Surface Modification of TiO₂ Photoanodes with Fluorinated Self-Assembled Monolayers for Highly Efficient Dye-Sensitized Solar Cells

Dye aggregation and electron recombination in TiO₂ photoanodes are the two major phenomena lowering the energy conversion efficiency of dye-sensitized solar cells (DSCs). Herein, we introduce a novel surface modification strategy of TiO₂ photoanodes by the fluorinated self-assembled monolayer (F-SAM) formation with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFTS), blocking the vacant sites of the TiO₂ surface after dye adsorption. The F-SAM helps to efficiently lower the surface tension, resulting in efficient repelling ions, e.g., I₃^-, in the electrolyte to decrease the electron recombination rate, and the role of F-SAM is characterized in detail by impedance spectroscopy using a diffusion-recombination model. In addition, the dye aggregates on the TiO₂ surface are relaxed by the F-SAM with large conformational perturbation (i.e., helix structure) seemingly because of steric hindrance developed during the SAM formation. Such multifunctional effects suppress the electron recombination as well as the intermolecular interactions of dye aggregates without the loss of adsorbed dyes, enhancing both the photocurrent density (11.9 → 13.5 mA cm^-2) and open-circuit voltage (0.67 → 0.72 V). Moreover, the combined surface modification with the F-SAM and the classical coadsorbent further improves the photovoltaic performance in DSCs.