High-efficiency lead-free tandem solar cells were numerically designed using \(\hbox {CaV}_{0.5}\hbox {Fe}_{0.5}\hbox {O}_{{3}}\) as a wide-bandgap top absorber and \(\hbox {Ag}_{{2}}\hbox {BeSnX}_{{4}}\) (X = Se, Te) kesterites as narrow-bandgap bottom absorbers. These materials combine non-toxicity, earth abundance, and complementary bandgaps suitable for tandem architectures. SCAPS-1D simulations, supported by material parameters from density functional theory and reported experimental data, were employed to optimize absorber thickness, carrier concentration, and band alignment. An optimal conduction band offset of approximately +0.2 eV at the absorber/CdS interface was identified, minimizing interfacial recombination and enhancing carrier extraction. Current matching was achieved at a top-cell thicknesses of 0.46 μm (Se-based) and 0.44 μm (Te-based), yielding matched short-circuit current density of \(\sim \) 17 mA \(\hbox {cm}^{-2}\) . The optimized \(\hbox {CaV}_{0.5}\hbox {Fe}_{0.5}\hbox {O}_{{3}}\) / \(\hbox {Ag}_{{2}}\hbox {BeSnSe}_{{4}}\) tandem device achieved power conversion efficiency of 32.56%, with open-circuit voltage of 2.05 V and a fill factor of 93.28%, while the Te-based configuration delivered 24.35% efficiency with \(V_{\textrm{oc}}\) of 1.57 V. Impedance analysis confirmed reduced recombination and improved charge transport under optimized conditions. These results demonstrate the strong potential of combining mixed B-site lead-free perovskites with kesterite absorbers for high-performance and environmentally sustainable tandem photovoltaic applications.
