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

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 (eta) 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 eta 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.