Detrimental Effect of Unreacted PbI₂ on the Long‐Term Stability of Perovskite Solar Cells

Excess/unreacted lead iodide (PbI 2 ) 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 PbI 2 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 PbI 2 crystals under continuous light illumination. Unreacted PbI 2 undergoes photodecomposition under illumination, resulting in the formation of lead and iodine in films. Thus, this photodecomposition of PbI 2 is one of the main reasons for accelerated device degradation. Therefore, this work reveals that carefully controlling the formation of unreacted PbI 2 crystals in perovskite films is very important to improve device operational stability for diverse opto‐electronic applications in the future.