Effects of TiCl4 Treatment of Nanoporous TiO2 Films on Morphology, Light Harvesting, and Charge-Carrier Dynamics in Dye-Sensitized Solar Cells

We report on the effects of treating TiO ₂ nanocrystalline films with different concentrations of TiCl ₄ (5-500 mM) on the film morphology, charge-carrier dynamics, and performance of dye-sensitized solar cells. Transport and recombination in the TiCl ₄-treated films were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. These studies showed that, at a low TiCl ₄ concentration (5 mM), the electron diffusion coefficient in the annealed film increased. At intermediate TiCl ₄ concentrations (15-50 mM), the surface area of the films increased, resulting in an increase of light harvesting and overall power conversion efficiency. At a high TiCl ₄ concentration (500 mM), light scattering in the film in the long wavelength region of the visible spectrum was enhanced, but the averaged pore size of the film became narrower, resulting in slower transport and loss of cell performance.