Improving hole injection ability using a newly proposed WO3/NiOx bilayer in solution processed quantum dot light-emitting diodes

We propose a novel device structure with a WO₃/NiO_x bilayer to improve the hole injection ability in QLEDs fabricated mainly by a solution-based process. First, we employed a spin-coated NiO_x thin film as a hole injection layer (HIL) to replace Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) PEDOT:PSS which corrodes indium tin oxide (ITO) used as an anode in QLEDs. We showed a simply optimized annealing process, instead of a rather complicated process like doping, can improve the electrical conductivity of the NiO_x thin film. The reason for the dependency of conductivity on the post-metallization annealing is because of the change of the total amount of Ni vacancy in the NiO_x thin film as a function of annealing temperature: The electrical conductivity of the NiO_x thin film annealed at 275 °C was the best in this work. Second, we inserted the WO₃ thin film in between ITO electrode and NiO_x HIL to form an ITO/WO₃/NiO_x structure which reduces the hole injection barrier to 0.35 eV, resulting in the excellent characteristic in view of charge balance. Finally, we measured the properties of QLEDs with the WO₃/NiO_x bilayer to check the effects of the proposed device structure and showed the substantial improvement of the electrical conductivity of NiO_x, the luminance, and the current efficiency of the QLEDs.