Experimental determination of band offsets of NiO-based thin film heterojunctions

The energy band diagrams of NiO-based solar cell structures that use various n-type oxide semiconductors such as ZnO, Mg0.3Zn0.7O, Zn0.5Sn0.5O, In2O3:Sn (ITO), SnO2, and TiO2 were evaluated by photoelectron yield spectroscopy. The valence band discontinuities were estimated to be 1.6 eV for ZnO/NiO and Mg0.3Zn0.7O/NiO, 1.7 eV for Zn0.5Sn0.5O/NiO and ITO/NiO, and 1.8 eV for SnO2/NiO and TiO2/NiO heterojunctions. By using the valence band discontinuity values and corresponding energy bandgaps of the layers, energy band diagrams were developed. Judging from the band diagram, an appropriate solar cell consisting of p-type NiO and n-type ZnO layers was deposited on ITO, and a slight but noticeable photovoltaic effect was obtained with an open circuit voltage (Voc) of 0.96 V, short circuit current density (Jsc) of 2.2 μA/cm2, and fill factor of 0.44.