Side-Chain Engineering in Small-Molecule Hole Transport Layer Materials for High-Performance QD-LEDs

By leveraging on the great advantage of low-cost solution processability, quantum-dot light-emitting diodes (QD-LEDs) based on polymeric hole transport layers (HTLs) have achieved great efficiency and stability. However, the polymeric HTL-based QD-LEDs still have many shortcomings: The mismatched energy levels between QDs and HTL layers would often result in a high hole injection barrier, and the inherent batch problem would also make the repeatability of device performance poor. Therefore, employing small-molecule (SM) HTL materials without batch problems and with suitable energy levels is a reasonable and ideal approach to address the aforementioned issues. Herein, a series of SM-HTL materials (FLCZ-0, FLCZ, FLCZ-2) are designed via side-chain engineering, specifically by varying the alkyl length. The three SM-HTL materials can be solution-processed to replace the polymeric HTL materials for QD-LEDs. The QD-LED based on FLCZ-2, having the longest alkyl side chain, achieves the highest external quantum efficiency (EQE) of 16.15% (PE: 17.17 lm W –1, CE: 22.04 cd A –1 ) among all SM-HTL-based QD-LEDs, even close to that of the state-of-the-art polymeric HTL-based QD-LEDs.