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 Sn 2+ 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.