Inhibiting Cation Segregation to Enable Highly Efficient Perovskite Light‐Emitting Diodes

Perovskite light‐emitting diodes (PeLEDs) present potential applications for next‐generation displays due to their excellent luminescent properties and solution processing. The formamidinium‐caesium lead bromide (FA x Cs 1− x PbBr 3 ) mixed cation strategy is one of the primary methods to achieve high‐performance of Cs‐dominate PeLEDs system. However, the inhomogeneous distribution of cations is detrimental to the photoelectronic performance of Cs‐dominated PeLEDs. Here, in mixed FA/Cs perovskite films, it is observed that Cs, Br and trace Pb elements precipitated on the surface of the perovskite films. It is found that the segregation of cations is induced by the uncontrollable crystallization process of Cs/FA cations. By introducing biuret additive, the hydrogen bonding interaction of biuret and FA cation balances crystallization rate, which therefore significantly improves the crystal quality and cations distribution of perovskite films. As a result, green PeLEDs with a high external quantum efficiency of 28.8% are obtained, and represent five times enhancement in half‐life time ( T 50 ) of 2.48 h at an initial luminance of 100 cd m −2 relative to the control device. The results help to understand the relationship between cationic distribution and PeLEDs properties, offering significant insights for modulating the performance of PeLEDs.