Enhancement in electrical properties of $NiO_x$ hole injection layer by $MoO_3$ charge transport doping for OLEDs

Abstract

Nickel oxide ($NiO_x$), which is one of the p-type metal oxide semiconductors, has excellent carrier selectivity because of wide bandgap (~3.7eV) and high work function. It has been widely used as hole injection layer (HIL) and studied in the field of optoelectronic devices based on these advantages in electrical property. However, $NiO_x$ has a problem of low electrical conductivity because the stoichiometric $NiO_x$ is Mott-Hubbard insulator. In this thesis, molybdenum trioxide ($MoO_3$) nanoparticles (2-3 nm) were doped to the $NiO_x$ matrix to control the nanostructure so that $MoO_3$ was uniformly distributed on the $NiO_x$. This structure enable to maximize the surface area between the materials and to induce charge transfer due to difference in the energy level at the interface between the two oxides. Consequently, suggested nanostructure of HIL provides not only increased work function and conductivity compared to pristine $NiO_x$ but also doping effect at heavy doping concentrations of more than 20 at%, while conventional doping shows limit of doping concentrations. OLEDs based on the solution processed $NiO_x:MoO_3$ HIL films exhibited both external quantum efficiency and current efficiency increased by 1.4 times compared to the devices with pristine $NiO_x$.