Tetracoordinated Bis-phenanthroline Copper-Complex Couple as Efficient Redox Mediators for Dye Solar Cells

A tetracoordinated redox couple, made by [Cu­(2-mesityl-4,7-dimethyl-1,10-phenanthroline)₂]­[PF₆], <b>1</b>, and its Cu­(II) form [Cu­(2-mesityl-4,7-dimethyl-1,10-phenanthroline)₂]­[PF₆]₂, <b>2</b>, has been synthesized, and its electrochemical and photochemical features have been investigated and compared with those of a previously published Cu²⁺/Cu⁺ redox shuttle, namely, [Cu­(2,9-dimethyl-1,10-phenanthroline)₂]­[PF₆], <b>3</b>, and its pentacoordinated oxidized form [Cu­(2,9-dimethyl-1,10-phenanthroline)₂Cl]­[PF₆], <b>4</b>. The detrimental effect of the fifth Cl^– ancillary ligand on the charge transfer kinetics of the redox shuttles has been exhaustively demonstrated. Appropriately balanced Cu-based electrolytes have been then formulated and tested in dye solar cells in combination with a π-extended benzothiadiazole dye. The bis-phenanthroline Cu-complexes, <b>1</b> and <b>2</b>, have been found to provide an overall 4.4% solar energy conversion efficiency, which is more than twice that of the literature benchmark couple, <b>3</b> and <b>4</b>, employing a Cl-coordinated oxidized species and even comparable with the performances of a I^–/I₃^<b>–</b> electrolyte of analogous concentration. A fast counter-electrode reaction, due to the excellent electrochemical reversibility of <b>2</b>, and a high electron collection efficiency, allowed through the efficient dye regeneration kinetics exerted by <b>1</b>, represents two major characteristics of these copper-based electron mediators and may constitute a pivotal step toward the development of a next generation of copper-based efficient iodine-free redox shuttles