Improving the photoresponse of copper(i) dyes in dye-sensitized solar cells by tuning ancillary and anchoring ligand modules

Abstract

The syntheses of five homoleptic copper(I) complexes [CuL₂][PF₆] are described in which L is a 4,4′-di(4-bromophenyl)-6,6′-dialkyl-2,2′-bipyridine ligand (compounds 1–4 with methyl, ⁿbutyl, ^isobutyl and hexyl substituents, respectively) or 4,4′-di(4-bromophenyl)-6,6′-diphenyl-2,2′-bipyridine (5). The new ligands 2–5 and copper(I) complexes [CuL₂][PF₆] (L = 1–5) have been fully characterized. The single crystal structures of 2{[Cu(1)₂][PF₆]}·3Me₂CO, [Cu(2)₂][PF₆], 2{[Cu(3)₂][PF₆]}·Et₂O and [Cu(5)₂][PF₆]·CH₂Cl₂ have been determined. The first three structures show similar distorted tetrahedral environments for the Cu⁺ ions with angles between the least squares planes of the bpy domains of 85.6, 86.4 and 82.9°, respectively; in contrast, the Cu⁺ ion in [Cu(5)₂][PF₆]·CH₂Cl₂ is in a flattened coordinate environment due to intra-cation face-to-face π-interactions. The solution absorption spectra of the complexes with ligands 1–4 are virtually identical with an MLCT band with values of λ_max = 481–488 nm. In contrast, the absorption spectrum of [Cu(5)₂][PF₆] shows two broad bands in the visible region. Cyclic voltammetric data show that oxidation of the copper(I) centre occurs at a more positive potential in [Cu(2)₂][PF₆], [Cu(3)₂][PF₆] and [Cu(4)₂][PF₆] than in [Cu(1)₂][PF₆] or [Cu(5)₂][PF₆] with the latter being oxidized at the lowest potential. The complexes have been used to prepare dye-sensitized solar cells (DSCs) incorporating heteroleptic dyes of type [Cu(L)(L_anchor)]⁺ where L is 1–5 and L_anchor is a 6,6′-dimethyl-2,2′-bipyridine functionalized in the 4- and 4′-positions with phosphonic acid groups with (L_anchor = 7) and without (L_anchor = 6) a spacer between the metal-binding and anchoring domains. The presence of the spacer results in enhanced performances of the dyes, and the highest energy conversion efficiencies are observed for the dyes [Cu(3)(7)]⁺ (η = 2.43% compared to 5.96% for standard dye N719) and [Cu(5)(7)]⁺ (η = 2.89% compared to 5.96% for N719). Measurements taken periodically over the course of a week indicate that the cells undergo a ripening process (most clearly seen for [Cu(5)(6)]⁺ and [Cu(5)(7)]⁺) before their optimum performances are achieved. IPCE (EQE) data are presented and confirm that, although the photo-to-current conversions are promising (37–49% for λ_max ≈ 480 nm), the copper(I) dyes do not realize the broad spectral response exhibited by N719.