Efficient electron injection due to a special adsorbing group’s combination of carboxyl and hydroxyl: dye-sensitized solar cells based on new hemicyanine dyes

Abstract

A series of new benzothiazolium hemicyanine dyes (HC-1, HC-2, HC-3, HC-4, and HC-5 in Scheme 1) were designed and synthesized for sensitization of nanocrystalline TiO₂ electrodes by introducing carboxyl, hydroxyl, or sulfonate anchoring groups onto the dyes' skeletons. A naphthothiazolium hemicyanine with both sulfonate and hydroxyl (HC-6) was also prepared for comparison. The photophysical and photoelectrochemical studies revealed that three kinds of efficiencies, i.e. the fluorescence quenching efficiencies of the dyes by colloidal TiO₂, the monochromatic incident photon-to-current conversion efficiencies (IPCEs) for the dye-sensitized TiO₂ electrodes, and the overall photoelectric conversion efficiencies (η) for the dye-sensitized solar cells (DSSCs) based on these hemicyanines, all depended strongly on the anchoring group types and decreased in the order: carboxyl + hydroxyl > carboxyl > sulfonate + hydroxyl, indicating the importance of the dyes' adsorbing groups for their sensitization effects in DSSCs. The combination of carboxyl and hydroxyl as anchoring groups led to highly efficient IPCEs over a wide spectrum region with the maximum IPCE of 73.6% and a η of 5.2% under AM1.5 Global simulated light (80 mW cm⁻²) for the HC-1 based DSSC, which may result from the complex formation between HC-1 and TiO₂ and the cathodic shift of the excited state oxidation potential.