Abstract Perovskite solar cells have emerged as a groundbreaking photovoltaic technology owing to their cost- effective production methods, customizable band gaps, and superior efficiency. In this work, we investigate the performance of CsGeI3, CsSn0.5Ge0.5I3, and a cascaded bilayer CsSn0.5Ge0.5I3/CsGeI3 device using numerical simulations. The influence of absorber thickness, junction properties, and interfacial alignment was systematically analyzed through J–V, EQE, capacitance–voltage (C–V), Mott–Schottky, apparent carrier concentration (Napp), and Nyquist impedance characteristics. Results show that CsGeI3 achieves optimal efficiency at 200–300 nm thickness, while CsSn0.5Ge0.5I3 performs best at 1000 nm, reflecting the balance between absorption and recombination. The cascaded bilayer device demonstrates superior photovoltaic performance, with PCE = 33.22%, FF = 88.35%, Jsc = 36.62 mA cm−2, and Voc = 1.03 V, surpassing both single-junction counterparts. This efficiency is a theoretical upper limit obtained under idealized SCAPS-1D assumptions (zero series resistance, infinite shunt resistance, low defect density of 1015 cm−3, and no ion migration) and does not represent an experimentally achievable value. Electrical analysis reveals built-in voltages of 1.40 V (CsGeI3), 1.02 V (CsSn0.5Ge0.5I3), and 1.16 V (cascaded bilayer), highlighting differences in junction field strength. Impedance spectra confirm reduced recombination and improved charge extraction in the cascaded bilayer configuration. These findings demonstrate that careful composi- tional engineering and cascaded bilayer stacking can significantly enhance device efficiency, while also providing insights into carrier dynamics and interfacial behavior critical for future optimization.
