LAUSR

Perovskite light‐emitting diodes (PeLEDs) have received great attention in recent years due to their narrow emission bandwidth and tunable emission spectrum. Efficient red emission is one of most important parts for lighting and displays. Quasi‐2D perovskites can deliver high emission efficiency due to the strong carrier confinement, while the external quantum efficiencies (EQE) of red quasi‐2D PeLEDs are inefficient at present, which is due to the complex distribution of different n‐value phases in quasi‐2D perovskite films. In this work, the phase distribution of the quasi‐2D perovskite is finely controlled by mixing two different large organic cations, which effectively reduces the amount of smaller n‐index phases, meanwhile the passivation of lead and halide defects in perovskite films is realized. Accordingly, the PeLEDs show 25.8% EQE and 1300 cd m−2 maximum brightness at 680 nm, which exhibits the highest performance for red PeLEDs up to now.