Homogenizing Sn-Pb Distribution Through A-Site MA Cations for Efficient All-Perovskite Tandem Solar Cells

Mixed tin-lead (Sn-Pb) perovskites are essential for high-efficiency tandem solar cells, typically employing a formamidinium (FA)-dominated composition with a small amount of MA. However, the regulatory role of such A-site mixing on mixed Sn-Pb perovskite properties remains underexplored. This work demonstrates that engineering the thermodynamic landscape of mixed Sn-Pb perovskite formation enables simultaneous regulation of phase homogeneity and defect chemistry. By controllably incorporating methylammonium (MA) cations, the crystallization kinetics are modulated to promote uniform nucleation, suppress Sn-Pb phase segregation, and inhibit Sn²⁺ oxidation, thereby mitigating non-radiative recombination losses. As a result, mix tin-lead perovskite solar cells achieved a certified power conversion efficiency (PCE) of 23.90%. By leveraging the improved compositional homogeneity and suppressed defect density, the optimized tin-lead absorber is further integrated into two-terminal monolithic all-perovskite tandem devices, delivering a PCE of 30.13% (certified 29.57%). The unencapsulated tandem devices maintained 90% of its initial PCE after 495 h of maximum power point operation under simulated one-sun illumination.