VO₂ As A High Work Function Hole Injection Electrode For Organic Semiconductors

Vanadium dioxide?VO₂?is a rather unique transition metal oxide as it exhibits a reversible first order metal-insulator transition?MI?from an insulating phase with monoclinic structure to a metallic phase with tetragonal rutile structure.The phase transition occurs at around 340 K and is accompanied by a dramatic change in the electrical conductivity as high as five orders of magnitude and optical variations in the infrared region.The energy gap in the insulator phase is 0.6 eV and monoclinic VO₂ can thus be regarded as a semiconductor from an application point of view.Meanwhile,the transition temperature can be altered over a wide range,even below the room temperature by incorporating ion doping or external strain.Due to these remarkable features VO₂ has a promising potential in the practical applications such as optical switching devices,memory devices and smart windows.For some metal oxides it is known that oxygen vacancies can also influence the work function,which can reach up to 6.9 eV for materials like MoO_x and WO_x.Such high work function materials are thus excellent hole injecting electrodes in organic electronic devices,as Fermi level pinning at the highest occupied molecular orbital?HOMO?level manifold of the organic semiconductor can enable ohmic contacts.For VO₂ the work function is also influenced by oxygen vacancies,but reported work function values are below 5.7 eV and thus little attention was paid to VO₂ as hole injecting electrodes for organic semiconductors.We used pulsed laser deposition method to fabricate 117 nm VO₂ thin film on?-Al₂O₃ substrate.We show with photoemission spectroscopy that the surface electronic structure of VO₂ is determined by the temperature dependent MI transition and the density of oxygen vacancies,which depends on temperature as well under ultrahigh vacuum?UHV?conditions.The atomically clean and stoichiometric VO₂ surface is insulating and features an ultrahigh work function of up to 6.7 e V.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 prototypicalorganicholetransportmaterial 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.After elaborately designed sputtering and annealing with oxygen or without oxygen procedures,ultraviolet photoemission spectroscopy?UPS?and X-ray photoemission spectroscopy?XPS?measurements were conducted to gain electronic structure of VO₂ thin film,which provided basic information for correlating the shifts of core levels,valence band evolution and work function changes.The impact of the atomic ratio between oxygen and vanadium?O/V?on valence band feature and work function has been revealed.The evolution of valence bands,core levels and work function of VO₂ thin films upon common sputtering and annealing with or without residual O₂ atmosphere procedures was monitored by UPS and XPS.Sputtering and subsequent annealing in vacuum introduce lower oxidation state V species,leading to an increased density of V3 d derived bands close to the Fermi level.However,annealing under O₂ results in stoichiometric VO₂ with a high work function of up to 6.7 eV and the work function of the VO₂ is dramatically affected by the O/V ratio on the surface.