Correlated Octahedral Rotation and Organic Cation Reorientation Assist Halide Ion Migration in Lead Halide Perovskites

Increasing the stability of lead halide perovskites (LHPs) is critical for their practical application in solar cells and other technologies. Halide ion migration is one of the main contributors to instability and hysteresis in LHP solar cells. Here, we employ a series of Gedankenexperiments to quantitatively establish the correlated effects of the A site cation motion, H bonding strength, and octahedral rotation on halide ion migration in APbBr3 (A = Cs or methylammonium/MA) LHPs. We find that in cubic CsPbBr3, the increase of PbBr6 octahedra rotation/tilting during ion migration lowers the halide ion migration barrier by at least 100 meV compared to the orthorhombic phase. In MAPbBr3, we show that halide ion migration is also assisted by MA cation rotation to re-establish H bonding, resulting in lower halide migration barriers. These results suggest that ``locking'' the organic cation via chemical and processing means can help mitigate halide migration-induced instability and reduced hysteresis in LHP solar cells.