Charge recombination kinetics in dye-sensitized nanocrystalline titanium dioxide films under externally applied bias

The kinetics of charge recombination were determined following low intensity optical excitation of Ru-II-(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%) similar to 200 ms for V = 0 V versus Ag/AgCl to < 30 ns for V less than or equal to -500 mV. This reduction in t(50%) was found to follow the equation t(50%), proportional to 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 I -400 mV, the rate of charge recombination was found to exceed the rate of rereduction of the dye cation by an I-/I-3-redox couple in solution.