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 (), 4,4'-bis(4-dimethylaminophenylethynyl)-6,6'-dimethyl-2,2'-bipyridine (), 4,4'-bis(4-diphenylaminophenyl)-6,6'-dimethyl-2,2'-bipyridine (), and 4,4'-bis(4-diphenylaminophenylethynyl)-6,6'-dimethyl-2,2'-bipyridine () are reported along with the preparations and characterisations of their homoleptic copper(I) complexes [CuL][PF] (L = -). 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()(L)] dyes in which is the anchoring ligand ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid) and L = - have been assembled on fluorine-doped tin oxide (FTO)-TiO 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()()] and [Cu()()]. The alkynyl spacers are not beneficial, leading to a decrease in the short-circuit current density (), confirmed by lower values of EQE. Addition of a co-absorbent (-decylphosphonic acid) to [Cu()()] lead to no significant enhancement of performance for DSCs sensitized with [Cu()()]. Electrochemical impedance spectroscopy (EIS) has been used to investigate the interfaces in DSCs; the analysis shows that more favourable electron injection into TiO is observed for sensitizers without the alkynyl spacer and confirms higher values for [Cu()()].