Unveiling the Role of Surface Iodine Vacancies in CsPbI3 Perovskite: Carrier Recombination Dynamics and Defect Passivation Mechanisms

Abstract

Lead-iodine perovskites are emerged as promising candidates for next-generation solar cells, yet a divergence persists between the theoretical and experimental realms regarding the impact of surface defect iodine vacancy (VI) on device performance. To elevate cell efficiency, a profound understanding and delicate control of VI and their passivation mechanisms are crucial. In this work, we study various VI 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 depend on the defect positions and configurations, as well as the efficacy of Lewis base additives in passivating defects. Despite the outermost layer VI creates no defect state within the band gap, the carrier recombination rate accelerates significantly by a factor of 2 compared to defect-free surface, owing to strong electron-phonon coupling. Subsurface defects create localized hole trapping state, enabling the swift capture of valence band holes, which subsequently accelerate recombination with the conduction band electrons by a factor of 6.5. Remarkably for Pb-dimer, this rate escalates 13-fold. Incorporating Lewis base molecule HCOO¯ forms the stable Pb-O bonds with lead ions, preventing the surface VI reconstruction (iodine migration), Pb-dimer formation, and in-band defect state. These effectively reduce the electron-phonon coupling, achieving performance comparable to defect-free surface. This work reconciles contradictory of surface VI on perovskite performance, and enriches our understanding of surface defect properties and their effects on carrier dynamics and device efficiency.