Theoretical investigation of Rb-based double halide perovskites for photovoltaic, photocatalytic water splitting, and CO2 reduction applications

Abstract

Understanding the intrinsic properties of emerging materials is essential for advancing next-generation optoelectronic and energy-related technologies, and DFT offers an effective route for this purpose. In this work, the structural, electronic, optical, photocatalytic, and photovoltaic properties of lead-free Rb₂NaRhX₆ (X = F, Cl, Br, I) double halide perovskites are systematically investigated using a combined DFT and SCAPS-1D simulation approach. Structural analysis confirms that all compounds crystallize in stable cubic phases and satisfy thermodynamic, mechanical, and dynamical stability criteria through negative formation energies, Born stability conditions, and the absence of imaginary phonon frequencies. Electronic band-structure calculations including PBE, spin–orbit coupling (SOC) and HSE06 hybrid functional corrections reveal direct band-gap semiconducting behavior with a systematic reduction in band gap from fluoride to iodide compositions. Optical analysis demonstrates a pronounced red shift in the absorption edge and enhanced visible-light absorption for the Br- and I-based compounds, indicating strong potential for solar-energy harvesting. Band-edge alignment calculations further show that Rb₂NaRhF₆, Rb₂NaRhCl₆, and Rb₂NaRhBr₆ possess suitable conduction- and valence-band positions for photocatalytic water splitting, while all investigated compounds exhibit favorable conduction-band potentials for photocatalytic CO₂ reduction reactions. In addition, pH-dependent band-edge analysis suggests stable redox capability across a broad pH range, particularly for the fluoride and chloride systems. Photovoltaic performance was evaluated using an FTO/SnS₂/Rb₂NaRhX₆/Au device architecture in SCAPS-1D, where halide substitution strongly influences carrier transport and device efficiency. Among the studied absorbers, Rb₂NaRhCl₆ exhibits the best overall photovoltaic performance, achieving a power conversion efficiency (PCE) of 27.03% with an open-circuit voltage (V_OC) of 0.893 V, a short-circuit current density (J_SC) of 35.41 mA cm⁻², and a fill factor (FF) of 83.56%. These findings establish Rb₂NaRhX₆ double perovskites as promising multifunctional materials for photovoltaic devices, photocatalytic hydrogen production, and CO₂ reduction applications.