Copper( i )-based dye-sensitized solar cells with sterically demanding anchoring ligands: bigger is not always better

The synthesis and characterization of ((6,6′-diphenyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid), 2 , are described. Compound 2 has been incorporated as an anchoring ligand in copper( i )-containing dyes in n-type dye-sensitized solar cells (DSCs), combined with 2,2′-bipyridine (bpy), 6-methyl-2,2′-bipyridine (6-Mebpy), 6,6′-dimethyl-2,2′-bipyridine (6,6′-Me 2 bpy), 4,4′-di(4-bromophenyl)-6,6′-dimethyl-2,2′-bipyridine ( 3 ) or 4,4′-di(4-bromophenyl)-6,6′-diphenyl-2,2′-bipyridine ( 4 ) as ancillary ligands (L ancillary ). Dyes were assembled on mesoporous TiO 2 using an on-surface assembly strategy which relies on ligand exchange between surface-anchored L anchor and [Cu(L ancillary ) 2 ] + ; 1 H NMR spectroscopy was used to confirm that the bulky phenyl substituents did not hinder ligand exchange. Comparison of values of the open-circuit voltages ( V OC ), short-circuit current densities ( J SC ) and external quantum efficiency (EQE) spectra for DSCs with model dyes [Cu( 2 )(bpy)] + , [Cu( 2 )(6-Mebpy)] + and [Cu( 2 )(6,6′-Me 2 bpy)] + confirm that methyl-substituents in L ancillary are beneficial. Performance data for DSCs with dyes [Cu( 1 )( 3 )] + , [Cu( 1 )( 4 )] + , [Cu( 2 )( 3 )] + and [Cu( 2 )( 4 )] + where 1 is the anchor ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid) show that dyes with anchor 2 (phenyl substituents in the 6- and 6′-positions) give relative conversion efficiencies ≤10% with respect to standard dye N719 set at η = 100%; this compares with relative efficiencies of up to 34.5% for the dyes [Cu( 1 )( 3 )] + and [Cu( 1 )( 4 )] + . The performance of [Cu( 2 )( 3 )] + can be improved by the addition of the co-adsorbant chenodeoxycholic acid. Although the phenyl ( versus methyl) substituents lead to enhanced light absorption to lower energies, dyes with anchor 2 quickly bleach when exposed to the I − /I 3 − electrolyte; bleaching also occurs after soaking in solutions of LiI. The dye can be regenerated by treatment of a bleached electrode with L ancillary , or with [Cu(NCMe) 4 ] + followed by L ancillary .