Dye-Sensitized Solar Cells Based on TiO₂ Nanotube and Shelled Arrayed Structures

Anatase TiO₂ nanostructure arrays were synthetized starting from a template made of self-standing ZnO NWs prepared by an electrodeposition technique. By controlling the liquid phase deposition step, the obtained structures could be varied from free-standing nanotube (NT) arrays with controlled morphology to hierarchical spiky radiating core-shell rods. The nanotubes were made of assembled nanocrystals with an average size of 7-8 nm. The structures were investigated as n-type layers in DSSCs. The efficiency was enhanced for the core-shell layer and by starting with longer initial ZnO NW templates. The limitation of the cell efficiency was shown related to the specific surface area and dye loading. The cell functioning was in-depth investigated by electrochemical impedance spectroscopy over a large applied voltage range and compared to a cell based on a nanoparticle TO₂ mesoporous layer. A slow recombination rate was found. The enhancement of electron transport with nanocrystallite size explained the conductivity results. We also found that the prepared structures presented a high charge collection efficiency.