The frequency-dependent photocurrent response of dye-sensitized TiO2 cells to modulated illumination is analyzed. Analytical expressions are derived that describe generation, collection, and recombination of electrons in a thin layer nanocrystalline solar cell under conditions of steady illumination and with a superimposed small amplitude modulation. The analysis considers illumination from the contact side and from the counter electrode side, and characteristic differences in the intensity-modulated photocurrent response are predicted for the two cases. The attenuation of the ac photocurrent by the RC time constant of the cell is also considered. The theoretical analysis shows that intensity modulated photocurrent spectroscopy (IMPS) can provide new insight into the dynamics of electron transport and collection in the dye-sensitized solar cell. Experimental IMPS data measured for high-efficiency dye-sensitized cells are fitted to the theoretical model using Bode plots in order to derive values of the lifetime (2 × 10-2 s) and diffusion coefficient (5 × 10-5 cm2 s-1) of photoinjected electrons.

We report a simple procedure for the transfer of a given nanostructure from a binary oxide semiconductor to binary chalcogenide compounds. The procedure involves two consecutive ion exchange steps performed in the gas phase and in solution. While there is a complete exchange of substance, a change in stoichiometry, and a change in crystal structure, the morphology is nearly perfectly maintained. Applying a partial exchange also allows a well-controlled manipulation of the nanostructure. Basic chemical guidelines for the ion exchange are discussed. The procedure is compatible with thin film applications.