| AVS 58th Annual International Symposium and Exhibition | |
| Surface Science Division | Tuesday Sessions |
| Session SS+EM-TuA |
| Session: | Organic Electronic Interfaces |
| Presenter: | Chih-I Wu, National Taiwan University, Republic of China |
| Authors: | C.I. Wu, National Taiwan University, Republic of China P. Wang, National Taiwan University, Republic of China I. Wu, National Taiwan University, Republic of China |
| Correspondent: | Click to Email |
In this paper, the mechanisms leading to the enhancement of organic light emitting diodes (OLEDs) with molybdenum oxide MoO3 incorporated in as hole injecting layers (HILs) will be discussed. The first one is the lowering of hole injection barrier between anodes and organic layers when a thin film of MoO3 is inserted. The high work function of MoO3 serves as a carrier ladder which decreases misalignment between the Fermi level of electrodes and highest occupied molecular orbital (HOMO) level of hole transport layers, such as N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidene (NPB). The second model is the formation of gap states above the valence band edge of MoO3 to the Fermi level of electrodes when NPB molecules are deposited on MoO3 layers. These gap states enhance the conductivity of MoO3 and provide transition paths of carrier to assist the injection of hole from indium tin oxide (ITO) anodes to NPB layers. The third mechanism is the p-type doping effect of MoO3 doped in NPB layers. This p-type doping increases hole concentration in NPB layers and reduces the energy difference between the Fermi level of electrodes and the HOMO of NPB.
We will also demonstrate an effective method to improve the current injection efficiency of OLEDs by modifying the oxidation states of as-deposited MoO3 as HILs with in-situ argon ion (Ar+) sputtering. The injection current of devices incorporating this method is enhanced by one order of magnitude, as compared to that of devices without sputter treatment. The luminance of the devices is also improved. Beside device characterization, X-ray photoemission spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS) were carried out to obtain the chemical and electronic information of MoO3 thin films treated with Ar+ sputter and to unveil the origins of improvement in device performance. It is found that, with slight sputter treatments, MoO3 layers represent lower oxidation states and show metallic characteristics in energy band structure, which remarkably elevates the carrier injection efficiency from ITO to NPB.