Dye-sensitized solar cells based on free-standing titanium dioxide nanotube arrays
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Dye-sensitized solar cells (DSSCs) are photoelectrochemical cells using wide band gap nanoporous oxide semiconductors sensitized by dey molecules. DSSCs generally consist of photosensitive dye molecules and a titanium dioxide (TiO 2 ) nanoparticle network. DSSCs convert light energy to electrical energy through photo-excitation of dye molecules anchored to the surface of TiO 2 nanoparticles, electron injection and transport in the TiO 2 nanoparticle network. DSSCs attracted great attention due to their respectable efficiency with very low fabrication cost, good performance under diffuse light conditions, semi-transparency and multi color range possibilities, and the ability to be fabricated on flexible substrates. Its main efficiency limiting factor is the random hopping of electrons within the TiO 2 nanoparticle network, which causes carrier trapping and recombination. The charge transport and collection can be enhanced by employing ordered nanostructures such as nanowire or nanotube arrays. However, DSSCs based on nanowire or nanotube arrays with power conversion efficiency higher than 11.18% achieved from the conventional DSSCs have yet to be demonstrated. This dissertation focuses on the development of DSSCs using highly crystalline free-standing TiO 2 nanotube (FSTNT) arrays to enhance charge transport and collection, and hence, power conversion efficiency. TiO 2 nanotube arrays were obtained by potentiostatic anodization of titanium foil in fluoride-based ethylene glycol electrolyte. TiO 2 nanotube arrays were detached from the titanium foil by chemical etching and annealed at high temperatures to obtain highly crystalline anatase phase FSTNT arrays. DSSCs based on FSTNT arrays revealed high power conversion efficiency of 13.2% and short-circuit photocurrent density comparable to that of monocrystalline silicon solar cell.