Oriented Single-Crystalline Rutile TiO2 Nanorods On Transparent Conducting Substrates for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) made from oriented, one-dimensional (1D) semiconductor nanostructures such as nanorods, nanowires, and nanotubes are receiving wide attention because direct connection of the point of photogeneration with the collection electrode using such 1D structures may improve the cell performance. Specifically, oriented single-crystalline TiO2 nanorods or nanowires on a transparent conductive substrate would be most desirable, but achieving these structures has been limited by the availability of synthetic techniques. In this presentation, we describe a facile hydrothermal method to grow oriented, single-crystalline rutile TiO2 nanorod films on transparent conductive fluorine-doped tin dioxide (FTO) substrates. The diameter, length, and density of the nanorods could be varied by changing the growth parameters such as growth time, growth temperature, initial reactant concentration, acidity and additives. The epitaxial relation between the FTO substrate and rutile TiO2 with a small lattice mismatch plays a key role in driving the nucleation and the subsequent growth of the rutile TiO2 nanorods. DSSCs were assembled using the TiO2 nanorods grown on FTO as the photoanode. A light-to-electricity conversion efficiency of 3% could be achieved by using 4 mm-long TiCl4-treated TiO2 nanorod films. We further compared the electron transport and recombination rates in these single-crystal rutile TiO2 nanorods with TiO2 nanoparticle films of similar thickness. Surprisingly, we find that the electron transport time constant is approximately the same and even slightly slower in single-crystal rutile TiO2 nanorods than in TiO2 nanoparticle films, suggesting that electron diffusion rate is still determined by the residence time in surface traps even in single-crystal TiO2 nanorods.