Lead-free perovskite solar cells (PSCs) offer a promising route to environmentally sustainable photovoltaics, but single-absorber devices often suffer from trade-offs between current density, voltage, and stability. Here, we systematically investigate single-junction and tandem absorber configurations using SCAPS-1D numerical sim- ulations, comparing Rb₂SnI₆ (vacancy-ordered double perovskite), MASnI₃ (tin-based perovskite), and a novel MASnI₃/Rb₂SnI₆ tandem structure. Device architecture was FTO/TiO₂ (50 nm)/absorber(s)/P3HT (200 nm)/Ag. For single absorbers, optimal thicknesses were 470 nm for Rb₂SnI₆ (PCE = 23.0%, JSC = 37.67 mA⋅cm⁻², VOC = 0.80 V, FF = 76.5%) and 500 nm for MASnI₃ (PCE = 17.2%, JSC = 33.17 mA⋅cm⁻², VOC = 0.73 V, FF = 71.1%). The tandem configuration (500 nm MASnI₃ / 470 nm Rb₂SnI₆) dramatically improved performance, achieving PCE = 27.6%, JSC = 39.7 mA⋅cm⁻², VOC = 0.90 V, and FF = 76.9%. Comprehensive electrical analyses—including J–V, EQE, C–V, Mott–Schottky, Nyquist impedance, conductance–frequency, and generation–recombination profiles—revealed that the tandem structure reduces charge transfer resistance, enhances built-in potential (Vbi = 0.63 V), suppresses Shockley–Read–Hall recombination, and improves carrier generation (peak ~2.4 × 10²⁷ cm⁻³⋅s⁻¹). These results establish the MASnI₃/Rb₂SnI₆ tandem as a high-performance, lead-free architecture that synergistically combines the high current density of MASnI₃ with the stability and voltage potential of Rb₂SnI₆, offering a viable pathway toward efficient and non-toxic next-generation PSCs
