Quantifying cooperative FA⁺/MA⁺ ion migration in mixed perovskites via nano-infrared imaging

Ion migration plays a critical role in the operational stability and efficiency of hybrid perovskite solar cells, yet direct and species-specific quantification of mobile cations remains challenging. Here, we employ infrared photo-induced force microscopy (IR-PiFM) to investigate the migration dynamics of MA⁺ and FA⁺ in MAPbI₃, FAPbI₃ and FA_0.5MA_0.5PbI₃. These techniques allow nanoscale mapping and real-time tracking of individual cations with high chemical specificity and 250 μs temporal resolution. Our results reveal composition- and location-dependent ion transport behavior; while MA⁺ exhibits higher mobility in pure MAPbI₃, its diffusion is significantly suppressed in the mixed-cation system. Surprisingly, FA⁺ and MA⁺ in FA_0.5MA_0.5PbI₃ exhibit similar migration rates, suggesting a cooperative migration mechanism. Density functional theory calculations support this interpretation, showing that A-site mixing alters lattice symmetry and raises migration barriers through steric and electrostatic interactions. Bias-dependent nano-infrared imaging and open-circuit voltage measurements further show that mixed-cation perovskites confine cation motion and suppress bulk migration, leading to stabilized internal fields and improved voltage performance. These results provide direct evidence for cooperative cation migration dynamics in mixed A-cation perovskites and demonstrate that controlled cation mobility, rather than complete immobilization, may be key to achieving both efficiency and operational durability in perovskite optoelectronics.