Understanding and Quantification of Ion Migration in Perovskite Solar Cells

Francesco Le Pera^a, Moritz C. Schmidt^a, Agustin O. Alvarez^a, Biruk Alebachew Seid^b, Felix Lang^b, Bruno Ehrler^a

^a AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands

^b Universität Potsdam, Capus Golm, Karl-Liebknecht-Str. 24/25, Germany

Due to their exceptional optoelectronic properties, perovskite solar cells (PSCs) have rapidly achieved power conversion efficiencies exceeding 26%. However, ion migration within the perovskite layer remains a critical bottleneck, degrading the crystal structure and screening the internal electric field upon accumulating at device interfaces. Here, we investigate Thermally Activated Ion Current (TAIC) measurements to quantify mobile ion density in PSCs. In this method, a voltage bias is applied to induce a homogeneous ion distribution across the bulk. The device is then cooled to 175 K using liquid nitrogen to immobilize the ions. Upon removing the bias, the temperature is increased in a controlled manner up to 300 K, allowing the ions to relax back to the interfaces and generate a measurable current transient.

The subsequent decay of this current peak can be governed by two distinct regimes: an ion-limited scenario, where bulk ion depletion stops the current before complete field screening occurs, allowing accurate ion density extraction via current integration, or a field-limited scenario, where severe interfacial ion accumulation screens the internal field, halting further migration and causing an underestimation of the true ion density.

To interpret these mechanisms, we employ drift-diffusion simulations. Our modeling dictates that devices should transition into the field-limited regime at ion densities exceeding 10¹⁷ cm^-3. Paradoxically, experimental TAIC measurements yield apparent ion densities on the order of 10¹⁸ cm^-3. To resolve this discrepancy, we evaluate TAIC profiles as a function of the initial voltage bias. Simulations reveal that increasing the initial bias yields two distinct behavioral regimes contingent on the total mobile ion concentration, elucidating how screening effects distort density extraction. Finally, we experimentally validate these simulation trends using stable triple-cation PSCs with a Cs_0.05(MA_0.05FA_0.95)_0.95Pb(I_0.95Br_0.05) composition, establishing TAIC as a powerful diagnostic tool for optimizing perovskite photovoltaics.