Nanocrystalline VO2 thin films were deposited onto glass slides by direct current magnetron sputtering and postoxidation. These films undergo semiconductor-metal transition at 70°C, accompanied by a resistance drop of two magnitude orders. The crystal structures and surface morphologies of the VO2 films were characterized by x-ray diffraction (XRD) and atomic force microscope (AFM), respectively. Results reveal that the average grain size of VO2 nanograins measured by XRD is smaller than those measured by AFM. In addition, Raman characterization indicates that stoichiometric VO2 and oxygen-rich VO2 phases coexist in the films, which is supported by x-ray photoelectron spectroscopy (XPS) results. Finally, the optical properties of the VO2 films in UV-visible range were also evaluated. The optical band gap corresponding to 2p-3d inter-band transition was deduced according to the transmission and reflection spectra. And the deduced value, E opt2p-3d = 1.81 eV, is in good agreement with that previously obtained by theoretical calculation.
We present the current-voltage characteristics of organic solar cells based on single and double heterojunction of copper phthalocyanine (CuPc) and C60 by introducing a constant JP instead of photo-current density Jph to represent the density of polaron-pairs generated from excitons at D/A interface. A diode Dext models polaron-pair dissociation, and a diode Drec stands for loss due to polaron-pair recombination. The photovoltaic response under AM 1.5 solar illumination at an intensity of 100 mW/cm2 is parameterized and modeled using the improved equivalent circuit model developed for inorganic pn-junction solar cells. The instinct mechanisms including dissociation, recombination of polaron-pairs and charge carrier collection process are explained by introducing Jph/JP as the dissociation rate of polaron-pairs and |J|/Jph as charge carrier application efficiency ηCA. Especially, we reveal the optimization mechanism for the fill factor FF and series resistance RS of organic solar cells.
