Efficient Blue Perovskite Light‐Emitting Diodes via Co‐Solvent and Additive Joint Engineering

Mixed chloride/bromide (Cl/Br) quasi‐two‐dimensional (quasi‐2D) perovskites have emerged as promising emitters for blue perovskite light‐emitting diodes (PeLEDs). However, suboptimal film morphology, defect‐assisted non‐radiative recombination, and inefficient interphase energy transfer limit device performance. To overcome these challenges, we develop a co‐solvent system comprising methylammonium acetate (MAAc) ionic liquid and dimethylformamide (DMF) for blue PeLEDs. The MAAc/DMF co‐solvent exhibits excellent solubility of perovskite precursors even at high Cl/Br ratios, thereby improving perovskite film morphology. Furthermore, incorporation of methanaminium 4‐methylbenzenesulfonate (MATsO) additive effectively passivates intrinsic defects and regulates the phase distribution to facilitate efficient energy transfer, resulting in enhanced radiative recombination. Consequently, the optimized blue PeLEDs achieve impressive peak external quantum efficiencies (EQEs) of 14.48% at 482 nm, 9.03% at 474 nm, and 6.29% at 468 nm with prolonged operational lifetime. This work establishes an effective strategy to improve the quality of mixed Cl/Br quasi‐2D perovskite films toward high‐performance blue PeLEDs.