Synthesis and Photovoltaic Properties of Powerful Electron-Donating Indeno[1, 2-b]thiophene-Based Green D-A-π-A Sensitizers for Dye-Sensitized Solar Cells

Three new near-infrared D-A-pi-A dyes (Si, S2, and S3) with 80 indeno[1,2-b]thiophene-based strong donor, containing 5,6-difluorobenzo[c][1,2,5]thiadiazole (DFBT), benzo[c][1,2,5]thiadiazole (BT), and 2,3diphenylpyrido[3,4-b]pyrazine (PP) as the auxiliary acceptor, respectively, were synthesized for the application of dye-sensitized solar cells. Introduction of powerful electron-donating unit of indeno[1,2-b]thiophene and the strong electron-withdrawing DFBT, BT, and PP groups into the conjugation bridge can enhance the absorption wavelength range into NIR region, where the maximum absorptions of the three compounds are all over 610 nm and S3 can reach 628 nm. Moreover, it is rare to see that the colors of the dyes are all green both in dichloromethane and on the TiO2 film. Via fine-tuning of the auxiliary moiety, the onset of incident-photon-conversion efficiency of S1-S3 can reach 850 nm and S3-based DSSCs in the presence of iodine and cobalt electrolytes show the best overall solar energy conversion efficiency of 6.29% and 7.23% under full sunlight (AM 1.5G, 100 mW cm(-2)) irradiation, which are relatively high values in organic NIR green dye-based DSSCs. Electrochemical impedance spectroscopy indicates that S3-based DSSCs can suppress charge recombination more efficiently, accounting for its higher open circuit voltage and short circuit current.