Controlling the Hole Injection Dynamics of Hole-Transporting Polymers for Efficient Quantum Dot Light-Emitting Diodes

Abstract

State-of-the-art quantum dot light-emitting diodes (QD-LEDs) have largely relied on poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4 '-(N-(4-sec-butylphenyl))diphenylamine)] (TFB) as the hole transport layer (HTL). However, the relatively high highest occupied molecular orbital (HOMO) energy level of the TFB hinders hole transfer into emitting layers (EMLs). In this study, we introduce fluorine (F) as a weak electron-withdrawing group (EWG) and the cyano group (CN) as a strong EWG into triphenylamine units of TFB to reduce the energy offset between HTLs and EMLs. Fluorination selectively downshifts HOMO energy levels and slightly improves the hole mobility of the TFB derivatives, leading to a significant enhancement of the external quantum efficiencies (EQEs) in red-emissive CdSe and blue-emissive ZnSe QD-LEDs. Specifically, using difluorinated TFB (2F-TFB) as HTLs in the red and blue QD-LED devices improves EQE to 17.4% and 10.6%, compared to the conventional TFB-based devices with EQEs of 13.3% and 5.0%, respectively. However, the devices with the TFB derivative incorporating CN (CN-TFB) show poor EQEs of 8.1% and 0.5% for the red and blue devices, respectively, due to the reduced hole mobility of CN-TFB and, in the blue devices, to enhanced electron leakage from the EMLs into the HTLs. The results show that beyond deepening the HTL HOMO to lower the hole injection barrier at the HTL/EML interface, effective HTL design must also balance hole mobility and electron blocking in order to maintain charge balance in QD-LEDs.