Organic Sensitizers with Extended Conjugation Frameworks as Cosensitizers of Porphyrins for Developing Efficient Dye-Sensitized Solar Cells.

Relatively high efficiencies have been achieved for porphyrin-based dye-sensitized solar cells. For the purpose of designing efficient cosensitizers, we herein report systematically optimized dyes XC1-XC5 employing a triphenylamine donor, a benzothiadiazole moiety as the auxiliary acceptor, and a benzoic acid acceptor. One hexyl and four hexyloxy groups were introduced, and an ethynylene moiety was introduced between the donor and the auxiliary acceptor to afford XC1. To further extend the absorption wavelength, a second ethynylene moiety was introduced between the acceptor and the auxiliary acceptor to afford XC2. XC3 and XC4 were designed by introducing one and two methyl substituents, respectively, into the meta-positions of the anchoring carboxyl group. XC5 was further synthesized by inserting a cyano substituent into one of the ortho-positions of the carboxyl group with the purpose to strengthen the intramolecular charge-transfer effect and thus broaden the absorption wavelength. As expected, compared with the Jsc (14.29 mA·cm-2) of XC1, the corresponding Jsc values for XC2-XC5 were enhanced to 16.50, 16.95, 16.73, and 17.74 mA·cm-2, respectively. Moreover, XC4 exhibits the highest Voc of 770 mV owing to the presence of a maximum of seven chains, which can effectively suppress dye aggregation. As a result, compared with XC1, XC2-XC5 exhibit improved efficiencies of 8.67, 9.05, 8.78, and 9.30%, respectively. In addition, the efficiencies of XC3 and XC5 were further enhanced by cosensitizing them with our previously reported porphyrin dye XW28 under various conditions. Finally, a remarkable efficiency of 10.50% was achieved for the cells cosensitized with XC5 and XW28. These results indicate that the combination of good planarity of the extended D-π-A framework with multiple alkoxy/alkyl chains may compose an effective optimizing strategy for designing and synthesizing excellent cosensitizers for porphyrins.