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-^tBuC₆H₄ (3), 4-MeOC₆H₄ (4), and 4-Me₂NC₆H₄ (5). The homoleptic copper(I) complexes [CuL₂][PF₆] (L = 1-5) were synthesized and characterized, and the single crystal structure of [Cu(1)₂][PF₆]^.Et₂O 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-TiO₂ 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 Me₂N 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.