Understanding why replacing I-3 (-)/I- by cobalt(II)/(III) electrolytes in bis(diimine) copper(I)-based dye-sensitized solar cells improves performance

摘要

The performances of dye-sensitized solar cells (DSCs) comprising heteroleptic bis(diimine) copper(I) based dyes combined with either [Co(bpy)(3)](2+/3+), [Co(phen)(3)](2+/3+) or I-3 (-)/I- redox mediators (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline) have been evaluated. The copper(I) dyes contain the anchoring ligand ((6,60-dimethyl-[2,2'-bipyridine]-4,4'-diyl) bis(4,1-phenylene)) bis(phosphonic acid), 1, and an ancillary ligand (2, 3 or 4) with a 2,9-dimethyl-1,10-phenanthroline metal-binding domain. Ligands 2 and 3 include imidazole 2'-functionalities with 4-(diphenylamino) phenyl (2) or 4-(bis(4-n-butoxy) phenylamino) phenyl (3) domains; in 4, the phen unit is substituted in the 4,7-positions with hole-transporting 4-(diphenylamino) phenyl groups. The photoconversion efficiency, h, of each of [Cu(1)(2)](+), [Cu(1)(3)](+) and [Cu(1)(4)](+) considerably improves by replacing the I-3 (-)/I- electrolyte by [Co(bpy)(3)](2+/3+) or [Co(phen)(3)](2+/3+), and after a change of electrolyte solvent (MeCN to 3-methoxypropionitrile). Due to the faster charge transfer kinetics and more positive redox potential, the cobalt-based electrolytes are superior to the I-3 = (-)/I- electrolyte in terms of open-circuit voltage (V-OC), short-circuit current (J(SC)) and h; values of V-OC = 594 mV, J(SC) = 9.58 mA cm(-2) and h = 3.69% (relative to h = 7.12% for N719) are achieved for the best performing DSC which contains [Cu(1)(4)](+) and [Co(bpy)(3)](2+/3+). Corresponding values for [Cu(1)(4)](+) and I-3 (-)/I- DSCs are 570-580 mV, 5.98-6.37 mA cm(-2) and 2.43-2.62%. Electrochemical impedance spectroscopy (EIS) has been used to study DSCs with [Cu(1)(4)](+) and the three electrolytes. EIS shows that the DSC with I-3 (-)/I- has the highest recombination resistance, whereas the [Co(phen)(3)](2+/3+) electrolyte gives the highest chemical capacitance and V-OC and, between [Co(bpy)(3)](2+/3+) and [Co(phen)(3)](2+/3+), the higher recombination resistance. The [Co(phen)(3)](2+/3+) electrolyte exhibits the highest mass transport restrictions which result in a lower J(SC) and DSC efficiency compared to the [Co(bpy)(3)](2+/3+) electrolyte.