Mixed-halide perovskite solar cells (PSCs) are known to be affected by ionic migration, which strongly influences device performance and stability. While ionic transport has been widely studied, the correlation between absorber stoichiometry and the characteristic ionic response remains poorly understood. In this work, we use impedance spectroscopy to systematically investigate how controlled stoichiometric perturbations affect the polarization dynamics of mobile ions in formamidinium–caesium lead mixed-halide perovskite absorbers.

Excess formamidinium halide salts (FAI, FABr, FACl) were introduced into the precursor solution to deliberately disturb absorber stoichiometry. We introduce a new variable, the characteristic ionic response frequency, which is a measure of how fast mobile ions can screen the electric field within the absorber. We find that even minor deviations from ideal stoichiometry lead to a significant increase of the ionic response frequency, suggesting more effective screening of the electric field due to increased ionic conductivity. While we show that an excess is necessary to induce this shift in frequency, the influence of the specific halide appears secondary compared to the overall stoichiometric offset.

To distinguish bulk incorporation from surface-driven effects, we further introduce a partial three-dimensional (3D) to two-dimensional (2D) conversion, therewith inducing surface-confined compositional modifications while largely preserving the pristine bulk absorber. In this case, a shift in ionic response frequency is also observed; however, in contrast to bulk incorporation, the specific halide plays a more pronounced role, linked to ion-exchange between the 2D and 3D layer during film crystallization.

Our results reveal two distinct regimes governing the relationship between the ionic response frequency and absorber stoichiometry. In a low-to-moderate ionic density regime, the response frequency is significantly affected by both ionic density and mobility, indicating coupled contributions from ion availability and transport dynamics. However, at higher ionic densities, the response frequency reaches a saturation limit, beyond which further shifting frequencies can only be achieved by increasing ionic mobility. This transition marks a shift from a density- and mobility-controlled regime to one dominated predominantly by ionic mobility.

Temperature-dependent measurements further allow extraction of activation energies associated with ionic transport processes, providing deeper insight into the underlying migration mechanisms. By correlating characteristic ionic response frequencies, activation energies, and absorber stoichiometry, this study highlight viable strategies to tailor ionic dynamics through both bulk composition control and surface-specific modifications, with significant possibilities for improving the stability and performance of perovskite solar cells.