Factors controlling the photoresponse of copper(I) diimine dyes containing hole-transporting dendrons in dye-sensitized solar cells

Two series of 2,2′-bipyridine (bpy) ligands bearing different 6,6′-substituents (Me, nBu, isoBu, hexyl, Ph and 2-naphthyl) and carrying first-generation (ligands 1–6) or second-generation (ligands 7–12) hole transporting dendrons in the 4,4′-positions are reported. They have been incorporated into homoleptic copper(I) complexes [CuL2][PF6]. FTO/TiO2 electrodes functionalized with the anchoring ligand ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid), 13, were dipped in either CH2Cl2 or acetone solutions of [CuL2][PF6] to produce two series of surface-bound heteroleptic dyes. Their performances in dye-sensitized solar cells (DSCs) are assessed. Solid-state absorption spectra of dye-functionalized electrodes show that dye uptake is greater if acetone is used in the dye-dipping cycle rather than CH2Cl2, and the DSCs made using acetone generally perform better than analogous DSCs made using CH2Cl2. Using acetone-dipping solutions, the best DSC efficiencies are obtained with the second-generation dyes [Cu(13)(L)]+ (L = 7–11 with Me, nBu, isoBu, hexyl, Ph groups); [Cu(13)(12)]+ (12 contains 2-naphthyl groups in the 6,6′-positions) and its first-generation analogue [Cu(13)(6)]+ perform poorly. When CH2Cl2 is used in the dipping cycle, DSCs with dyes [Cu(13)(1)]+ and [Cu(13)(7)]+ (6,6′-Me2-substituted) show the highest VOC, JSC and η values, and EQE spectra confirm electron injection over a wider energy range than for other dyes. For CH2Cl2 in the dipping cycle (but not for acetone), [Cu(13)(5)]+ (6,6′-Ph2-substituted) performs as well as [Cu(13)(1)]+. The overall results of the study indicate that a combination of small 6,6′-substituents and acetone in the dye-dipping cycle lead to the best performing dyes.