Unveiling the Photovoltaic Potential of Rhombohedral RbGeX ₃ (X = Cl, Br, I) Perovskites Via Combined DFT and SCAPS‐1D Study

In pursuit of efficient, non‐toxic all‐inorganic perovskite solar cells (PSCs), we have investigated the rhombohedral phase of the rubidium germanium halide perovskites (X = Cl, Br, I) using density functional theory (DFT). The band structures and partial density of states are computed using PBE and TB‐mBJ functionals, both with and without spin‐orbit coupling (SOC), to accurately estimate the bandgaps. Optical properties, including the dielectric function, absorption coefficient, and refractive index, are evaluated within the PBE framework. Among the halides, is identified as the most promising absorber material, exhibiting the narrowest bandgap (0.96 eV with TB‐mBJ + SOC) and superior optical absorption characteristics. SCAPS‐1D simulations are carried out using DFT‐derived input parameters including bandgap, effective density of states, and carrier mobilities. Device performance is optimized by exploring various inorganic hole and electron transport layers (HTL/ETL). The influence of the absorber layer (AL) thickness, doping levels, defect densities at AL, ETL/AL, and AL/HTL interfaces, back contact materials, as well as series and shunt resistance is examined. The optimized all‐inorganic, non‐toxic device structure FTO///CuI/Au achieves a power conversion efficiency (PCE) of 25.76% with a fill factor (FF) of 79.81%.