Multifunctional Ionic‐Ligand Regulated High‐Quality γ‐CsPbI₃ Thin Films for Efficient and Bright Deep‐Red Light‐Emitting Diodes

All‐inorganic CsPbI 3 perovskite semiconductor is attractive for deep‐red light‐emitting diodes (LEDs) because of its high photoluminescence, good thermal stability, excellent charge transport, and solution processability. However, the metastable phase of optically active CsPbI 3 hinders the fabrication of efficient and bright deep‐red LEDs. Herein, a multifunctional ionic ligand regulation strategy using (2‐hydroxy‐5‐methylphenyl) triphenyl‐phosphonium iodide and 3‐(N, N‐dimethyloctylammonio) propanesulfonate is reported to regulate the optically active CsPbI 3 crystallization and thus obtain high‐quality γ‐CsPbI 3 thin films. The high crystallinity and preferential orientation of γ‐CsPbI 3 arise from the strong bonding between the ionic ligands and ionic lead halide clusters. The obtained γ‐CsPbI 3 thin films also possess suppressed defect densities, high luminescence, and high stability. Using this high‐quality thin film, an efficient and bright deep‐red LED with a peak external quantum efficiency of 13.34% and a maximum luminance of 2110 cd m −2 is successfully fabricated. The operational lifetime of the fabricated LED (time to half of the initial brightness, T 50 ) reaches up to 40 min under a high current density of 50 mA cm −2 (corresponding to the initial brightness of 200 cd m −2 ).