Exploring lead-free A₂AgRhF₆ fluoride double perovskites for photovoltaic applications: a first-principles and device...

Lead-free fluoride double perovskites have emerged as promising candidates for stable and environmentally benign optoelectronic applications. In this work, a comprehensive first-principles and device-level investigation of A₂AgRhF₆ (A = K, Rb, Cs) double perovskites is performed using density functional theory (DFT) within the CASTEP framework, combined with SCAPS-1D solar cell simulations. Structural optimization confirms cubic phase stability with a systematic lattice expansion from K to Cs, while negative formation energies verify thermodynamic stability. Electronic structure analysis reveals direct band gap semiconducting behavior, with band gaps decreasing along the series due to enhanced lattice expansion and modified d-p hybridization. Effective mass calculations indicate improved electron transport with increasing A-site ionic radius, whereas hole transport remains nearly composition-independent. Optical investigations demonstrate strong ultraviolet absorption, moderate visible-light activity, high dielectric response, and pronounced plasmonic features, highlighting their suitability for UV optoelectronic and photonic applications. Charge density and population analyses reveal dominant ionic character of A-site cations and strong covalent Rh-F/Ag-F interactions, confirming that the electronic structure is primarily governed by the AgRhF₆ octahedral framework. Mechanical stability is verified through elastic constants satisfying Born criteria, while anisotropy analysis demonstrates moderate directional dependence of elastic behavior. Ab initio molecular dynamics (AIMD) simulations further confirm thermal stability at elevated temperatures, validating structural robustness under operating conditions. Device simulations under AM1.5G illumination demonstrate promising photovoltaic performance with optimized absorber thickness and doping concentration, where Cs₂AgRhF₆ exhibits the highest power conversion efficiency among experimentally feasible compositions. This combined DFT-device-level study establishes a direct correlation between intrinsic bonding, mechanical stability, thermal robustness, and photovoltaic performance, positioning A₂AgRhF₆ double perovskites as promising candidates for stable lead-free solar energy conversion and advanced optoelectronic devices.