Enhancing the performance of front-illuminated dye-sensitized solar cells with highly [001] oriented, single-crystal-like TiO2 nanotube arrays

Highly ordered, vertically aligned TiO2 nanotube arrays (TNAs) have been prepared by a modified two-step anodic oxidation. The second as-prepared TNAs show smoother surface, fewer cracks and more uniform nanotubes on large scale compared with the first as-grown one. With different content of water in anodizing electrolyte, highly oriented TNAs and arbitrarily oriented TNAs were obtained. The narrow dominant peak of (004) of XRD shows that the TNAs are highly crystallized. Moreover, the interplanar distance of 0.237 nm, the marked angle in fast Fourier transform of 68.3° and the diffraction fringe on the margin of tube wall by high-resolution transmission electron microscopy demonstrate that the highly oriented TNAs are single-crystal-like anatase in nature with highly exposed (001) facets which grow along the [001] direction. The TNAs have been detached and transferred to Fluorine-doped Tin Oxide substrate successfully as transparent photo-anodes for assembling dye-sensitized solar cells. Under front-side illumination, DSSCs based on highly [001] oriented TNAs show a remarkable enhancement of 29% in power conversion efficiency and faster electron transport compared to that of arbitrarily oriented TNAs. The influence of the thickness on the performance of the DSSCs based on highly [001] oriented TNAs is investigated and the best power conversion efficiency of 5.73% is achieved.