Atomistic Mechanism of Passivation of Halide Vacancies in Lead Halide Perovskites by Alkali Ions

Intrinsic defects in perovskite films strongly influence carrier dynamics by introducing nonradiative recombination centers, limiting the performance of perovskite solar cells. Extensive "trail-and-error" experimental efforts have been devoted to defect passivation, requiring fundamental understanding and rational guidance. Using state-of-the-art ab initio quantum dynamics simulations, we demonstrate suppression of nonradiative energy losses in lead halide perovskites with the introduction of monovalent alkali ions. We show that alkali doping of iodine vacancies, the most common defect, eliminates trap states in MAPbI3 and extends charge carrier lifetimes. Negative formation energy is found when alkali cations occupy the B site of the perovskite lattice, identifying the location of the alkali dopants. Iodine vacancy introduces a sub-gap state capable of trapping holes. The state is supported by Pb-p orbitals that interact across the vacancy site. Alkali doping eliminates the trap state by weakening the interaction of Pb-p orbitals across the vacancy and removing extraneous electrons from the conduction band. We demonstrate that the lifetimes grow in the order unpassivated -> Li -> Na -> K-passivated, as rationalized by symmetry breaking, charge localization, and participation of low-frequency phonon modes that lead to changes in electronic structure, nonadiabatic electron-phonon coupling, and quantum coherence time. The atomistic understanding of the various factors contributing to the defect passivation guides development of high-efficiency perovskite devices.