Visible, Far-Red, and Near-Infrared Active Unsymmetrical Squaraine Dyes Based on Extended Conjugation within the Polymethine Framework for Dye-Sensitized Solar Cells

Alkyl group wrapped visible, far-red, and NIR active unsymmetrical squaraine dyes with π-extension in the polymethine framework-based AM4-AM7 have been designed, synthesized, and utilized as sensitizers for dye-sensitized solar cells. To extend the π-conjugation within the polymethine framework, thiophene moieties have been incorporated between the donor and acceptor moieties. Absorption spectroscopic studies revealed that π-extension with each −C═C– unit resulted ∼100 nm of redshift in the charge transfer transition with the λ maximum of 541, 643, 747, and 833 nm for AM4, AM5, AM6, and AM7 dyes, respectively, with the molar extinction coefficient of >10 5 M –1 cm –1 . The π-extended conjugation-based AM6 and AM7 dyes showed improved light-harvesting efficiency (LHE), where the AM7 dye showed an LHE of 386 nm at 60%. Electrochemical studies of AM dyes revealed that the HOMO energy level of the sensitizers has been modulated systematically. Further, π-extension within the polymethine framework showed a dramatic effect on V OC, J SC, and device efficiency when move from visible active AM4 to far-red active to NIR active AM7 dyes. The DSSC efficiencies of 7.35, 5.18, 0.08, and 0.053% have been achieved with the I – /I 3 – electrolyte (Z-50) for the AM4, AM5, AM6, and AM7 dyes, respectively. Further, AM4 dye has been cosensitized with AM5, AM6, and AM7 dyes, where AM4:AM5 (1:1) composition achieved the maximum efficiency of 8.12% with I – /I 3 – electrolyte (Z-50) compared to the other cosensitization compositions.