Change of interface dipole energy with interfacial layer thickness and O2 plasma treatment in metal/organic interface

The authors determined the interface dipole energies between interfacial layers with different thicknesses coated on indium tin oxides (ITOs) and 4,4'-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl using ultraviolet and synchrotron radiation photoemission spectroscopy. The interface dipole energy increased as a function of interfacial layer thickness up to 4 nm. After O2 plasma treatment on thick-metal (>4 nm) coated ITO, the work function and interface dipole energy increased. In thin-metal (<2 nm) coated ITO, no change in the interface dipole energy was found though the work function increased. Thus, the O2 plasma treated thin (<2 nm) interfacial layer reduced the hole injection barrier