Long Vertically Aligned TiO₂ Nanotube Electrodes Prepared via Two-Step Anodization for Highly Efficient Photovoltaics

Vertically aligned long TiO₂ nanotube (NT) electrodes with optimized structures for photovoltaic application were prepared by a two-step anodization method. The structures of the conventional TiO₂ NT electrodes prepared by one-step anodization became severely bundled and disordered for tube lengths exceeding 20 μm. On the other hand, two-step anodization resulted in uniform and bundle-free TiO₂ NT electrodes, with tube lengths of up to 38 μm. When used in dye-sensitized solar cells (DSSCs) as photoanodes, the two-step TiO₂ NT electrodes exhibited enhanced short-circuit current density (J_sc) with increasing tube length, owing to an increased roughness factor, which resulted in high dye loading. However, the dark current and open-circuit voltage decay (OCVD) curves confirmed that the electron recombination rate increased, and the electron lifetime decreased, with increasing tube length. This reduced electron lifetime resulted in sharp reductions in the open-circuit voltage (V_oc) with increasing tube length. Owing to the trade-off between the enhanced current density and the reduced electron lifetime, an optimal conversion efficiency of 4.56 % was obtained for a tube length of 29 μm.