Intramolecular Exciton-Coupled Squaraine Dyes for Dye-Sensitized Solar Cells

V-shaped squaraine dyes with carbazole cores in the center and indolenine groups at the end were designed to broaden the light-harvesting range of dye-sensitized solar cells (DSSCs) by taking advantage of intramolecular exciton coupling. V-shaped squaraines containing carboxy group on the indolenine moiety and 4-phenyl-2-cyanoacrylic acid groups on the carbazole cores as anchoring groups for chemical adsorption onto TiO2 were synthesized by the utilization of Stille cross-coupling as a key reaction. Although conventional squaraine dyes show typically narrow absorption bands, V-shaped squaraine dyes exhibited split absorption bands due to the intramolecular exciton coupling and thereby enabled the absorption of a broad range of light. The exciton coupling between squaraine chromophores obliquely installed in the molecule was theoretically supported by the Kasha exciton coupling model, in which the coupling energy was well-fitted with the experimental absorption spectra. As the result of intramolecular exciton coupling, the DSSCs based on exciton-coupled squaraine dyes exhibited a spectral response in a much wider range compared to that of a conventional linear counterpart, giving the higher energy conversion efficiency in comparison to that of the single chromophoric counterpart.