The versatile SALSAC approach to heteroleptic copper(I) dye assembly in dye-sensitized solar cells

Abstract

Surface-bound heteroleptic copper(I) dyes [Cu(Lanchor)(Lancillary)]+ are assembled using the "surfaces-as-ligands, surfaces as complexes" (SALSAC) approach by three different procedures. The anchoring and ancillary ligands chosen are (6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)-bis(4,1-phenylene)bis(phosphonic acid) (3) and4,4′-bis(4-iodophenyl)-6,6′-diphenyl-2,2′-bipyridine (4), respectively. In the first SALSACprocedure, the FTO/TiO2 electrode is functionalizedwith 3 in the first dye bath, and then undergoes ligand exchangewith the homoleptic complex [Cu(4)2][PF6] to give surface-bound [Cu(3)(4)]+. In the secondmethod, the FTO/TiO2 electrode functionalizedwith 3 is immersed in a solution containing a 1:1mixture of [Cu(MeCN)4][PF6] and 4 to give surface-anchored [Cu(3)(4)]+. In the third procedure, the anchor 3, copper(I) ion and ancillary ligand 4 are introduced in a sequential manner. The performances of the DSSCs show a dependence on the dye assembly procedure. The sequentialmethod leads to the best-performing DSSCswith the highest values of JSC (7.85 and 7.73mAcm-2 for fullymasked cells) and overall efficiencies (h = 2.81 and 2.71%, representing 41.1 and 39.6%relative to anN719 referenceDSSC). Use of the 1:1mixture of [Cu(MeCN)4][PF6] and 4 yieldsDSSCs with higher VOC values but lower JSC values compared to those assembled using the sequential approach; values of h are 2.27 and 2.29%versus 6.84%for theN719 reference DSSC. The ligand exchange procedure leads to DSSCs that performrelatively poorly. The investigation demonstrates the versatile and powerful nature of SALSAC in preparing dyes for copper-basedDSSCs, allowing the photoconversion efficiency of dye to be optimized for a given dye. The SALSAC strategy provides alternative hierarchical strategieswhere the isolation of the homoleptic [Cu(Lancillary)2]+ is difficult or time-consuming; stepwise strategies aremore atom-economic than ligand exchange involving the homoleptic [Cu(Lancillary)2]+.