Direct Evidence of Hole Injection Enhanced by O-2 Plasma Treatment on Indium Tin Oxide

Abstract

We have investigated interface dipole energies and the energy alignment between 4,4'-bis[N-(1-naphtyl)-N-phenyl-amino] biphenyl (alpha-NPD) and bare indium tin oxide (ITO)/O-2-plasma-treated ITO (O-2-ITO) anodes using synchrotron radiation photoemission spectroscopy (SRPES). SRPES spectra showed that the work function of O-2-ITO is 0.6 eV higher than that of ITO. After deposition of alpha-NPD, the dipole energy and amount of band bending were calculated to be - 0.1 and - 0.5 eV for ITO and - 0.3 and - 0.3 for O-2-ITO, respectively. It is thereupon concluded that the work function of O-2-ITO is still 0.6 eV higher than that of ITO, resulting in a decrease of the turn-on voltage via reduction of the hole injection barrier.