Influence of a co-adsorbent on the performance of bis(diimine) copper(I)-based dye-sensitized solar cells

In the bis(diimine) copper(I) dyes, [Cu(1)(3)](+) and [Cu(1)(4)](+) (1 = ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid), the ancillary ligands 3 and 4 contain sterically demanding second-generation hole-transporting dendrons with methyl or phenyl substituents adjacent to the N-donor atoms of the 2,2'-bipyridine metal-binding domain. The performances of DSCs containing [Cu(1)(3)](+) and [Cu(1)(4)](+) depend on both the solvent (acetone or CH2Cl2) used in the dye-bath and on the presence of a co-adsorbent. Irrespective of solvent, the dye [Cu(1)(4)](+) (6,6'-Ph-2-substituted) only performs well if chenodeoxycholic acid (cheno) is added as a co-adsorbent; for [Cu(1)(3)](+), cheno has a noticeable effect when the dye assembly is carried out in CH2Cl2. Overall, the results indicate that a combination of small 6,6'-substituents in the ancillary ligand and acetone in the dye-dipping cycle lead to the best performing dyes, and for the second-generation dyes, the addition of cheno is essential. Conditions to form TiO2-bound [Cu(1)(5)](+) (5 = 4,4'-bis(4-iodophenyl)-6,6'-dimethyl-2,2'-bipyridine) in a stepwise manner have been optimized and the effects of introducing cheno at different points during the dye-assembly process have been investigated. When cheno is added to the [Cu(MeCN)(4)][PF6]/5 dye-bath, the DSCs exhibit values of J(SC), V-OC and eta values that are similar to those with no co-adsorbent. However, competitive binding of 1 and cheno in the first dipping-cycle leads to lower values of J(SC) and lower photoconversion efficiencies.