Zn-Ni-Co-O wide-band-gap p-type conductive oxides with high work functions

Co3O4-based spinels are a new class of wide-band-gap p-type conductive oxides with high work functions. We examined the structures, conductivities, work functions, and optical spectra of quaternary Zn-Ni-Co-O thin films across the entire spinel region of the ZnO-NiO-Co3O4 diagram using a high-throughput combinatorial approach. We found that the conductivity of as-deposited films is maximized (100 S/cm) and optical absorption (at 1.8 eV) is minimized in different regions of the diagram, while the work function of annealed films is high and relatively constant (5.8 +/- 0.1 eV). These properties made Zn-Ni-Co-O thin films applicable as p-type interlayers in solar cells. As an example, amorphous Zn-Co-O hole transport layers had good performance in bulk heterojunction organic photovoltaic devices.