Very low turn-on voltage and high brightness tris-(8-hydroxyquinoline) aluminum-based organic light-emitting diodes with a MoOx p-doping layer

We have demonstrated an organic light-emitting diode based on molybdenum oxide $\left(\mathrm{Mo}{\mathrm{O}}_x\right)$ doped $4,4^{\prime },4^{″}$ -tris(3-methylphenylphenylamino)triphenylamine $\left(m\text{-}\mathrm{MTDATA}\right)$ as a $p$ -type doping hole injection layer. The tris-(8-hydroxyquinoline) aluminum $\left(\mathrm{Al}{q}_3\right)$ -based organic light-emitting diodes show high brightness at very low operating voltage, $100\mathrm{cd}∕{\mathrm{m}}^2$ at $3.2\mathrm{V}$ and $1000\mathrm{cd}∕{\mathrm{m}}^2$ at $4.4\mathrm{V}$ , corresponding to a low turn-on voltage of $2.4\mathrm{V}$ . Such improved properties are attributed to the formation of the charge transfer complex produced by doping $\mathrm{Mo}{\mathrm{O}}_x$ into $m\text{-}\mathrm{MTDATA}$ , which provides much more free hole carriers, and the introduction of an efficient electron-injecting layer to improve the performance.