Atomic Model for Alkali Metal Passivation of Point Defects at Perovskite Grain Boundaries

Experiments show that grain boundaries (GBs) are detrimental to MAPbI3 (MA = CH3NH3+) optoelectronic properties, while passivation with alkali metals greatly improves performance. Using ab initio nonadiabatic (NA) molecular dynamics, we demonstrate that common defects, which are benign in MAPbI3 bulk, create deep traps at GBs and accelerate carrier losses. Thus, common substitution of Pb with MA in bulk MAPbI3 leaves the bandgap and nonradiative electron–hole recombination unchanged, while it creates a deep electron trap at GBs by forming an I-dimer. K and Rb dopants eliminate the midgap state by breaking the I-dimer, restore the bandgap to the pristine MAPbI3 value, decrease electron–hole overlap, reduce the NA coupling, and accelerate quantum decoherence. Ultimately, the charge carriers' lifetime is doubled even compared to pristine MAPbI3. The detailed mechanistic understanding of the synergy in the negative influence of GB and point defects on perovskite properties and defect elimination by alkali metal doping provides valuable guidelines for solar and electro-optic applications.