Are High-Potential Cobalt Tris(bipyridyl) Complexes Sufficiently Stable to Be Efficient Mediators in Dye-Sensitized Solar Cells? Synthesis, Characterization, and Stability Tests

Abstract

We report the synthesis of three new, high-oxidation-potential cobalt complexes. The ligand lability of cobalt complexes, especially those of higher redox potential, has been scantly addressed in the current literature on the use of these complexes in dyesensitized solar cells (DSSCs). In characterizing our high-potential complexes, multiple pieces of evidence reveal a trend of decreasing complex stability with increasing redox potential. These include the appearance of multiple waves in the cyclic voltammetry, a color change upon addition of 4-tert-butylpyridine indicating the formation of a new species, direct NMR evidence of free dissociated ligand in acetonitrile solution, and potential-independent DSSC recombination currents paralleling the stability trends as determined by NMR. We take advantage of a simple quantitative NMR experiment to determine the approximate ligandbinding equilibrium (stability) constants of our complexes in acetonitrile at room temperature. With the above information, we propose alternative, clathrochelating ligand structures for the future study of high potential cobalt mediators for application in DSSCs.

Coordinating Ability of Anions, Solvents, Amino Acids, and Gases towards Alkaline and Alkaline‐Earth Elements, Transition Metals, and Lanthanides

“…Electrochemical studies of [Co({NC} 2 bipy) 3 ] 2+ indicated the presence of a second species in acetonitrile, that was interpreted as possibly resulting from the substitution of one bidentate ligand by two solvent molecules. The use of the less coordinating solvent nitromethane resulted in the absence of the second species, thus confirming the importance of choosing solvents with low coordinating ability indices for electrochemical experiments on transition metal complexes …”

“…Electrochemical studies of [Co({NC} 2 bipy) 3 ] 2 + indicated the presence of as econd species in acetonitrile, that was interpreted as possibly resulting from the substitution of one bidentate ligand by two solventm olecules.The use of the less coordinating solventn itromethane resulted in the absence of the second species, thus confirming the importance of choosing solvents with low coordinatinga bility indices for electrochemical experiments on transition metal complexes. [70] In as tudy of binuclear cycloplatinated(II) complexes,S angari et al found in DFTcalculations that the positive chargea tt he Pt atom was smaller for complexesw ith the chloride than with the trifluoroacetate ligand, and associated such ad ifference to the higher donor ability of the former. [71] An aerosol-assisted chemical vapour deposition (AACVD) study of CdS thin films at 400 8Cf ound that chloroform, with a much lower coordinating ability and lower heat of combustion than THF evaporates faster causing af aster growth of porous thin films with irregular texture and poorer adhesion to the substrate.…”

Light-induced electrolyte improvement in cobalt tris(bipyridine)-mediated dye-sensitized solar cells

“…47 This effect has also been noted by Kirner and Elliot suggesting a Co(III) ligand exchange involving TBP. 48 Based on our previous work, 2 the adsorption of TBP molecules could inuence the surface property of the Pt electrode and thus the charge transport kinetics. Aer light exposure, CVs of the TBP-containing system show no scan-rate dependence and a 3-fold limiting current.…”

“…The effects of adding TBP to a solution/electrolyte containing [Co(bpy) 3 ] 3+ must involve a change in the coordination geometry of [Co(bpy) 3 ] 3+ . 48 There are however no clues in the spectroscopic results to what type of coordination that may take place. Considering the fast TBP exchange, a general expression of ligand effect can be formulated (eqn (2)).…”

“…TBP as a strong Lewis base is a likely competitive ligand for coordination to the cobalt centres based on evidence regarding the instability of cobalt tris(bipyridine) complexes through ligand exchange. 10,11,22,23 In addition, TBP was claimed by Salim et al to exhibit a shielding effect with respect to the [Co(bpy) 3 ] 3+ ions via weak ion-dipole attraction accounting for the retarding role in the recombination loss reactions. 21 This work will further explore the interaction between the two necessary electrolyte components in a lithium-ion-free [Co(bpy) 3 ] 2+/3+ -based DSSC system, where a high concentration of [Co(bpy) 3 ] 3+ is used since it has been demonstrated benecial for long-term device stability.…”