Linearly π-extended squaraine dyes enable the spectral response of dye-sensitized solar cells in the NIR region over 800 nm

Squarylium dyes with linearly pi-extended structures involving three cyclobutene cores and four heterocyclic components were designed as metal-free sensitizing dyes that can enhance spectral responses in the range over 800 nm and successfully synthesized through the combination of Pd-catalyzed cross-coupling using stannylcyclobutenediones and condensation reactions. The dyes exhibited intense absorption in the NIR region with maxima at around 850 nm and onsets at around 950 nm. Although the HOMO levels remained at the same levels as the corresponding standard squaraine dye, the LUMO levels of the dyes were at lower energies owing to the lowering of the bandgaps of the dyes. Dye-sensitized solar cells (DSSCs) based on these dyes exhibiting significantly wide spectral response including the NIR region showed enhanced short-circuit current density. Co-sensitization of the NIR-absorbing dyes with a standard far-red absorbing squaraine dye produced a complementary effect in spectral response, resulting in the increase in short-circuit current density.