Influence of grain boundaries and interfaces on the electronic structure of polycrystalline CuO thin films

CuO thin films were grown by reactive magnetron sputtering and analyzed with respect to their structural, chemical, optical, and electrical properties. The films are strongly p-type with carrier concentrations of similar to 10(20) cm(-3). Carrier accumulation at grain boundaries is evident from temperature and gas phase dependent conductivity measurements. The films show high absorption coefficients with a band gap of approximately 1.5 eV. Interface formation with TiO2 and Auwas studied by photoelectron spectroscopy. Regarding the TiO2/CuO interface, a strong band bending in the TiO2 substrate and a valence band off-set of similar to 2.4 eV were observed. The TiO2 conduction band is therefore approximately midgap of CuO at the interface. CuO is chemically reduced upon Au deposition but an ideal ohmic contact is formed with a Fermi level at the valence band maximum of CuO. Solar cell device structures were prepared of fluorine-doped SnO2/TiO2 substrates and Au back contacts, revealing open circuit voltages of <0.15V and photocurrent densities of <0.1 mA/cm(2). The photoactivity of the structures is ascribed mostly to the TiO2 substrate. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim