Effect of Posttreatment
of Titania Mesoscopic Films
by TiCl<sub>4</sub> in Solid-State Dye-Sensitized Solar Cells: A Time-Resolved
Spectroscopy Study
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Abstract
Posttreatment of mesoporous titanium dioxide films by
TiCl<sub>4</sub> solutions is commonly applied during the fabrication
of solid-state
dye-sensitized solar cells (ssDSCs), as this operation markedly improves
the performance of the photovoltaic device. The effect of the posttreatment
upon the charge carrier dynamics was scrutinized in an ssDSC aiming
at unraveling its mechanism. Kinetic studies carried out using femtosecond
and nanosecond transient absorption spectroscopies showed that a biphasic
electron injection from the dye excited state is observed, for both
treated and nontreated films, whose kinetics is not significantly
affected by the surface modification step. However, hole injection
in the hole transport material (HTM) spiro-OMeTAD and charge recombination
were found to be markedly slower in TiCl<sub>4</sub>-treated films.
These findings are rationalized by a model describing the interaction
at the interface between TiO<sub>2</sub>, the dye-sensitizer, and
spiro-OMeTAD. Rather than resulting from a modification of the energetics
of the conduction band of the oxide, the effect of the TiCl<sub>4</sub> posttreatment appears to be associated with a subtle change of the
film morphology. Results emphasize the importance of controlling the
contact at the heterojunction between the HTM and the sensitized semiconductor
oxide network