Utilization of Nanoporous Nickel Oxide as the Hole Injection Layer for Quantum Dot Light-Emitting Diodes

Nickel oxide (NiO_x) has been extensively investigated as the hole injection layer (HIL) for many optoelectronic devices because of its excellent hole mobility, high environmental stability, and low-cost fabrication. In this research, a NiO_x thin film and nanoporous layers (NPLs) have been utilized as the HIL for the fabrication of quantum dot light-emitting diodes (QLEDs). The obtained NiO_x NPLs have spongelike nanostructures that possess a larger surface area to enhance carrier injection and to lower the turn-on voltage as compared with the NiO_x thin film. The energy levels of NiO_x were slightly downshifted by incorporating the nanoporous structure. The amount of Ni₂O₃ species is higher than that of NiO in the NiO_x NPL, confirming its good hole transport ability. The best QLED was achieved with a 30 nm thick NiO_x NPL, exhibiting a maximum brightness of 68 646 cd m^-2, a current efficiency of 7.60 cd A^-1, and a low turn-on voltage of 3.4 V. More balanced carrier transport from the NiO_x NPL and ZnO NPs/polyethylenimine ethoxylated (PEIE) is responsible for the improved device performance.