over a wide range by substitutional doping and external strain, etc. [3] Due to these remarkable features, VO 2 has in addition to its potential use as electrode, [4] applications such as optical switching devices, [5] memory devices, [6] and smart windows. [7] At room temperature (RT), undoped VO 2 is in the insulating state and for most VO 2 thin films the work function is moderately high (up to 5.55 eV). [8] 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. [9] 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. [4] 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. [10] Consequently, also the underlying reasons that enable achieving high work function are largely unexplored. It can be speculated that for VO 2 , as for other transition metal oxides, the work function is related to the stoichiometry. [11] Vanadium, with an electronic configuration of [Ar]3d 3 4s 2 , can form multivalent oxides with different valence states, like VO, V 2 O 5 , V 2 O 3 , V 6 O 13 , and VO 2 . [12] It is thus essential to have a comprehensive understanding of the electronic properties of an actual surface in order to use it in practical devices. [13] The evolution of electron valence bands, core levels, and work function of vanadium dioxide (VO 2 ) thin films upon argon ion sputtering and annealing, as commonly done to obtain atomically clean surfaces, with and without low-pressure oxygen atmosphere during annealing is investigated by ultraviolet and X-ray photoemission spectroscopy. Both sputtering and annealing in vacuum introduce lower oxidation state V species, leading to an increased intensity of V 3d derived bands close to the Fermi level. Such oxygen deficient surfaces exhibit low work function values as low as 4.40 eV. Annealing the sample under low-pressure oxygen atmosphere (few times 10 −4 mbar partial O 2 pressure) results in stoichiometric VO 2 surfaces with a high work function of up to 6.70 eV. Moreover, the work function of the VO 2 can be continuously tuned between the high and low limits by adjusting the atomic ratio of oxygen and vanadium at the surface. Appropriately adjusted VO 2 can thus be employed as moderate electron as well as superior hole injecting electrode material in electronic and optoelectronic devices. Figure 6. Correlation of work function and atomic ratio (O to V). www.advancedsciencenews.com

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.

CH3NH3PbI3 thin films are fabricated using several representative synthesis methods such as spin‐coating, evaporation, and a combination of the two. These methods, which frequently occur in reported literatures, use the same precursors PbI2 and CH3NH3I but differ in how the two are mixed. It is found that the latter plays a vital role in determining the surface morphology, composition, and grain size of the films, even when the same stoichiometric ratio of the precursors is used. X‐ray photoelectron spectroscopy reveals that the amount of CH3+‐type defects, which results from CH3NH3I dissociation, is sensitive to both the physical state of CH3NH3I and the order of mixing sequence. The variation of the CH3NH3+:CH3+ ratio also affects the valence band and the work function of the corresponding films, as revealed by ultraviolet photoelectron spectroscopy. Furthermore, the energy‐level alignment between the perovskite film and a model hole transport layer, N,N′‐di(1‐naphthyl)‐N,N′‐diphenylbenzidine (NPB) is examined. It is found that the CH3NH3+:CH3+ ratio correlates with the offsets between the valence band maximum of perovskite film and the highest occupied molecular orbital of NPB as well, and the energy‐level alignment with the dual‐source, coevaporated CH3NH3PbI3 film is most suitable for efficient hole transport.