The importance of optical resonance in enhancing light outcoupling efficiency (OCE) is frequently overlooked in conventional bottom-emitting quantum-dot light-emitting diodes (QLEDs) due to their weak microcavity effect. Herein, we show that by synergistically optimizing the optical and the electrical performances, QLEDs with efficiency approaching the theoretical limit can be realized. By introducing a high refractive index indium zinc oxide (IZO) electrode and optimizing its thickness, the light OCE is significantly improved and consequently the red QLEDs exhibit an external quantum efficiency (EQE) of 33.2%, which is 1.4-fold higher than that of the reference devices with conventional indium tin oxide (ITO) electrodes. Moreover, with a high refractive index plastic substrate and a microlens array, the EQE can further be improved to a record value of 37.5%. Similar results are obtained in green and blue devices, which show an EQE of 18.8% and 14.4%, respectively. We also predict that the theoretical EQE limit of red, green, and blue QLEDs can reach 35.4%–36.5%, 24.8%–34.0%, and 25.1%–35.8%, respectively, without using any light outcoupling structures. The proposed synergistic optimization strategy enables the efficiencies of red, green, and blue QLEDs to approach their theoretical limits.

ZnSeTe blue Cd-free quantum dot (QD) has emerged as a promising emitter for display applications due to its nontoxicity, tunable wavelength, and high efficiency. However, ZnSeTe-based quantum-dot light-emitting diodes (QLEDs) usually exhibit unsaturated emissions with broad spectra. Herein, a top-emitting structure, equipped with a transparent indium-zinc-oxide (IZO) top electrode and an IZO phase tuning layer (PTL), is developed to modulate the emission spectra and the efficiency of the devices. Saturated blue emissions with color coordinates beyond Recommendation ITU-R BT.709 (Rec.709) and near Rec.2020 standards are achieved. Moreover, benefiting from the improved outcoupling efficiency and the enhanced charge balance, the top-emitting QLED demonstrates a high external quantum efficiency of 15.14%, which is further improved to 18.16% by capping the devices with SiO₂ nanospheres. Simulation analysis reveals that the surface plasmon polariton (SPP) losses are effectively reduced by applying a 100 nm PTL, leading to an outcoupling efficiency of 41.2% at a wavelength of 478 nm. Due to the simultaneously enhanced color saturation and efficiency, a high chroma efficiency (current efficiency/y coordinate in Commission Internationale de l'Eclairage chart) of 123 is obtained. The developed top-emitting architecture could enable the realization of efficient and saturated QLEDs for wide color gamut high-definition display applications.

The aging characteristics, e.g., the evolution of efficiency and luminance of quantum-dot light-emitting diodes (QLEDs) are greatly affected by the encapsulation. When encapsulated with ultraviolet curable resin, the efficiency is increased over time, a known phenomenon termed as positive aging which remains one of the unsolved mysteries. By developing a physical model and an analytical model, we identify that the efficiency improvement is mainly attributed to the suppression of hole leakage current that is resulted from the passivation of ZnMgO defects. When further encapsulated with desiccant, the positive aging effect vanishes. To fully take the advantage of positive aging, the desiccant is incorporated after the positive aging process is completed. With the new encapsulation method, the QLED exhibits a high external quantum efficiency of 20.19% and a half lifetime of 1,267 h at an initial luminance of 2,800 cd·m^-2, which are improved by 1.4 and 6.0 folds, respectively, making it one of the best performing devices. Our work provides an in-depth and systematic understanding of the mechanism of positive aging and offers a practical encapsulation way for realizing efficient and stable QLEDs.