High-efficiency quantum dot light-emitting diodes employing lithium salt doped poly(9-vinlycarbazole) as a hole-transporting layer

For the purpose of fabricating solution-processed quantum-dot light-emitting diodes (QLEDs) with high performance, the efficient hole-electron recombination at low current density is particularly pivotal. Herein, to enhance the charge balance of the QLED device, we employed lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI) as a p-type dopant into the hole-transporting material (HTM) of poly(9-vinlycarbazole) (PVK). In the experiment, the increased conductivity and the enhanced charge mobility of the Li-TFSI-doped PVK layer were confirmed by the J-V curves of the hole-only devices and conductive atomic force microscopy (c-AFM). Furthermore, on combining ultraviolet photoelectron spectroscopy (UPS) and the absorption spectra, it was found that the highest occupied molecular orbital (HOMO) of the Li-TFSI-doped PVK layers gradually shifted closer to the Fermi level upon increasing the doping ratios from 0 to 4.5 wt%. Therefore, the hole-injecting barrier decreases from 1.17 eV to 0.64 eV. As a result, the maximum current efficiency and the highest external quantum efficiency (EQE) of our fabricated QLED devices can reach as high as 15.5 cd A^-1 and 11.46%, respectively. It was demonstrated that the p-type dopant Li-TFSI in the HTM can contribute to the fabrication of high-performance solution-processed light-emitting diodes.