Approaching the theoretical efficiency limit of quantum-dot light-emitting diodes via synergistic optimization

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.