Excess/unreacted lead iodide (PbI2) has been commonly used in perovskite films for the state‐of‐the‐art solar cell applications. However, an understanding of intrinsic degradation mechanisms of perovskite solar cells (PSCs) containing unreacted PbI2 has been still insufficient and, therefore, needs to be clarified for better operational durability. Here, it is shown that degradation of PSCs is hastened by unreacted PbI2 crystals under continuous light illumination. Unreacted PbI2 undergoes photodecomposition under illumination, resulting in the formation of lead and iodine in films. Thus, this photodecomposition of PbI2 is one of the main reasons for accelerated device degradation. Therefore, this work reveals that carefully controlling the formation of unreacted PbI2 crystals in perovskite films is very important to improve device operational stability for diverse opto‐electronic applications in the future.

Organic-inorganic halide perovskites are promising for use in solar cells because of their efficient solar power conversion. Current-voltage hysteresis and degradation under illumination are still issues that need to be solved for their future commercialization. However, why hysteresis and degradation occur in typical perovskite solar cell structures, with an electron transport layer of metal oxide such as SnO 2 , has not been well understood. Here we show that one reason for the hysteresis and degradation is because of the localization of positive ions caused by hydroxyl groups existing at the SnO 2 surface. We deactivate these hydroxyl groups by treating the SnO 2 surface with a self-assembled monolayer. With this surface treatment method, we demonstrate hysteresis-less and highly stable perovskite solar cells, with no degradation after 1000 h of continuous illumination.

Quasi-2D perovskites have attracted wide attention as the emitter of light-emitting diodes (LEDs) in recent years because of the ease of obtaining high external quantum efficiencies (EQEs). However, the quick degradation under continuous operation and significant EQE roll-off at high current densities are issues that need to be overcome for future practical applications using quasi-2D perovskite LEDs (PeLEDs). In this context, we discuss the mechanism of the degradation and EQE roll-off on the basis of ion migration. The migration of ligand cations though domain boundaries of quasi-2D perovskite films induces the gradual loss of defect passivation at the boundaries, which results in the reversible PeLED degradation and severe EQE roll-off. When the device operation time is long, the mobile cations enter and interact with the electron transport layer, leading to the stage of irreversible PeLED degradation. The device degradation mechanisms we discovered here are constructive for developing quasi-2D PeLEDs with better operational durability.