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 CsPbBrxCl3-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 CsPbBrxCl3-x NC-based LEDs. Our approach involves the solution-processed treatment of CsPbBrxCl3-x NCs by the complex of tributylphosphine oxide and calcium tetrafluoroborate, which yields pseudohalide BF4--passivated CsPbBrxCl3-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 CsPbBrxCl3-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 BF4--passivated NCs strategy will pave the way for the development of efficient, stable, and pure blue perovskite LEDs.