New thieno[3,2-b][1]benzothiophene-based organic sensitizers containing π-extended thiophene spacers for efficient dye-sensitized solar cells

Three new thieno[3,2-b][1]benzothiophene (TBT)-based D–π–A organic sensitizers containing the thiophene π-spacer (SGT-121, 123 and 125) have been synthesized for the application of dye-sensitized solar cell (DSSC), where TBT was employed as a new fused π-bridge unit using the advantages of good co-planarity with the linkage between the thiophene unit and the phenyl unit of the triphenylamine group. Specifically, the combination of a dihexyloxyphenyl-substituted biphenylamine donor and the TBT π-bridge plays multifunctional roles, e.g., the enhanced ability of the π-bridge and donor, slow charge recombination and prevention of dye aggregation in the D–π–A sensitizer. The photophysical, electrochemical and photovoltaic properties of the SGT sensitizers were systematically investigated. As a strategy for the improvement of absorption abilities, the various thiophene derivatives, e.g., those with thiophene (T, SGT-121), bithiophene (BT, SGT-123) and thienothio[3,2-b]thiophene (TT, SGT-125) moieties, were incorporated as π-spacers between the TBT π-bridge and the acceptor unit. The introduction of thiophene π-spacers significantly improved the photovoltaic performance (in particular, in terms of the photocurrent Jsc and open-circuit voltage Voc) compared to SGT-127 without the thiophene unit. The SGT sensitizers were systematically evaluated for DSSCs based on the Co(bpy)32+/3+ (bpy = 2,2′-bipyridine) redox couple. Among the four SGT sensitizers, SGT-123-based DSSC including the BT moiety exhibited the highest power conversion efficiency of 9.69%, Jsc of 16.16 mA cm−2, Voc of 830 mV and fill factor of 0.72. These results present the impact of thiophene π-spacers for enhancing the photovoltaic performances of a D–π–A organic sensitizer.