Ligand Engineering of Ultrasmall CsPbI 3 Quantum Dots via In Situ S N 2 Substitution Enables Bright Rec. 2020 Pure‐Red Perovskite LEDs with Exceptional Current Efficiency

ABSTRACT Ultrasmall‐sized cesium lead iodide (CsPbI 3 ) quantum dots (QDs) are promising candidates for achieving spectrally stable pure‐red perovskite light‐emitting diodes (PeLEDs) meeting Rec. 2020 standards. However, the corresponding devices hardly achieve satisfactory external quantum efficiency (EQE), current efficiency (CE), and luminance simultaneously because of the use of largely excessive insulating long‐chain ligands and additional difficulties in the defect control of ultrasmall CsPbI 3 QDs. Herein, we develop an alkyl iodide‐assisted ligand modulation strategy for CsPbI 3 QDs toward high‐efficiency and bright pure‐red PeLEDs. We elucidate an in‐situ nucleophilic bimolecular (S N 2) substitution reaction between the oleylamine and additionally incorporated short‐chain 1‐iodooctane (IO) molecules during the materials synthesis. The reaction‐generated hydriodic acid (HI) induces non‐destructive surface etching of QDs, enabling exceptional luminescent properties of the strongly confined products. In addition, the S N 2 reaction‐derived secondary amine strongly adsorbs at the surface of QDs, which stabilizes the products with a reduced ligand density, simultaneously enhancing photoluminescence stability and electrical properties of the assembled emissive layers. The resultant devices emitting at 632 nm demonstrate a peak EQE of 21.56%, an impressive luminance of 13,132 cd m −2 , and an exceptional CE of 20.73 cd A −1 , which outperforms state‐of‐the‐art Rec. 2020 pure‐red PeLEDs utilizing ultrasmall‐sized colloidal CsPbI 3 QDs.