Stabilizing Perovskite Light‐Emitting Diodes by Incorporation of Binary Alkali Cations

The poor stability of perovskite light‐emitting diodes (PeLEDs) is a key bottleneck that hinders commercialization of this technology. Here, the degradation process of formamidinium lead iodide (FAPbI 3 )‐based PeLEDs is carefully investigated and the device stability is improved through binary‐alkalication incorporation. Using time‐of‐flight secondary‐ion mass spectrometry, it is found that the degradation of FAPbI 3 ‐based PeLEDs during operation is directly associated with ion migration, and incorporation of binary alkali cations, i.e., Cs + and Rb + , in FAPbI 3 can suppress ion migration and significantly enhance the lifetime of PeLEDs. Combining experimental and theoretical approaches, it is further revealed that Cs + and Rb + ions stabilize the perovskite films by locating at different lattice positions, with Cs + ions present relatively uniformly throughout the bulk perovskite, while Rb + ions are found preferentially on the surface and grain boundaries. Further chemical bonding analysis shows that both Cs + and Rb + ions raise the net atomic charge of the surrounding I anions, leading to stronger Coulomb interactions between the cations and the inorganic framework. As a result, the Cs + –Rb + ‐incorporated PeLEDs exhibit an external quantum efficiency of 15.84%, the highest among alkali cation‐incorporated FAPbI 3 devices. More importantly, the PeLEDs show significantly enhanced operation stability, achieving a half‐lifetime over 3600 min.