Copper complexes coordinated with a hexadentate or pentadentate ligand have been investigated as potential redox shuttles to be used in dye-sensitized solar cells (DSSCs). Copper complexes have had a growing interest as redox shuttles in DSSCs, however, it has been revealed that Lewis base additives such as 4-tert-butylpyridine (TBP), in the DSSC electrolyte can coordinate with copper (II) species, displacing the ligand bound the copper (I) species. This alters the solution potential, reorganization energy, and electron transfer kinetics, which can restrict the overall performance and stability of the device. To prevent this substitution two multidentate ligands predicted to have high stability due to the chelate effect were chosen. The studied copper complexes, [Cu(bpyPY4)]2+/+, where bpyPY4 is 6,6’-bis(1,1-di(pyridine-2-yl)ethyl)-2,2’-bipyridine, and [Cu(PY5Me2)]2+/+, where PY5Me2 is the pentadentate ligand 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine, are shown to be stable in the presence of TBP through the use of cyclic voltammetry, UV-Vis and 1H-NMR spectroscopy. However, the 5-coordinate square pyramidal structure of [Cu(PY5Me2)]2+/+ leaves an open binding site allowing for Lewis bases to coordinate. This allows for the potential of the redox shuttle to be tuned to match the requirements of the sensitizer. The tunability of the [Cu(PY5Me2)]2+/+ complex along with the low redox potential of both complexes allows for an interesting opportunity to investigate and optimize devices utilizing near infrared absorbing sensitizers that have a narrow band gap and low oxidation potentials.

Investigation of low potential copper redox shuttles

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