Exploring simple ancillary ligands in copper-based dye-sensitized solar cells: effects of a heteroatom switch and of co-sensitization

The copper(i) complexes [Cu(1)2][PF6], [Cu(2)2][PF6], [Cu(3)2][PF6] and [Cu(4)2][PF6] (1 = 2-(1H-imidazol-2-yl)-6-methylpyridine, 2 = 2-(6-methylpyridin-2-yl)oxazole, 3 = 2-(6-methylpyridin-2-yl)thiazole and 4 = 2-methyl-6-(1-methyl-1H-imidazol-2-yl)pyridine) are reported. The crystal structures of [Cu(2)2][PF6].0.5CH2Cl2 and [Cu(3)2][PF6] confirm N,N'-chelation modes for 2 and 3, and tetrahedral copper(i). In the solution absorption spectra, the MLCT band shifts to lower energy with a change in heteroatom (O, 424 nm; NH, 435 nm; NMe, 446 nm; S, 465 nm). [Cu(1)2][PF6] and [Cu(4)2][PF6] undergo copper-centred oxidative processes at lower potential than the complexes with O or S heteroatoms. Heteroleptic complexes [Cu(5)(L)]+ (5 = ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bi s(4,1-phenylene)bis(phosphonic acid)), L = 1-4) were assembled on FTO/TiO2 electrodes. The shift in the MLCT band (O similar to NH < NMe < S) in the solid-state absorption spectra of the dye-functionalized electrodes parallels that of solution. The photoconversion efficiencies ( eta ) of masked, dye-sensitized solar cells (DSCs) containing [Cu(5)(L)]+ (L = 1-4) dyes and an I-/I3- redox shuttle follow the order [Cu(5)(1)]+ (3.03%) > [Cu(5)(3)]+ (2.88%) > [Cu(5)(4)]+ (2.71%) > [Cu(5)(2)]+ (2.62%) relative to 7.55% for N719. Ancillary ligand 1 (with NH) leads to the highest open-circuit voltage (VOC = 608 mV) whilst 3 (S-heteroatom) gives the highest short-circuit current density (JSC = 7.76 mA cm-2). The performances of [Cu(5)(1)]+ and [Cu(5)(3)]+ are understood with the aid of electrochemical impedance spectroscopy (EIS). The DSC with [Cu(5)(1)]+ exhibits a high chemical capacitance (C mu ) and a low recombination resistance (Rrec); since the latter is offset by a low transport resistance (Rtr), a high JSC and VOC are observed for [Cu(5)(1)]+. DSCs with [Cu(5)(3)]+ have the lowest Rtr of all four devices. The performance of DSCs sensitized by a combination of [Cu(5)(1)]+ and [Cu(5)(3)]+ were assessed in order to capitalize upon the high VOC of [Cu(5)(1)]+ and the high JSC of [Cu(5)(3)]+. After FTO/TiO2 functionalization with anchor 5, the electrodes were treated with a 1 : 1 mix of [Cu(5)(1)]+ and [Cu(5)(3)]+ or sequentially with [Cu(5)(3)]+ then [Cu(5)(1)]+, or [Cu(5)(1)]+ then [Cu(5)(3)]+. The DSC performances and the EIS parameters are consistent with competition between 1 and 3 for surface binding-sites; 1 dominates over 3, both in binding and in contribution to the overall photores