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

This study synthesizes TiO2 anatase colloids at 250 degrees 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 TiO2 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 Ti4+ sites in the TiO2 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 TiO2 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 TiO2 crystallization pressure affects the density of defects in the produced TiO2 films and, therefore, the electron-conveying performance in DSSCs.