Enhancing the incorporation compatibility of molybdenum oxides in organic light emitting diodes with gap state formations

The enhancement of injection current and luminance in organic light emitting diodes is achieved by annealing molybdenum oxide (MoO3) hole injecting layers prior to the deposition of hole transport layers. While there is no benefit by the incorporation of non-annealed MoO3 in devices using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as the hole transport layers, the annealed MoO3 layers exhibit a significant improvement in hole injection from indium tin oxide anodes to TAPC. X-ray photoemission spectroscopy reveals the change of oxidation states of Mo atoms in MoO3 films due to the annealing process. The gap state formation is verified by ultra-violet photoemission spectroscopy. A more energetically favorable band alignment is obtained at the interface between the annealed MoO3 and TAPC, resulting in improved hole injection efficiency. The overall performance of OLEDs can be enhanced by adopting annealed MoO3 in most of the hole transport layers.