Copper-Based Redox Shuttles Supported by Pentadentate Ligands for Dye-Sensitized Solar Cells with Enhanced Efficiencies under Low-Light Conditions

Two copper-based redox shuttles (RSs) featuring tunable pentadentate polypyridyl ligands, 2,6-bis(1,1-di(pyridin-2-yl)ethyl)pyrazine (PY4PZMe2) and 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine (PY5Me2), have been developed and tested in dye-sensitized solar cell (DSC) devices with cosensitized commercial dyes XY1b and Y123. In the solid state, the CuI complexes of each ligand exhibit four-coordinate geometries, whereas the CuII complexes have five-coordinate geometries. Interestingly, the [Cu(PY4PZMe2)]+ ion crystallizes as a coordination polymer with the pyrazine group serving as a bridging ligand between CuI centers. A higher theoretical maximum VOC is predicted for the [Cu(PY4PZMe2)]2+/+ RS compared to the [Cu(PY5Me2)]2+/+ RS (840 vs 740 mV, respectively) in the DSC devices studied here based on the more positive CuII/I redox couple of the former relative to the latter. However, the [Cu(PY4PZMe2)]2+/+ based devices produced a lower VOC of 494 mV compared to a VOC of 642 mV with [Cu(PY5Me2)]2+/+ under 1-sun (1000 W/m2) illumination. Electrochemical impedance spectroscopy (EIS) measurements reveal that detrimental recombination processes are more favorable with [Cu(PY4PZMe2)]2+/+ where pyrazine is proposed to bind to the TiO2 surface, shifting the TiO2 conduction band to higher energies. This behavior is not observed with the [Cu(PY5Me2)]2+/+ based DSC devices, which display higher performance in terms of peak IPCE, JSC, and VOC values. Given the outstanding performance of Cu-based redox shuttles under ambient light conditions, low-light studies were pursued with the [Cu(PY5Me2)]2+/+ RS achieving an exceptional power conversion efficiency (PCE) of 21.2% at 2400 lx (4.7 W/m2) while [Cu(PY4PZMe2)]2+/+ based devices gave a PCE of 11.4%; these PCE values under low-light conditions represent 4- and 5-fold improvements in PCE, respectively, relative to full-sun conditions.