Interface properties of organic/indium–tin oxide and organic/GeS(001) studied using photoemission spectroscopy

We present an investigation of interface properties of the hole transport material N,N′-diphenyl-N,N′-bis (3-methylphenyl)–(1,1′)–biphenyl-4,4′diamine (TPD) on indium–tin oxide (ITO) and on GeS(001) using x-ray and ultraviolet photoelectron spectroscopy. On both the TPD/ITO interface and the TPD/GeS(001) heterojunction we found shifts of the vacuum level indicating a formation of an interfacial dipole barrier. We obtained a barrier for the hole injection for TPD on Ar ion sputter-cleaned ITO of 1.5 eV which is significantly higher than the commonly cited value of 0.5 eV for ozone-oxidized ITO, whereas the interface dipole barrier of 0.3 eV is in good agreement with reported data. At low coverages of TPD on ITO additional features in the C 1s core level spectra were found, which can be ascribed to a partially oxidized TPD species at the interface.