J. Phys. Chem. C 2008, 112, 11063–11067
11063
Tetrahydrothiophenium-Based Ionic Liquids for High Efficiency Dye-Sensitized Solar Cells Chengcheng Xi,†,‡ Yiming Cao,† Yueming Cheng,† Mingkui Wang,§ Xiaoyan Jing,‡ Shaik M. Zakeeruddin,§ Michael Gra¨tzel,§ and Peng Wang*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, Key Laboratory of Superlight Materials and Surface Technology, Harbin Engineering UniVersity, Harbin 150001, China, and Laboratory for Photonics and Interfaces, Swiss Federal Institute of Technology, CH 1015, Lausanne, Switzerland ReceiVed: April 1, 2008
Binary melts of S-ethyltetrahydrothiophenium iodide and dicyanoamide (or tricyanomethide) have been employed for dye-sensitized solar cells with high power conversion efficiencies up to 6.9% under the illumination of AM 1.5G full sunlight. We have further shown that the transport of triiodide in ionic liquids with high iodide concentration is viscosity-dependent in terms of a physical diffusion coupled bond exchange mechanism apart from the simple physical diffusion. In addition, we have found that some anions of ionic liquid electrolytes such as dicyanoamide have a significant influence on surface states and electron transport in the mesoporous semiconducting film. Introduction As a potentially low-cost candidate for future photovoltaic markets, the mesoscopic dye-sensitized solar cell1 (DSC) is attracting a large amount of academic and industrial interest due to its high efficiency2 and good stability under prolonged thermal and light-soaking stress.3 It is widely recognized that the use of any volatile solvents in DSC may be prohibitive for practical solar panels in view of the need for robust encapsulation. During the past years, solvent-free room temperature ionic liquid electrolytes of imidazolium melts4 have been actively pursued as a very attractive solution to this dilemma, and over 7% efficiencies measured under the air mass 1.5 global (AM 1.5G) illumination have been achieved. Other ionic liquids with cations such as sulfonium,5 guanidinium,6 ammonium,7 or phosphonium8 have also been explored as solvent-free electrolytes but show low device efficiencies due to mass transport limitation of the photocurrent under operation in full sunlight.6 Until now the highest efficiency measured under standard AM 1.5G full sunlight of the latter systems was only 1.2%, very recently achieved with a binary phosphonium melt.8 Herein we report a remarkable enhancement of device efficiency by employing high fluidity tetrahydrothiophenium melts, for the first time demonstrating that nonimidazolium ionic liquids can also be used for high efficiency DSC. Results and Discussion Tetrahydrothiophene is an ideal odorant in natural gas due to its low toxicity and low corrosiveness to gas pipes and valves. In comparison to the key starting material for imidazoliumbased ionic liquids, it is more cost-effective, motivating us to develop tetrahydrothiophenium based ionic liquids for DSC application. In the family of tetrahydrothiophenium dicyanoamides (TnDCA, where T is tetrahydrothiophenium and n denotes * To whom correspondence should be addressed. E-mail: peng.wang@ ciac.jl.cn. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Harbin Engineering University. § Swiss Federal Institute of Technology.
the number of carbons in the linear alkyl chain substituent), T2DCA has the lowest viscosity of 37.3 cP at 25 °C. As shown in Figures S1 and S2, in the Supporting Information, the conductivity trend observed with T2DCA, T3DCA, T4DCA, and T5DCA is consistent with that of their fluidities, indicating that the van der Waals force between Tn cations becomes weak upon shortening the linear alkyl chain, which can be further empirically rationalized in terms of molecular volumes9 calculated from our density measurements shown in Figure S3, in the Supporting Information. However, the higher viscosity of T1DCA compared with T2DCA and T3DCA is unxpected. This could be caused by the presence of a strong Coulombic attraction due to the close distance between the T1 cation and dicyanoamide, compensating the decrease of van der Waals force.10 Based on the physical properties of TnDCA, we further synthesized S-ethyl-tetrahydrothiophenium tricyanomethide (T2TCM), which has an even lower room temperature viscosity of 32.5 cP compared with T2DCA, indicative of the weaker electrostatic force between anion and cation due to the highly delocalized negative charge on tricyanomethide. Data of temperature-dependent conductivities and fluidities for all these ionic liquids presented in the Supporting Information can be wellfitted to the Vogel-Fulcher-Tammann (VFT) equation.11 In our previous work,4i we have shown that it is necessary to have a high concentration of iodide in the ionic liquid electrolytes for DSC, to efficiently intercept the recombination between oxidized sensitizer and photoinjected electron in the titania film. Thus, we made two melts with low viscosity T2DCA and T2TCM to evaluate the potential application of tertrahydrothiophenium based ionic liquids. Melt I: T2I/T2DCA/I2 (6:4:1, molar ratio); Melt II: T2I/T2TCM/I2 (6:4:1, molar ratio). We measured the temperature-dependent viscosities, conductivities, and densities of these two melts, which have been dried at 60 °C under a vacuum of ∼3 Torr for 6 h. As depicted in Figure 1, the dependence of molar conductivity on the fluidity of the T2DCA, T2TCM and melts I and II can be described by the fractional Walden rule: ΛηR ) constant, where R is the slope of the line in the Walden plot and reflects the decoupling degree of ions.12 The slopes of four fitted lines are all slightly less than
10.1021/jp802798k CCC: $40.75 2008 American Chemical Society Published on Web 06/26/2008
11064 J. Phys. Chem. C, Vol. 112, No. 29, 2008
Xi et al.
Figure 1. Walden plots of molar conductivity versus fluidity. (a) melt I; (b) melt II; (c) T2DCA; and (d) T2TCM. The dashed “ideal” Walden line is also included.
Figure 2. Temperature-dependent plots of diffusion coefficient versus fluidity in the Stokes-Einstein coordinate. (a) melt I; (b) melt II; and (c) melt III. The dashed line is calculated from the Stokes-Einstein relation with a rH of 2.1 Å for triiodide.
one predicted by the “ideal” Walden rule, indicating progressive augmentation in the population of less conductive ion-pairs with the increase of temperature. Compared with pure T2DCA and T2TCM ionic liquids, melts I and II with iodine doping both show higher molar conductivities than expected from their fluidities (η-1). This anomalous conduction behavior will be further scrutinized below by analyzing the triiodide diffusion coefficients measured with ultramicroelectrode voltammetry. As shown in Figure 2, the temperature-dependent apparent triiodide diffusion coefficients (D) in melts I and II are plotted versus fluidity (η-1) according to the Stokes-Einstein equation13
D ) kBT/6πrHη
(1)
where kB is the Boltzmann constant, T is the absolute temperature, rH is the effective hydrodynamic radius, and η is the dynamic viscosity. It is noted that, although log(D/T) increases linearly with log(η-1), the fitted slopes (0.73 and 0.77) are less than unity, departing considerably from the description of the Stokes-Einstein relation. The rH of triiodide derived from the fitted intercepts are unrealistically small (
