Symmetrical and Unsymmetrical Polydentate Pyridyl Ligands for Redox Mediators Based on Co(II)/(III) Complexes

Muhammad Kashif^a, Udo Bach^a, Jordan Axelson^b, Leone Spiccia^b, Michael Nippe^b, Jeffrey Long^b, Christopher Chang^d, Noel Duffy^e

^aMonash University, Department of Materials Engineering, Australia, Clayton VIC 3800, Australia, Clayton, Australia

^bSchool of Chemistry, University of California, Berkeley, California 94720, EE. UU., Berkeley, United States

^cMonash University, AU, Calyton, 3800, Australia

^dDepartments of Chemistry and Molecular and Cell Biology and the Howard Hughes Medical Institute, University of California, Berkeley, California 94720, EE. UU., Berkeley, United States

^eCSIRO, Materials Science and Engineering, Clayton South, Victoria 3169, Australia

International Conference on Hybrid and Organic Photovoltaics
Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)

Ecublens, Switzerland, 2014 May 11th - 14th

Organizers: Michael Graetzel and Mohammad Nazeeruddin

Oral, Muhammad Kashif, presentation 185
Publication date: 1st March 2014

Cobalt (II)/(III) complexes have attracted considerable attention as redox mediator in dye-sensitized solar cells (DSCs) following the successful application of [Co(bpy)₃]^2+/3+. Most of these complexes are based on bidentate or tridentate polypyridyl ligands. One aspect of our research into alternative redox mediators is focused on the application of cobalt(II)/(III) complexes with polypyridyl ligands of higher denticity (pentadentate and hexadentate) as DSC redox mediators. Cobalt complexes with the general formula [Co(L₁)(Y)]^2+/3+ based on a pentadentate ligand L₁^1,2 and a weakly bound ligand, Y, have been synthesized. Lewis bases (B), which are important electrolyte components, have been shown to replace the monodentate ligand Y. The redox properties of the resulting complex [Co(L₁)(B)]^2+/3+ are dependent on the Lewis basicity of the base B. We show that this approach can be used to fine-tune the energetics of the dye regeneration process. The influence of ligand symmetry on the mediator performance will also be studied². Using an organic sensitizer we have attained efficiencies of 8.4% and 9.4% at simulated light intensity of 100% sun (1,000 Wm^-2 AM1.5) and at 10% sun, respectively, with an open circuit voltage (V_OC) in excess of 1V at 100% sun^a. The influence of denticity of the complexing ligands in cobalt-based mediators on the longevity of the DSC under illumination will also be addressed.

Diagram illustrating the DSC components and the cobalt(II)/(III) complexes used as redox mediators.
[1] Kashif, M.K.; Nippe, M.; Duffy, N. W.; Forsyth, C. M.; Chang, C. J.; Long, J. R.; Spiccia, L.; Bach, U. “Stable Dye-Sensitized Solar Cell Electrolytes Based on Cobalt (II)/(III) Complexes of a Hexadentate Pyridyl Ligand” Angew. Chem. Int. Ed., 2013, 52, 5527-5531 [2] Kashif, M.K.; Axelson, J.C.; Duffy, N. W.; Forsyth, C. M.; Chang, C. J.; Long, J. R.; Spiccia, L.; Bach, U. “A New Direction in Dye-Sensitized Solar Cells Redox Mediator Development: In Situ Fine-Tuning of the Cobalt (II)/(III) Redox Potential through Lewis Base Interactions” Journal of the American Chemical Society, 2012, 134, 16646-16653.

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