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All‐inorganic CsPbI3 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 CsPbI3 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 CsPbI3 crystallization and thus obtain high‐quality γ‐CsPbI3 thin films. The high crystallinity and preferential orientation of γ‐CsPbI3 arise from the strong bonding between the ionic ligands and ionic lead halide clusters. The obtained γ‐CsPbI3 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, T50) 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). [ABSTRACT FROM AUTHOR]

Inorganic cesium lead bromide nanocrystals (CsPbBr 3 NCs) hold promising prospects for high performance green light-emitting diodes (LEDs) due to their exceptional color purity and high luminescence efficiency. However, the common ligands employed for passivating these indispensable NCs, such as long-chain organic ligands like oleic acid and oleylamine (OA/OAm), display highly dynamic binding and electronic insulating issues, thereby resulting in a low efficiency of the as-fabricated LEDs. Herein, we report a new zwitterionic short-branched alkyl sulfobetaine ligand, namely trioctyl(propyl-3-sulfonate) ammonium betaine (TOAB), to in situ passivate CsPbBr 3 NCs via a feasible one-step solution synthesis, enabling efficiency improvement of CsPbBr 3 NC-based LEDs. The zwitterionic TOAB ligand not only strengthened the surface passivation of CsPbBr 3 NCs with a high photoluminescence quantum yield (PLQY) of 97%, but also enhanced the carrier transport in the fabricated CsPbBr 3 NC thin films due to the short-branched alkyl design. Consequently, CsPbBr 3 NCs passivated with TOAB achieved a green LED with an external quantum efficiency (EQE) of 7.3% and a maximum luminance of 5716 cd m -2 , surpassing those of LEDs based on insulating long-chain ligand-passivated NCs. Our work provides an effective surface passivation ligand design to enhance the performance of CsPbBr 3 NC-based LEDs.