Cation‐Anion‐Pair‐Assisted Synthesis Strategy Enables High‐Performance Pure Blue Perovskite Quantum Dot‐Based Light‐Emitting Diodes

The mixed‐halide CsPbCl 3‐x Br x perovskite quantum dots (QDs) have emerged as promising candidates for pure‐blue light‐emitting diodes (LEDs). However, the widely existing defects in these mixed‐halide perovskites, arising from uncontrollable crystallization and low Cl vacancy formation energy, significantly restrict the device performance. Herein, a cation‐anion pair‐assisted synthesis strategy is proposed to obtain high‐quality CsPbCl 3‐x Br x QDs. Guanidinium sulfate (GA 2 SO 4 ) is introduced during the synthesis process, in which sulfate anions passivate undercoordinated Pb 2+ defects, effectively reducing the non‐radiative recombination, and the guanidinium cations interact with halides on the QD surface via hydrogen bonds, inhibiting the formation of Cl vacancy defects. Benefiting from this synergistic passivation effect of the cation‐anion pair, the photoluminescence quantum yield of the QDs increases from 42% to 86%. Meanwhile, the QDs exhibit high crystalline quality, which facilitates the transport of charge carriers. Resultantly, the QLEDs achieve a record external quantum efficiency of 24.9% at 470 nm, showing a 1.6‐fold improvement over that of the control devices. This work demonstrates that a synergistic ion‐pair passivation strategy is a practical design approach for realizing efficient and stable pure‐blue perovskite LEDs.