Influence of Surface-Attachment
Functionality on the
Aggregation, Persistence, and Electron-Transfer Reactivity of Chalcogenorhodamine
Dyes on TiO<sub>2</sub>
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Abstract
Chalcogenorhodamine dyes bearing phosphonic acids and
carboxylic
acids were compared as sensitizers of nanocrystalline TiO<sub>2</sub> in dye-sensitized solar cells (DSSCs). The dyes were constructed
around a 3,6-bis(dimethylamino)chalcogenoxanthylium core and varied
in the 9 substituent: 5-carboxythien-2-yl in dyes <b>1-E</b> (E = O, Se), 4-carboxyphenyl in dyes <b>2-E</b> (E = O, S),
5-phosphonothien-2-yl in dyes <b>3-E</b> (E = O, Se), and 4-phosphonophenyl
in dyes <b>4-E</b> (E = O, Se). All dyes adsorbed to TiO<sub>2</sub> as mixtures of H aggregates and monomers, which exhibited
broadened absorption spectra relative to those of purely amorphous
monolayers. Surface coverages of dyes and the extent of H aggregation
varied minimally with the surface-attachment functionality, the structure
of the 9-aryl group, and the identity of the chalcogen heteroatom.
Carboxylic acid-functionalized dyes <b>1-E</b> and <b>2-E</b> desorbed rapidly and completely from TiO<sub>2</sub> into acidified
CH<sub>3</sub>CN, but phosphonic acid-functionalized dyes <b>3-E</b> and <b>4-E</b> persisted on TiO<sub>2</sub> for days. Short-circuit
photocurrent action spectra of DSSCs corresponded closely to the absorptance
spectra of dye-functionalized films; thus, H aggregation did not decrease
the electron-injection yield or charge-collection efficiency. Maximum
monochromatic incident photon-to-current efficiencies (IPCEs) of DSSCs
ranged from 53 to 95% and were slightly higher for carboxylic acid-functionalized
dyes <b>1-E</b> and <b>2-E</b>. Power-conversion efficiencies
of DSSCs under white-light illumination were low (<1%), suggesting
that dye regeneration was inefficient at high light intensities. The
photoelectrochemical performance (under monochromatic or white-light
illumination) of <b>1-E</b> and <b>2-E</b> decayed significantly
within 20–80 min of the assembly of DSSCs, primarily because
of the desorption of the dyes. In contrast, the performance of phosphonic
acid-functionalized dyes remained stable or improved slightly on similar
timescales. Thus, replacing carboxylic acids with phosphonic acids
increased the inertness of chalcogenorhodamine–TiO<sub>2</sub> interfaces without greatly impacting the aggregation of dyes or
the interfacial electron-transfer reactivity