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

Iodine vacancies (VI) 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 VI- 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 VI-, 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 VI may not be as detrimental as previously thought and offer new guidance for defect engineering in perovskites.