Tetrafluoroborate-Passivated CsPbBr_xCl_3–x Nanocrystals for Spectrally Stable Pure Blue Perovskite Light-Emitting Diodes

Mixed halide perovskite nanocrystals show great promise for high-performance light-emitting diodes (LEDs) due to their tunable band gaps, high photoluminescence efficiency, and excellent color purity. However, one major challenge in using mixed halide CsPbBr_xCl_3–x NCs for pure blue LEDs is the instability of their electroluminescence (EL) caused by halide segregation. Here, we report an effective pseudohalide tetrafluoroborate (BF₄^–) passivation approach to improve the EL stability of CsPbBr_xCl_3–x NC-based LEDs. Our approach involves the solution-processed treatment of CsPbBr_xCl_3–x NCs by the complex of tributylphosphine oxide and calcium tetrafluoroborate, which yields pseudohalide BF₄^–-passivated CsPbBr_xCl_3–x NCs. The BF₄^– fills the surface halide ion vacancy, providing dual benefits of surface passivation and halide migration suppression. By using these BF₄^–-passivated CsPbBr_xCl_3–x NCs as the emitting layer, we have successfully fabricated spectrally stable and pure blue LEDs with an emission peak at 468 nm, a maximum luminance of 275 cd/m², and a maximum external quantum efficiency of 3.2%. Our proposed BF₄^–-passivated NCs strategy will pave the way for the development of efficient, stable, and pure blue perovskite LEDs.