The beneficial effects of trifluoromethyl-substituents on the photoconversion efficiency of copper(I) dyes in dye-sensitized solar cells

The synthesis and characterization of [Cu(2)(2)][PF6] and [Cu(3)(2)][PF6] in which 2 = 6,6'-bis(trifluoromethyl)2,2'- bipyridine and 3 = 6-trifluoromethyl-2,2'- bipyridine are reported. The single crystal structure of [Cu(2)(2)][PF6] confirms that the copper(I) centre is sterically protected by the four CF3 groups in a near regular tetrahedral environment. The Cu+/Cu2+ oxidation potential is shifted from +0.44 to +0.72 V on going from [Cu(1)(2)][PF6] to [Cu(2)(2)][PF6] (1 = 6,6'-dimethyl- 2,2'- bipyridine), in keeping with the electron-withdrawing effects of the CF3 groups. H-1 and F-19 NMR spectroscopic data for a CH2Cl2 solution containing [Cu(1)(2)][PF6] and [Cu(2)(2)][PF6] demonstrate that the ligands in [Cu(2)(2)][PF6] remain highly labile. An on-surface procedure was used to assemble the dyes [Cu(4)(2)](+) and [Cu(4)(3)](+) (4 = anchoring ligand ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl) bis(4,1-phenylene)) bis(phosphonic acid)) on TiO2; solid-state absorption spectra show that dye coverage for [Cu(4)(3)][PF6] exceeds that of [Cu(4)(2)][PF6]. The performances of the dyes in dye-sensitized solar cells (DSCs) are compared with that of [Cu(4)(1)](+). DSCs containing the fluorinated ancillary ligands exhibit photoconversion efficiencies of approximate to 30-34% relative to N719 set at 100%; this is a significant enhancement with respect to DSCs containing [Cu(4)(1)](+) with no fluoro-groups. Density functional theory (DFT) calculations for the heteroleptic dyes demonstrate that the HOMO is stabilized upon introduction of the CF3 groups, but that the compositions of the molecular orbitals in the HOMO-LUMO manifold are little changed. The improved DSC performance is associated with enhanced short-circuit current density (J(SC)) for [Cu(4)(3)](+) and [Cu(4)(2)](+) versus [Cu(4)(1)](+); this is also seen in high EQE(max) values of 46% for [Cu(4)(2)](+) and 51% for [Cu(4)(3)](+) (both at lmax = 480 nm).