Synthesis of Doped and Undoped ZnO Nanostructures via Chemical Vapor Deposition for Applications in Dye-Sensitized Solar Cells

Nanostructure synthesis of wide band gap transparent conductive oxides (TCO) has been garnering significant interest in optoelectronics. They have different characteristics than their bulk counterparts, particularly larger surface area to volume ratio, leading to increased performance. Zinc oxide is a wide band gap TCO that is non-toxic, cheap, and has impressive electron mobility. The effects of Sb doping and Cu doping on ZnO nanowires grown by a novel chemical vapor deposition method that uses an inner reaction tube to promote uniformity and scalability were investigated. Undoped ZnO nanowires were first synthesized and grown in the wurtzite crystal phase. They had a diameter of ~130 nm and expressed a lower bandgap than bulk ZnO. The undoped nanowires expressed oxygen-vacancy (VO ) defects as well as evidence of additional defects including zinc vacancies (VZn) that may be responsible for p-type conductivity. The Sb-doped ZnO nanostructures were much larger in diameter being 289 nm and 306 nm for the two separate oxidation states of the antimony precursor. X-ray diffraction (XRD) showed decreased crystallinity with slight peak shifts, which could be a direct result of Sb doping. Photoluminescence data showed a decrease in the bandgap and electron-hole (e--h+) lifetime. Similar defects were expressed in Sb(III)-doped ZnO as the undoped nanowires, but Sb(V)-doped ZnO showed much less evidence of VO. Cu-doped ZnO had a decrease in diameter to 80 nm. XRD showed no shifts in the Bragg angles, while energy dispersive Xray spectroscopy (EDS) showed 1.11% - 1.71% incorporation of Cu into the ZnO crystal lattice. A decrease in the bandgap and an increase in e--h+ lifetime were determined from photoluminescence data. There is also evidence of a significant decrease in VO defects. The undoped ZnO nanowires were used as a working electrode for a dye-sensitized solar cell. Initial experiments did not show any significant power conversion efficiencies for the developed cells, which could be due to several factors. However, continuing efforts are being made to develop “functioning” dye-sensitized solar cells using metal oxide nanostructured electrodes prepared in this project.