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 (TiO 2 ) 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 TiO 2 films were compared with those obtained for control dye-coated ZrO 2 films, as the high conduction band edge of ZrO 2 prevents electron injection. Adsorption of the dye onto the TiO 2 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 e10 -12 s. Detailed analysis indicates the injection is at least biphasic, with ∼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.

The kinetics of charge recombination were determined following low intensity optical excitation of RuII(2,2‘-bipyridyl-4,4‘dicarboxylate)2(NCS)2 sensitized nanocrystalline TiO2 films as a function of externally applied bias voltage V. These kinetics were found to be multiexponential. The median time (t 50%) for charge recombination was found to depend strongly upon bias voltage, reducing from t 50% ∼ 200 ms for V = 0 V versus Ag/AgCl to < 30 ns for V ≤ −500 mV. This reduction in t 50% was found to follow the equation t 50% ∝ exp(qV/1.7k B T). These results are consistent with the charge recombination kinetics being strongly dependent upon the accumulation of electrons in intraband/conduction band states of the TiO2 film. For biases V ≤ −400 mV, the rate of charge recombination was found to exceed the rate of rereduction of the dye cation by an I-/I3- redox couple in solution.