Subpicosecond interfacial charge separation in dye-sensitized nanocrystalline titanium dioxide films

We have employed subpicosecond transient absorption spectroscopy to study the rate of electron injection following optical excitation of the ruthenium dye Ru-II(2,2'-bipyridyl-4,4'-dicarboxylate)(2)(NCS)(2) (1) adsorbed onto the surface of nanocrystalline titanium dioxide (TiO2) films. This sensitizer dye is of particular interest as it is the most efficient sensitizer dye reported to date and is receiving considerable attention for applications in photoelectrochemical solar energy conversion. Transient data collected for 1 adsorbed onto TiO2 films were compared with those obtained for control dye-coated ZrO2 films, as the high conduction band edge of ZrO2 prevents electron injection. Adsorption of the dye onto the TiO2 film was found to result in a rapid (<500 ps) quenching of the dye excited-state luminescence. Absorption difference spectra collected for the two dye-coated films were assigned by comparison with the spectroscopy of the dye excited and cation states in solution. These transient absorption data indicated that electron injection in these films occurs in less than or equal to 10-12 s. Detailed analysis indicates the injection is at least biphasic, with similar to 50% occurring in <150 fs (instrument response limited) and 50% in 1.2+/-0.2 ps. These ultrafast electron injection kinetics are contrasted with the charge recombination reaction, which occurs on the microsecond-millisecond time scales. The ultrafast rate of electron injection observed here is critical both for the high energy conversion efficiencies obtained with this sensitizer dye, and for the excellent long-term stability of this dye in photoelectrochemical solar cells.