Nanostructured ZnO/CuO photovoltaic cell with power conversion efficiency of 2.11%.

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The effects of annealing on the stoichiometry and electronic structure of Codoped ZnO nanorods (NRs), as prepared by chemical bath deposition (CBD), were investigated by photoemission spectroscopies (XPS and UPS) and X-ray diffraction (XRD). The Co 2p core level state of the as-grown Co-doped ZnO NRs is consistent with a Co 3 O 4 stoichiometry, independent of the Co concentration. Upon annealing of the as-grown material above 865 K, a Co 2p core level satellite state forms, which is consistent with the formation of CoO. The valence band density of states also exhibits changes with annealing, where a new density of states forms between the Fermi level and 1 eV below. The new valence band states are also consistent with the formation of CoO. This change in the cobalt oxide stoichiometry from Co 3 O 4 to CoO results in Zn − due to reduced O coordination, consistent with the Auger analysis. In addition, the evolution of Co 3 O 4 to CoO is irreversible upon exposure to air. The oxidation state of Co is a key parameter in the material's performance when used for solar cells.

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