Charge Recombination Kinetics in Dye-Sensitized Nanocrystalline Titanium Dioxide Films under Externally Applied Bias

Abstract

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 (t50%) for charge recombination was found to depend strongly upon bias voltage, reducing from t50% ∼ 200 ms for V = 0 V versus Ag/AgCl to < 30 ns for V ≤ −500 mV. This reduction in t50% was found to follow the equation t50% ∝ exp(qV/1.7kBT). 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.