Abstract
Posttreatment of mesoporous titanium dioxide films by TiCl4 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 TiCl4-treated films. These findings are rationalized by a model describing the interaction at the interface between TiO2, 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 TiCl4 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.