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

Abstract

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 [CuL₂][PF₆] (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)-TiO₂ 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 TiO₂ is observed for sensitizers without the alkynyl spacer and confirms higher J_SC values for [Cu(5)(1)]⁺.

Keywords: 2,2′-bipyridine; alkynyl group; copper; dye-sensitized solar cell; phosphonic acid anchor.