Unveiling the role of surface iodine vacancies in CsPbI3 perovskite: carrier recombination dynamics and defect passivation mechanisms†

Abstract

Lead–iodine perovskites are emerging as promising candidates for next-generation solar cells, yet a divergence persists between the theoretical and experimental realms regarding the impact of surface iodine vacancies (V_I) on device performance. To elevate cell efficiency, a profound understanding and delicate control of V_I and their passivation mechanisms are crucial. In this work, we studied various V_I defects near the surface of all-inorganic CsPbI₃ perovskite using ab initio non-adiabatic molecular dynamics. The results show that the electron–hole (e–h) recombination lifetime highly depends on the defect positions and configurations, as well as the efficacy of Lewis base additives in passivating defects. Despite the outermost layer V_I creating no defect state within the band gap, the carrier recombination rate accelerates significantly by a factor of 2 compared to that with the defect-free surface, owing to strong electron–phonon coupling. Subsurface defects create a localized hole trapping state, enabling swift capture of valence band holes, which subsequently accelerate recombination with the conduction band electrons by a factor of 6.5. Remarkably for Pb-dimers, this rate escalates 13-fold. Incorporating the Lewis base molecule HCOO⁻ forms the stable Pb–O bonds with lead ions, preventing surface V_I reconstruction (iodine migration), Pb-dimer formation, and an in-band defect state. These effectively reduce the electron–phonon coupling, achieving performance comparable to that of the defect-free surface. This work reconciles contradictory of surface V_I on perovskite performance, and enriches our understanding of surface defect properties and their effects on carrier dynamics and device efficiency.