Quantum‐Dots Matrix Encapsulated CsPbI 3 Polycrystal Composite Films for Efficient and Bright Deep‐Red Light‐Emitting Diodes

All‐inorganic CsPbI 3 perovskite is a promising emitter for deep‐red light‐emitting diodes (LEDs). However, presently fabricated CsPbI 3 polycrystalline films are composed of island‐like polycrystals, encountering the problems of serious interface current leakage and low‐efficiency carrier radiative recombination. Here, a CsPbI 3‐x Br x quantum dots (QDs) matrix encapsulated CsPbI 3 polycrystal film is reported to address the low‐efficiency issue of island‐like CsPbI 3 polycrystalline film applied in deep‐red LEDs. The developed QDs matrix encapsulation strategy has two benefits. One is the filling of void space in the island‐like CsPbI 3 polycrystal film to suppress the interface current leakage. The other more important benefit is utilizing the strong carrier confinement effect of QDs to strengthen the formation of excitons in the composite film and thus improve the electroluminescence efficiency. Moreover, interesting grain growth is found between CsPbI 3‐x Br x QDs and CsPbI 3 polycrystals, which further enhances the exciton transfer effect brought by the QDs matrix and optoelectronic properties of the fabricated composite films. Based on the obtained high‐quality QDs/polycrystal composite films, efficient and bright deep‐red LEDs are achieved with a peak external quantum efficiency of 15.24% and a maximum luminance of 3691 cd m −2 .