Conductivity via Thermally Induced Gap-States in a Polyoxometalate Thin Layer

We report a study of alpha-[P2W18O62](6-), Wells-Dawson polyoxometalate (POM) layers deposited on indium tin oxide (ITO)-coated glass substrates. A variety of techniques have been used including atomic force microscopy for surface topography characterization, current mapping, and current-voltage characteristics, X-ray photo emission spectroscopy for chemical analysis, UV-visible photoemission spectroscopy for determination of band line-ups, and energy dispersive X-ray reflectivity for determination of layer thicknesses and scattering length densities. The conditions of film deposition and subsequent thermal annealing strongly affect the film characteristics. In particular, we show that nanostriped films a few tens of rim thick can be obtained in a reproducible manner and that such structuring is accompanied by the appearance of gap states and by a switch from an insulating to a conductive state. Current-voltage characteristics demonstrate that highly ordered films of K-6[P2W18O62] allow electron flow only from ITO to [P2W18O62](6-), thus showing a rectifying effect. Finally, we integrate the POM layer into an organic photovoltaic device and show the conduction through it thanks to favorable band alignment between ITO, the gap states, and the active photovoltaic layers.