Ion migration in perovskite solar cells

Metal halide perovskite solar cells have considerable potential for next-generation solar power production. However, if not controlled, the migration of mobile ions can hamper the stability of perovskite solar cells. Intensive research efforts have devised methods of suppressing ion migration and degradation in perovskite materials, resulting in solar cells that are stable over thousands of hours during accelerated ageing testing. Here, we review the chemical origins of ion migration, its effect on material and device performance and stability, and strategies to mitigate its impact. Ion migration originates in the soft lattice of the halide perovskite framework and its low defect-formation energy, but there are many different strategies to reduce its effects, from compositional engineering of materials and device architecture changes to additives and strain engineering. The field has made great progress in understanding the origin and properties of mobile ions in halide perovskites and has improved operational stability beyond expectations. Nonetheless, there are still ample opportunities to further improve the long-term durability of perovskite solar cells, either by reducing ion migration or its effect on solar cell efficiency.