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 (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.

Parameters Influencing Charge Recombination Kinetics in Dye-Sensitized Nanocrystalline Titanium Dioxide Films

“…For the purposes of this paper, consideration of the time taken for 50% of the initially formed dye cations to decay (t 50% ) provides a reasonable quantification of the recombination time. 23 Figure 4 shows a plot of log t 50% versus applied bias determined from the data shown in Figures 2 and 3. It is evident that the graph exhibits a plateau region at positive biases (+0.1 and +0.2V), and a linear dependence of log t 50% versus V for more negative potentials (one decade change of rate every 100 mV).…”

“…The acceleration in charge recombination under negative bias has been reported previously and attributed to the increased occupation of conduction band/ trap states of the TiO 2 film under negative bias. 23 The data shown in Figures 2 and 3 were collected with an ethanolic electrolyte. Comparable data were collected for a range of different redox inactive electrolytes, including protic and aprotic solvents (ethanol and acetonitrile, respectively).…”

“…Nanosecond to millisecond transient absorption spectroscopy was performed as described previously. 23 The excitation wavelength was set at 560 nm, except for studies at high excitation intensities (Figure 1), where the excitation wavelength was 532 nm (pulse durations of ∼600 ps and 10 ns, respectively; repetition rate of 2 Hz). At these excitation wavelengths sample optical density was dominated by sensitizer dye absorption.…”

“…1,[26][27][28][29][30] We have suggested that both the nonexponential recombination kinetics and the strong dependence upon applied bias may be associated with these trap states. 23 The acceleration in the charge recombination kinetics at negative applied potentials was tentatively attributed to an increase in the electron occupancy of these trap states as the semiconductor Fermi level was raised. 23 Spectroelectrochemical techniques have been used to investigate the dependence of the potential of the conduction band edge upon the electrolyte composition.…”

“…23 The acceleration in the charge recombination kinetics at negative applied potentials was tentatively attributed to an increase in the electron occupancy of these trap states as the semiconductor Fermi level was raised. 23 Spectroelectrochemical techniques have been used to investigate the dependence of the potential of the conduction band edge upon the electrolyte composition. This edge has been shown to exhibit a Nernstein shift as a function of electrolyte proton activity, shifting by 59 meV per pH of the electrolyte.…”