Blue Perovskite Nanocrystal Light‐Emitting Diodes: Overcoming RuddlesdenPopper Fault‐Induced Nonradiative Recombination via Post‐Halide Exchange

Abstract Metal halide perovskites (MHPs) have gained traction as emitters owing to their excellent optical properties, such as facile bandgap tuning, defect tolerance, and high color purity. Nevertheless, blue‐emitting MHP light‐emitting diodes (LEDs) show only marginal progress in device efficiency compared with green and red LEDs. Herein, the origin of the drop in efficiency of blue‐emitting perovskite nanocrystals (PNCs) by mixing halides and the genesis of Ruddlesden–Popper faults (RPFs) in CsPbBr X Cl 3‐X nanocrystals is investigated. Using scanning transmission electron microscopy and density functional theory calculations, the authors have found that RPFs induce possible nonradiative recombination pathways owing to the high chloride vacancy concentration nearby. The authors further confirm that the blue‐emitting PNCs do not show RPFs post‐halide exchange in the CsPbBr 3 nanocrystals. By introducing the post‐halide exchange treatment, high‐efficiency pure blue‐emitting (464 nm) PNC‐based LEDs with an external quantum efficiency of 2.1% and excellent spectral stability with a full‐width at half‐maximum of 14 nm are obtained.