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 fluorinedoped 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 mu m 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.