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 TiO 2 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 /1.7 k 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 TiO 2 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 - /I 3 - redox couple in solution.