title: Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage.
abstract: 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 electron back transfer between TiO2and the oxidized dye. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With the introduction of our 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, dmby = 6,6'-dimethyl-2,2'-bipyridine) and Cu(II/I)(tmby)2TFSI2/1(0.87 V vs SHE, tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine), 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, these copper complexes can sufficiently regenerate the oxidized dye molecules with close to unity yield at driving force potentials as low as 0.1 V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. Despite the small driving forces for dye regeneration, fast and efficient dye regeneration (2-3 μs) was observed for both complexes. As another advantage, the electron back transfer (recombination) rates were slower with Cu(II/I)(tmby)2TFSI2/1as 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 10.3%, 10.0%, and 10.3%, respectively, using the organic Y123 dye under 1000 W m-2AM1.5G 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.
authors.forename: Yasemin
authors.initials: Y
authors.lastname: Saygili
authors.forename: Magnus
authors.initials: M
authors.lastname: Söderberg
authors.forename: Norman
authors.initials: N
authors.lastname: Pellet
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Giordano
authors.forename: Yiming
authors.initials: Y
authors.lastname: Cao
authors.affiliation: Department of Chemical Sciences, University of Naples Federico II , 80126 Naples, Italy.
authors.forename: Ana Belen
authors.initials: AB
authors.lastname: Muñoz-García
authors.forename: Shaik M
authors.initials: SM
authors.lastname: Zakeeruddin
authors.forename: Nick
authors.initials: N
authors.lastname: Vlachopoulos
authors.affiliation: Department of Chemical Sciences, University of Naples Federico II , 80126 Naples, Italy.
authors.forename: Michele
authors.initials: M
authors.lastname: Pavone
authors.affiliation: Department of Chemistry, Ångström Laboratory, Uppsala University , 751 20 Uppsala, Sweden.
authors.forename: Gerrit
authors.initials: G
authors.lastname: Boschloo
authors.affiliation: J. Heyrovsky Institute of Physical Chemistry , 1823 Prague, Czech Republic.
authors.forename: Ladislav
authors.initials: L
authors.lastname: Kavan
authors.forename: Jacques-E
authors.initials: JE
authors.lastname: Moser
authors.forename: Michael
authors.initials: M
authors.lastname: Grätzel
authors.forename: Anders
authors.initials: A
authors.lastname: Hagfeldt
authors.affiliation: Department of Chemistry, Ångström Laboratory, Uppsala University , 751 20 Uppsala, Sweden.
authors.forename: Marina
authors.initials: M
authors.lastname: Freitag