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

## TL;DR

Nano-infrared imaging shows how MA+ and FA+ ions move together in mixed perovskites, helping improve solar cell stability.

## Contribution

The study provides direct evidence of cooperative cation migration in mixed perovskites using nano-infrared imaging.

## Key findings

- MA+ and FA+ show similar migration rates in FA0.5MA0.5PbI3, indicating cooperative migration.
- Mixed-cation perovskites suppress bulk ion migration, stabilizing internal fields and improving voltage.
- DFT calculations show that A-site mixing increases migration barriers through steric and electrostatic effects.

## Abstract

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 FA0.5MA0.5PbI3. 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 FA0.5MA0.5PbI3 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.

Nano-infrared imaging reveals how two common organic ions move cooperatively inside mixed perovskites, offering a mechanistic basis for engineering perovskite devices with improved operational stability.