Effective suppression of interfacial charge recombination by a 12-crown-4 substituent on a double-anchored organic sensitizer and rotating disk electrochemical evidence

Mono/double-anchored phenothiazine-based dyes (DCE1-DCE4) containing various N-substituents (12-crown-4-substituted phenyl, 4-hexoxyphenyl, and bare phenyl) were synthesized and applied as sensitizers for dye-sensitized solar cells (DSSCs). The incorporation of a 12-crown-4-substituted phenyl moiety on the double-anchored dye effectively suppressed dark current via Li⁺ chelation in the case of LiI redox mediator or steric blockade of Co-phen³⁺ in the case of cobalt redox mediator. The suppression of dark current was further confirmed via rotating disk electrode technique from the recombination rate constant for the reduction of the oxidized redox mediators at the photoanode/electrolyte interface. The DCE2-based DSSC with iodide-based electrolyte exhibited a good cell efficiency up to 8.82%, which surpassed that of the N719-based DSSC (8.20%). With cobalt-based electrolyte, higher cell efficiencies of 10.12% and 11.17% were obtained at 1 sun and 0.1 sun, respectively, using a Pt-free and TCO-free counter electrode.