of SrZrS₃ chalcogenide perovskite via substitutional doping for photovoltaic applications: a...

Strontium zirconium sulfide (SrZrS₃) has garnered significant attention for photovoltaic (PV) applications due to its excellent optoelectronic properties, high chemical and moisture stability, and non-toxicity. However, the bandgaps of both the α- and β-phases lie outside the optimum ranges for both single-junction solar cells (SJSCs) and tandem solar cells (TSCs), thereby limiting their applications in PV technologies. In this study, we employed hybrid density functional theory to engineer the band gaps of α- and β-SrZrS₃ through substitutional doping. We considered three dopants (Hf, Sn, and Ti) at the Zr site of SrZrS₃ with varying doping concentrations and investigated their effects on the structural, electronic, and optical properties of the materials. We found that Sn and Ti doping effectively lowers the band gaps of both α- and β-SrZrS₃, whereas Hf doping increases them. For x values up to 0.25, the band gaps of the α-SrZr_1-xSn_xS₃ and α-SrZr_1-xTi_xS₃ are within the optimum range for SJSCs, and those of β-SrZr_1-xSn_xS₃ and β-SrZr_1-xTi_xS₃ lie within the optimum range for Si/perovskite as well as perovskite/perovskite TSCs. The three dopants exhibited significant effects on the optical properties of both α- and β-SrZrS₃, including the absorption coefficient, energy-loss functions, reflectivity, and refractivity spectra. Thermodynamic stability analysis revealed that for both phases, SrZr_1-xHf_xS₃ can be synthesized via exothermic processes, whereas the formation of SrZr_1-xTi_xS₃ and SrZr_1-xSn_xS₃ is endothermic and hence, not thermodynamically favorable. Further analysis showed that SrZr_1-xTi_xS₃ in both α- and β-phases are stable under thermodynamic equilibrium conditions, whereas SrZr_1-xSn_xS₃ is prone to dissociation into ternary phases (SrZrS₃ and SrSnS₃), especially at higher doping concentrations. These results show that Ti doping is effective in tuning the band gaps of α- and β-SrZrS₃ toward the optimal values for PV applications.