ZnO nanorods, sensitised to visible light using dyes, quantum dots, or thin films of low-bandgap semiconductors are commonly used as photoanodes in novel solar cells . They have a number of exciting properties for such an application. These include a direct conducting path for electrons, with few grain boundaries, and an increased surface area offering enhanced optical absorption. They are also able to sustain a depletion layer, the electric field this creates can effectively separate charge carriers at the nanorod/sensitiser interface, reducing recombination. However, despite these potential benefits, devices made using 1-D ZnO nanostructures are so far unable to match the performance of devices constructed using mesoporous Ti02 films. One of the reasons given for this is the presence of mid-bandgap states at the surface, which are investigated in this work. ZnO nanorods were grown using a hydrothermal growth technique. Using cyclic voltammetry and photocurrent measurements, mid-band gap trap states were identified and their position found to be centred around 0.8V below the conduction band. Visible luminescence from defect states identified by photoluminescence may be associated with these states. The effects of annealing in air at 180°C, 350°C and 450°C were investigated, all three experimental techniques showed that annealing at temperatures equal to or greater than 350°C was effective in reducing the density of these states. Annealing was also found to have a critical effect on doping density of the nanorods, with implications for both conductivity and the characteristics of the depletion layer that forms at the interface of the nanorods. The doping density was measured using a modified form of MottSchottky analysis and found to be high (> 1020cm-3) for as-grown rods, and only slightly reduced by annealing at temperatures up to 350°C. Annealing at 450°C however reduced the doping density by over two orders of magnitude. These results are consistent with previously reported evidence that incorporated hydrogen acts as a donor for ZnO. A study of the structure of alloyed CdSel-xTex quantum dots was also carried out using a combination of high resolution transmission electron microscopy, selected area electron diffraction and X-ray diffraction. This showed that both wurtzite and zinc blende QDs were present at all compositions, with no clear phase transition between different compositions. CdSe QDs were then successfully used to sensitise annealed nanorods to visible light. Electron transport within the rods was shown to be efficient. The results in this work should prove useful in understanding how best to utilise 1D ZnO nanostructures in solar cells, and provide insights into the nature of defect states which have so far limited the efficiency of such devices.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:681734 |
| Date | January 2015 |
| Creators | Parker, David |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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