Published: April 26, 2011 r2011 American Chemical Society 9772 dx.doi.org/10.1021/jp110473n | J. Phys. Chem. C 2011, 115, 9772–9779 ARTICLE pubs.acs.org/JPCC Cobalt Polypyridyl-Based Electrolytes for p-Type Dye-Sensitized Solar Cells Elizabeth A. Gibson, ||,† Amanda L. Smeigh, ‡ Loïc Le Pleux, § Leif Hammarstr€ om, ‡ Fabrice Odobel, § Gerrit Boschloo,* ,† and Anders Hagfeldt † † Department of Physical and Analytical Chemistry, Uppsala University, Box 259, SE-751 05, Uppsala, Sweden ‡ Department of Photochemistry and Molecular Science, Uppsala University, Box 532, SE-751 20, Uppsala, Sweden § CEISAM, Universit  e de Nantes, CNRS, 2 rue de la Houssini  ere, 44322 Nantes cedex 3, France b S Supporting Information ’ INTRODUCTION Over recent years, great effort has been focused on improving the efficiency of low-cost electrochemical devices for solar energy to electricity conversion, namely, the dye-sensitized solar cell (DSC). 1,2 Nonetheless, the record efficiency still lags behind bulk semiconductor and thin-film devices. One way to improve the device efficiency is to incorporate a photocathode in tandem with the standard TiO 2 -based n-type photoanode to produce a device whereby two electrodes are photoactive, rather than just one, as is the case of Gr € atzel-type DSCs. 35 In these devices, sensitizers can be matched to absorb the high energy photons on one electrode and low energy photons on the other, thereby utilizing the full solar spectrum. In order to realize this, work must be focused on improving the efficiency of p-type devices, but until recently, these systems have been relatively unexplored. 6 A number of improvements to the devices and more understanding of the complex system are needed before competitive efficiencies are reached. Dye-sensitized p-type semiconductors generate cathodic photocurrents on visible light excitation. A number of mesopor- ous, NiO-based p-type devices have been reported, in which the iodide/triiodide redox couple 7 and dyes including coumarin 343, perylene derivatives, 8 and diphenylaniline derivatives 9 have been incorporated, in some cases giving maximum incident photon-to- current conversion efficiency (IPCE) values of up to 64%. 10 In the past, much of our focus has been driven toward preparing dyes which promote charge separation and limit recombination between the sensitizer and the semiconductor and, also, absorb light toward the red region of the visible spectrum. 1113 During our investigations, we have found that much of the limitations to this system arise from the use of the iodide/triiodide redox couple. 14,15 Not only is the corrosive and volatile nature of this standard redox electrolyte impractical for the upscale and commercialization of DSCs but also it limits the obtainable photovoltage in the cell because of its complex redox chemistry. Furthermore, since our aim is to prepare tandem devices, where light must travel through the electrolyte to the second photo- electrode, the high optical density of the redox couple will cause the light to be filtered, thereby lowering the performance. To avoid this, we have made use of alternative redox couples, such as tris(bipyridyl)cobalt derivatives, which, when used in conjunc- tion with a suitable sensitizer such as the perylenemonoimide naphthalenediimide (PMI-NDI) dyad, give a 3-fold increase in photovoltage over triiodide/iodide p-type DSCs. 8,14,15 The chemical stability, nonvolatility, and optical transparency in the visible region (although bis(4,4 0 ,4 00 -tri-tert-butyl-2,2 0 :6 0 ,2 00 -ter- pyridine) cobalt(II) has an extinction coefficient at its absorption maximum of 1.4  10 3 M 1 cm 1 , 1 order of magnitude larger than tris(4,4 0 -di-tert-butyl-2,2 0 -bipyridine)cobalt(II), but slightly lower than triiodide 16 ), electrochemical reversibility, tunability of physical chemical properties (e.g., redox potential, solubility, size), and noncorrosive nature of these redox shuttles make them favorable alternatives to the triiodide/iodide system. Further- more, their redox chemistry is simple and studies of DSCs incorporating one-electron shuttles can provide insight to the complex mechanism of these photoelectrochemical systems. Received: November 2, 2010 Revised: March 2, 2011 ABSTRACT: A series of polypyridyl cobalt complexes with different substitu- ents was applied as redox mediators in p-type dye-sensitized solar cells (p-DSCs), consisting of mesoporous NiO sensitized with a perylenemonoimi- denaphthalenediimide (PMI-NDI) dyad. The photocurrent and photovol- tages of the devices were found to depend on the steric bulk of the redox species rather than their electrochemical potential. Bulky substituents were found to slow the detrimental charge recombination reactions between holes in the NiO semiconductor and the reduced form of the redox couple. The open-circuit potential (V OC ) of each of the devices was superior to the equivalent PMI-NDI- sensitized p-DSCs containing the triiodide/iodide redox couple.