Uncovering the Influence of Cation Composition Engineering on the Ion Migration Kinetics in Perovskite Solar Cells

Cation composition engineering of perovskite polycrystals, such as substituting the typically used methylamine (MA+) with formamidine (FA+) wholly or partially, is a reliable protocol for fabricating high-efficiency perovskite solar cells (PSCs). Hitherto, it is still in debate how and to what extent the FA+ substitution influences the ion migration, which restricts the further optimization of PSCs. In this work, ion migration kinetics in MAPbI3-based and FAPbI3-based PSCs was systematically studied by a series of time-resolved photoelectric measurements. We find that by replacing MA+ with FA+, owing to the steric effect of the large-sized FA+, the cation migration is significantly suppressed, and the migration rate of halide anions is reduced. However, FA+ results in a structural tolerance factor exceeding the ideal range, which leads to a drastic increase in halide defects and eventually aggravates the ion migration in FAPbI3 PSCs. These findings provide unique insights into the manipulation of ion migration in PSCs via the strategy of cation composition engineering.