Photoassisted spatially resolved STM measurements of dye-sensitized nanocrystalline TiO₂ films

Photoassisted scanning tunneling microscopy was used to investigate photoinduced currents in a dye-sensitized nanoporous TiO₂ network in a locally resolved experiment. The light-induced tunneling current (LITC) was studied with respect to its dependence on the modulation frequency of the exciting light as well as on the externally applied bias. By this, two main contributions to the LITC were identified and assigned to both a tunneling current of photoelectrons from the TiO₂ conduction band to the tip and a tunneling current driven by a photoinduced change of the voltage drop over the tunneling gap. Additionally, the observed frequency dependence of the LITC components is in agreement with the time scales expected for a hopping transport via localized energy states. Lateral variations in the LITC signal are found between aggregates of TiO₂ particles, directly reflecting different electronic properties. These results might be important for further optimization of porous materials in applications such as dye-sensitized solar cells.