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

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, (n)butyl, (iso)butyl 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]}center dot 3Me(2)CO, [Cu(2)(2)][PF6], 2{[Cu(3)(2)][PF6]}center dot Et2O and [Cu(5)(2)][PF6]center dot 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 degrees, respectively; in contrast, the Cu+ ion in [Cu(5)(2)][PF6]center dot CH2Cl2 is in a flattened coordinate environment due to intra-cation face-to-face pi-interactions. The solution absorption spectra of the complexes with ligands 1-4 are virtually identical with an MLCT band with values of lambda(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)](+) (eta = 2.43% compared to 5.96% for standard dye N719) and [Cu(5)(7)](+) (eta = 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 lambda(max) approximate to 480 nm), the copper(I) dyes do not realize the broad spectral response exhibited by N719.