Chapter 7 Time-resolved spectroscopy and microscopy on nanocrystalline TiO2 and ZnO films

Abstract

6. Summary We have investigated electron-hole dynamics in nanocrystalline TiO2 film by means of femtosecond and nanosecond transient absorption spectroscopy in the visible nd near-IR wavelength regions. Electron and hole spectra were assigned, and charge-carrier dynamics such as trapping and recombination were revealed. Free electrons showed a very broad absorption band whose intensity increased monotonously with increasing wavelength. Trapped electrons and holes were found to have absorption bands around 800 nm and 500 nm, respectively. The trapping and thermalization processes occurred in a complex fashion over a wide temporal range from a few hundred femtoseconds to several hundred picoseconds. Electron-injection processes in dye-sensitized ZnO films were also investigated with a newly developed transient absorption microscope and the femtosecond spectroscopic system. Microcrystals formed by aggregation of the Ru complex dye (N3) molecules and Zn ions were observed in the fluorescence image, and these were found not to function as sensitizers, whereas adsorbed N3 dyes in monomeric form seem to inject electrons efficiently into the film at any position. Femtosecond spectroscopy of ZnO films with an adsorbed coumarin derivative revealed that part of the electron-injection process proceeds stepwise through exciplex states as intermediates. The rate of electron injection for the stepwise process was slower than that for direct injection (without an intermediate). The overall injection rate was slower for the dye having a lower LUMO energy.