Stoichiometric and Oxygen-Deficient VO2 as Versatile Hole Injection Electrode for Organic Semiconductors

Using photoemission spectroscopy, we show that the surface electronic structure of VO2 is determined by the temperature-dependent metal insulator phase transition and the density of oxygen vacancies, which depends on the temperature and ultrahigh vacuum (UHV) conditions. The atomically clean and stoichiometric VO2 surface is insulating at room temperature and features an ultrahigh work function of up to 6.7 eV. Heating in UHV just above the phase transition temperature induces the expected metallic phase, which goes in hand with the formation of oxygen defects (up to 6% in this study), but a high work function >6 eV is maintained. To demonstrate the suitability of VO2 as hole injection contact for organic semiconductors, we investigated the energy-level alignment with the prototypical organic hole transport material N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB). Evidence for strong Fermi-level pinning and the associated energy-level bending in NPB is found, rendering an Ohmic contact for holes.