Migration and evolution of iodine in perovskite solar cells

Perovskite solar cells (PSCs) have rapidly gained prominence in photovoltaics, achieving impressive advancements in power conversion efficiency (PCE), increasing from 3.8 % to over 26 % in just a decade. We have seen many ideas and additives one after another on the way to improving device efficiency. A feature of perovskite solar cells is that once a certified power conversion efficiency has been reported, it appears that most plausible additives can accomplish the same job in increasing device performance. A familiar story with graphene. There is an old saying in China about a black bear in a corn forest that will never get the super corn it wanted. We must focus on the critical issues of the PSCs and find a suitable solution for them. Otherwise, strategies or methods of temporary relief, regardless of the consequences, would not provide a boost to the development of this emerging technology. For example, the long-term stability of PSCs remains a major challenge, particularly due to the migration of iodine ions, which can lead to degradation through redox reactions and the formation of corrosive iodine species, such as I₂ and I₃⁻. Chemically reactive iodine species can further damage the perovskite layer and adjacent components, shortening the device longevity. Here, we first examine the origin of iodine ion migration and the development of iodine defects in perovskites. The migration of iodine ions and the formation of their byproducts can trigger self-catalyzed degradation reactions during the operation of PSCs. We summarize strategies to address this issue, including composition regulation, grain boundary passivation, crystallization control, and the use of redox-active additives and interfacial barrier layers. These methods show promising potential for resolving iodine defects and improving the operational durability of PSCs. By developing multifunctional additives or using multiple strategies in combination, the migration and evolution of iodine ions can be controlled more effectively. Finally, we propose the introduction of new approaches from other scientific fields to inhibit ion migration and capture volatile iodine, and discuss their applicability in PSCs to achieve long-term operational stability.