Carrier transport in dye-sensitized solar cells using single crystalline TiO₂ nanorods grown by a microwave-assisted hydrothermal reaction

Single crystalline rutile nanorod was grown directly on top of fluorine-doped tin oxide (FTO) substrate via a microwave assisted hydrothermal reaction which dramatically increased a growth rate over a conventional hydrothermal method. In addition, the introduction of thin TiO₂ seed layer to FTO substrates promotes heterogeneous nucleation and increases the density. Dye-sensitized solar cells (DSSCs) were fabricated using the rutile nanorods that were differently treated with TiCl₄ solution and the carrier transport mechanism in the nanorod-based DSSCs was systematically examined. When the nanorods were treated with TiCl₄, more dye was adsorbed on the TiO₂ films and the energy conversion efficiency increased to 3.7% for a 2.5 μm thick TiO₂ film. Stepped light induced-transient measurement of photocurrent and voltage measurements showed that the role of the nanorods in DSSCs is to increase an electron diffusion coefficient in TiO₂ mesoporous films. In contrast to the diffusion coefficient, the lifetime of electron is not dependent on the presence of the nanorods. To explain the experimental observations, we propose a surface diffusion model for electrons that are injected into the rutile nanorods from dye molecules. This surface diffusion may originate from the high crystallinity of nanorods and the homogeneous contact between nanorod and coated nanoparticle layer.