Electrically Induced Directional Ion Migration in Two-Dimensional Perovskite Heterostructures

Understanding ion migration in halide perovskite materials is key to enhancing both device performance and stability. The introduction of two-dimensional (2D) perovskites has improved stability, spurring increased interest in understanding ion migration in these materials. Hindered by the challenge in building a reliable device platform, prior studies primarily focused on heat and light induced ion migration. Here, we construct a high-quality single crystal 2D perovskite heterostructure, free from grain boundaries and impurities. Using a device platform fabricated through a multi-step dry transfer process to realize near defect-free van der Waals contact, we achieve real-time visualization and imaging of directional ion migration. Notably, aside from the expected directional ion migration driven by the electric field, we discover the unique behavior of halide anions inter-diffusing against the field under strong and prolonged bias. Spatial confocal photoluminescence mapping reveals a halide migration channel that aligns with the crystal edge of high electrical conductivity, highlighting the role that electric current plays in ion migration. Finally, after a sustained mild bias, stable junction diodes exhibiting ~1000-fold forward to reverse current ratio are realized due to directional ion migration-induced asymmetricity. This study unveils important fundamental insights on halide migration under electrical bias, paving the way toward high-performance electronic and optoelectronic devices.