Deciphering the Interplay between Tin Vacancies and Free Carriers in the Ion Transport of Tin-Based Perovskites

Mixed ionic-electronic conduction is a prevalent phenomenon in metal halide perovskites, having a critical impact in multiple optoelectronic applications. In Sn-based halide perovskites, their higher hole density ([p]) owing to the facile formation of Sn vacancies (V_Sn^-2) induces substantial electronic transport differences versus their Pb-based analogues. However, the influence of [p] and V_Sn^-2 on their ionic transport properties remains elusive. Herein, the link between electronic and ionic transport is unravelled in a compendium of Sn-based perovskite compositions. Specifically, ionic and electronic conductivities are found to concomitantly rise with higher Sn content. Using a combination of electrical characterization techniques, a rise in [p] and V_Sn^-2 is demonstrated to increase mobile ion density, enhancing lateral ion migration and ionic conductivity. First-principles simulations reveal that [p] and V_Sn^-2 jointly lower the energy barrier for iodide migration from 0.38 eV to 0.12 eV. Chemical mapping techniques support these observations by identifying the bias-induced migration of iodide and formamidinium ions in compositions with higher [p] and V_Sn^-2. These fundamental insights on the ionic-electronic coupling will enable next-generation of Sn-based perovskite technologies with improved performance and stability.