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Highly Efficient Pure‐Blue Perovskite Light‐Emitting Diode Leveraging CsPbBrxCl3−x/Cs4PbBrxCl6−x Nanocomposite Emissive Layer with Shallow Valence Band

Metal‐halide perovskite light‐emitting diodes (PeLEDs) have shown great advancement in green, red, and near‐infrared regions with external quantum efficiencies (EQEs) exceeding 20%. However, blue PeLEDs, an essential part of displays and lightings, show limited progress compared to the other color counterparts. Herein, a highly efficient pure‐blue PeLED is demonstrated by leveraging a novel CsPbBrxCl3‐x/Cs4PbBrxCl6‐x nanocomposite perovskite film as an emissive layer. The Cs4PbBrxCl6‐x phase, the derived phase of CsPbBr3 perovskite with a mixed halide system, effectively passivates defects in CsPbBrxCl3‐x, leading to high luminescence efficiency due to the significant reduction of nonradiative recombination. Furthermore, experimental and computational results confirmed that the compositionally optimized nanocomposite layer possesses a shallower valence band maximum (≈5.5 eV) than the pristine perovskite layer (≈5.9 eV), which is very advantageous in hole injection for device operation. The combined effects of the CsPbBrxCl3‐x/Cs4PbBrxCl6‐x nanocomposite render the fabricated blue PeLED to exhibit a pure‐blue emission at 470 nm with a maximum EQE of 5.3%.