In recent years, copper(ii/i) complexes have attracted growing interest as redox mediators in dye-sensitized solar cells. Here, we report a new class of copper complexes bearing a bis(2-pyridylmethyl)-1,2-ethanedithiol ligand, coded as [Cu(N2S2)]2+/+, with tetrafluoroborate ([BF4]-) and hexafluorophosphate ([PF6]-) counterions. Strikingly, it is discovered that the solubility and diffusion coefficients of Cu(i) species are dramatically improved by changing the counterion from [BF4]- to [PF6]-. As a result, DSCs incorporating the latter redox system show a much enhanced short-circuit current density and fill factor, which in turn lead to a superior overall power conversion efficiency (PCE) of 10.3% under one sun illumination (100 mW cm-2, AM 1.5G). This has been the highest PCE reported so far for copper redox mediators with sulfur-coordinating ligands in DSCs. Thus, this work provides an effective strategy for further design of more efficient redox mediators in DSCs.