Conductivity via Thermally Induced Gap States in a Polyoxometalate Thin Layer

We report a study of ?-[P2W18O62]6-, Wells-Dawson polyoxometalate layers deposited on ITOcoated glass substrates. A variety of techniques has been used including atomic forcemicroscopy for surface topography characterization, current mapping and current-voltagecharacteristics, X-ray photoemission spectroscopy for chemical analysis, UV-visiblephotoemission spectroscopy for determination of band line-ups and energy dispersive X-rayreflectivity for determination of layer thicknesses and scattering length densities. Theconditions of film deposition and subsequent thermal annealing strongly affect the filmcharacteristics. In particular, we show that nanostriped films a few tens of nm thick can beobtained in a reproducible manner and that such structuring is accompanied by the appearanceof gap-states and by a switch from an insulating to a conductive state. Current-voltagecharacteristics demonstrate that highly ordered films of K6[P2W18O62] allow electron flow onlyfrom ITO to [P2W18O62]6-, thus showing a rectifying effect. Finally, we integrate the POM layerinto an organic photovoltaic device and show the conduction through it thanks to favorableband alignment between ITO, the gap states and the active photovoltaic layers.