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Growth of Polymer/TiO2 Nanostructures by Layer-by-Layer Self Assembly Method as Electrodes of Photoelectrochemical Cells

Rahman, Masoud | 2011

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 41975 (48)
  4. University: Sharif University of Technology
  5. Department: Institute for Nanoscience and Nanotechnology
  6. Advisor(s): Taghavinia, Nima; Shahrokhian, Saeed
  7. Abstract:
  8. Dye sensitized solar cells (DSCs) are among the 3rd generation solar cells which their mechanism of light absorption is similar to chlorophyll in green leafs and is based on photo-excitation of dyes. The electron-hole pairs of photo-excited dye separate at the interface of mesoporous semiconductor-dye-electrolyte. One of the approaches to increase the light path length and therefore the light harvesting efficiency in DSC is the addition of light scatterers inside the mesoporous semiconductor (photoanode).In this research, nanoparticulated hollow TiO2 fibers are introduced as novel light scatterers in DSCs with simultaneous light scattering, light trapping and high surface area. Light scattering of these hollow fiber were modeled analytically and it was found that maximum of external light scattering occurs for fibers with diameters around 500-1000 nm and wall thickness of around 150-200 nm. Both the wavelength of incident light and its polarization affect the scattering. However, by increasing the diameter of fiber the sensitivity of scattering to polarization decreases. A weighted-average based on solar radiation spectrum (Air mass 1.5) was proposed to remove the dependence of scattering to the wavelength.Layer-by-layer self assembly method (LbL) which is based on alternative adsorption of a positively charged polyelectrolyte and negatively charge TiO2 nanoparticles were employed for preparation of hollow fibers. Cellulose fibers were chosen as removable templates for deposition of TiO2 nanoparticles. The LbL parameters such as the dip-time, deposition trend versus number of dip-cycles and molecular weight of polyelectrolyte were investigated on quartz sheets. A simple method for determination of the quantity of adsorbed polyelectrolyte and nanoparticles based on UV-Visible method was proposed. It was noticed that the saturation time is around 10 min however; the dip-time of 3-5 would be the optimized condition. It was also noticed that in the case of short molecular weight polyelectrolyte the deposition decrease by around 25% and the increase of dip-time change the deposition mode from 2-dimentional to 3-dimensional.LbL preparation of hollow fibers showed that low molecular weight polyelectrolyte can better penetrate the cellulose fibers and hence increases the deposition. Two dispersions of TiO2 nanoparticles with different size and crystallinity were investigated and it was noticed that the presence of active peroxotitanium complexes on the surface of the nanoparticles is vital for preservation of uniformity of fibers upon thermal removal of template.The scattering coefficient of fibers was measured by diffuse reflection method and in comparison with P25 commercial nanoparticles showed around six times improvement. Composites of fiber-P25 were prepared by electrophoretic method with various weight percentages of fibers and were employed as the photoanode of DSCs. The maximum conversion efficiency (around 65% enhancement) was obtained by addition of 15-20 wt% of fibers in the electrophoresis bath. It was noticed that addition of fibers decreased the cracks of the electrophoresis film which occur during the evaporation of the solvent. It was shown by electrochemical impedance spectroscopy (EIS) that the crack reduction is concurrent with the decrease of layer-substrate delamination. The increase of life time by addition of fibers was related to the decrease of electron traps inside the fibers
  9. Keywords:
  10. Polyelectrolyte ; Titanium Oxide ; Nanoparticles ; Dye Sensitized Solar Cells ; Photoelectrochemical Cell ; Light Scattering ; Layer-by-Layer Self Assembly Method

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