Halide perovskite light emitting diodes (LEDs) have gained great progress in recent years. However, mixed-halide perovskites for blue LEDs usually suffer from electroluminescence (EL) spectra shift at a high applied voltage or current density, limiting their efficiency. In this work, we report a strategy of using single-layer perovskite quantum dots (QDs) film to tackle the electroluminescence spectra shift in pure-blue perovskite LEDs and improve the LED efficiency by co-doping copper and potassium in the mixed-halide perovskite QDs. As a result, we obtained pure-blue halide perovskite QD-LEDs with stable EL spectra centred at 469 nm even at a current density of 1,617 mA·cm⁻². The optimal device presents a maximum external quantum efficiency (EQE) of 2.0%. The average maximum EQE and luminance of the LEDs are 1.49% and 393 cd·m⁻², increasing 62% and 66% compared with the control LEDs. Our study provides an effective strategy for achieving spectra-stable and highly efficient pure-blue perovskite LEDs.

Perovskite light-emitting diodes (PeLEDs) are attracting increasing attention owing to their impressive efficiencies and high luminance across the full visible light range. Further improvement of the external quantum efficiency (EQE) of planar PeLEDs is limited by the light out-coupling efficiency. Introducing perovskite emitters with directional emission in PeLEDs is an effective way to improve light extraction. Here, we report that it is possible to achieve directional emission in mixed-dimensional perovskites by controlling the orientation of the emissive center in the film. Multiple characterization methods suggest that our mixed-dimensional perovskite film shows highly orientated transition dipole moments (TDMs) with the horizontal ratio of over 88%, substantially higher than that of the isotropic emitters. The horizontally dominated TDMs lead to PeLEDs with exceptional high light out-coupling efficiency of over 32%, enabling a high EQE of 18.2%.