LAUSR

Physicochemical property matched inorganic carrier transport layers are highly desirable for bright and efficient perovskite light‐emitting diodes (PeLEDs). However, mismatched physicochemical properties of the currently used zinc oxide (ZnO) electron transport layer (ETL), including unfavorable deprotonation reaction and misaligned conduction band, impose performance constraints. Herein, a ternary zinc stannate (ZnSnO3) nanocrystal ETL fabricated through a nanoscale solid‐state diffusion of stannic oxide (SnO2) and ZnO nanocrystal layers at a mild sintering temperature is reported. This ZnSnO3 ETL exhibits well‐matched physicochemical properties with CsPbI3 to achieve ultrabright, efficient, and stable deep‐red PeLEDs. Its deprotonation–inert property enables the in‐situ growth of a high‐quality and stable CsPbI3 layer. Notably, it features a slightly higher‐lying conduction band than ZnO ETL, ensuring efficient electron confinement within the CsPbI3 emitter for radiative recombination. Consequently, the fabricated deep‐red CsPbI3 PeLEDs based on ZnSnO3 nanocrystal ETL exhibit a peak external quantum efficiency (EQE) of 22% and maintain a high EQE of ≈20% at a current density of 1000 mA cm−2. Furthermore, the PeLED achieves a maximum luminance of 11 012 cd m−2 at an emission peak wavelength of 701 nm, superior to previously reported iodide‐based deep‐red PeLEDs.