Combining phosphonic acid-functionalized anchoring ligands with asymmetric ancillary ligands in bis(diimine)copper(i) dyes for dye-sensitized solar cells

Abstract

A ‘surfaces-as-ligands’ strategy is used to assemble heteroleptic copper(I) dyes [Cu(L_anchor)(L_ancillary)]⁺ on FTO/TiO₂ electrodes for dye-sensitized solar cells (DSCs). The anchoring domain, L_anchor, is either ((6,6′-diphenyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid) (1) or ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid) (2). Asymmetric ancillary ligands with a 2,2′-bipyridine metal-binding domain are used to counter the sterically-demanding 6,6′-diphenyl-substitution pattern in anchor 1; L_ancillary = 6-methyl-4-phenyl-2,2′-bipyridine (3), 6-methyl-4-(4-bromophenyl)-2,2′-bipyridine (4), 6-methyl-4-(4-methoxyphenyl)-2,2′-bipyridine (5) or 6-methyl-4-(3,4,5-trimethoxyphenyl)-2,2′-bipyridine (6). Solid-state absorption spectra of adsorbed [Cu(1)(L_ancillary)]⁺ and [Cu(2)(L_ancillary)]⁺, and external quantum efficiency (EQE) spectra of DSCs containing these dyes confirm that incorporation of 6,6′-diphenyl-substituted 1 leads to a broadened spectral response at lower energies compared to dyes with anchor 2; dye-loading is higher for [Cu(2)(L_ancillary)]⁺ than for [Cu(1)(L_ancillary)]⁺, and EQE_max is >41% for [Cu(2)(L_ancillary)]⁺ compared to <12% for [Cu(1)(L_ancillary)]⁺. Enhanced values of the short-circuit current density (J_SC) are observed on going from anchor 1 to 2, independent of L_ancillary. For the series of [Cu(2)(L_ancillary)]⁺ dyes, photoconversion efficiencies (confirmed using four DSCs per dye) vary with L_ancillary in the order 3 ∼ 5 > 6 > 4 on the day of DSC assembly, and 5 > 3 ∼ 6 > 4 after a week. The best performing DSCs achieve efficiencies of ∼37% relative to N719 set at 100%.