Improved Hole Injection by Enhancing Electron Extraction of Solution-Processed MoO_x in Quantum Dot Light-Emitting Diodes

Hole injection layers (HILs) are pivotal for the performance of quantum dot light-emitting diodes (QLEDs), with solution-processed inorganic HIL materials being a primary means for the commercial application of QLEDs. Transition metal oxides (TMOs), due to their excellent stability, have been widely employed as inorganic HILs by thermal evaporation in QLEDs; however, the hole injection ability of solution-processed TMO film necessitates further enhancement owing to inferior film quality. In this study, a solution-processed molybdenum oxide (MoO_x) film was used as a HIL, and its hole injection ability in the QLED was improved by tuning the oxygen states. Oxidation treatments on the MoO_x layer can effectively mitigate oxygen vacancies, and consequently, the conduction band minimum (CBM) of MoO_x is elevated, which enhances the hole injection through easier electron extraction from the highest occupied molecular orbitals (HOMO) of the hole transport layer. The MoO_x-based red QLED exhibits significantly longer working lifetimes (T50@100 cd·m^-2 of ∼66,892 h) and comparable current efficiencies (13.7 cd·A^-1) compared to the Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)-based QLEDs. Our research not only proposes a promising approach to high-performance MoO_x-based QLEDs but also paves the way for further applications of TMOs in QLEDs.