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

Abstract

Using photoemission spectroscopy, we show that the surface electronic structure of VO 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 VO 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 VO 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.

Oxygen Vacancies Allow Tuning the Work Function of Vanadium Dioxide

“…Notably, a finite DOS at the Fermi level ( E F ) is visible in the spectra after the ASA process (Figure c) although the measurements were performed at room temperature and thus in the insulating state of VO 2 . However, the DOS at E F and the pronounced increase of the V 3d‐derived band intensity is not due to the phase transition of VO 2 , but due to the presence of oxygen vacancies as evidenced by the high abundance of lower oxidation state V (Figure ). In contrast, for stoichiometric VO 2 after ASAO process the intensity at E F is not visible and V 3d band appears as well‐defined peak, similar to previous VO 2 single crystal[23b,25] and thin film results.…”

“…The transition temperature was 335 K. More details about the VO 2 thin film fabrication can be found in refs. and .…”

“…[2a] The transition temperature can be altered over a wide range by substitutional doping and external strain, etc . Due to these remarkable features, VO 2 has in addition to its potential use as electrode, applications such as optical switching devices, memory devices, and smart windows…”

“…Thus, VO 2 has not been considered as a suitable hole injecting electrode as for related hole injecting transition metal oxides, the work function is usually >6.0 eV . However, recently we could show that the work function of VO 2 can reach up to 6.70 eV and, moreover, even in the insulating state at RT, the conductivity is sufficient to allow for adequate charge transport . VO 2 is yet a complex material and although the MIT was discovered already in 1959,[2a] it is still under debate whether it is induced by Mott–Hubbard correlation or due to a structural Peierls‐type transition, or both .…”

“…In this work, we used pulsed laser deposition (PLD) to fabricate 120 nm thick VO 2 epitaxial films on α‐Al 2 O 3 substrates . After different sputtering and annealing procedures, with and without low‐pressure oxygen atmosphere (during annealing), UPS and XPS measurements provide the desired correlation of surface composition, valence electronic structure, and work function, dependent on the treatment conditions.…”