General Strategy for Fabricating Transparent TiO2 Nanotube Arrays for Dye-Sensitized Photoelectrodes: Illumination Geometry and Transport Properties

We report on the preparation of transparent oriented titania nanotube (NT) photoelectrodes and the effect of illumination direction on light harvesting, electron transport, and recombination in dye-sensitized solar cells (DSSCs) incorporating these electrodes. High solar conversion efficiency requires that the incident light enters the cell from the photoelectrode side. However, it has been synthetically challenging to prepare transparent TiO₂ NT electrodes by directly anodizing Ti metal films on transparent conducting oxide (TCO) substrates because of the difficulties of controlling the synthetic conditions. We describe a general synthetic strategy for fabricating transparent TiO₂ NT films on TCO substrates. With the aid of a conducting Nb-doped TiO₂ (NTO) layer between the Ti film and TCO substrate, the Ti film was anodized completely without degrading the TCO. The NTO layer was found to protect the TCO from degradation through a self-terminating mechanism by arresting the electric field-assisted dissolution process at the NT-NTO interface. The illumination direction and wavelength of the light incident on the DSSCs were shown to strongly influence the incident photon-to-current conversion efficiency, light-harvesting, and charge-collection properties, which, in turn, affect the photocurrent density, photovoltage, and solar energy conversion efficiency. Effects of NT film thickness on the properties and performance of DSSCs were also examined. Illuminating the cell from the photoelectrode substantially increased the conversion efficiency compared with illuminating it from the counter-electrode side.