Dye-anchored nanocatalysts for reduced voltage loss with inexpensive iodide/triiodide electrolytes

Guangliang Liu^a^,^b, Bowen Yang^b^,^d, Alexander G. Agrios^b^,^d, Ian Weiss^c, Andrew Kopecky^c, Elena Galoppini^c

^aDepartment of Chemical and Biomolecular Engineering, University of Connecticut, 191 Auditorium Road, Storrs, CT 06269

^bCenter for Clean Energy Engineering, University of Connecticut, 44 Weaver Rd, Storrs, CT, 06269

^cRutgers University Chemistry Department, Warren Street, 73, Newark, United States

^dDepartment of Civil & Environmental Engineering, University of Connecticut, 261 Glenbrook Road, Storrs, CT 06269

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Oral, Alexander G. Agrios, presentation 269
Publication date: 5th February 2015

The iodide/triiodide redox was for many years the dominant system for hole transport in dye-sensitized solar cells. The large overpotential (ca. 0.5 V) required for iodide to reduce an oxidized dye molecule led to an intense search for alternatives, with cobalt(II/III) complexes now favored. However, apart from the overpotential problem, iodide/triiodide is an excellent system. Both the oxidized and reduced forms are very cheap, highly soluble, small (for good mass transport), and have minimal light absorbance. The system has uniquely low rates of recombination with electrons in the photoanode (e.g. TiO₂), which is often credited to the fact that even the oxidized species carries a negative charge. Alternatives tend to be bulky and to exhibit rapid recombination, requiring very thin photoanode films. We will present a scheme to overcome the overpotential problem by anchoring a platinum nanoparticle to a dye molecule. We have synthesized Pt nanoparticles 1-2 nm in diameter [1] and a bifunctional dye molecule [2] capable of binding to both TiO₂ and to Pt. The N3-type dye has one bipyridine ligand substituted with the usual carboxylic acid groups for anchoring to TiO_2, and a second bipyridine ligand having a lipoate ester terminating in a dithiolane moiety, containing two sulfur atoms for attachment to Pt. Cyclic voltammetry of the dye on a Pt wire working electrode, compared to CV of a reference dye without sulfur atoms, confirms that the dye attaches to the Pt exclusively via the dithiolane group. The TiO₂/dye/Pt structure has been successfully assembled as demonstrated by SEM/EDX, XPS, and electrochemical measurements. Future work will include adjusting the dye HOMO to arrive at an overpotential sufficient to enable regeneration from iodide/triiodide only when a Pt nanoparticle is attached, thereby maximizing the harvestable range of the solar spectrum without sacrificing cell voltage.
Schematic of bifunctional dye bound to both TiO2 and a Pt nanoparticle.
[1] Liu, G.; Arellano-Jiménez, M.J.; Carter, C.B.; Agrios, A.G. Preparation of functionalized platinum nanoparticles: a comparison of different methods and reagents. J. Nanopart. Res. 2013, 15, 1744-1756. [2] Kopecky, A.; Liu, G.; Agushi, A.; Agrios, A.G.; Galoppini, E. Synthesis of bifunctional Ru complexes with 1,2-dithiolane and carboxylate-substituted ligands. Tetrahedron 2014, 70, 6271-6275.

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