Electron transport in porous nanocrystalline TiO2 photoelectrochemical cells

The photocurrent response of dye-sensitized, porous nanocrystalline TiO2 cells was studied as a function of light intensity, in both the time domain (photocurrent transient measurements) and the frequency domain (intensity-modulated photocurrent spectroscopy). The photocurrent transients are characterized by a fast and a slow component. The rise time of the transients was in the range of milliseconds to seconds and exhibited a power law dependence on light intensity with an exponent of -0.6 to -0.8. The response to a modulated light intensity is characterized by a depressed semicircle in the complex plane. The time constant obtained from these spectra exhibits the same power law dependence on light intensity. The transient response of these cells is dominated by electron transport in the TiO2 film, and the results are shown to be consistent with a diffusion model where the diffusion coefficient for electrons in the particle network is a function of the light intensity.