Improved Routes to Nanocrystalline Metal Oxide Films for Dye-Sensitised Solar Cells and Related Applications
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Improved Routes to Nanocrystalline Metal Oxide Films for Dye-Sensitised Solar Cells and Related Applications
Iain P. O’Hare, Kuvasani Govender and Paul O’Brien The University of Manchester, Department of Chemistry and the Materials Science Centre, Oxford Road, Manchester, M13 9PL, U.K. ABSTRACT
Nanoporous metal oxide thin films are currently attracting interest for a wide range of electronic applications, including sensors and dye-sensitised photovoltaic cells. However, limited, and poorly controlled, film fabrication routes represent a key factor impeding the development of such devices. To date, device applications have largely been limited to sol-gelfabricated nanocrystalline films of titanium dioxide (TiO2). Such studies have recently been extended to the application of an alternative film fabrication technique, notably that of chemical bath deposition (CBD), for the growth of zinc oxide (ZnO). One interesting feature of CBDfabricated films of ZnO is that, under specific conditions of supersaturation, highly reticulated layers may be obtained, such an observation suggesting that control of morphology is possible. Thick nanoporous films of ZnO have been deposited, upon conducting glass substrates, under both acidic and alkaline conditions, from a solution containing the metal ion, added acid or base, and either a chelating agent, such as ethylenediamine or triethanolamine, or a buffer, hexamethylenetetraamine (HMT). The deposition rate is controlled by systematic adjustment of both temperature and pH, together with the nature, and relative concentration, of the reactants in the solution (chelating agent or metal ion). The material properties of the resulting films have been characterised through the use of a range of techniques, including Scanning Electron Microscopy (SEM), X-Ray Powder Diffraction (XRD) and Energy Dispersive Analysis by XRays (EDAX), and the results are discussed within the context of the suitability of the deposited films for incorporation within nanocrystalline devices.
INTRODUCTION
To date, research on mesoporous, nanocrystalline metal oxide films has focused primarily upon films of titanium dioxide (TiO2). Thin films of this wide band gap material retain favorable conduction band energetics, and exhibit excellent stability. Nanocrystalline, thin, porous, optically transparent films of TiO2, composed of densely-packed nanometre-sized crystallites (diameter 5-50 nanometres (nm)), may have a surface area up to three orders of magnitude greater than that of the flat surface. Sintering of the prepared layers, at temperatures within the range of 723-823 K, results in electrical contact between neighbouring particles, thus allowing current to be transported. The fabrication of such films, by sol-gel methods, has led to considerable growth in studies of both the fundamental properties of, and potential applications for, this type of material. The development of photoelectrochemical (PEC) solar cells, first reported by Grätzel et al. H8.14.1
[1], has sought to utilise the inherent properties of
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