Electron transport in the nanostructured titanium dioxide electrodes in the application of solar cells.

摘要

The high efficiency of dye sensitized nanostructured TiO 2 photoelectrochemical solar cells underlines the high charge transfer efficiency at the semiconductor/dye interface and the ability of the nanometer sized TiO2 particle network in intimate contact with the electrolyte to transport the injected electrons without significant losses. The operating mechanisms of these photoelectrochemical systems are reviewed. The focus of this dissertation is the photoelectrical properties of nanostructured TiO 2 films and the associated charge transport process. Transient techniques, both in the time domain and frequency domain were used to probe the electron transport in the TiO2 electrodes.;The conductivity of nanostructured TiO2 thin films increases several orders of magnitude under illumination yet the conductivity is very low in the dark, illustrating the fact that the conductivity is strongly dependent on the trapped charges in these films. Due to the lack of a strong electrical field, charge transport only becomes efficient after the build up of a concentration gradient to drive the electron transport process, resulting in slow photocurrent and photovoltage transients in the photoelectrochemical cells. The open circuit photovoltages follow the conventional diode equation. This can be understood in terms of the Fermi level at the TiO2/tin oxide contact. The injection of electrons leads to the increase of the Fermi level at the contact. The time constants obtained follow a simple power law relationship with the light intensity reflecting the kinetics of electron transport in the nanostructured electrodes.;The essential features of the nonsteady state response can be described by a diffusion model where the electron diffusion coefficient is dependent on light intensity. The lack of electron migration can be rationalized by the fact that any electrical field in the porous electrodes may be neutralized by the mobile ions in the electrolyte.