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.