Understanding the Role of Small Cations on the Electroluminescence Performance of Perovskite Light‐Emitting Diodes

The delicate engineering of monovalent cations in perovskite material has led to continuous performance breakthroughs and stability improvement for the perovskite light‐emitting diodes (PeLEDs). However, the exact role of A‐site cations on the electroluminescence (EL) performance and degradation mechanism of PeLEDs has not been systematically answered yet. Herein, it is demonstrated that the most commonly used methylammonium cation (MA+) has an adverse effect on the electrochemical reaction at the interface between perovskite and metal‐oxide layer, leading to deteriorated EL performance as compared to that of the formamidinium cation (FA+)‐based perovskite. It reveals that the accelerated deprotonation process of MA+ under an electric field will aggravate the reaction between iodide and metal ion in oxide layer. The further substitution of a small portion of FA+ with inorganic cesium cation (Cs+) results in much enhanced crystallinity and enlarged crystal size, leading to an optimized peak external quantum efficiency of 21.3%. The ion migration process in the PeLEDs can be significantly suppressed with Cs+ incorporation, leading to a smaller roll‐off under large current density and an elongated half‐lifetime of 190.1 h under a current density of 20 mA cm‐2, representing one of the most stable PeLEDs based on 3D perovskite layer. High‐efficiency perovskite light‐emitting diode (PeLED) with an optimized external quantum efficiency of 21.3% is achieved through delicate engineering of monovalent cations. The inorganic cation Cs+ in the perovskite leads to suppressed cation accumulation and mitigated electrochemical reactions in different functional layers. The device exhibits a long half‐lifetime T50 of 190.1 h, representing the most stable PeLEDs based on 3D perovskite layer.