Effects of Introducing Methoxy Groups into the Ancillary Ligands in Bis(diimine) Copper(I) Dyes for Dye-Sensitized Solar Cells

A systematic investigation of four heteroleptic bis(diimine) copper(I) dyes in n-type Dye-Sensitized Solar Cells (DSSCs) is presented. The dyes are assembled using a stepwise, on-surface assembly. The dyes contain a phosphonic acid-functionalized 2,2-bipyridine (bpy) anchoring domain (5) and ancillary bpy ligands that bear peripheral phenyl (1), 4-methoxyphenyl (2), 3,5-dimethoxyphenyl (3), or 3,4,5-trimethoxyphenyl (4) substituents. In masked DSSCs, the best overall photoconversion efficiency was obtained with the dye [Cu(5)(4)](+) (1.96% versus 5.79% for N719). Values of J(SC) for both [Cu(5)(2)](+) (in which the 4-MeO group is electron releasing) and [Cu(5)(4)](+) (which combines electron-releasing and electron-withdrawing effects of the 4- and 3,5-substituents) and are enhanced with respect to [Cu(5)(1)](+). DSSCs with [Cu(5)(3)](+) show the lowest J(SC). Solid-state absorption spectra and external quantum efficiency spectra reveal that [Cu(5)(4)](+) benefits from an extended spectral range at higher energies. Values of V-OC are in the order [Cu(5)(4)](+) > [Cu(5)(1)](+) > [Cu(5)(2)](+) > [Cu(5)(3)](+). Density functional theory calculations suggest that methoxyphenyl character in MOs within the HOMO manifold in [Cu(5)(2)](+) and [Cu(5)(4)](+) may contribute to the enhanced performances of these dyes with respect to [Cu(5)(1)](+).