Nucleophilic Reaction‐Enabled Chloride Modification on CsPbI3 Quantum Dots for Pure Red Light‐Emitting Diodes with Efficiency Exceeding 26 %

High-performance pure red perovskite light-emitting diodes (PeLEDs) with an emission wavelength shorter than 650 nm are ideal for wide-color-gamut displays, yet remain an unprecedented challenge to progress. Mixed-halide CsPb(Br/I)₃ emitter-based PeLEDs suffer spectral stability induced by halide phase segregation and CsPbI₃ quantum dots (QDs) suffer from a compromise between emission wavelength and electroluminescence efficiency. Here, we demonstrate efficient pure red PeLEDs with an emission centered at 638 nm based on PbCl_x -modified CsPbI₃ QDs. A nucleophilic reaction that releases chloride ions and manipulates the ligand equilibrium of the colloidal system is developed to synthesize the pure red emission QDs. The comprehensive structural and spectroscopic characterizations evidence the formation of PbCl_x outside the CsPbI₃ QDs, which regulates exciton recombination and prevents the exciton from dissociation induced by surface defects. In consequence, PeLEDs based on PbCl_x -modified CsPbI₃ QDs with superior optoelectronic properties demonstrate stable electroluminescence spectra at high driving voltages, a record external quantum efficiency of 26.1 %, optimal efficiency roll-off of 16.0 % at 1000 cd m^-2 , and a half lifetime of 7.5 hours at 100 cd m^-2 , representing the state-of-the-art pure red PeLEDs. This work provides new insight into constructing the carrier-confined structure on perovskite QDs for high-performance PeLEDs.