Effect of oxygen plasma treatment on air exposed MoOx thin film

Oxygen plasma (OP) treatment on air exposed molybdenum oxide (MoOx) has been investigated with ultraviolet photoemission spectroscopy (UPS) and angle resolved X-ray photoemission spectroscopy (AR-XPS). It was found that the work function (WF) reduction of MoOx by air exposure can be recovered partially by OP treatment on the surface. The overall recovery was measured to be slightly more than 64%, which was adequate to provide a hole extraction layer to many hole-conducting organic materials. The incompleteness of the WF recovery could be attributed to the formation of a very thin layer of oxygen rich absorbents on top of the evaporated MoOx film after OP treatment. AR-XPS showed that OP treatment shifted the core levels of oxygen and molybdenum about 0.1 eV toward the lower binding energy (BE), and confirmed the existence of oxygen deficiency in the evaporated MoOx film. We also investigated the electronic energy level evolution of copper phthalocyanine (CuPc) on MoOx/ITO by UPS. With the deposition of CuPc on OP treated MoOx we observed band bending and the highest occupied molecular orbital (HOMO) level of CuPc was almost pinned to the Fermi level, which indicates the possibility of efficient hole injection with the OP treated MoOx film.