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Novel hydrophobic ionic liquids electrolyte based on cyclic sulfonium used in dye-sensitized solar cells
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Solar Energy 85 (2011) 7–11 www.elsevier.com/locate/solener
Novel hydrophobic ionic liquids electrolyte based on cyclic sulfonium used in dye-sensitized solar cells Lei Guo, Xu Pan, Meng Wang, Changneng Zhang, Xiaqin Fang, Shuanghong Chen, Songyuan Dai ⇑ Key Lab of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, Anhui 230031, PR China Received 12 August 2010; received in revised form 28 October 2010; accepted 11 November 2010 Available online 4 December 2010 Communicated by: Associate Editor Sam-Shajin Sun
Abstract A novel series of hydrophobic room temperature ionic liquids based on six cyclic sulfonium cations were first time synthesized and applied in dye-sensitized solar cells as pure solvents for electrolyte system. The chronoamperograms result showed that the length of substituent on sulfonium cations could inhibit the I 3 diffusion and the five-ring structure of sulfonium was benefit for fast triiodide ion diffusion. The electrochemical impendence spectra measurement of dye-sensitized solar cells with these ionic liquid electrolytes was carried out and the result indicated that the cations’ structure had indeed influence on the cells’ performance especially for the fill factor, which was further proved by the measurement result of I–V curves of these dye-sensitized solar cells. The conclusion was obtained that the electron exchange reaction on Pt counter electrode/electrolyte interface dominated the cells’ performance for these ionic liquid electrolyte-based DSCs. Ó 2010 Elsevier Ltd. All rights reserved. Keywords: Dye-sensitized solar cell; Ionic liquids; Electrolyte; Cyclic sulfonium
1. Introduction Dye-sensitized solar cell (DSC) has gained considerable interest over the past decade as the new generation of solar cells because of their unique properties such as low-cost, simple assembling technology and high efficiency up to 11% (Oregan and Gratzel, 1991; Gratzel, 2004). An important component of DSC is electrolyte system. The conventional electrolyte solvents used in DSC are some organic solvents such as c-butyrolactone (Lan et al., 2006), acetonitrile (Hao et al., 2006) and 3-methoxypropionitrile (Agrell et al., 2003). All of them are usually poisonous and volatile, which restricts DSC’s industrialization. The air and water stable room temperature ionic liquids (RTILs) are attractive as an alternative to organic solvents due to their ⇑ Corresponding author. Tel./fax: +86 551 5591377.
E-mail address: [email protected] (S. Dai). 0038-092X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2010.11.010
unique characteristics such as chemical and thermal stability, negligible vapor pressure, nonflammability, high ionic conductivity and wide electrochemical window (Wilkes and Zaworotko, 1992). Recently, sulfonium-based molten salts have been applied in DSC as electrolyte solvents and shown some exciting results (Paulsson et al., 2003; Xi et al., 2008), which gave us an encouragement in exploring nonimidazolium ionic liquids as solvent-free electrolytes for high performance devices. However, the works involving cyclic sulfonium-based ionic liquids are still very rare (Xi et al., 2008; Guo et al., 2010; Zhang et al., 2009a) and most of the developed cyclic sulfonium-based ILs exhibited strong hydrophilicity, which restricted them to be used as electrolytes because the water in air can pollute these ILs inevitably (Zhang et al., 2009a). Moreover, the research about the influence of cyclic structure on the performance of DSC based on these cyclic sulfonium electrolytes had not been reported.
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L. Guo et al. / Solar Energy 85 (2011) 7–11
Hence, we synthesized a new series of hydrophobic room temperature ionic liquids based on six cyclic sulfonium cations, as shown in Fig. 1, and succeeded in assembling dye-sensitized solar cells directly using these cyclic sulfonium-based ILs as electrolyte solvents. The investiga tion about the I redox electrochemical behavior was 3 /I carried out, as well as the performance of the corresponding DSC. 2. Experimental 2.1. Reagents and materials Tetrahydrothiopyran, 1-iodopropane, 1-iodopentane, lithium bis(trifluoromethysulfonyl) imide (LiTFSI), anhydrous lithium iodide (LiI) and 4-tert-butylpyridine (TBP) were purchased from Alfa Aesar. Tetrahydrothiophene, iodine (I2), 1,2-dichloroethane and 1-butyliodide were obtained from J&K Chemical Ltd. All solvents and reagents were of analytical grade and used as received. 1Methyl-3-propylimidazolium iodide (MPII) was synthesized according to previous report (Shi et al., 2008). 2.2. Synthesis and characterization of cyclic sulfonium-based room temperature ionic liquids A series of cyclic sulfonium iodide were prepared according to the reference method (Xi et al., 2008; Zhang et al., 2009a). About 100 mmol tetrahydrothiopyran or tetrahydrothiophene was in a round bottle covered with tinfoil. An equimolar amount of the corresponding alkyl iodide was added. The reaction atmosphere was protected from oxygen and water by a continuous flow of dry nitrogen gas. The reaction mixture was left for 2–3 days at temperature 333 K. After that the yellow rough products were dissolved with hot acetone, and then the mixture was dropped in cold diethyl ether, crystalline white solid or yellow liquid precipitated. The products were separated from the solvent followed by thorough washing with diethyl ether. The final products were dried under vacuum. The produced iodide and LiTFSI (1:1.1 M rates) were dissolved
in deionized water and mixed for 24 h at ambient temperature under stirring vigorously. The crude ILs were dissolved with 1,2-dichloroethane and washed with deionized water until no residual iodide anions in the deionized water were detected with use of AgNO3. The 1,2-dichloroethane was removed by rotating evaporation. All the ionic liquids were dried under high vacuum for 24 h at 100 °C. The structures and purity of products were confirmed by 500-MHz 1H NMR (DMX-600, Bruker, Switzerland). Chemical shifts were reported downfield in parts per million (ppm, d) from a tetramethylsilane. The characterization data are as follows: S53TFSI 1H NMR: (DMSO-d6, 500 MHz): 3.489 3.281(m, 4H), 3.150 3.120(t, 2H), 2.231 2.079(m, 4H), 1.747 1.673(m, 2H), 1.004 0.974(t, 3H); S54TFSI: 1H NMR: (DMSO-d6, 500 MHz): 3.486 3.327(m, 4H), 3.167 3.137(t, 2H), 2.218 2.087(m, 2H), 1.685 1.625(m, 2H), 1.414 1.369(m, 2H), 0.922 0.892(t, 3H); S55TFSI 1H NMR: (acetone-d6, 500 MHz): 3.870 3.800(m, 2H), 3.743 3.677(m, 2H), 3.508 3.468(t, 2H), 2.561 2.417(m, 4H), 1.990 1.914(m, 2H), 1.561 1.486(m, 2H), 1.456 1.365(m, 2H), 0.943 0.906(t, 3H); S63TFSI: 1H NMR: (acetone-d6, 500 MHz): 3.799 3.761(t, 2H), 3.601 3.570(t, 2H), 3.467 3.422(t, 2H), 2.341 2.283(m, 2H), 1.992 1.923(t, 2H), 1.878 1.701(m, 4H), 1.136 1.106(t, 3H); S64TFSI: 1H NMR: (acetone-d6, 500 MHz): 3.807 3.769(t, 2H), 3.628 3.597(t, 2H), 3.348 3.433(t, 2H), 2.345 2.287(m, 2H), 1.934 1.873(m, 2H), 1.860 1.721(m, 4H), 1.567 1.522(m, 2H), 0.981 0.952(t, 3H); S65TFSI: 1H NMR: (DMSO-d6, 500 MHz): 3.514 3.466(t, 2H), 3.214 3.165(m, 4H), 2.077 2.017(m, 2H), 1.785 1.725(m, 2H), 1.718 1.658(m, 2H), 1.616 1.581(m, 2H), 1.538 1.493(m, 2H), 1.3801 1.300(m, 2H), 0.891 0.863(t, 3H).
2.3. Measurement
Fig. 1. Structures of cyclic sulfonium-based room temperature ILs prepared in this work.
Conductivity measurement of neat ILs was performed by lab conductivity meter (DDSJ-308A, REX, Shanghai Precision and Scientific Instrument Co., Ltd., PR China) with two platinum black electrodes conductance cell (DJS-1C, the area of each platinum black electrode is about 0.25 cm2, the distance between two platinum black electrodes is about 0.50 cm, the cell constant is 1.021, determined by aqueous KCl standard solution). Chronoamperometry measurement was carried out by using an electrochemical workstation (CHI660A, CH Instruments, Inc., USA). The working electrode was a 5-lm-radius Pt disk (CHI107) and platinum
L. Guo et al. / Solar Energy 85 (2011) 7–11
23
S53TFSI 22
S54TFSI S55TFSI
21
Current/nA
wire acted as the reference electrode and auxiliary electrode. Due to small currents (nA-range) and resulting problems of electrical interference, all measurements with microelectrodes were performed within a Faraday cage. Impedance measurements were performed with a computer-controlled potentiostat (IM6e, Zahner, Germany) in the frequency range from 60 mHz to 1000 kHz. The magnitude of the alternative signal was 5 mV. The impedance measurements were carried out in dark, and the obtained spectra were fitted with Sim software in terms of appropriate equivalent circuits. For I–V characteristics of the DSC, a 450 W xenon light source (Oriel, USA) was used to provide an intensity of 100 mW cm2 (the equivalent of 1.0 sun at global AM 1.5) at the surface of the solar cell and measuring the generated photocurrent with a Keithley model 2400 digital source meter (Keithley, USA).
9
S63TFSI S64TFSI
20
S65TFSI
19 18 17 16 0.3
0.4
0.5
0.6
0.7
0.8
0.9
-1/2
1/square(time)/s
Fig. 2. Plot of the current recorded at a Pt disk microelectrode (5-lmradius disk) during a chronoamperometric measurement vs. t1/2 for ionic liquid electrolytes prepared this work.
2.4. DSC assembly The preparation of DSC was similar to the previous reports (Zhang et al., 2009b). The TiO2 film was about 13 lm thick and a 2-lm-thick light scattering layer was used. The films were immersed in anhydrous ethanol solution with 5.0 104 mol L1 N719 dye 12 h. The total active electrode area of DSC was 0.25 cm2.
Table 1 Fitting and calculating results for chronoamperograms according Eq. (1). Electrolytea
Intercept
R2
DI3 (1E-6acm2/s)
3. Results and discussion
S53TFSI S54TFSI S55TFSI S63TFSI S64TFSI S65TFSI
1.96E08 1.93E08 1.60E08 1.92E08 1.60E08 1.48E08
0.993 0.996 0.997 0.995 0.997 0.996
0.17 0.17 0.14 0.17 0.14 0.13
3.1. Diffusion of triiodide ion
The electrolyte is composed of 0.3 mol L1 I2, 1.5 mol L1 MPII, 0.1 mol L1 LiI and 0.5 mol L1 TBP in pure SxxTFSI. a
Chronoamperometry is the second established and naturally the fastest method for determination of diffusion coefficients with microelectrodes. At a disk microelectrode, it leads to a steady-state current at infinite long times, after the application of a potential step to a value where a diffusion-controlled process is occurring. Zistler et al. has reported the solution of the chronoamperometric response at a 5-lm-radius disk microelectrode with the current I expressed as a function of t (Zistler et al., 2006): 8 nFAD1=2 c I¼ þ 4nFDcr ð1Þ p2 p1=2 t1=2 where n is the electron number per molecule, F is the Faraday constant, A is the electrode area, c is the bulk concentration of I 3 , t is the time since the potential step and r is the radius of Pt disk. Notably, the use of Eq. (1) was the only adequate for currents recorded at the 5-lm-radius disk microelectrode. For these measurements, data points at times less than 2 s were discarded, to ensure that Eq. (1) is valid. In that case, the diffusion coefficient D can be determined from the intercept of a plot of I vs. t1/2, since n, c and r are known, as shown in Fig. 2. The fitting results and calculated values of the I 3 diffusion coefficients of each electrolyte were listed in Table 1. The good fitting results (R2 > 0.99) were obtained indicating that the time was long enough to keep a diffusion-controlled process occurring. As
shown in Table 1, the apparent diffusion coefficients of triiodide both for five-ring and six-ring cations based electrolyte slightly decreased with the increase of the length of cations’ substitutes, because the longer substitutes could block the movement of triiodide ion. It meant that the shorter substitute should be benefit of triiodide ion diffusion. Besides, for the same alkyl substitution, the movement of triiodide ion in five-ring-sulfonium-based electrolyte was a little faster than that in the six-ring especially when the length of cations’ substitutes was longer. It may be also because the six-ring owing larger volume exhibited more blocking ability than the smaller five-ring. In a word, smaller volume and shorter substitution for cation were benefit for the ion triiodide diffusion. 3.2. The electron exchange reactions on interfaces in DSC Fig. 3 shows the Electrical Impedance Spectra (EIS) of DSCs based on the RTILs electrolyte prepared in this work. Experimental data is represented by symbols while the solid lines correspond to the fit result using the equivalent circuit shown in Fig. 4 (van de Lagemaat et al., 2000; Wang et al., 2005) and the parameters obtained by fitting the experimental spectra with the equivalent circuit are shown in Table 2.
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L. Guo et al. / Solar Energy 85 (2011) 7–11
100
S53TFSI S53TFSI S53TFSI
60
Z''/Ohm
Current Density/mA⋅cm2
S53TFSI
80
S53TFSI S53TFSI
40
S53TFSI
16
20
S54TFSI S55TFSI
12
S63TFSI S64TFSI
8
S65TFSI
4 0
0 0
40
80
120
160
200
240
0.0
280
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage/V
Z'/Ohm Fig. 3. EIS spectra of DSCs based on six RTILs electrolytes under dark at 298 K.
Fig. 4. Equivalent circuits for EIS. Rs: serial resistance; Rct: chargetransfer resistance; CPE: constant phase element.
Here, we mainly focused on the two interfaces: Pt/electrolyte and TiO2/electrolyte, and corresponding charge transfer reactions are 2ePt þ I and 3 ! 3I 2eTiO2 þ I3 ! 3I , respectively. The latter reaction, which generates the dark current, is usually called as back reaction. From Table 2, for the Sþ 5 -based electrolyte, the Rct values of the two interfaces (Pt/electrolyte and TiO2/electrolyte) were increased with the length of substitutes increasing, which meant the longer substitute could inhibit the adsorption of triiodide ion on the two interfaces, resulting in the corresponding charge-transfer reactions mentioned before became more difficult. For the Sþ 6 -based electrolyte, the changing regularity of Rct correlated with the two interfaces was not obvious compared with the Sþ 5 -based electrolyte. In general, the Rct values of two interfaces for six-ring based electrolytes were much larger than that for five-ring, which indicated that the Sþ 6 was unfavorable to electron exchanging reaction on the two interfaces, however, the Sþ 64 was the special case. The Rct value on the two interfaces in S64TFSI was much lower than other
Fig. 5. The I–V curves under irradiation and dark for DSCs using RTILs electrolyte.
six-ring based electrolytes, which meant that the Sþ 64 was benefit for the electron exchange reaction on the two interfaces, the reason was worth to be further studied. Besides, the higher Rct values for TiO2/electrolyte interface were þ observed for Sþ 63 and S64 based electrolyte, meaning that the two cations-based electrolytes had stronger ability to inhibit back reaction, resulting in lower dark current of corresponding cells, as shown in Fig. 5. In summary, the five-ring structure was benefit for electron exchange reaction on Pt/electrolyte interface and the six-ring structure was useful for restraining dark current. 3.3. Photovoltaic performances of DSCs Fig. 5 showed the photovoltaic curves both under irradiation and dark, the corresponding data were summarized in Table 3. In general, the five-ring sulfonium-based electrolytes worked little better than those six-member sulfonium-based salts because they showed faster ion diffusion in solvent and better electron exchange reaction on counter electrode. The current density of DSCs with Sþ 6 -based electrolyte was much lower than that of S þ in spite of some of 5 the Sþ series showing good ion diffusion (S TFSI) and 63 6 lower dark current (S63TFSI and S65TFSI), suggesting that the electron exchange reaction on the counter electrode dominated the charge transporting progress in these DSCs. The bad electron exchange reaction on counter electrode for cells based on S63TFSI and S65TFSI led to the bad fill factor (Wang et al., 2010), resulting in bad cell performance
Table 2 The parameters obtained by fitting the experimental spectra with the equivalent circuit. Solvents
S53TFSI S54TFSI S55TFSI S63TFSI S64TFSI S65TFSI
Pt/electrolyte interface
TiO2/electrolyte interface
Rct (X)
CPE-C (lF)
n
Rct (X)
CPE-C (lF)
n
25.44 30.91 35.04 64.89 26.82 47.53
8.990 18.683 10.974 19.043 8.180 10.725
0.8 0.8 0.8 0.8 0.8 0.8
93.10 105.2 125.3 159.7 108.8 160.0
276.7 433.6 304.5 356.0 307.3 257.2
0.9 0.9 0.9 0.9 0.9 0.9
L. Guo et al. / Solar Energy 85 (2011) 7–11 Table 3 The DSC’s performance with different sulfonium-based electrolyte under one sun intensity. Solvent
Open voltage (V)
Current density (mA cm2)
Fill factor
Efficiency (%)
S53TFSI S54TFSI S55TFSI S63TFSI S64TFSI S65TFSI
0.64 0.65 0.65 0.66 0.64 0.65
8.58 8.58 8.64 8.16 7.98 8.22
0.60 0.58 0.56 0.51 0.60 0.55
3.27 3.23 3.12 2.72 3.09 2.97
(Table 3). The cell based on S64TFSI showed the lowest current density among all cells because of its highest dark current (Fig. 6 and Table 2), but good fill factor due to the lower Rct on the counter electrode could be offset against bad cell performance. It was also proved that the electron exchange reaction on the counter electrode dominated the cells’ performance based on these sulfoniumbased electrolytes. From the result of cells’ performance, we should focus on not only the viscosity of product but also the electron exchange reaction on interfaces when designing and synthesizing a new tape of ionic liquids; for cyclic sulfonium-based ionic liquid, more attention should be paid on five-ring structure. 4. Conclusions A novel family of hydrophobic room temperature ionic liquids based on cyclic sulfonium cations with TFSI were synthesized and applied in dye-sensitized solar cells as pure solvents for electrolyte successfully. The result of chronoamperometry measurement showed that the shorter substitute on the cyclic cations was benefit for the ion diffusion and the triiodide ion diffusion was a little faster in five-ring based electrolyte than in the six-ring based. Electrical Impedance Spectra measurement indicated that the Sþ 6 was negative to the electron exchange reaction on the two interfaces except for S64TFSI and that longer substitute could inhibit the electron exchange reaction on the two interfaces. The final results of dye-sensitized solar cells with these sulfonium-based electrolytes suggested that the electron exchange reaction on the counter electrode dominated the performance of DSCs. Based on all results obtained this paper, the Sþ 5 showed optimistic application prospect compared with Sþ 6 , the next work should focus on modification on five-ring-based sulfonium ionic liquids such as introducing some function group in order to further improve the cells’ performance. Acknowledgements This work was financially supported by the National Basic Research Program of China under Grant No.
11
2011CB201600, the National High Technology Research and Development Program of China under Grant No. 2009AA050603, Funds of the Chinese Academy of Sciences for Key Topics in Innovation Engineering under Grant No. KGCX2-YW-326, and the National Natural Science Foundation of China (Grant No. 20703046). References Agrell, H.G., Lindgren, J., Hagfeldt, A., 2003. Degradation mechanisms in a dye-sensitized solar cell studied by UV–VIS and IR spectroscopy. Solar Energy 75, 169–180. Gratzel, M., 2004. Conversion of sunlight to electric power by nanocrystalline dye-sensitized solar cells. Journal of Photochemistry and Photobiology A – Chemistry 164, 3–14. Guo, L., Pan, X., Zhang, C., et al., 2010. Ionic liquid electrolyte based on S-propyltetrahydrothiophenium iodide for dye-sensitized solar cells. Solar Energy 84, 373–378. Hao, S.C., Wu, J.H., Huang, Y.F., et al., 2006. Natural dyes as photosensitizers for dye-sensitized solar cell. Solar Energy 80, 209–214. Lan, Z., Wu, J.H., Wang, D.B., et al., 2006. Quasi-solid state dyesensitized solar cells based on gel polymer electrolyte with poly(acrylonitrile-co-styrene)/NaI + I2. Solar Energy 80, 1483–1488. Oregan, B., Gratzel, M., 1991. A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature 353, 737–740. Paulsson, H., Hagfeldt, A., Kloo, L., 2003. Molten and solid trialkylsulfonium iodides and their polyiodides as electrolytes in dye-sensitized nanocrystalline solar cells. Journal of Physical Chemistry B 107, 13665–13670. Shi, C.W., Ge, Q., Han, S.K., et al., 2008. An improved preparation of 1methyl-3-propylimidazolium iodide and its application in dye-sensitized solar cells. Solar Energy 82, 385–388. van de Lagemaat, J., Park, N.G., Frank, A.J., 2000. Influence of electrical potential distribution, charge transport, and recombination on the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystalline TiO2 solar cells: a study by electrical impedance and optical modulation techniques. Journal of Physical Chemistry B 104, 2044–2052. Wang, Q., Moser, J.E., Gratzel, M., 2005. Electrochemical impedance spectroscopic analysis of dye-sensitized solar cells. Journal of Physical Chemistry B 109, 14945–14953. Wang, M.K., Chamberland, N., Breau, L., et al., 2010. An organic redox electrolyte to rival triiodide/iodide in dye-sensitized solar cells. Nature Chemistry 2, 385–389. Wilkes, J.S., Zaworotko, M.J., 1992. Air and water stable 1-ethyl-3methylimidazolium based ionic liquids. Journal of the Chemical Society – Chemical Communications 965, 967. Xi, C.C., Cao, Y.M., Cheng, Y.M., et al., 2008. Tetrahydrothiopheniumbased ionic liquids for high efficiency dye-sensitized solar cells. Journal of Physical Chemistry C 112, 11063–11067. Zhang, Q.H., Liu, S.M., Li, Z.P., et al., 2009a. Novel cyclic sulfoniumbased ionic liquids: synthesis, characterization, and physicochemical properties. Chemistry – A European Journal 15, 765–778. Zhang, C.N., Huang, Y., Huo, Z.P., et al., 2009b. Photoelectrochemical effects of guanidinium thiocyanate on dye-sensitized solar cell performance and stability. Journal of Physical Chemistry C 113, 21779– 21783. Zistler, M., Wachter, P., Wasserscheid, P., et al., 2006. Comparison of electrochemical methods for triiodide diffusion coefficient measurements and observation of non-Stokesian diffusion behaviour in binary mixtures of two ionic liquids. Electrochimica Acta 52, 161–169.
Solar Energy 85 (2011) 7–11 www.elsevier.com/locate/solener
Novel hydrophobic ionic liquids electrolyte based on cyclic sulfonium used in dye-sensitized solar cells Lei Guo, Xu Pan, Meng Wang, Changneng Zhang, Xiaqin Fang, Shuanghong Chen, Songyuan Dai ⇑ Key Lab of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, Anhui 230031, PR China Received 12 August 2010; received in revised form 28 October 2010; accepted 11 November 2010 Available online 4 December 2010 Communicated by: Associate Editor Sam-Shajin Sun
Abstract A novel series of hydrophobic room temperature ionic liquids based on six cyclic sulfonium cations were first time synthesized and applied in dye-sensitized solar cells as pure solvents for electrolyte system. The chronoamperograms result showed that the length of substituent on sulfonium cations could inhibit the I 3 diffusion and the five-ring structure of sulfonium was benefit for fast triiodide ion diffusion. The electrochemical impendence spectra measurement of dye-sensitized solar cells with these ionic liquid electrolytes was carried out and the result indicated that the cations’ structure had indeed influence on the cells’ performance especially for the fill factor, which was further proved by the measurement result of I–V curves of these dye-sensitized solar cells. The conclusion was obtained that the electron exchange reaction on Pt counter electrode/electrolyte interface dominated the cells’ performance for these ionic liquid electrolyte-based DSCs. Ó 2010 Elsevier Ltd. All rights reserved. Keywords: Dye-sensitized solar cell; Ionic liquids; Electrolyte; Cyclic sulfonium
1. Introduction Dye-sensitized solar cell (DSC) has gained considerable interest over the past decade as the new generation of solar cells because of their unique properties such as low-cost, simple assembling technology and high efficiency up to 11% (Oregan and Gratzel, 1991; Gratzel, 2004). An important component of DSC is electrolyte system. The conventional electrolyte solvents used in DSC are some organic solvents such as c-butyrolactone (Lan et al., 2006), acetonitrile (Hao et al., 2006) and 3-methoxypropionitrile (Agrell et al., 2003). All of them are usually poisonous and volatile, which restricts DSC’s industrialization. The air and water stable room temperature ionic liquids (RTILs) are attractive as an alternative to organic solvents due to their ⇑ Corresponding author. Tel./fax: +86 551 5591377.
E-mail address: [email protected] (S. Dai). 0038-092X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2010.11.010
unique characteristics such as chemical and thermal stability, negligible vapor pressure, nonflammability, high ionic conductivity and wide electrochemical window (Wilkes and Zaworotko, 1992). Recently, sulfonium-based molten salts have been applied in DSC as electrolyte solvents and shown some exciting results (Paulsson et al., 2003; Xi et al., 2008), which gave us an encouragement in exploring nonimidazolium ionic liquids as solvent-free electrolytes for high performance devices. However, the works involving cyclic sulfonium-based ionic liquids are still very rare (Xi et al., 2008; Guo et al., 2010; Zhang et al., 2009a) and most of the developed cyclic sulfonium-based ILs exhibited strong hydrophilicity, which restricted them to be used as electrolytes because the water in air can pollute these ILs inevitably (Zhang et al., 2009a). Moreover, the research about the influence of cyclic structure on the performance of DSC based on these cyclic sulfonium electrolytes had not been reported.
8
L. Guo et al. / Solar Energy 85 (2011) 7–11
Hence, we synthesized a new series of hydrophobic room temperature ionic liquids based on six cyclic sulfonium cations, as shown in Fig. 1, and succeeded in assembling dye-sensitized solar cells directly using these cyclic sulfonium-based ILs as electrolyte solvents. The investiga tion about the I redox electrochemical behavior was 3 /I carried out, as well as the performance of the corresponding DSC. 2. Experimental 2.1. Reagents and materials Tetrahydrothiopyran, 1-iodopropane, 1-iodopentane, lithium bis(trifluoromethysulfonyl) imide (LiTFSI), anhydrous lithium iodide (LiI) and 4-tert-butylpyridine (TBP) were purchased from Alfa Aesar. Tetrahydrothiophene, iodine (I2), 1,2-dichloroethane and 1-butyliodide were obtained from J&K Chemical Ltd. All solvents and reagents were of analytical grade and used as received. 1Methyl-3-propylimidazolium iodide (MPII) was synthesized according to previous report (Shi et al., 2008). 2.2. Synthesis and characterization of cyclic sulfonium-based room temperature ionic liquids A series of cyclic sulfonium iodide were prepared according to the reference method (Xi et al., 2008; Zhang et al., 2009a). About 100 mmol tetrahydrothiopyran or tetrahydrothiophene was in a round bottle covered with tinfoil. An equimolar amount of the corresponding alkyl iodide was added. The reaction atmosphere was protected from oxygen and water by a continuous flow of dry nitrogen gas. The reaction mixture was left for 2–3 days at temperature 333 K. After that the yellow rough products were dissolved with hot acetone, and then the mixture was dropped in cold diethyl ether, crystalline white solid or yellow liquid precipitated. The products were separated from the solvent followed by thorough washing with diethyl ether. The final products were dried under vacuum. The produced iodide and LiTFSI (1:1.1 M rates) were dissolved
in deionized water and mixed for 24 h at ambient temperature under stirring vigorously. The crude ILs were dissolved with 1,2-dichloroethane and washed with deionized water until no residual iodide anions in the deionized water were detected with use of AgNO3. The 1,2-dichloroethane was removed by rotating evaporation. All the ionic liquids were dried under high vacuum for 24 h at 100 °C. The structures and purity of products were confirmed by 500-MHz 1H NMR (DMX-600, Bruker, Switzerland). Chemical shifts were reported downfield in parts per million (ppm, d) from a tetramethylsilane. The characterization data are as follows: S53TFSI 1H NMR: (DMSO-d6, 500 MHz): 3.489 3.281(m, 4H), 3.150 3.120(t, 2H), 2.231 2.079(m, 4H), 1.747 1.673(m, 2H), 1.004 0.974(t, 3H); S54TFSI: 1H NMR: (DMSO-d6, 500 MHz): 3.486 3.327(m, 4H), 3.167 3.137(t, 2H), 2.218 2.087(m, 2H), 1.685 1.625(m, 2H), 1.414 1.369(m, 2H), 0.922 0.892(t, 3H); S55TFSI 1H NMR: (acetone-d6, 500 MHz): 3.870 3.800(m, 2H), 3.743 3.677(m, 2H), 3.508 3.468(t, 2H), 2.561 2.417(m, 4H), 1.990 1.914(m, 2H), 1.561 1.486(m, 2H), 1.456 1.365(m, 2H), 0.943 0.906(t, 3H); S63TFSI: 1H NMR: (acetone-d6, 500 MHz): 3.799 3.761(t, 2H), 3.601 3.570(t, 2H), 3.467 3.422(t, 2H), 2.341 2.283(m, 2H), 1.992 1.923(t, 2H), 1.878 1.701(m, 4H), 1.136 1.106(t, 3H); S64TFSI: 1H NMR: (acetone-d6, 500 MHz): 3.807 3.769(t, 2H), 3.628 3.597(t, 2H), 3.348 3.433(t, 2H), 2.345 2.287(m, 2H), 1.934 1.873(m, 2H), 1.860 1.721(m, 4H), 1.567 1.522(m, 2H), 0.981 0.952(t, 3H); S65TFSI: 1H NMR: (DMSO-d6, 500 MHz): 3.514 3.466(t, 2H), 3.214 3.165(m, 4H), 2.077 2.017(m, 2H), 1.785 1.725(m, 2H), 1.718 1.658(m, 2H), 1.616 1.581(m, 2H), 1.538 1.493(m, 2H), 1.3801 1.300(m, 2H), 0.891 0.863(t, 3H).
2.3. Measurement
Fig. 1. Structures of cyclic sulfonium-based room temperature ILs prepared in this work.
Conductivity measurement of neat ILs was performed by lab conductivity meter (DDSJ-308A, REX, Shanghai Precision and Scientific Instrument Co., Ltd., PR China) with two platinum black electrodes conductance cell (DJS-1C, the area of each platinum black electrode is about 0.25 cm2, the distance between two platinum black electrodes is about 0.50 cm, the cell constant is 1.021, determined by aqueous KCl standard solution). Chronoamperometry measurement was carried out by using an electrochemical workstation (CHI660A, CH Instruments, Inc., USA). The working electrode was a 5-lm-radius Pt disk (CHI107) and platinum
L. Guo et al. / Solar Energy 85 (2011) 7–11
23
S53TFSI 22
S54TFSI S55TFSI
21
Current/nA
wire acted as the reference electrode and auxiliary electrode. Due to small currents (nA-range) and resulting problems of electrical interference, all measurements with microelectrodes were performed within a Faraday cage. Impedance measurements were performed with a computer-controlled potentiostat (IM6e, Zahner, Germany) in the frequency range from 60 mHz to 1000 kHz. The magnitude of the alternative signal was 5 mV. The impedance measurements were carried out in dark, and the obtained spectra were fitted with Sim software in terms of appropriate equivalent circuits. For I–V characteristics of the DSC, a 450 W xenon light source (Oriel, USA) was used to provide an intensity of 100 mW cm2 (the equivalent of 1.0 sun at global AM 1.5) at the surface of the solar cell and measuring the generated photocurrent with a Keithley model 2400 digital source meter (Keithley, USA).
9
S63TFSI S64TFSI
20
S65TFSI
19 18 17 16 0.3
0.4
0.5
0.6
0.7
0.8
0.9
-1/2
1/square(time)/s
Fig. 2. Plot of the current recorded at a Pt disk microelectrode (5-lmradius disk) during a chronoamperometric measurement vs. t1/2 for ionic liquid electrolytes prepared this work.
2.4. DSC assembly The preparation of DSC was similar to the previous reports (Zhang et al., 2009b). The TiO2 film was about 13 lm thick and a 2-lm-thick light scattering layer was used. The films were immersed in anhydrous ethanol solution with 5.0 104 mol L1 N719 dye 12 h. The total active electrode area of DSC was 0.25 cm2.
Table 1 Fitting and calculating results for chronoamperograms according Eq. (1). Electrolytea
Intercept
R2
DI3 (1E-6acm2/s)
3. Results and discussion
S53TFSI S54TFSI S55TFSI S63TFSI S64TFSI S65TFSI
1.96E08 1.93E08 1.60E08 1.92E08 1.60E08 1.48E08
0.993 0.996 0.997 0.995 0.997 0.996
0.17 0.17 0.14 0.17 0.14 0.13
3.1. Diffusion of triiodide ion
The electrolyte is composed of 0.3 mol L1 I2, 1.5 mol L1 MPII, 0.1 mol L1 LiI and 0.5 mol L1 TBP in pure SxxTFSI. a
Chronoamperometry is the second established and naturally the fastest method for determination of diffusion coefficients with microelectrodes. At a disk microelectrode, it leads to a steady-state current at infinite long times, after the application of a potential step to a value where a diffusion-controlled process is occurring. Zistler et al. has reported the solution of the chronoamperometric response at a 5-lm-radius disk microelectrode with the current I expressed as a function of t (Zistler et al., 2006): 8 nFAD1=2 c I¼ þ 4nFDcr ð1Þ p2 p1=2 t1=2 where n is the electron number per molecule, F is the Faraday constant, A is the electrode area, c is the bulk concentration of I 3 , t is the time since the potential step and r is the radius of Pt disk. Notably, the use of Eq. (1) was the only adequate for currents recorded at the 5-lm-radius disk microelectrode. For these measurements, data points at times less than 2 s were discarded, to ensure that Eq. (1) is valid. In that case, the diffusion coefficient D can be determined from the intercept of a plot of I vs. t1/2, since n, c and r are known, as shown in Fig. 2. The fitting results and calculated values of the I 3 diffusion coefficients of each electrolyte were listed in Table 1. The good fitting results (R2 > 0.99) were obtained indicating that the time was long enough to keep a diffusion-controlled process occurring. As
shown in Table 1, the apparent diffusion coefficients of triiodide both for five-ring and six-ring cations based electrolyte slightly decreased with the increase of the length of cations’ substitutes, because the longer substitutes could block the movement of triiodide ion. It meant that the shorter substitute should be benefit of triiodide ion diffusion. Besides, for the same alkyl substitution, the movement of triiodide ion in five-ring-sulfonium-based electrolyte was a little faster than that in the six-ring especially when the length of cations’ substitutes was longer. It may be also because the six-ring owing larger volume exhibited more blocking ability than the smaller five-ring. In a word, smaller volume and shorter substitution for cation were benefit for the ion triiodide diffusion. 3.2. The electron exchange reactions on interfaces in DSC Fig. 3 shows the Electrical Impedance Spectra (EIS) of DSCs based on the RTILs electrolyte prepared in this work. Experimental data is represented by symbols while the solid lines correspond to the fit result using the equivalent circuit shown in Fig. 4 (van de Lagemaat et al., 2000; Wang et al., 2005) and the parameters obtained by fitting the experimental spectra with the equivalent circuit are shown in Table 2.
10
L. Guo et al. / Solar Energy 85 (2011) 7–11
100
S53TFSI S53TFSI S53TFSI
60
Z''/Ohm
Current Density/mA⋅cm2
S53TFSI
80
S53TFSI S53TFSI
40
S53TFSI
16
20
S54TFSI S55TFSI
12
S63TFSI S64TFSI
8
S65TFSI
4 0
0 0
40
80
120
160
200
240
0.0
280
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage/V
Z'/Ohm Fig. 3. EIS spectra of DSCs based on six RTILs electrolytes under dark at 298 K.
Fig. 4. Equivalent circuits for EIS. Rs: serial resistance; Rct: chargetransfer resistance; CPE: constant phase element.
Here, we mainly focused on the two interfaces: Pt/electrolyte and TiO2/electrolyte, and corresponding charge transfer reactions are 2ePt þ I and 3 ! 3I 2eTiO2 þ I3 ! 3I , respectively. The latter reaction, which generates the dark current, is usually called as back reaction. From Table 2, for the Sþ 5 -based electrolyte, the Rct values of the two interfaces (Pt/electrolyte and TiO2/electrolyte) were increased with the length of substitutes increasing, which meant the longer substitute could inhibit the adsorption of triiodide ion on the two interfaces, resulting in the corresponding charge-transfer reactions mentioned before became more difficult. For the Sþ 6 -based electrolyte, the changing regularity of Rct correlated with the two interfaces was not obvious compared with the Sþ 5 -based electrolyte. In general, the Rct values of two interfaces for six-ring based electrolytes were much larger than that for five-ring, which indicated that the Sþ 6 was unfavorable to electron exchanging reaction on the two interfaces, however, the Sþ 64 was the special case. The Rct value on the two interfaces in S64TFSI was much lower than other
Fig. 5. The I–V curves under irradiation and dark for DSCs using RTILs electrolyte.
six-ring based electrolytes, which meant that the Sþ 64 was benefit for the electron exchange reaction on the two interfaces, the reason was worth to be further studied. Besides, the higher Rct values for TiO2/electrolyte interface were þ observed for Sþ 63 and S64 based electrolyte, meaning that the two cations-based electrolytes had stronger ability to inhibit back reaction, resulting in lower dark current of corresponding cells, as shown in Fig. 5. In summary, the five-ring structure was benefit for electron exchange reaction on Pt/electrolyte interface and the six-ring structure was useful for restraining dark current. 3.3. Photovoltaic performances of DSCs Fig. 5 showed the photovoltaic curves both under irradiation and dark, the corresponding data were summarized in Table 3. In general, the five-ring sulfonium-based electrolytes worked little better than those six-member sulfonium-based salts because they showed faster ion diffusion in solvent and better electron exchange reaction on counter electrode. The current density of DSCs with Sþ 6 -based electrolyte was much lower than that of S þ in spite of some of 5 the Sþ series showing good ion diffusion (S TFSI) and 63 6 lower dark current (S63TFSI and S65TFSI), suggesting that the electron exchange reaction on the counter electrode dominated the charge transporting progress in these DSCs. The bad electron exchange reaction on counter electrode for cells based on S63TFSI and S65TFSI led to the bad fill factor (Wang et al., 2010), resulting in bad cell performance
Table 2 The parameters obtained by fitting the experimental spectra with the equivalent circuit. Solvents
S53TFSI S54TFSI S55TFSI S63TFSI S64TFSI S65TFSI
Pt/electrolyte interface
TiO2/electrolyte interface
Rct (X)
CPE-C (lF)
n
Rct (X)
CPE-C (lF)
n
25.44 30.91 35.04 64.89 26.82 47.53
8.990 18.683 10.974 19.043 8.180 10.725
0.8 0.8 0.8 0.8 0.8 0.8
93.10 105.2 125.3 159.7 108.8 160.0
276.7 433.6 304.5 356.0 307.3 257.2
0.9 0.9 0.9 0.9 0.9 0.9
L. Guo et al. / Solar Energy 85 (2011) 7–11 Table 3 The DSC’s performance with different sulfonium-based electrolyte under one sun intensity. Solvent
Open voltage (V)
Current density (mA cm2)
Fill factor
Efficiency (%)
S53TFSI S54TFSI S55TFSI S63TFSI S64TFSI S65TFSI
0.64 0.65 0.65 0.66 0.64 0.65
8.58 8.58 8.64 8.16 7.98 8.22
0.60 0.58 0.56 0.51 0.60 0.55
3.27 3.23 3.12 2.72 3.09 2.97
(Table 3). The cell based on S64TFSI showed the lowest current density among all cells because of its highest dark current (Fig. 6 and Table 2), but good fill factor due to the lower Rct on the counter electrode could be offset against bad cell performance. It was also proved that the electron exchange reaction on the counter electrode dominated the cells’ performance based on these sulfoniumbased electrolytes. From the result of cells’ performance, we should focus on not only the viscosity of product but also the electron exchange reaction on interfaces when designing and synthesizing a new tape of ionic liquids; for cyclic sulfonium-based ionic liquid, more attention should be paid on five-ring structure. 4. Conclusions A novel family of hydrophobic room temperature ionic liquids based on cyclic sulfonium cations with TFSI were synthesized and applied in dye-sensitized solar cells as pure solvents for electrolyte successfully. The result of chronoamperometry measurement showed that the shorter substitute on the cyclic cations was benefit for the ion diffusion and the triiodide ion diffusion was a little faster in five-ring based electrolyte than in the six-ring based. Electrical Impedance Spectra measurement indicated that the Sþ 6 was negative to the electron exchange reaction on the two interfaces except for S64TFSI and that longer substitute could inhibit the electron exchange reaction on the two interfaces. The final results of dye-sensitized solar cells with these sulfonium-based electrolytes suggested that the electron exchange reaction on the counter electrode dominated the performance of DSCs. Based on all results obtained this paper, the Sþ 5 showed optimistic application prospect compared with Sþ 6 , the next work should focus on modification on five-ring-based sulfonium ionic liquids such as introducing some function group in order to further improve the cells’ performance. Acknowledgements This work was financially supported by the National Basic Research Program of China under Grant No.
11
2011CB201600, the National High Technology Research and Development Program of China under Grant No. 2009AA050603, Funds of the Chinese Academy of Sciences for Key Topics in Innovation Engineering under Grant No. KGCX2-YW-326, and the National Natural Science Foundation of China (Grant No. 20703046). References Agrell, H.G., Lindgren, J., Hagfeldt, A., 2003. Degradation mechanisms in a dye-sensitized solar cell studied by UV–VIS and IR spectroscopy. Solar Energy 75, 169–180. Gratzel, M., 2004. Conversion of sunlight to electric power by nanocrystalline dye-sensitized solar cells. Journal of Photochemistry and Photobiology A – Chemistry 164, 3–14. Guo, L., Pan, X., Zhang, C., et al., 2010. Ionic liquid electrolyte based on S-propyltetrahydrothiophenium iodide for dye-sensitized solar cells. Solar Energy 84, 373–378. Hao, S.C., Wu, J.H., Huang, Y.F., et al., 2006. Natural dyes as photosensitizers for dye-sensitized solar cell. Solar Energy 80, 209–214. Lan, Z., Wu, J.H., Wang, D.B., et al., 2006. Quasi-solid state dyesensitized solar cells based on gel polymer electrolyte with poly(acrylonitrile-co-styrene)/NaI + I2. Solar Energy 80, 1483–1488. Oregan, B., Gratzel, M., 1991. A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature 353, 737–740. Paulsson, H., Hagfeldt, A., Kloo, L., 2003. Molten and solid trialkylsulfonium iodides and their polyiodides as electrolytes in dye-sensitized nanocrystalline solar cells. Journal of Physical Chemistry B 107, 13665–13670. Shi, C.W., Ge, Q., Han, S.K., et al., 2008. An improved preparation of 1methyl-3-propylimidazolium iodide and its application in dye-sensitized solar cells. Solar Energy 82, 385–388. van de Lagemaat, J., Park, N.G., Frank, A.J., 2000. Influence of electrical potential distribution, charge transport, and recombination on the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystalline TiO2 solar cells: a study by electrical impedance and optical modulation techniques. Journal of Physical Chemistry B 104, 2044–2052. Wang, Q., Moser, J.E., Gratzel, M., 2005. Electrochemical impedance spectroscopic analysis of dye-sensitized solar cells. Journal of Physical Chemistry B 109, 14945–14953. Wang, M.K., Chamberland, N., Breau, L., et al., 2010. An organic redox electrolyte to rival triiodide/iodide in dye-sensitized solar cells. Nature Chemistry 2, 385–389. Wilkes, J.S., Zaworotko, M.J., 1992. Air and water stable 1-ethyl-3methylimidazolium based ionic liquids. Journal of the Chemical Society – Chemical Communications 965, 967. Xi, C.C., Cao, Y.M., Cheng, Y.M., et al., 2008. Tetrahydrothiopheniumbased ionic liquids for high efficiency dye-sensitized solar cells. Journal of Physical Chemistry C 112, 11063–11067. Zhang, Q.H., Liu, S.M., Li, Z.P., et al., 2009a. Novel cyclic sulfoniumbased ionic liquids: synthesis, characterization, and physicochemical properties. Chemistry – A European Journal 15, 765–778. Zhang, C.N., Huang, Y., Huo, Z.P., et al., 2009b. Photoelectrochemical effects of guanidinium thiocyanate on dye-sensitized solar cell performance and stability. Journal of Physical Chemistry C 113, 21779– 21783. Zistler, M., Wachter, P., Wasserscheid, P., et al., 2006. Comparison of electrochemical methods for triiodide diffusion coefficient measurements and observation of non-Stokesian diffusion behaviour in binary mixtures of two ionic liquids. Electrochimica Acta 52, 161–169.