A Competitive Electron Transport Mechanism in Hierarchical Homogeneous Hybrid Structures Composed of TiO2 Nanoparticles and Nanotubes

We prepared well-defined hierarchical structures comprising doubly open-ended TiO2 nanotube (NT) arrays covered with various layers of few-nanometer-sized TiO2 nanoparticles (NPs) to investigate the electron collection mechanisms in homogeneous hybrid structures. We found that competitive electron transport pathways (direct transport through the NT and randomized transport through the NPs) are present in the homogeneous hybrid structures. Photoinduced electrons generated at the few-nanometer-sized TiO2 NPs directly connected with TiO2 NTs (e.g., isolated and single-layer NPs on the surface of NTs) dominantly traveled to the NTs. With an increasing number of TiO2 NP layers, photoinduced electrons are randomly transported through the TiO2 NP layers. Enhanced light harvesting and efficient charge collection (similar to 95%) caused by the increased amounts of dye loading and the direct transport through the NT, respectively, are achieved in a structure with similar to 1.4 layers of few-nanometer-sized TiO2 NPs, resulting in a power conversion efficiency of 11.3% with a J(SC) value (22.9 mA/cm(2)) close to the theoretical value (similar to 26 mA/cm(2)) of a N719-based dye-sensitized solar cell.