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

Abstract

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. ¹H and ¹⁹F 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 ≈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 λ_max = 480 nm).