Copper Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage

- -, Anders Hagfeldt^a, Nick Vlachopoulos^a, Shaik M. Zakeeruddin^a, Yasemin Saygili^a, Marina Freitag^a, Yiming Cao^b, Norman Pellet^b, Michael Graetzel^b, Fabrizio Giordano^b, Magnus Soderberg^c, Jacques-E. Moser^c, Ana Belen Munoz-Garcia^d, Michele Pavone^d, Gerrit Boschloo^e, Ladislav Kavan^f

^aEcole Polytechnique Fédérale de Lausanne EPFL, Laboratory of Photomolecular Science, Institute of Chemical Sciences and Engineering, Station, 6, Lausanne, Switzerland

^bLaboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, Station 6, CH-1015 Lausanne, Lausanne, Switzerland

^cEcole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland

^dDepartment of Chemical Sciences, University of Naples Federico II

^eUppsala University, Ångström Laboratory, Sweden, Lägerhyddsvägen, 1, Uppsala, Sweden

^fJ. Heyrovsky Institute of Physical Chemistry

Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)

Yokohama-shi, Japan, 2017 February 2nd - 4th

Organizers: Tsutomu Miyasaka and Iván Mora-Seró

Oral, Yasemin Saygili, presentation 096
Publication date: 7th November 2016

Redox mediators play a major role determining the photocurrent and the photovoltage in dye-sensitized solar cells (DSCs). To maintain the photocurrent, the reduction of oxidized dye by the redox mediator should be significantly faster than the back electron transfer between TiO2 and the oxidized dye. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With our introduction of new copper complexes as promising redox mediators in DSCs both criteria are satisfied to enhance power conversion efficiencies. In this study, two copper bipyridyl complexes Cu(II/I)(dmby)2TFSI2/1 (0.97 V vs SHE) and Cu(II/I)(tmby)2TFSI2/1 (0.87 V vs SHE) are presented as new redox couples for DSCs. They are compared to previously reported Cu(II/I)(dmp)2TFSI2/1 (0.93 V vs SHE, dmp =bis(2,9-dimethyl-1,10-phenanthroline). Due to the small reorganization energy between Cu(I) and Cu(II) species, this copper complexes can sufficiently regenerate the oxidized dye molecules with close to unity yield at driving force potentials as low as 0.1V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. Despite of the small driving forces for dye regeneration fast and efficient dye regeneration (2-3 μs) was observed for both complexes. As another advantage, the back electron transfer (recombination) rates were slower Cu(II/I)(tmby)2TFSI2/1 as evidenced by longer lifetimes.The solar-to-electrical power conversion efficiencies for [Cu(tmby)2]2+/1+, [Cu(dmby)2]2+/1+ and [Cu(dmp)2]2+/1+ based electrolytes were 10.2%, 10.0% and 10.2%, respectively, using the organic Y123 dye under 1000 W m-2 AM1.5G illumination. The high photovoltaic performance of Cu based redox mediators underlines the significant potential of the new redox mediators and points to a new research and development direction for DSCs.

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