Ion migration in halide perovskite solar cells: Mechanism, characterization, impact and suppression

Metal halide perovskites are emerging as the most promising candidate for the next-generation Photovoltaics (PV) materials, due to their superior optoelectronic properties and low cost. However, the resulting Perovskite solar cells (PSCs) suffer from poor stability. In particular, the temperature and light activated ionic defects within the perovskite lattice, as well as electric-field-induced migration of ionic defects, make the PSCs unstable at operating condition, even with device encapsulation. There is no doubt that the investigation of ion migration is crucial for the development of PSCs with high intrinsic stability. In this review, we first briefly introduce the origin and pathways of ion migration, and also the essential characterization methods to identify ion migration. Next, we discuss the impact of ion migration on the perovskite films and cells with respect to photoelectric properties and stability. Then, several representative strategies to suppress ion migration are systematically summarized in the context of composition engineering, additive engineering and interface engineering, with an in-depth understanding on the underlying mechanisms which may provide more clues for further fabrication of PSCs with improved stability. Finally, a perspective with some suggestion on future research directions and chemical approaches are provided to alleviate ion migration in perovskite materials and the entire devices.