Effect of Posttreatment of Titania Mesoscopic Films by TiCl4 in Solid- State Dye-Sensitized Solar Cells: A Time-Resolved Spectroscopy Study

Post-treatment 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 post-treatement upon the charge carrier dynamics was scrutinized in ssDSC aiming at unraveling its mechanism. Kinetic studies carried out using femtosecond and nanosecond transient absorption spectroscopy, showed that a biphasic electron injection from the dye excited state is observed, for both treated and non-treated films, which 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 post-treatment 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.