Spiro-sulfone-based Auxiliary Acceptor in D-A-pi-A Dye-sensitized Solar Cells Application under Indoor/Outdoor Light

Novel D-A-pi-A type organic sensitizers (YS-1 & YS-2) with spiro[fluorene-9,9 '-thioxanthene]-10 ',10 '-dioxide as an auxiliary acceptor were synthesised and effectively fabricated in dye-sensitised solar cells (DSSCs). The sensitizer structures were modified by introducing an auxiliary acceptor of spiro[fluorene-9,9 '-thioxanthene]-10 ',10 '-dioxide between the donor and the pi-bridge moieties. The bulky auxiliary acceptor can suppress intermolecular aggregation, and the sulfone group can enhance Li+ ions trapping to retard charge recombination. The sensitizer YS-2 performed power conversion efficiency of 4.21% under AM1.5 solar condition. Both YS-1 and YS-2 displayed higher light conversion efficiency than N719 in the wavelength range of 300 similar to 500 nm. Therefore, mixing N719 with YS-2 in a co-sensitized test displayed an efficiency of 8.66% under one sun (AM 1.5G). While operated under CWF and TL84 indoor light, the efficiency was boosted to 22.45% and 24.72%, respectively, at 2500 lux illumination. The high co-sensitization performance of mixing N719 and YS-2 can be ascribed to the high absorption efficiency of YS-2 at short-wavelength, which is a shortcoming of N719 sensitizer. The high performance of N719 and YS-2 co-sensitization is mainly due to a better surface coverage on TiO2 surface. The durability test of the device co-sensitized with N719 and YS-2 showed a mild decay of PCE from 24.7% to 16.8% (68% maintenance) after 336 h in ambient atmosphere without encapsulation. In a comparison, the device made with bare-N719 starting with a PCE of 19.49% dropped to 3.87% (20% maintenance) under the same condition.