A-Site-Dependent Oxidative Stability in Tin–Lead Halide Perovskites Reveals Kinetic Origins via Gradient Oxidation Behavior

Tin-lead (Sn-Pb) hybrid perovskites are promising candidates for low-bandgap subcells in tandem solar cells but suffer from rapid degradation due to Sn²⁺ oxidation. While A-site cation composition is known to influence oxidation stability, conventional thermodynamic analyses fail to fully capture these differences. Here, through combined in situ spectroscopic experiments and first-principles modeling, we reveal that surface reaction kinetics govern the oxidation behavior of Sn-Pb perovskites with varying A-site compositions. Cs-rich perovskites exhibit higher activation energy barriers for Sn oxidation and Sn vacancy migration, enabling the formation of a dense SnO _<i>x</i> surface layer that passivates and protects the bulk material. In contrast, perovskites with predominantly organic A-site cations (MA, FA) undergo more uniform bulk oxidation. This kinetically controlled, composition-dependent gradient oxidation mechanism provides new insights beyond conventional thermodynamic perspectives and highlights the critical role of kinetics and A-site engineering in improving the long-term stability of Sn-Pb perovskites for optoelectronic applications.