Tetrafluoroborate-Passivated CsPbBr x Cl 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 x Cl 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 (BF 4 – ) passivation approach to improve the EL stability of CsPbBr x Cl 3– x NC-based LEDs. Our approach involves the solution-processed treatment of CsPbBr x Cl 3– x NCs by the complex of tributylphosphine oxide and calcium tetrafluoroborate, which yields pseudohalide BF 4 – -passivated CsPbBr x Cl 3– x NCs. The BF 4 – fills the surface halide ion vacancy, providing dual benefits of surface passivation and halide migration suppression. By using these BF 4 – -passivated CsPbBr x Cl 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/m 2 , and a maximum external quantum efficiency of 3.2%. Our proposed BF 4 – -passivated NCs strategy will pave the way for the development of efficient, stable, and pure blue perovskite LEDs.