TiO ₂ nanostructures by electrochemical anodization for dye-sensitized solar cells

Highly ordered TiO ₂ nanotube (NT) array can be formed by electrochemical anodic oxidation process in fluoride-containing electrolyte. The NT length, diameter, and wall thickness can be tailored by controlling the anodization process. Its facile fabrication, relatively large surface area, and especially the 1-D charge carrier transport property of the tubular geometry, have drawn intensive interest in using NT array for TiO ₂ photovoltaic and photocatalysis applications. In this article, we briefly review our recent works in the study of TiO ₂ NT array fabrication by anodic oxidation, and their application for dye-sensitized solar cells. The effects of anodic voltage and fluorine concentration in the electrolyte were investigated to achieve highly ordered TiO ₂ NTs. With controlled tubular pore diameter, TiO ₂ NTs of different tube length were obtained by varying the growth time. Dye sensitized solar cells were subsequently fabricated using NTs and current-voltage characteristics were tested for comparison. It was found that photoelectrons in NTs has long diffusion length for charge collection, and the overall energy conversion efficiency is dependent on the tube length due to limited dye loading. To further improve the efficiency through enhanced light harvesting, mixed NT and nanoparticle (NP) structures were studied by embedding NPs into nanotubes through infiltration process. The mixed structures exploit the combined benefits of large surface area of NPs for high dye-loading and light harvesting, and the fast electron transport in the 1-D NTs for high electron collection efficiency. The result is improved solar energy conversion efficiency. The kinetics of charge transfer and recombination was also investigated via electrochemical impedance spectroscopy to determine the efficiency limited by charge collection or by light harvesting.