Enhanced efficiency in dye-sensitized solar cells based on TiO2 nanocrystal/nanotube double-layered films

A new bilayer-structured film with TiO₂ nanocrystals as underlayer and TiO₂ nanotubes as overlayer was fabricated. The resultant double-layer TiO₂ (DL-TiO₂) film could significantly improve the efficiency of dye-sensitized solar cells (DSSCs) owing to its synergic effects, i.e. effective dye adsorption mainly originated from TiO₂ nanocrystal layer and rapid electron transport in one-dimensional TiO₂ nanotube layer. The overall energy-conversion efficiency (η) of 6.15% was achieved by the formation of DL-TiO₂ film, which is 44.7% higher than that formed by pure nanocrystalline TiO₂ (NC-TiO₂) film and far larger than that formed by nanotube TiO₂ (NT-TiO₂) film (η = 0.37%). The charge recombination behavior of cells was investigated by electrochemical impedance spectra, and the results showed that DL-TiO₂ film-based cell possessed the lowest transfer resistance and the longest electron lifetime. The incident-photon-to-current efficiency spectra indicated that the broad bands covered almost the entire visible spectrum from 400 to 700 nm with the maxima of 57.3%, 40.3%, and 2.2% at a wavelength of ∼530 nm for DL-TiO₂-, NC-TiO₂-, and NT-TiO₂-based solar cells, respectively. It is expected that the double-layer film electrode can be extended to other composite films with different layer structures and morphologies for enhancing the efficiencies of DSSCs.