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 ][PF 6 ] and [Cu( 3 ) 2 ][PF 6 ] 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 ][PF 6 ] confirms that the copper( i ) centre is sterically protected by the four CF 3 groups in a near regular tetrahedral environment. The Cu + /Cu 2+ oxidation potential is shifted from +0.44 to +0.72 V on going from [Cu( 1 ) 2 ][PF 6 ] to [Cu( 2 ) 2 ][PF 6 ] ( 1 = 6,6′-dimethyl-2,2′-bipyridine), in keeping with the electron-withdrawing effects of the CF 3 groups. 1 H and 19 F NMR spectroscopic data for a CH 2 Cl 2 solution containing [Cu( 1 ) 2 ][PF 6 ] and [Cu( 2 ) 2 ][PF 6 ] demonstrate that the ligands in [Cu( 2 ) 2 ][PF 6 ] 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 TiO 2 ; solid-state absorption spectra show that dye coverage for [Cu( 4 )( 3 )][PF 6 ] exceeds that of [Cu( 4 )( 2 )][PF 6 ]. 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 CF 3 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).