Are Alkynyl Spacers in Ancillary Ligands in Heteroleptic Bis(diimine)copper(I) Dyes Beneficial for Dye Performance in Dye-Sensitized Solar Cells?

The syntheses of 4,4′-bis(4-dimethylaminophenyl)-6,6′-dimethyl-2,2′-bipyridine ( 1 ), 4,4′-bis(4-dimethylaminophenylethynyl)-6,6′-dimethyl-2,2′-bipyridine ( 2 ), 4,4′-bis(4-diphenylaminophenyl)-6,6′-dimethyl-2,2′-bipyridine ( 3 ), and 4,4′-bis(4-diphenylaminophenylethynyl)-6,6′-dimethyl-2,2′-bipyridine ( 4 ) are reported along with the preparations and characterisations of their homoleptic copper(I) complexes [CuL 2 ][PF 6 ] (L = 1 – 4 ). The solution absorption spectra of the complexes exhibit ligand-centred absorptions in addition to absorptions in the visible region assigned to a combination of intra-ligand and metal-to-ligand charge-transfer. Heteroleptic [Cu( 5 )(L ancillary )] + dyes in which 5 is the anchoring ligand ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid) and L ancillary = 1 – 4 have been assembled on fluorine-doped tin oxide (FTO)-TiO 2 electrodes in dye-sensitized solar cells (DSCs). Performance parameters and external quantum efficiency (EQE) spectra of the DSCs (four fully-masked cells for each dye) reveal that the best performing dyes are [Cu( 5 )( 1 )] + and [Cu( 5 )( 3 )] + . The alkynyl spacers are not beneficial, leading to a decrease in the short-circuit current density ( J SC ), confirmed by lower values of EQE max . Addition of a co-absorbent ( n -decylphosphonic acid) to [Cu( 5 )( 1 )] + lead to no significant enhancement of performance for DSCs sensitized with [Cu( 5 )( 1 )] + . Electrochemical impedance spectroscopy (EIS) has been used to investigate the interfaces in DSCs; the analysis shows that more favourable electron injection into TiO 2 is observed for sensitizers without the alkynyl spacer and confirms higher J SC values for [Cu( 5 )( 1 )] + .