Influence of Hydrothermal Pressure during Crystallization on the Structure and Electron-Conveying Ability of TiO₂ Colloids for Dye-Sensitized Solar Cells

This study synthesizes TiO 2 anatase colloids at 250 °C under varying pressures of 57−120 bar by adjusting the residual volume in the autoclaving chamber. Transmission electron microscopy and X-ray diffraction analyses showed that the amorphous phase content of TiO 2 powders and films obtained from calcining the colloids increased with the pressure during crystallization, whereas the Ti vacancy in the crystalline phase decreased. This illustrated a trade-off between lattice distortion and vacancy reduction as a result of an increase in pressure during crystallization. X-ray absorption fine structure spectroscopic analysis showed that the coordination number of the Ti 4+ sites in the TiO 2 increased with the pressure during crystallization to reach a maximum value at 100 bar and then decreased with further increases in pressure. A dye-sensitized solar cell assembled with a TiO 2 film from a 100-bar synthesis exhibited the highest solar energy conversion efficiency. Electrochemical impedance spectroscopy analysis showed that the 100-bar film had the highest charge collection efficiency for photogenerated electrons. From these results, we concluded that the TiO 2 crystallization pressure affects the density of defects in the produced TiO 2 films and, therefore, the electron-conveying performance in DSSCs.