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

Zn(II)-porphyrin dyes (SGT-030and SGT-031) 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 (ZnP)–ethynyl–benzothiadiazole-acceptor platform, but their donor groups varied from phenylene (Ph) in SGT-053as a reference dye to the thieno[3,2-b]benzothiophene (TBT) and 4-hexyl-4H-thieno[3,2-b]indole (TI) moieties in SGT-030and 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 the 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 TBTand TIpromoted 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 4-(3,6-bis(4-((2-ethylhexyl)oxy)phenyl)-9H-carbazol-9-yl)benzoic acid in conjunction with the [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 Jscand Vocoriginates mainly from the twisted orientation between D and ZnPby the introduction of TBTand 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 the light-harvesting ability and hindering charge recombination on the TiO2photoanode.