Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
In the pursuit of high stability redox mediators, we have developed novel hemicage cobalt complexes for dye sensitised solar cells.
Cobalt complexes have a negligible extinction coefficient, and their redox properties can be tuned by synthetic modifications of the ligand. This approach offers an attractive alternative to the traditional I3−/I− redox shuttle used as dye solar cells with cobalt, which have demonstrated the highest efficiencies with liquid electrolytes.1,2
An innovative synthetic approach for controlling the electronic and structural properties of the mediator couple systems was used. We combined all six nitrogen-donors in one multi-dentate ligand. This chelating ligand reduces the entropy losses during metal binding and suppresses dissociation of the Ligands as reported for [Co(bpy)3]2+/3+, leading to a high binding constant.3,4 The well-known 1,3,5-tri-ethylphenyl-platform was the base for attachment of three bipyridine-ligands in the 2, 4, and 6-position.5 The proposed rigid and symmetric bipyridine ligand system of the hemicaged complex can effectively shield the cobalt(II)-core, and provide an unprecedented control of the geometrical preorganization and binding geometry for exploring the electrochemical processes.
The cobalt complex using a hemicage ligand [Co(ttb)]2+/3+ TFSI3/2 and [Co(teb)]2+/3+ TFSI3/2 as redox mediators in combination with a cyclopentadithiophene-bridged donor-acceptor dye (LEG 4), adsorbed on TiO2, yielded a power conversion efficiency of over 6% at 100 mW cm−2. This result indicates that the molecularly engineered cobalt redox shuttle is also a legitimate alternative to the commonly used cobalt redox mediators.
General structure of a new ligand class for cobalt based mediators for DSCs
(1)Feldt, S. M.; Gibson, E. A.; Gabrielsson, E.; Sun, L.; Boschloo, G.; Hagfeldt, A. A cobalt complex redox shuttle for dye-sensitized solar cells with high open-circuit potentials. JACS 2010, 132, 16714. (2)Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Graetzel, M. Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency. Science 2011, 334, 629. (3)Karunadasa, H. I.; Montalvo, E.; Sun, Y.; Majda, M.; Long, J. R.; Chang, C. J. A molecular MoS₂ edge site mimic for catalytic hydrogen generation. Science 2012, 335, 698. (4)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. Angewandte Chemie Int. Ed. 2013, 52, 5527. (5) Wang, J.; Oyler, K. D.; Bernhard, S. Synthesis and characterization of hemicage 8-hydroxyquinoline chelates with enhanced electrochemical and photophysical properties. Inorganic Chemistry 2007, 46, 5700.
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