Fabrication and Characterization of Anodic Titanium Oxide Nanotube Arrays of Controlled Length for Highly Efficient Dye-Sensitized Solar Cells

The performance of dye-sensitized solar cells (DSSC), made of highly ordered anodic titanium oxide (ATO) nanotube (NT) arrays produced directly on Ti foil, depends on the length of these arrays. We controlled these lengths L from 4 to 41 μm while varying the concentration (0.1, 0.25, 0.5, and 0.8 wt %) of the electrolyte (NH 4 F) in ethylene glycol in the presence of H 2 O (2 vol %) with anodization for various periods ( t = 0.5−8 h). The compact and bundle layers introduced during anodization were effectively removed upon simple ultrasonic cleaning in deionized water containing submicrometer particles of Al 2 O 3 in a small proportion. The photovoltaic performance of the NT-DSSC devices (NH 4 F at 0.5 wt %) made of ATO films, as prepared, increased from 3.0% to 5.2% as L was increased from 6 μm ( t = 0.5 h) to 30 μm ( t = 8 h). After treatment of the ATO films with TiCl 4 in a two-step annealing process, the optimized NT-DSSC device attained an overall efficiency of 7.0% power conversion.