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

Abstract

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′-Ph₂-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 η 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.