Evolution of the Stoichiometry and Electronic Structure of Cobalt Oxide in Thermally Treated Co-Doped ZnO Nanorods for Solar Cells

Abstract

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.

Enhancement in the performance of nanostructured CuO–ZnO solar cells by band alignment

“…The tunneling process mediated by defect densities could cause shunting of the device and result in a lower reverse breakdown voltage. 39 This type of behavior can be seen in Fig. 9 with sizeable reverse leakage current and low photocurrent.…”

“…For 10%, 15%, and 20% cobalt doping, we observe an additional spinel ZnCo 2 O 4 phase, which is a typically found spinel structure with Zn ++ ions in the tetrahedral sites and Co 3+ occupying the octahedral sites. In our previous studies, 39 we have discussed the detail Rietveld analysis of these nanomaterials and found that the percentage of the wurtzite ZnO phase in samples decreases and the secondary phase of ZnCo 2 O 4 increases with excessive cobalt concentration. However, for 10% cobalt doped samples, the existence of the ZnCo 2 O 4 secondary phase is low (<5%) in comparison to other higher doped samples.…”

“…The detailed process of fabricating ZnO and cobalt doped ZnO nanorods using chemical bath deposition (CBD) was discussed in detail in our previous studies. [35][36][37][38][39][40][41] For CuO nanostructures, the growth mechanism was a modied chemical bath deposition (CBD) method in which equimolar 0.1 M of copper nitrate trihydrate (Cu(NO) 3 $3H 2 O) and hexamethylenetetramine (C 6 H 12 N 4 ) was dissolved in deionized water under constant stirring. Few drops of ammonium hydroxide solution (30-33% NH 3 in H 2 O) was added to the resulting precursor solution until the pH reached 8.…”

ZnO–ferromagnetic metal vertically aligned nanocomposite thin films for magnetic, optical and acoustic metamaterials

“…, Co diffuses easily into ZnO to form phases of Co-doped ZnO. 25–28 The Co pillars were observed to grow substantially larger than those in the Ni-doped ZnO, with an average height and width of 134.0 ± 2.4 nm and 31.5 ± 4.6 nm, respectively, in the 500 °C sample. The microstructure STEM cross-section image of the ZnO–Co composite grown at 700 °C is shown in Fig.…”

Spectrally selective coating of nanoparticles (Co3O4:Cr2O3) incorporated in carbon to captivate solar energy

“…The nanocomposite was used in the electronic sensor . On the other hand, Co 3 O 4 was doped with manganese (Mn) to produce a unique mesoporous by a one‐stage hydrothermal process, which used an electrode of supercapacitors.…”