Multiple linker half-squarylium dyes for dye-sensitized solar cells; are two linkers better than one?

The synthesis and full characterization of new half-squaraine dyes (Hf-SQ) containing two or three carboxylate-based linker units is reported and these dyes tested in dye-sensitized solar cell (DSC) devices. The data show improved device efficiency for a Hf-SQ dye with two linkers (eta = 5.5%) compared to the highest efficiency Hf-SQ previously reported which had only a single linker (eta = 5.0%); this is mainly due to improved V-oc. To understand the effects of using multiple dye linker groups, device I-V data have been correlated with single crystal X-ray structural analysis and dye electrical properties (both in solution and adsorbed to TiO2) using UV-visible and ATR-IR spectroscopy along with cyclic voltammetry, and also theoretical studies using density functional theory (DFT) calculations. These data show that positioning the linkers near the dye LUMO and so that this enables complete linker chemisorption are key factors for device performance.