Interfacial Electronic Structure In MoO₃-based Organic Semiconductor Devices

Extensive studies have been conducted on molybdenum oxide since it has outstanding properties as an insertion layer for efficient charge injection and extraction in organic semiconductor devices. Efficient charge transfer at semiconductor and electrode interface is one of the most crucial issues for the performance of organic electronic device. A lot of efforts have been spent to address this issue but there are still many unclarified issues to understand the physical mechanisms. 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 than64%, 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 about0.1eV toward the lower binding energy (BE), and confirmed the existence of oxygen deficiency in the evaporated MoOx film.The electronic structure evolution and energy level alignment have been investigated at interfaces comprising fullerene (C6o)/4,40-cyclohexylidenebis[N,N-bis(4-methylphenyl) benzenamine](TAPC)/molybdenum oxide (MoOx)/indium tin oxide. With deposition of TAPC upon MoOx, a dipole of1.58eV was formed at the TAPC/MoOx interface due to electron transfer from TAPC to MoOx. The downward band bending and the resulting built-in field in TAPC were favorable for the hole transfer toward the TAPC/MoOx interface. With subsequent deposition of C60on TAPC, a dipole of0.27eV was observed at the C6o/TAPC heterojunction due to the electron transfer from TAPC to C60. This led to a drop of the HOMO of TAPC near the C60/TAPC interface, and hence further enhanced the band bending in TAPC. The band bending behavior was also observed in C60, similarly creating a built-in field in C60film and improving the electron transfer away from the C6o/TAPC interface.