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