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

Abstract

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)bis(4,1-phenylene)bis(phosphonic acid)), L = 1–4) were assembled on FTO/TiO₂ electrodes. The shift in the MLCT band (O ∼ NH < NMe < S) in the solid-state absorption spectra of the dye-functionalized electrodes parallels that of solution. The photoconversion efficiencies (η) 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 (V_OC = 608 mV) whilst 3 (S-heteroatom) gives the highest short-circuit current density (J_SC = 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_μ) and a low recombination resistance (R_rec); since the latter is offset by a low transport resistance (R_tr), a high J_SC and V_OC are observed for [Cu(5)(1)]⁺. DSCs with [Cu(5)(3)]⁺ have the lowest R_tr 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 V_OC of [Cu(5)(1)]⁺ and the high J_SC 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 photoresponse.