Schiff Base Ancillary Ligands in Bis(diimine) Copper(I) Dye-Sensitized Solar Cells

Five 6,6′-dimethyl-2,2′-bipyridine ligands bearing N-arylmethaniminyl substituents in the 4- and 4′-positions were prepared by Schiff base condensation in which the aryl group is Ph (1), 4-tolyl (2), 4-(t)BuC(6)H(4) (3), 4-MeOC(6)H(4) (4), and 4-Me(2)NC(6)H(4) (5). The homoleptic copper(I) complexes [CuL(2)][PF(6)] (L = 1–5) were synthesized and characterized, and the single crystal structure of [Cu(1)(2)][PF(6)]·Et(2)O was determined. By using the “surfaces-as-ligands, surfaces-as-complexes” (SALSAC) approach, the heteroleptic complexes [Cu(6)(L(ancillary))](+) in which 6 is the anchoring ligand ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid)) and L(ancillary) = 1–5 were assembled on FTO-TiO(2) electrodes and incorporated as dyes into n-type dye-sensitized solar cells (DSCs). Data from triplicate, fully-masked DSCs for each dye revealed that the best-performing sensitizer is [Cu(6)(1)](+), which exhibits photoconversion efficiencies (η) of up to 1.51% compared to 5.74% for the standard reference dye N719. The introduction of the electron-donating MeO and Me(2)N groups (L(ancillary) = 4 and 5) is detrimental, leading to a decrease in the short-circuit current densities and external quantum efficiencies of the solar cells. In addition, a significant loss in open-circuit voltage is observed for DSCs sensitized with [Cu(6)(5)](+), which contributes to low values of η for this dye. Comparisons between performances of DSCs containing [Cu(6)(1)](+) and [Cu(6)(4)](+) with those sensitized by analogous dyes lacking the imine bond indicate that the latter prevents efficient electron transfer across the dye.