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

Abstract

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′-Me₂bpy), 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 TiO₂ using an on-surface assembly strategy which relies on ligand exchange between surface-anchored L_anchor and [Cu(L_ancillary)₂]⁺; ¹H 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′-Me₂bpy)]⁺ 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⁻/I₃⁻ 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.