Fine-tuning the coordination atoms of copper redox mediators: an effective strategy for boosting the photovoltage of dye-sensitized solar cells

Abstract

Natural systems have marvelously utilized copper complexes featuring sulfur-coordinating ligands, known as blue copper proteins, as efficient electron-transfer mediators in biological processes. Copper complexes with sulfur-coordinating ligands have been attempted as redox mediators in dye-sensitized solar cells (DSCs), the performance of which is not yet satisfactory and still remains less well explored. Herein, we report the application of new copper complexes bearing a tetradentate polythioether ligand, [(S₄)Cu]^2+/+ (1^2+/+, S₄ = 1,4,8,11-tetrathiocyclotetradecane), as a redox mediator in DSCs in comparison with its N₄-tetradentate counterpart [(N₄)Cu]^2+/+ (2^2+/+, N₄ = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). Impressively, the changes of coordination atoms from N to S positively shift the formal redox potential of the copper complexes by 600 mV, leading to a remarkably high photovoltage approaching 1.0 V. This is one of the highest photovoltage values reported thus far for DSCs based on copper redox mediators.