Improving the performance of Cd-free quantum dot light-emitting diodes by incorporating the thermally activated delayed fluorescence molecules

Abstract

Cadmium-free quantum-dot light-emitting diodes (QLEDs) are the potential candidate for next generation displays due to its non-toxicity, tunable colors, saturated color emission, high luminescence efficiency, and simple fabrication process. Several methods have been proposed to improve the performance of QLEDs through synthetizing new quantum dots and charge transporting materials, optimizing the device architecture, modifying the device interfaces, and engineering the fabrication process. On the other hand, the full utilization of triplet excitons would be another important pathway to improve the efficiency of QLEDs since most triplet excitons are usually lost in fluorescence light-emitting diodes. Both the phosphorescent materials and thermally activated delayed fluorescence (TADF) materials have been proposed to realize the utilization of triplet excitons. However, in QLEDs, the use of TADF materials to achieve the use of triplet excitons to improve the luminous efficiency is still rare. Here, the TADF molecule of 2CzPN was incorporated into PVK hole-transporting layer (HTL) with the ratio of 1:5 to achieve hybrid HTL PVK: 2CzPN, through which QLEDs with the architecture ITO/PEDOT:PSS/PVK:2CzPN/InP/ZnS QDs/ZnO/Al were fabricated and measured. The results show that the incorporation of 2CzPN into PVK HTL enhances the hole transport efficiency, leading to a more balance of electrons and holes in the device. Moreover, the use of triplet excitons is achieved by the reverse intersystem crossing process in 2CzPN, and the efficiency of InP/ZnS cadmium-free QLEDs is improved by the following Forster energy transfer process between the doped HTL and quantum dots InP/ZnS. Its maximum luminous brightness is 513 cd/m2. A 26% increase was achieved compared to the maximum luminance of the undoped control device (407 cd/m2). At the same time, the maximum current efficiency is increased by 4 times compared to the undoped control device, increasing to 1.6 cd/A.