Pseudohalide BF4– Doping Suppresses Lattice Anharmonicity and Passivates the Pb–Pb Dimer in Defective CH3NH3PbI3 Perovskites

Excess electron-induced Pb-Pb dimer formation critically degrades perovskite solar cell performance by converting shallow defects into deep trap states and generating highly localized small polarons that accelerate nonradiative recombination. Using density functional theory and nonadiabatic molecular dynamics, we demonstrate that pseudohalide BF₄^- doping can passivate such degradation by stabilizing the PbX₆ octahedral framework and maintaining delocalized electrons without small polaron formation. This structural stabilization reduces lattice anharmonicity and atomic thermal fluctuations, while enhanced rigidity prevents the large atomic displacements required for dimer formation. The BF₄^- doping further weakens electron-vibration interaction and decreases nonadiabatic coupling strength, synergistically extending the carrier lifetime by approximately 6 times. Our findings establish pseudohalide doping as an effective strategy to inhibit excess electron-induced degradation through anharmonicity suppression, providing a broadly applicable approach for enhancing perovskite stability.