D-π-A-Structured Porphyrins with Extended Auxiliary π-Spacers for Highly Efficient Dye-Sensitized Solar Cells.

Zn(II)-porphyrin dyes ( and ) with extended auxiliary π-spacers in the donor (D) part have been prepared and applied to dye-sensitized solar cells (DSSCs). The porphyrin dyes contained the same D-ethynyl-zinc porphyrinyl ()-ethynyl-benzothiadiazole-acceptor platform, but their donor groups varied from phenylene () in as a reference dye to the thieno[3,2-]benzothiophene () and 4-hexyl-4-thieno[3,2-]indole () moieties in and , respectively. The effects of the extended auxiliary π-spacer in the D-π-A-structured porphyrin sensitizers on the molecular and photovoltaic properties were investigated via photophysical and electrochemical experiments as well as theoretical calculations. With the trend in conjugation length ( < ≈ ) and the donating ability of the π-spacer ( < < ), the absorption maxima and molecular absorptivity increased in the order () () (). The incorporation of and promoted significant enhancements in the light-harvesting properties by reducing the energy gap and efficiently improving electronic communication. The DSSCs based on (10.80%) and (10.89%) with coadsorption of 4-(3,6-bis(4-((2-ethylhexyl)oxy)phenyl)-9-carbazol-9-yl)benzoic acid in conjunction with the [Co(bpy)]-based electrolyte showed better power conversion efficiency than that of (9.10%). Electrochemical impedance spectroscopy analysis unveiled that the difference in and originates mainly from the twisted orientation between D and by the introduction of and . This result indicated that the introduction of an extended auxiliary π-spacer in the donor part is a rational molecular design approach to improve photovoltaic performance by enhancing the light-harvesting ability and hindering charge recombination on the TiO photoanode.