Double-layer composite film based on sponge-like TiO2 and P25 as photoelectrode for enhanced efficiency in dye-sensitized solar cells

A TiO2 double-layer composite film consisting of hierarchically sponge-like macro-/mesoporous TiO2 (HSMM–TiO2) as overlayer and commercial-grade TiO2 nanoparticles (P25) as underlayer is designed as the photoelectrode of dye-sensitized solar cells (DSSCs). The photoelectric conversion performances of DSSCs based on P25 nanoparticle film, HSMM–TiO2 film, and P25/HSMM–TiO2 double-layer composite film are investigated and compared. It is found that the overall energy-conversion efficiency of 5.48% is achieved by the formation of P25/HSMM–TiO2 double-layer composite film, which is 51.4% higher than that formed by P25 nanoparticle film (η = 3.62%) and 27.1% higher than that formed by HSMM–TiO2 film (η = 4.31%) under identical film thickness (ca. 20 μm) at a constant irradiation of 100 mWcm−2. The enhanced conversion efficiency of TiO2 double-layer composite film can be attributed to the combined effect of the following factors. The HSMM–TiO2 overlayer enhances light-harvesting efficiency due to its intense light scattering and the P25 nanoparticle underlayer ensures good electronic contact between TiO2 film and the F-doped tin oxide (FTO) glass. Furthermore, the high specific surface area and special pore-wall structure of HSMM–TiO2 are respectively beneficial to adsorption of dye molecules and transport of both electrons and electrolytes.

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► A novel TiO2 double-layer composite film is successfully fabricated.
► The overlayer enhances light-harvesting efficiency and electron transport.
► The underlayer ensures good electronic contact between TiO2 film and FTO.
► TiO2 double-layer composite film cells have the maximum conversion efficiency.