Tetrafluoroborate-Passivated\nCsPbBr_<i>x</i>Cl_3–<i>x</i> Nanocrystals for Spectrally\nStable Pure Blue Perovskite Light-Emitting Diodes

Abstract

Mixed halide perovskite nanocrystals show great promise\nfor high-performance\nlight-emitting diodes (LEDs) due to their tunable band gaps, high\nphotoluminescence efficiency, and excellent color purity. However,\none major challenge in using mixed halide CsPbBr_<i>x</i>Cl_3–<i>x</i> NCs for pure blue\nLEDs is the instability of their electroluminescence (EL) caused by\nhalide segregation. Here, we report an effective pseudohalide tetrafluoroborate\n(BF₄^–) passivation approach to improve\nthe EL stability of CsPbBr_<i>x</i>Cl_3–<i>x</i> NC-based LEDs. Our approach involves the solution-processed\ntreatment of CsPbBr_<i>x</i>Cl_3–<i>x</i> NCs by the complex of tributylphosphine oxide and\ncalcium tetrafluoroborate, which yields pseudohalide BF₄^–-passivated CsPbBr_<i>x</i>Cl_3–<i>x</i> NCs. The BF₄^– fills the surface halide ion vacancy, providing dual\nbenefits of surface passivation and halide migration suppression.\nBy using these BF₄^–-passivated CsPbBr_<i>x</i>Cl_3–<i>x</i> NCs\nas the emitting layer, we have successfully fabricated spectrally\nstable and pure blue LEDs with an emission peak at 468 nm, a maximum\nluminance of 275 cd/m², and a maximum external quantum\nefficiency of 3.2%. Our proposed BF₄^–-passivated NCs strategy will pave the way for the development of\nefficient, stable, and pure blue perovskite LEDs.