Synthesis and characterization of ensembles containing zinc oxide nanocrystals and organic or transition metal dyes to probe the early events in a dye-sensitized solar cell.

Abstract

The synthesis of 3',4'-dibutyl-2-phenyl-2,2':5',2"-terthiophene-5"-carboxylic acid, and its behavior with monodispersed ZnO nanocrystals (NCs) having diameters from 2.7 to 3.2 nm are reported. The excited state of the dye (E0* = -1.61 V vs NHE) was quenched upon binding to ZnO Ncs. Adsorption isotherms were measured for the terthiophene dye in ethanol and fit with a Langmuir model, which gave a size-independent Kads of 2.3 ± 1.0 x 105 M-1. The maximum number of attached dyes per nanocrystal depended on the diameter and was consistent with each dye occupying 0.5 ± 0.1 nm2 at maximum coverage. Deviation from the Langmuir model observed at low dye concentrations was attributed to a small amount of free zinc ion present in solution that bind the carboxylate ions more strongly than do ZnO NCs. Incorporation of the equilibrium expression between zinc ion and free carboxylate into the model provided a satisfactory fit for both the adsorption isotherm experiments and the complex shape of the Stern-Volmer graphs. Treatment of the terthiophene dye-nanocrystal dyads with increasing concentrations of sodium acetate in ethanol resulted in gradual displacement of the dye. Time-resolved fluorescence and time- and frequency-resolved pump-probe spectroscopy confirm and characterize electron injection from the dye to the semiconductor nanocrystals in room temperature ethanol dispersions at a series of dye:ZnO NC concentration ratios. The spectrum of the oxidized dye was determined by spectroelectrochemistry. The singlet excited state of the dye (190 ps lifetime in ethanol) is quenched almost exclusively by electron transfer to the ZnO NC, and the electron transfer dynamics exhibit a single time scale of 3.5 ( 0.5 ps at all concentration ratios. In the measured transient responses at different dye:ZnO NC ratios, gain in the amplitude of the electron injection component is anticorrelated with loss of amplitude from unperturbed excited state dye molecules. The dependence of this amplitude on dye:ZnO NC ratio deviates significantly from the prediction of a standard Stern-Volmer model. This observation is in agreement with the static quenching studies. By identifying electron transfer as the quenching mechanism at all ratios, the work presented here helps to exclude concentration quenching as the basis for the complicated quenching results, and supports the model that incorporates competitive binding between ZnO NC s and free Zn2+ cations in solution.