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.