Spin-Doctoring Cobalt Redox Shuttles for Dye-Sensitized Solar Cells

Abstract

A new low-spin (LS) cobalt(II) outer-sphere redox shuttle (OSRS) [Co(PY5Me 2 )(CN)] + , where PY5Me 2 represents the pentadentate ligand 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine, has been synthesized and characterized for its potential application in dye-sensitized solar cells (DSSCs). Introduction of the strong field CN − ligand into the open axial coordination site forced the cobalt(II) complex, [Co-(PY5Me 2 )(CN)] + , to become LS based upon the complex's magnetic susceptibility (1.91 ± 0.02 μ B ), determined by the Evans method. Interestingly, dimerization and subsequent cobalt hexacyanide cluster formation of the [Co(PY5Me 2 )(CN)] + monomer was observed upon long-term solvent exposure or addition of a supporting electrolyte for electrochemical characterization. Although long-term stability of the [Co(PY5Me 2 )-(CN)] + complex made it difficult to fabricate liquid electrolytes for DSSC applications, short-term stability in neat solvent afforded the opportunity to isolate the self-exchange kinetics of [Co(PY5Me 2 )(CN)] 2+/+ via stopped-flow spectroscopy. Use of Marcus theory provided a smaller than expected self-exchange rate constant of 20 ± 5.5 M −1 s −1 for [Co(PY5Me 2 )(CN)] 2+/+ , which we attribute to a Jahn−Teller effect observed from the collected monomer crystallographic data. When compared sideby-side to cobalt tris(2,2′-bipyridine), [Co(bpy) 3 ] 3+ , DSSCs employing [Co(PY5Me 2 )(CN)] 2+ are expected to achieve superior charge collection, which result from a smaller rate constant, k et , for recombination based upon simple dark J−E measurements of the two redox shuttles. Given the negative redox potential (0.254 V vs NHE) of [Co(PY5Me 2 )(CN)] 2+/+ and the slow recombination kinetics, [Co(PY5Me 2 )(CN)] 2+/+ becomes an attractive OSRS to regenerate near IR absorbing sensitizers in solid-state DSSC devices.

Molecular Switch Cobalt Redox Shuttle with a Tunable Hexadentate Ligand

“…19 The self-exchange rate constant for the related [Co(PY5Me 2 ) (CN)] 2+/+ complex was determined to be an intermediate value of 20 ± 5.5 M −1 s −1 , which was necessarily measured without a supporting electrolyte and therefore represents an underestimated basis of comparison. 21 In contrast, [Co(bpy) 3 ](PF 6 ) 3/2 has an order of magnitude slower self-exchange rate constant of 0.27 ± 0.06 M −1 s −1 , which was attributed to a large inner sphere reorganization energy from the structural changes associated with low-spin to high-spin Co(III/II) electronic configurations. 19 Values of λ i were calculated by subtracting the outersphere reorganization energy, λ o , from the total reorganization energy of the self-exchange reaction, λ se , following the procedure previously reported.…”

“…Another low-spin Co(II) redox shuttle, [Co(PY5Me 2 )(CN)] 2+/1+ , demonstrated a suitably negative redox couple but suffered from stability issues and the formation of dimer and cluster complexes in solution. 21 Herein, a series of cobalt redox shuttles are synthesized with the goal of increased stability, redox potential tunability, and a strong-field ligand framework for low-spin Co(II) redox couples. Inspiration came from the PY5Me 2 , PY4Im, and bpyPY4 ligands previously coordinated to cobalt.…”

Toward Sustainable, Colorless, and Transparent Photovoltaics: State of the Art and Perspectives for the Development of Selective Near‐Infrared Dye‐Sensitized Solar Cells

“…All cobalt-based redox couples are characterized by Co(III) species having a closed-shell electron structure without unpaired electrons, which results in a lowspin state and translates into low absorption in the blue part visible region. [150] For Co(II) species, the situation is not straightforward because the energy difference between the HS and LS configuration is relatively low. As such, spin-crossover processes can take place, which leads to a decreasing of formal redox potential and higher internal reorganization energy.…”

“…[171] The approach led by Hamann et al to obtain LS states Co(II)-based mediators was really meaningful in lowering reorganization energy by hampering the spin crossing. [150] Unfortunately, DSSCs employing [Co(ttcn) 2 ] 3+/2+ (Table 5 entry 15) suffered from faster nongeminate recombination dynamic compared to [Co(bpy) 3 ] 3+/2+ . As aforementioned, the first and sole real attempt to tailor a Co-based selective NIR-DSSC was made with VG20 dye and [Co(bpy) 3 ] 3+/2+ as redox mediator in the electrolyte, delivering a 2.3% PCE, 68% AVT, and a CRI value of 92%.…”

Electronic and geometric structure effects on one-electron oxidation of first-row transition metals in the same ligand framework

“…[7][8][9] This team also reported structural parameters, magnetic susceptibility, electrochemical behaviour, and optical properties of the series of [M(Py5OMe)Cl] + (M = Mn, Fe, Co, Ni, Cu and Zn) complexes, but only in their reduced M II oxidation state. 4 During the following two decades, Py5 complexes have also been applied as anti-tumour agents, 10,11 redox mediators for dye-sensitized solar cells, 12,13 and materials with tunable magnetic properties. [14][15][16] In the field of the artificial photosynthesis, Py5-type ligands have been reported to be a suitable scaffold for synthesizing both proton reduction [17][18][19][20] and water oxidation catalysts.…”