Cation Exchange at Semiconducting Oxide Surfaces: Origin of Light- Induced Performance Increases in Porphyrin Dye-Sensitized Solar Cells Matthew J. Griffith,* ,†,‡ Kenji Sunahara, §,⊥ Akihiro Furube, §,⊥ Attila J. Mozer, † David L. Officer, † Pawel Wagner, † Gordon G. Wallace, † and Shogo Mori* ,‡ † ARC Centre of Excellence for Electromaterials Science and Intelligent Polymer Research Institute, Innovation Campus, University of Wollongong, Northfields Avenue, Wollongong, New South Wales 2522, Australia § National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, Tsukuba, Ibaraki 305-8565, Japan ‡ Division of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University, Ueda, Nagano 386-8567, Japan ⊥ Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan * S Supporting Information ABSTRACT: The origin of simultaneous improvements in the short-circuit current density (J sc ) and open-circuit voltage (V oc ) of porphyrin dye-sensitized TiO 2 solar cells following white light illumination was studied by systematic variation of several different device parameters. Reduction of the dye surface loading resulted in greater relative performance enhancements, suggesting open space at the TiO 2 surface expedites the process. Variation of the electrolyte composition and subsequent analysis of the conduction band potential shifts suggested that a light-induced replacement of surface-adsorbed lithium (Li + ) ions with dimethylpropylimidazolium (DMPIm + ) ions was responsible for an increased electron lifetime by decreasing the recombination with the redox mediator. Variation of the solvent viscosity was found to affect the illumination time required to generate increased performance, while similar performance enhancements were not replicated by application of negative bias under dark conditions, indicating the light exposure effect was initiated by formation of dye cation molecules following photoexcitation. The substituents and linker group on the porphyrin chromophore were both varied, with light exposure producing increased electron lifetime and V oc for all dyes; however, increased J sc values were only measured for dyes containing binding moieties with multiple carboxylic acids. It was proposed that the initial injection limitation and/or fast recombination process in these dyes arises from the presence of lithium at the surface, and the improved injection and/or retardation of fast recombination after light exposure is caused by the Li + removal by cation exchange under illumination. 1. INTRODUCTION Over the past two decades dye-sensitized solar cells (DSSCs) have emerged as an innovative technology for the development of low-cost, renewable, and environmentally acceptable energy production. 1-3 Efficient charge separation in these devices is achieved by photoinduced electron injection from a sensitizing dye into the conduction band of a metal oxide electrode to which it is chemically anchored. The resulting dye cations are subsequently reduced by a redox electrolyte, which also conducts holes to the cathode. The solar-to-electric power conversion efficiencies of DSSCs depend on a balance of the kinetics for charge injection, collection, recombination, and dye regeneration processes, 4 with the best devices currently exhibiting power conversion efficiencies of 11-12% under AM 1.5 illumination. 5-8 The efficient light-harvesting potential of porphyrins, exemplified by their primary role in photosynthesis, makes them promising candidates for photosensitizers within DSSCs. 7,9 Their synthesis is relatively straightforward, and their optical and electronic properties can be easily tuned via chemical modification of the porphyrin core, 10 the number of porphyrin units, 11,12 and the linker between the core and the inorganic oxide. 13 The capability of porphyrin sensitizers was recently highlighted by a report demonstrating a new DSSC benchmark efficiency of 12.3% under AM 1.5 full sunlight for a device with a donor-π-acceptor zinc porphyrin sensitizer coupled with a cobalt-based redox mediator. 7 That report demonstrated the remarkable potential of this class of chromophores, although the majority of porphyrin dyes have not approached such impressive efficiencies despite major progress in the development of innovative design strat- egies. 14-17 It is therefore clearly important that the factors Received: July 9, 2012 Revised: April 19, 2013 Published: April 26, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 11885 dx.doi.org/10.1021/jp3067712 | J. Phys. Chem. C 2013, 117, 11885-11898