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

Novel Zn(II)-porphyrin dyes (SGT-030 and SGT-031) with extended auxiliary π-spacers in the donor 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 (Ph) in SGT-053 as a reference dye to the thieno[3,2-b]benzothiophene (TBT) and 4-hexyl-4H-thieno[3,2-b]indole (TI) moieties in SGT-030 and SGT-031, 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 (Ph < TBT ≈ TI) and the donating ability of π-spacer (Ph < TBT < TI), the absorption maxima and molecular absorptivity increased in the order SGT-053 (Ph) < SGT-030 (TBT) < SGT-031 (TI). The incorporation of TBT and TI promoted significant enhancements in the light-harvesting properties by reducing the energy gap and efficiently improving electronic communication. The DSSCs based on SGT-030 (10.80%) and SGT-031 (10.89%) with coadsorption of HC-A1 in conjunction of [Co(bpy)3]2+/3+ based electrolyte showed better power conversion efficiency than that of SGT-053 (9.10%). Electrochemical impedance spectroscopy analysis unveiled that the difference in Jsc and Voc originates mainly from the twisted orientation between D and ZnP by the introduction of TBT and TI. 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 light-harvesting ability and hindering charge recombination on TiO2 photoanode.