Molecular Switch Cobalt Redox Shuttle with a Tunable Hexadentate Ligand

Abstract

Strong-field hexadentate ligands were synthesized and coordinated to cobalt metal centers to result in three new lowspin to low-spin Co(III/II) redox couples. The ligand backbone has been modified with dimethyl amine groups to result in redox potential tuning of the Co(III/II) redox couples from −200 to −430 mV versus Fc +/0 . The redox couples surprisingly undergo a reversible molecular switch rearrangement from five-coordinate Co(II) to six-coordinate Co(III) despite the ligands being hexadentate. The complexes exhibit modestly faster electron selfexchange rate constants of 2.2−4.2 M −1 s −1 compared to the highspin to low-spin redox couple [Co(bpy) 3 ] 3+/2+ at 0.27 M −1 s −1 , which is attributed to the change in spin state being somewhat offset by this coordination switching behavior. The complexes were utilized as redox shuttles in dye-sensitized solar cells with the near-IR AP25 + D35 dye system and exhibited improved photocurrents over the [Co(bpy) 3 ] 3+/2+ redox shuttle (19.8 vs 18.0 mA/cm 2 ). Future directions point toward pairing the low-spin to low-spin Co(II/III) tunable series to dyes with significantly more negative highest occupied molecular orbital potentials that absorb into the near-IR where outer sphere redox shuttles have failed to produce efficient dye regeneration.

Benzothiophene-Based Cross-Conjugated Organic Dyes for High-Photocurrent Dye-Sensitized Solar Cells with Photocurrents up to 27 mA/cm2

“…Values this high have rarely been reached by ruthenium dyes . A decrease in photovoltage using the E3 electrolyte is expected and observed since TFMP does not increase the TiO 2 CB energy as high as with TBP …”

“…TFMP has weaker sigma electron donation to Ti­(IV) atoms due to the trifluoromethyl group acting as an electron-withdrawing group, which causes a reduction in the conduction band shift relative to that of TBP. Therefore, a V OC decrease and a J SC increase in E3 are expected compared to E1 and E2 …”

Review on Functional Electrolyte, Redox Polymers, and Solar Conversions in 3G Emerging Photovoltaic Technologies: Progress and Outlook

“…Redox electrolytes based on cobalt complexes have defined potential ranges ( E 0 = 0.3–1.0 V vs NHE), which may be due to structural variation of the organic ligands . The redox potential varies with position and number of redox centers. , Photovoltage can be maximized by controlling the redox center or redox potential of electrolyte. , In general, low V oc originates from the relatively high redox potential of pristine polysulfide electrolyte, which needs high overpotential for QD regeneration. Therefore, V oc can be maximized through a downshift of redox potential of polysulfide electrolyte in the potential energy levels.…”

“…65,67 Photovoltage can be maximized by controlling the redox center or redox potential of electrolyte. 68,69 In general, low V oc originates from the relatively high redox potential of pristine polysulfide electrolyte, which needs high overpotential for QD regeneration. Therefore, V oc can be maximized through a downshift of redox potential of polysulfide electrolyte in the potential energy levels.…”

Halogen functionalized D–A–D-type unsymmetrical squaraine dyes for dye-sensitized solar cells

“…26,27 Additionally, a pyridyl-donor-based D-p-A dye on TiO 2 showed enhanced regeneration kinetics with a cobalt electrolyte for improved device performance by improving the interaction between the dye and the cobalt electrolyte through the pyridyl unit in the dye. [33][34][35] Hence, integration of a functional moiety that helps to promote the dye-regeneration process by either halogen-bonding or halogen-bonding (s-hole in the halogen-atom-containing dyes) with a Lewis base are very important to enhance the DSSC device J SC and efficiency. 25,34 As there are only a few chromophores that harvest photons in the far-red and NIR regions of the solar spectrum, such as porphyrin-and phthalocyanine-based dyes, it is important to develop metal-free organic dyes for dye-sensitized solar cells.…”