Designing Nanostructured One-Dimensional TiO2 Nanotube and TiO2 Nanoparticle Multilayer Composite Film as Photoanode in Dye-Sensitized Solar Cells to Increase the Charge Collection Efficiency

A photoanode consisting of hydrothermally synthesized TiO₂ nanotubes (TNT) and TiO₂ nanoparticles (TNP) was designed for efficient charge collection in dye-sensitized solar cells. TNT and TNP films were fabricated on a conductive glass substrate by using electrophoretic deposition and doctor-blade methods, respectively. The TNP, TNT, and TNT/TNP bi-layer electrodes exhibit solar cell efficiencies of 5.3, 7.4, and 9.2%, respectively. Solar cell performance results indicate a higher short-circuit current density (J_sc) for the TNT/TNP bi-layer electrode when compared to a TNT or TNP electrode alone. The open-circuit voltages (V_oc) of TNT/TNP and TNT electrodes are comparable while the V_oc of TNP electrode is inferior to that of the TNT/TNP electrode. Fill factors of TNT/TNP, TNT, and TNP electrodes also exhibit similar behaviors. The enhanced efficiency of the TNT/TNP bi-layer electrode is found to be mainly due to the enhancement of charge collection efficiency, which is confirmed by the charge transport parameters measured by electrochemical impedance spectroscopy (EIS). EIS analyses also revealed that the TNT/TNP incurs smaller charge transport resistances and longer electron life times when compared to those of TNT or TNP electrodes alone. It was demonstrated that the TNT/TNP bi-layer electrode can possess the advantages of both rapid electron transport rate and a high light scattering effect.