Improvement of Current Efficiency in Inverted Top-Emitting Quantum Dot Light-Emitting Diodes Using p-Doped Hole Transporting Layer

Quantum-dot light-emitting diodes (QLEDs) have attracted considerable interest as next-generation displays due to low-cost processing using solution processes, high color purity by narrow full-width half maximum of quantum dots (QDs), and potential for high luminance and stability based on inorganic emitting materials (EML) [1]. In QLEDs, injecting holes from the hole transporting layer (HTL) into the QD EML is considered challenging due to the presence of a relatively high hole injection barrier at the HTL/QD interface. On the other hand, the electron from the electron transporting layer (ETL) is relatively easily inject to QD EML compared with the hole injection. This causes charge imbalance in the QD EML, resulting in low efficiency of QLEDs [2]. The p-dopant is widely used in the organic light-emitting diode for lowering hole injection barrier and enhancing hole charge carrier concentration. Although p-doping effect is used for improving device performance in the QLEDs, it has not been as extensively studied in QLEDs as in OLEDs.<br/>In this study, we fabricated inverted top-emitting red QLEDs using indium-tin-oxide (ITO)/Ag/ITO as a reflective cathode, ZnMgO as an ETL, CdSe/CdZnSe/ZnSeS red QDs as an EML, TcTa as a HTL, 2-(7-dicyanomethylene-1,3,4,5,6,8,9,10-octafluoro-7H-pyrene-2-ylidene) malononitrile (NDP-9)-doped tris(4-carbazoyl 9-ylphenyl)amine (TcTa) as hole injction layer (HIL), and thin Ag as a top anode. NDP-9 is a representative p-type dopant owing to its strong electron affinity and hole generation property. We investigated effect of NDP-9-doped TcTa in the inverted top-emitting red QLEDs. By applying NDP-9-doped TcTa instead of molybdenum oxides typically used as the HIL in the inverted QLEDs, the current efficiency of the device dramatically increases approximately two times. In addition, we analyzed device characteristics depending on NDP-9 doping ratios. P-type doping can be an effective strategy for improving efficiency of QLEDs.<br/><br/>Acknowledgement<br/>This research was supported by an Institute of Information & Communications Technology Planning & Evaluation (IITP) grant funded by the MSIT (No. 2022-0-00026).<br/><br/>References<br/>[1] J. Kim, J. Roh, M. Park, C. Lee, Adv. Mater. 36, 2212220 (2024).<br/>[2] S. Lei, Y. Xiao, K. Yu, B. Xiao, M. Wan, L. Zou, Q. You, R. Yang, Adv. Funct. Mater. 33, 2305732 (2023).<br/>[3] W.-h. Park, D.-p. Park, S. S. Kim, J. Inf. Disp. 23, 45 (2022).