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Novel Near-Infrared Squaraine Sensitizers for Stable and Efficient Dye-Sensitized Solar Cells



A new molecular design strategy for tuning the energy levels of cis-configured squaraine sensitizers for dye-sensitized solar cells is described. The Hammett substituent constant and the π-conjugation length are used as quantitative indicators to modify the central squarate moiety of the sensitizer dyes; specifically, novel near-infrared squaraine dyes HSQ3 and HSQ4 are synthesized by incorporation of an electron-withdrawing and π-extending ethyl cyanoacetate unit on the central squarate moiety. The solution absorption maximum of HSQ4 occurs at 703 nm, and the energy levels of the lowest unoccupied molecular orbital and the highest occupied molecular orbital are in the ideal range for energetically efficient electron injection and regeneration of the oxidized dye. A solar cell sensitized with HSQ4 exhibits a broad incident photo­n-to-current conversion efficiency spectrum, extending into the near-infrared region with a maximum value of 80% at 720 nm, which is is the highest value reported for a squaraine dye–based dye-sensitized solar cell. The HSQ4-sensitized solar cell also exhibits excellent durability during light soaking, owing to the double anchors attaching the dye to the TiO2 surface and to the long alkyl chains extending outward from the surface.