1-Alkenyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids: novel and low-viscosity ionic liquid electrolytes for dye-sensitized solar cells

Dye-sensitized Solar Cells (DSCs) based on ruthenium complex N719 as sensitizer have received much attention due to their affordability and high efficiency. However, their best performance is only achieved when using volatile organic solvents as electrolyte solutions, which are unstable under prolonged thermal stress. Thus, we developed a new series of 1-alkenyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids used as robust DSC electrolytes. These ionic liquids exhibit low viscosity, high conductivity, and thermal stability. The implementation of 1-but-3-enyl-3-methyl-imidazolium trifluoromethanesulfonate, [ButMIm]OTf, into DSCs gave the best photovoltaic performance. The results are fairly comparable to those reports for other popular ionic liquid electrolytes currently used in DSC field. An insightful discussion on the relationship between the structure of these new ionic liquids and the J–V characterization as well as electrochemical impedance measurement of DSCs will give more interesting information. The results are useful for large-scale outdoor application of DSCs.


Introduction
Dye-sensitized solar cells (DSCs) have been developed as promising photovoltaic devices since their rst invention by Gratzel and O'Regan. [1][2][3] The three basic components constructing a simple DSC are mesoporous metal oxide semiconductor layers deposited on one out of two conductive transparent electrodes, a photosensitizer (or dye) thoroughly loaded onto the mesoporous lm, and an electrolyte solution lled up the space between the as-prepared electrode and the other one to assemble a functional DSC. As one of most common electrolytes, the solution of triiodide/iodide redox couple in a high dielectric constant organic solvent is usually employed in high performance DSCs. 2 An efficiency of 12% has been so far reported as the highest power conversion using this kind of liquid electrolytes in DSC devices applied Ruthenium dyes. 4 However, high volatility and unavoidable leakage of organic solvents exert a negative impact on the longevity and performance of DSCs. 5,6 Such so-called "non-robust" electrolytes also caused a dramatically fast thermal degradation of Ruthenium dyescrucial components in DSCs, leading to a loss of the cell performance. 7,8 By replacing traditional electrolytes with ionic liquid ones which are known as "robust electrolytes", thermal stability of the dyes were notably improved to guarantee the passing of serious damn heat test. 9,10 Hence, more and more researches on the application of ionic liquids as an alternative electrolytes in DSC devices due to their easy synthesis, unique properties, thermal stability, highly ionic conductivity, nonvolatility, broad electrochemical potential window, relative non-ammability, and low toxicity have been reported. [11][12][13][14][15][16][17] The ionic liquids are entirely composed of cations (ammonium, phosphonium, guanidinium, pyridinium, and sulfonium) and anions (halides and complex anions such as BF 4 , PF 6 , OTf, NTf 2 , etc.) with low melting point below 100 C. [18][19][20][21] By virtue of this property, ionic liquids have gained widespread applications in DSCs led as solvents in liquid electrolytes and organic salts in quasi-solid-state electrolytes. 22-27 1-Hexyl-3methylimidazolium iodide was rstly used in DSCs by Papageorgiou's group with the aim of reducing the volatility of electrolyte. 28 However, the traditional 1-alkyl-3methylimidazolium iodide is too viscous and the high concentration of iodide preventing them from the practical application of DSCs. Recently, the use of some binary ionic liquid electrolytes allows the reduction of electrolyte viscosity. Wang and coworkers reported a solvent-free electrolyte 1-methyl-3ethylimidazolium dicyanamide, 29 1-ethyl-3-methylimidazolium thiocyanate, 30 and 1-ethyl-3-methylimidazolium selenocyanate, 31 and 1-ethyl-3-methylimidazolium tricyanomethanide, 32 1-ethyl-3-methylimidazolium tetracyanoborate 33 blending with standard iodide-based ionic liquids achieved a high conversion efficiency of 6.5 to 8.3%. More latterly, Bidikoudi reported the use of the electrolytes prepared by blending a low viscosity ionic liquid 1-ethyl-3-methylimidazolium dicyanamide with methylimidazolium iodide to enable the devices to attain the efficiencies of 4.4 to 6.5%. 34 Although these DSCs devices showed the high conversions, the long-term stability was not good. Therefore, the search for alternative electrolytes in DSCs devices has been still studied extensively.
In this paper, we proceed with the synthesis of low-viscosity 1-alkenyl-3-methyimidazolium triuoromethanesulfonate ionic liquids whose alkenyl chain consists of three to ve carbon atoms. The synthesized ionic liquids were structurally identied as well as further characterized for physical properties such as viscosities, conductivities, and thermal stabilities. Then, these low-viscosity ionic liquids were used to prepare electrolytes to be implemented to the functional DSCs. The cells were nally characterized by J-V curve and electrochemical impedance measurement.
The results from Table 1 showed that the ionic liquids were obtained in excellent yield (68-90%) with high purity. Compared to conventional heating, microwave irradiation was a better method to afford the ionic liquids in higher yields.
The fact that ionic liquids are more viscous than traditional organic solvents can impede them from being used as electrolytes in DSCs. Fortunately, the viscosity of a given ionic liquid can be greatly governed by the nature of its anion as reported by Willner. 35 For example, the replacement of three uorine atoms in the [PF 6 ] anion by pentauoroethyl groups tremendously decreases the viscosity of imidazolium ionic liquids from 548 to 74 cP. 35 Consequently, we developed a new series of lowviscosity imidazolium triate ionic liquids bearing different alkenyl substituents. As expected, the replacement of halide anions with triate anion reduced the viscosity of imidazolium ionic liquids remarkably. The viscosity of synthesized ionic liquids was given in Table 2 36 In the current work, we found that the replacement of alkyl substituents by alkenyl substituents led to decrease the viscosity of ionic liquids. The viscosity of [ButMIm] [OTf] is only 16.9 cP (Table 2). Generally, the viscosity of ionic liquids increases with the length of alkyl side chain due to the increase of the van der Waals interaction. 36 Fig. 2 shows current-voltage J-V characteristics of the cells using ionic liquid electrolytes mixed with PMII in comparison with the popular one 1-ethyl-3-methylimidazolium tetracyanoborate (EMITCB) under AM 1.5G illumination. The respectively photovoltaic parameters are involved in Table 3. In general, all the J-V curves show similar shape in all three cases of  a Isolated yield. b Microwave heating: the reaction was performed in closed vessels using a CEM Discover monomode oven with strict control of pressure and temperature (power 10 W). c Conventional heating: the reaction was performed in a thermostat-controlled oil bath. OTf and [AMIm]OTf, respectively. The viscosity and ionic conductivity can be explained as the main factors responsible for these differences. The results of photovoltaic performance correspond reasonably with the viscosity and conductivity values of our ionic liquids in which [ButMIm]OTf has the lowest viscosity (16.9 cPs) and the highest conductivity (12.13 mS cm À1 ), thus leading to the best voltage and current. Further data summarized in Table S1 (ESI †) shows that DSCs with our ionic liquids are well comparable to the results reported elsewhere with different ionic liquids. Fig. 3 shows an analysis of the electrochemical impedance spectroscopy (EIS) of the DSCs using the synthesized ionic liquid electrolytes. The measurement was performed in the dark under a forward bias of À0.60 V. As shown in Fig. 3; three arcs can be observed in the Nyquist plots in the frequency range of 0.01 Hz to 100 kHz. The rst arc at the high-frequency range (10 3 to 10 5 Hz) is related to the charge transfer processes at counter electrode|electrolyte interface, the second arc at the middle frequency range (1-10 3 Hz) is related to carrier transport resistance (R CT ) in TiO 2 |dye|electrolyte interfaces, and the nal one at the low-frequency range (2 Â 10 À2 to 1 Hz) is associated with ionic diffusion in the electrolyte.
As can be seen from Fig. 3    From the calculation and simulated data, the values of R t , R CT and capacitance were extracted. We also obtained further analysis on Bode plots. From the Bode plots, the electron life times (s) of these cells were estimated using eqn (1), where the f max value was read off the Bode plot at the phase angle peak observed from the second peak. Fig. 4 shows linear curves of R CT ; R t ; capacitance; as well as s values when the applied potentials increase in all the three ionic liquids, indicating that all the cells respond with the same behaviour at different potentials. The data again conrm the less back electron transfer between electrolyte mediators and photo-anodes, as well as better electrolyte ionic diffusion, leading to the best photovoltaic performance with [

Experimental section
General procedure for the preparation of 1-allyl-3methylimidazolium triuoromethanesulfonate ([AMIm]OTf) under microwave irradiation In a 5 mL round-bottom ask, a mixture of 1-methylimidazole (2 mmol, 0.1640 g) and allyl bromide (2 mmol, 0.2409 g) was irradiated under microwave heating at 100 C for 20 min. Then, LiOTf (2 mmol, 0.3120 g) was added and the resulting mixtures were further irradiated at 100 C for 15 min. The reaction mixture was cooled to ambient temperature, and then diluted with 5 mL of acetonitrile. Aer ltration through celite, the solvent was removed. The crude products were washed with diethyl ether and concentrated by rotary evaporator to obtain [AMIm]OTf whose structure was conrmed by 1 H, 13 C NMR spectroscopy, and HRMS (ESI).

DSCs fabrication and characterization
The dye-sensitized solar cells were constructed by two electrodes prepared from glass substrates (Pilkington À8 U cm À2 ) coated with F-doped SnO 2 (FTO) as previously described in our publications elsewhere. 37 Photovoltaic measurements of DSCs were performed following the protocol as described 37 and the DSCs were masked with a 0.1444 cm 2 active area of the anode electrode.
Electro impedance spectroscopy (EIS) was carried out using an electrochemical interface workstation (Schlumberger SI- 1286) and a HF frequency response analyzer (Schlumberger SI-1255). Different bias potentials from À0.3 V to open-circuit voltage value, synchronized with a modulated voltage of 10 mV with a frequency range of 100 kHz to 10 MHz, were applied in the dark. The data was analyzed by using Z-View soware with the appropriate equivalent circuit.

Conclusions
Three low-viscosity ionic liquids, based on 1-alkenyl-3methyimidazolium cations and triuoromethane (OTf-) anion, where the alkenyl chain length was 3 to 5 carbon atoms, were synthesized and implemented in DSC electrolytes.
[ButMIm]OTf shows the lowest viscosity although its structure does not have the shortest alkenyl chain. The solar cells using [ButMIm]OTf in electrolytes also respond the best photovoltaic performance due to its best conductivity and low viscosity. The cells using [AMIm]OTf were obtained better short circuit current values than the ones with [Pent-MIm]OTf. All the DSCs applied these three ionic liquids show comparable performance to the present popular ones demonstrating their high potential for the use in DSCs. For further application of these ionic liquids in DSC devices, the long term stability of DSCs with these ionic liquid electrolytes should be investigated.

Conflicts of interest
There are no conicts to declare.