Optimizing the central steric hindrance of cross-linkable hole transport materials for achieving highly efficient RGB QLEDs

Abstract

Cross-linking strategies of hole transport materials (HTMs) have been widely investigated and used in various optoelectronic devices, mainly owing to their excellent solvent resistance and thermal stability. More importantly, the energy level and charge mobility of cross-linkable HTMs can be fine-tuned through precise molecular design, which offers great opportunities to explore and screen efficient HTMs. Currently, the investigations of cross-linkable HTMs are limited in the research of quantum dot light emitting diodes (QLEDs), which could not realize the full advantages of the superior emissive properties of QDs and benefits of solution-processing fabrication techniques. Thus, we synthesized two cross-linkable materials DV-SFCZ and DV-FLCZ with different center cores to explore the influence of the central steric hindrance on the properties. With a smaller central steric hindrance, the cross-linking temperature of DV-FLCZ is 75 °C lower than that of DV-SFCZ, and the cross-linking time is shortened by 2/3 times, which is of great significance in future practical application. DV-FLCZ shows a better hole transport property, which favors hole injection into the QD layer. Notably, red, green and blue QLEDs with DV-FLCZ as the HTM achieved a maximum EQE of 20.5%, 16.6% and 8.5%, respectively, which are much better than the devices based on DV-SFCZ as the HTM. To the best of our knowledge, the results represent the highest EQE values of QLED performance using cross-linkable materials alone as hole transport layers.