Dye-sensitized solar cells (DSCs) are an attractive technology for producing renewable electricity from sunlight due to their high photon-to-current conversion efficiencies and low manufacturing cost. In a DSC, light absorption by a surface-anchored dye results in electron transfer from the excited dye into the conduction band of a metal oxide semiconductor, a redox shuttle (RS) then regenerates the oxidized dye. The injected electron traverses as an external circuit to the counter electrode where the oxidized RS collects the electron to complete the circuit. Transition metal complexes have been widely used as RSs in DSCs because they are capable of giving better device efficiencies than the conventional I–/I3– redox couple. Metal-based RSs can be finely tuned by modifying the coordination environment to match dye energetics and they serve as one-electron transfer agents that can avoid the excess driving force required of the two-electron I–/I3– couple. Indeed, Cu-based RSs have shown promising results with low driving forces for dye regeneration (~100 mV) and efficiencies as high as 13% for light-to-electric energy conversion. Most Cu RSs reported in the literature feature bipyridine or phenanthroline ligands with bulky groups adjacent to the nitrogen donor atoms to reduce the high inner-sphere electron transfer reorganization energies that are common for these complexes. We recently initiated a program to develop Cu-based RSs supported by preorganized multidentate ligands with the goal to impart lower inner-sphere reorganization energies and high stability via the chelate effect. Here, we report the synthesis, characterization, and DSC performance metrics of a novel Cu-based RS bearing a pentadentate mixed-donor ligand. The Cu(II/I) redox couple is at 0.40 V vs NHE (in CH3CN / 0.1 M Bu4NPF6), which makes it suitable for pairing with dyes absorbing out to ~950 nm on TiO2, an ideal location for harvesting light in the near infrared (NIR) region of the solar spectrum. Using dye Y123, a current density of 11.4 mA/cm2 was observed while I–/I3– gives 12 mA/cm2 with the same configuration. The retention of high photocurrent indicates that this is a promising RS for NIR DSC devices.

Copper based redox shuttle for high performance dye sensitized solar cells

ACS Fall 2021

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