Beyond Conventional Views on Iodine Vacancies: Spin–Orbit Coupling-Mediated Structural Dynamics and Photophysical Implications in Bulk Perovskites

Iodine vacancies (V_I) are widely regarded as the dominant recombination centers, limiting the photovoltaic performance of halide perovskites. However, most studies neglect the influence of spin-orbit coupling (SOC) on the stability of defect configurations during structural modeling. Here, we investigate the impact of SOC on the structure and carrier dynamics of the V_I^- in the CsPbI₃ system by using ab initio nonadiabatic molecular dynamics with time-dependent density functional theory. Our study demonstrates that SOC significantly alters the stable configuration of V_I^-, allowing the nondimer to replace the conventional Pb dimer as the lower-energy defect structure. Such a transition, governed by the competition between the electronic energy and lattice strain energy, leads to an effective suppression of nonadiabatic electron-phonon coupling and mitigates sublattice distortions, thereby extending the lifetime by approximately 4 times, which approaches that of the defect-free system. These findings reveal that the role of bulk V_I may not be as detrimental as previously thought and offer new guidance for defect engineering in perovskites.