Combined DFT and SCAPS-1D Investigation of Novel Rb 3 SBr Anti-Perovskite for Photovoltaic Performance and Solar-to-Hydrogen Generation

Rb 3 SBr is proposed in this study as a lead-free anti-perovskite candidate for multifunctional solar-energy applications and is evaluated through an integrated first-principles and device-modeling approach. Its structural, mechanical, vibrational, electronic, and optical characteristics were examined using density functional theory (DFT), followed by photovoltaic and solar-driven hydrogen assessments using SCAPS-1D. The optimized cubic phase satisfies the examined elastic stability requirements, while the phonon spectrum contains no unstable modes, supporting its mechanical and dynamical viability. Also, negative formation energy suggests that Rb 3 SBr is thermodynamically stable. A Pugh ratio (B/G) of 1.73 places the compound slightly below the conventional ductile–brittle threshold of 1.75, suggesting a moderately brittle response close to the transition boundary. Electronic calculations identify a direct Γ-point transition, with energy gaps of 1.316 eV from PBE and 2.193 eV from HSE06. The material also exhibits pronounced visible–UV photon absorption, with α spanning approximately 0.5×10 5 −3.5×10 5 cm -1 , together with weak reflectance and favorable refractive behavior, indicating promising interaction with incident solar radiation. Device-level performance was examined using a lead-free Al/FTO/SnS 2 /Rb 3 SBr/CuO/Se heterojunction. With an Rb 3 SBr thickness of 0.6 µm, shallow acceptor concentration of 1×10 17 cm -3 , bulk defect concentration of 1×10 14 cm -3 , series resistance of 1 Ώ.cm 2 , and shunt resistance of 10 6 Ώ.cm 2 , the optimized SCAPS-1D model delivers J SC = 32.996 mA/cm 2 , FF = 83.68%, PCE = 26.24%, and V OC = 0.951 V. Beyond electricity generation, a conceptual series-connected photovoltaic configuration was evaluated for water splitting, predicting a solarto-hydrogen (STH) efficiency of 19.785%. Overall, the combination of favorable optoelectronic properties, calculated stability, and lead-free nature identifies Rb 3 SBr as a theoretically promising candidate for further investigation in photovoltaic, optoelectronic, and solar-driven hydrogen-production applications.