Optical, Electrochemical, and Photovoltaic Effects of an Electron-Withdrawing Tetrafluorophenylene Bridge in a Push–Pull Porphyrin Sensitizer Used for Dye-Sensitized Solar Cells

We have prepared a novel push–pull porphyrin with an electron-donating diarylamino group at the meso-position and an electron-withdrawing 4-carboxy-2,3,5,6-tetrafluorophenylethynyl anchoring group at the opposite meso-position to address the substitution effects of the electron-withdrawing fluorine atoms on photovoltaic properties for the first time. The fluoro-substituted porphyrin showed slightly improved light-harvesting properties when compared to the reference porphyrin in solution, due to the enhancement of charge-transfer character. The fluoro-substituted porphyrin-sensitized TiO 2 cell exhibited a moderate power conversion efficiency of 4.6% but was lower than the corresponding value (6.9%) of the reference porphyrin-sensitized TiO 2 cell. The carboxylic group in the reference porphyrin binds to the TiO 2 surface with bidentate coordination, leading to formation of the densely packed monolayer on the TiO 2 surface, whereas both of the electron-withdrawing fluorine atoms and the carboxylic group of the fluoro-substituted porphyrin interact with the TiO 2 surface, adopting relatively parallel orientation to the TiO 2 surface with monodentate binding of the carboxylic acid to the TiO 2, eventually yielding to the small surface coverage and low cell performance due to the fast charge recombination and small amount of the long-lived porphyrin radical cation. Such fundamental information may be useful for the molecular design of highly efficient dye-sensitized solar cells based on push–pull porphyrins.