Influence of Ionic Liquid on Recombination and Regeneration Kinetics in Dye-Sensitized Solar Cells Feng Li, † James Robert Jennings, † Xingzhu Wang, † Li Fan, † Zhen Yu Koh, † Hao Yu, ‡ Lei Yan, ‡ and Qing Wang* ,† † Department of Materials Science and Engineering, Faculty of Engineering, NUSNNI-NanoCore, National University of Singapore, Singapore 117576 ‡ College of Chemistry, Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan Province, P. R. China * S Supporting Information ABSTRACT: Nonvolatile electrolyte solutions are necessary for dye-sensitized solar cells (DSCs) with good long-term stability. Such electrolytes usually contain room-temperature ionic liquids (RTILs) and consequently possess higher viscosity and ionic strength than the volatile electrolytes used in current champion cells. In this study, we systematically investigated the effect of an RTIL additive on the performance of DSCs employing either a classical Ru-complex dye or a recently developed organic D-A-π-A dye, in combination with either I - /I 3 - or [Co(bpy) 3 ] 2+/3+ as redox mediator. Using impedance spectroscopy and transient absorption measurements under various background illumination intensities, recombination and regeneration kinetics were examined. Recombination is accelerated in the I - /I 3 - devices upon addition of RTIL, regardless of the dye used, but it is retarded in the [Co(bpy) 3 ] 2+/3+ devices. Addition of RTIL slowed regeneration in I - /I 3 - devices for both sensitizers, marginally accelerated it for [Co(bpy) 3 ] 2+/3+ with the Ru-complex dye, and did not significantly affect it for [Co(bpy) 3 ] 2+/3+ with the D-A-π-A dye. We show that these findings cannot be explained by diffusion limitations caused by increased solution viscosity or by a shift in the TiO 2 conduction band relative to the electrolyte redox level. These findings should be useful for future optimization of RTIL-based DSCs. ■ INTRODUCTION Dye-sensitized solar cells (DSCs) are a credible alternative to conventional silicon-based photovoltaics, having now achieved more than 12% power conversion efficiency for laboratory test cells. 1,2 A typical DSC consists of a nanocrystalline mesoporous film of a wide band gap metal oxide, usually TiO 2 , supported by a transparent and conducting substrate where photogenerated electrons are eventually extracted into the external circuit. Onto the mesoporous film a monolayer of a transition metal com- plex 3,4 or organic sensitizer 5,6 is chemically adsorbed. The sensitizer absorbs photons and becomes oxidized after injecting electrons from its excited states into the semiconductor film. As a regenerative-type photoelectrochemical cell, 7 the oxidized sensitizer is regenerated by reduced species in the electrolyte, while at the same time the oxidized species diffuse to the cathode and become reduced after accepting electrons that have flowed through the external circuit. Although the performance of champion DSCs is encourag- ing, to be applied commercially, DSCs must be reasonably stable, which will be problematic if volatile liquid solvents are used. With the impetus for long-term operation of DSCs, re- search into solvent-free ionic liquids, polymer electrolytes, and all-solid-state hole-transporting materials has recently surged. 8-14 Specifically, studies of solvent-free room-temperature ionic liquids (RTILs) have increased dramatically thanks to their electrochemical stability, nonvolatility, and high conductivity. DSCs with RTIL I - /I 3 - electrolytes have achieved over 9% power conversion efficiency (PCE) and good stability under long-term light-soaking and thermal stress tests. 9 One major feature of RTILs is high viscosity, which may result in in- efficient mass transport in DSCs operating under full sunlight, thus limiting the overall PCE. This is probably the main reason why all the high-efficiency RTIL DSCs adopt I - /I 3 - as redox mediator rather than [Co(bpy) 3 ] 2+/3+ , as the transport of I - /I 3 - is more efficient because of not only its higher physical diffusion coefficient but also the acceleration from the Grotthuss ex- change mechanism in highly concentrated electrolyte. 8,15,16 Nevertheless, I - /I 3 - is far from perfect considering the possible corrosion of metal substrates in commercial modules, high free energy loss resulting from the large driving force for dye regeneration, 17 and competitive light absorption in the visible spectrum. To further improve the performance of RTIL DSCs, it is essential to investigate charge-transfer processes and to identify Special Issue: Michael Grä tzel Festschrift Received: March 7, 2014 Revised: April 8, 2014 Article pubs.acs.org/JPCC © XXXX American Chemical Society A dx.doi.org/10.1021/jp502341a | J. Phys. Chem. C XXXX, XXX, XXX-XXX