Structure and electron-conducting ability of TiO ₂ films from electrophoretic deposition and paste-coating for dye-sensitized solar cells

An electrophoretic deposition (EPD) method, consisting of repetitive short-term depositions with intermediate drying, was developed to prepare nanocrystalline TiO ₂ films for dye-sensitized solar cells (DSSCs). After calcination, the EPD TiO ₂ films exhibited a more compact TiO ₂ network than films derived from the conventional paste-coating (PC) method. X-ray absorption fine structure spectroscopic analysis showed that the EPD films had a higher density of defect states than the PC films because of the higher number of interparticle necking regions created in the EPD films. However, the DSSCs assembled with the EPD films outperformed those with the PC films by 20% in photocurrent and 15% in solar energy conversion efficiency. Intensity-modulated photocurrent spectroscopic analysis showed that the EPD films had a shorter electron transit time than the PC films. Under one-sun illumination on the cells at open-circuit, impedance analysis showed that the EPD films had a constant charge collection efficiency of 95% for thicknesses ranging from 4 to 13 μm, whereas the efficiency of the PC films was not greater than 90% and showed a decreasing trend with increasing film thickness. Concerning the porosity dependence of the electron transport dynamics, the electron diffusivity had much weaker dependence than one would expect from the percolation model with hard spheres. This may result from the fact that interparticle necking causes greater lattice distortion for more compact TiO ₂ films. The present study demonstrates that an optimized EPD process can construct a nanocrystalline TiO ₂ architecture with a minimized void fraction to shorten the electron traveling distance and to effectively collect photogenerated charges, even for films with large thicknesses.