Influence of conjugated pi-linker in D-D-pi-A indoline dyes: towards long-term stable and efficient dye-sensitized solar cells with high photovoltage

The judicious choice of conjugated pi-linkers is a critical strategy towards the energy-level engineering of donor-pi-acceptor (D-pi-A) sensitizers. Given that the vinyl bond on pi-bridge thiophene segment can deteriorate its intrinsic photo-stability, we systematically study three D-D-pi-A indoline dyes C-CA, WBS-1T and WBS-1F with different conjugated pi-linkers (vinylthiophene, thiophene and furan) for high efficiency, long-term stable dye-sensitized solar cells (DSSCs). Compared with the vinylthiophene unit in C-CA, the conjugated pi-linker of the thiophene or furan group in WBS-1T and WBS-1F can improve the solar cell performance with a great enhancement in the open-circuit photovoltage (V-OC). As an overall result of the upshift of the TiO2 conduction band (CB) edge and the slow charge recombination, the V-OC values are in the order WBS-1F (779 mV) > WBS-1T (756 mV) > C-CA (670 mV). Moreover, the CB edge shift of TiO2 is the major contribution to the large difference in V-OC, accounting for 80% of the enhancement. Both the stepped light-induced transient measurements (SLIT) and the molecular dipole simulation are accounted for well by the observed superior photovoltage upon removal of the vinyl group in the conjugated pi-linker. The higher molecular dipole moments can bring forth a more effective charge separation between donor and acceptor units, resulting in a remarkable increase in V-OC. Using a liquid electrolyte, WBS-1F shows an impressively high efficiency of 9.49% with a high photovoltage of 779 mV. Its efficiency reaches 8.03% with ionic-liquid electrolyte while it reduces to 7.60% after a 1000 h aging test. Our work has shown that for D-pi-A organic dyes, the vinyl unit in the conjugated pi-linker is detrimental to the molecular dipole moment, the upshift of TiO2 CB edge, and the suppression of charge recombination, as well as the photo-stability.