By using femtosecond transient absorption spectroscopy with visible pump and IR probe to observe generation of injected electrons, we could directly observe plasmon-induced electron transfer from 10 nm gold nanodots to TiO2 nanocrystalline film. It was revealed that the reaction time was within 240 fs and the yield was about 40%.

Reactive species, holes, and electrons in photoexcited nanocrystalline TiO 2 films were studied by transient absorption spectroscopy in the wavelength range from 400 to 2500 nm. The electron spectrum was obtained through a hole-scavenging reaction under steady-state light irradiation. The spectrum can be analyzed by a superposition of the free-electron and trapped-electron spectra. By subtracting the electron spectrum from the transient absorption spectrum, the spectrum of trapped holes was obtained. As a result, three reactive speciess trapped holes and free and trapped electronsswere identified in the transient absorption spectrum. The reactivity of these species was evaluated through transient absorption spectroscopy in the presence of hole-and electronscavenger molecules. The spectra indicate that trapped holes and electrons are localized at the surface of the particles and free electrons are distributed in the bulk.

The efficiency of electron injection from excited N3 dye (cis-bis-(4,4′-dicarboxy-2,2′-bipyridine) dithiocyanato ruthenium(II), Ru(dcbpy) 2 (NCS) 2 ), into various nanocrystalline semiconductor (ZrO 2 , TiO 2 , ZnO, Nb 2 O 5 , SnO 2 , In 2 O 3 ) films was studied by transient absorption spectroscopy. For TiO 2 , ZnO, Nb 2 O 5 , SnO 2 , or In 2 O 3 films, injection efficiencies were found to be very high; for ZrO 2 film, the efficiency was very low. These findings indicate that electron injection occurs efficiently if the LUMO level of N3 dye is located sufficiently far above the bottom of the conduction band of the semiconductor film. On the basis of the results, we discuss the reason TiO 2 exhibits higher solar cell performance than other materials.

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