Suppressed efficiency roll-off in blue light-emitting diodes by balancing the spatial charge distribution

Abstract

Quantum dot-based light-emitting diodes (QLEDs) have been considered as a highly promising candidate for lighting and display applications. Their peak external quantum efficiencies (EQEs) are now approaching the theoretical values for red/green/blue QLEDs. However, these peak values are commonly obtained at low current densities (low brightness), whereas their efficiency usually exhibits a quick roll-off with increasing current densities, especially for blue QLEDs, limiting the operation lifetime and applications. In this study, we demonstrated suppressed efficiency roll-off and bright blue QLEDs by using propanethiol-modified ZnCdSeS QDs as the emitters. A peak EQE of 9.9% is achieved at a luminance of 1040 cd m⁻², along with a high luminance up to 52 360 cd m⁻². Moreover, the reduction of the EQE is less than 50% of the peak value until the luminance is as high as 28 260 cd m⁻², highly superior to the control device. Most importantly, higher than 75% of the peak EQE value is sustained within the luminance range from 100 to 10 000 cd m⁻², which has already reached the threshold values for applications in both displays and lighting. We attribute these major advances to the transport-enhancement of short-chain ligands outside the QDs, which improves both conductivity and injection balance of the emissive layer, reducing the quenching processes induced by accumulated charges.