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Fast charge transport of titania nanotube arrays in dye-sensitized solar cells

Abstract Vertically oriented nanotubular titania arrays with a length of ca. 12 μm were fabricated by anodisation of titanium foils. Rietveld refinements of grazing incidence and wide angle XRD patterns for the annealed nanotube titania array showed that they consist of anatase crystals with a pronounced size anisotropy and a moderate preferred orientation. The energy conversion efficiency of dye sensitized solar cells based on the annealed nanotubular anatase array exceeds 6% under standard AM1.5 100 mW/cm2 irradiance and thus outperforms solar cells based on anatase nanoparticles when comparing analogous back illumination cell designs. Electrochemical impedance spectroscopy was employed to clarify the origin of the improved short circuit current and conversion efficiency. It was found that the effective diffusion length in the nanotube cells reaches >100 μm and no longer limits the onversion efficiency. The faster electron transport can be traced back to the larger crystallite size and the reduced intergrain orientation mismatch in the titania nanotubes.