Ion migration is widely recognized as a key factor influencing the performance and stability of halide perovskite solar cells, yet its quantification remains challenging. In particular, separating the total mobile ion density from the fraction redistributed under built-in electric fields is essential for understanding device behavior—a point underscored by recent work highlighting mobile ions as a critical factor in device stability and loss mechanisms [1, 2].

In this work, we introduce a capacitance–frequency approach that enables extraction of both quantities in a controlled and reproducible manner. By carefully designing a capacitor-like device structure (suppressing electronic injection), the high-frequency response probes the dielectric properties of the perovskite, while the low-frequency response reflects mobile-ion redistribution.

Our device consists of the perovskite layer sandwiched between thin insulating layers, forming a symmetric insulator–perovskite–insulator capacitor. At high frequencies, the measured capacitance corresponds to the geometrical stack capacitance, from which we determine a dielectric constant of ε ≈ 46 - 47 for our triple-cation perovskite. High relative permittivity in perovskites, particularly CsFAMA, is supported by reports of substantial low-frequency dielectric response and polarization contributions tied to ionic and molecular dipoles [3].

Such a high dielectric constant, combined with very small Debye lengths, makes ionic capacitance in typical solar-cell stacks difficult to detect—consistent with findings that transport layers and device geometry can obscure ionic contributions in standard devices.

At low frequencies, mobile ions follow the oscillating field, screening the perovskite bulk and confining the electric field to roughly one Debye length (L_D). The total ion density is thus accessible via the classical Debye relation.

To probe ions displaced by built-in potential, asymmetric contacts are introduced to induce internal electric fields. Modeling the accumulation and depletion regions as effective slabs enables estimation of the moved ion density from their combined widths. Importantly, drift-diffusion simulations, which have been used to interpret ion migration and capacitance behavior in perovskites, confirm these distinct width trends in depletion vs. accumulation regions.

This methodology, supported by simulation and precise measurement, enables direct access to both bulk mobile-ion populations and the fraction redistributed under field bias. The framework is versatile and applicable to other perovskite compositions, offering new insights into ion migration and stability in perovskite optoelectronics.