High Efficiency Dye-Sensitized Solar Cells based on a Bi-Layered Photoanode Made of TiO2 Nanocrystallites and Microspheres with High Thermal Stability

Submicrometer-sized monodispersed TiO2 spheres were synthesized by a controlled hydrolysis of titanium tetraisopropoxide (TTIP) and subsequent solvothermal treatment. The X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-visible spectroscopy analyses revealed that aqueous ammonia concentration and calcining temperature significantly influence the morphology, crystallization, dye loading and light scattering capacity of TiO2 microspheres. And it was found that the TiO2 microspheres prepared by this method showed a good thermal stability of phase. Bilayered dye-sensitized solar cells (DSSCs) composed of TiO2 microspheres as the scattering cover layers and TiO2 nanocrystallines as the underlayer exhibited a remarkable improvement in the power conversion efficiency (8.25%) compared with the nanocrystalline TiO2 DSSC (6.38%), owing to high light scattering efficiency and the dye-loading capacity of the microsphere cover layer. In addition, electrochemical impedance spectra (EIS) and open-circuit photo-voltage decay curves (OCVD) testing indicated that the microsphere cover layer passivated the surface states, increased the density of bulk traps, enlarged the lifetime of electrons, and promoted more efficient charge-transfer, which was also an important reason for the improved power conversion efficiency of double-layered DSSCs.