Nanocrystalline lead halide perovskites are promising as emissive layers for light-emitting diodes due to their bright, tunable emission with very narrow linewidths. Blue perovskite light-emitting diodes, in the wavelength range useful for display applications (460-470 nm), could be made with CsPb(Br/Cl)3 nanocrystals (NCs). However, mixed halide perovskites suffer from color instability, foremost, due to the segregation of halide ions. In this study, we address this issue with several measures. First, we show that thinner CsPb(Br/Cl)3 NC layers are less prone to color instability. Additionally, inefficient hole injection due to the deep-lying valence band of CsPb(Br/Cl)3 NCs detrimentally affects the device performance, and we mitigate this problem by stepwise hole injection using two hole-transporting materials. Next, we employ NCs capped with zwitterionic ligands that allow for a more thorough washing of the NC solutions. Furthermore, our new device layout explores the use of polystyrene in the emitting layer to limit the current leakage. Undertaking these steps, we show light-emitting diodes with a stable electroluminescence peak wavelength of 463 nm over the lifetime of the device and a peak external quantum efficiency of over 1%. The results prove that perovskite NCs are a viable contender in the development of blue-emissive, active pixel displays.
Keywords: efficiency; electroluminescence; ionic diffusion; lead halides; light-emitting diode; nanocrystal; perovskite; stability.