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 TiO 2 nanotube (NT) arrays covered with various layers of few-nanometer-sized TiO 2 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 TiO 2 NPs directly connected with TiO 2 NTs (e.g., isolated and single-layer NPs on the surface of NTs) dominantly traveled to the NTs. With an increasing number of TiO 2 NP layers, photoinduced electrons are randomly transported through the TiO 2 NP layers. Enhanced light harvesting and efficient charge collection (∼95%) caused by the increased amounts of dye loading and the direct transport through the NT, respectively, are achieved in a structure with ∼1.4 layers of few-nanometer-sized TiO 2 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 (∼26 mA/cm 2 ) of a N719-based dye-sensitized solar cell.