Kinetics
of the Regeneration by Iodide of Dye Sensitizers Adsorbed on Mesoporous
Titania
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Abstract
Regeneration of dye sensitizer molecules
by reducing species contained in the electrolyte is a key mechanism
in liquid dye-sensitized solar cells because it competes kinetically
with a detrimental charge recombination process. Kinetics of the reduction
by iodide ions of the oxidized states (S<sup>+</sup>) of two Ru<sup>II</sup> complex dyes and four organic π-conjugated bridged
donor–acceptor sensitizers were examined as a function of the
electrolyte concentration. Results show that two different cases can
be distinguished. A sublinear behavior of the regeneration rate and
a plateau value reached at high bulk iodide concentrations were found
for N820 ruthenium dye and interpreted as being due to an associative
interaction involving the formation of (S<sup>+</sup>, I<sup>–</sup>)···I<sup>–</sup> surface complexes prior to
the reaction. On the other hand, feeble reaction rates at low electrolyte
concentrations and a superlinear behavior are observed predominantly
for the organic dyes, pointing to a repulsive interaction between
the dyed surface and iodide anions. At higher iodide bulk concentration,
a linear behavior is reached, providing an estimate of a second-order
rate constant. A correlation of these two opposite behaviors with
the structure of the dye is observed, emphasizing the role of sulfur
atoms in the association of I<sup>–</sup> anions in the dye-sensitized
layer. These findings allow for a better understanding of the dye–electrolyte
interaction and of the effect of the iodide concentration on the photovoltaic
performances of dye-sensitized solar cells