Novel cyclic sulfonium iodide containing siloxane high performance electrolyte for dye-sensitized solar cell

Journal of Industrial and Engineering Chemistry 19 (2013) 322–326

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Novel cyclic sulfonium iodide containing siloxane high performance electrolyte for dye-sensitized solar cell Soon Ho Lee, Young Don Lim, Dong Wan Seo, Md. Awlad Hossain, Ho Hyoun Jang, Hyun Chul Lee, Whan Gi Kim * Department of Applied Chemistry, Konkuk Univ., 322 Danwol, Chungju 380-701, Republic of Korea

A R T I C L E I N F O

Article history: Received 10 March 2012 Accepted 22 August 2012 Available online 29 August 2012 Keywords: Ionic liquid Electrolyte Dye sensitized solar cell Sulfonium iodide DSSC

A B S T R A C T

A new type of solid and gel state electrolytes based on siloxane cyclic sulfonium iodides was synthesized and used in dye-sensitized solar cells. The resulting electrolytes were characterized by 1H NMR, TGA, diffusion coefficient, and ion conductivity. The thermo-oxidative stabilities of the prepared ionic species are lower than that of DMII because the bond strength of S+–C in thiophenium is lower than that of N+–C in imidazolium. Among the three SiCSIs based electrolytes, SiCSI3 showed a maximum photoconversion efficiency of 7.3%. In addition, the performance of the DSSCs showed relatively reasonable compared with the imidazolium type DMII electrolyte. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction The great interest in room temperature ionic liquids (RTILs) has recently rendered them to one of the most attractive materials since the discovery of the first ambient temperature ionic liquid by Wier [1]. There are numerous attempts to gain the fundamental understanding of the unique characteristics of the RTILs include combinations of new anions and cations to replace novel ionic liquids [2–4]. In recent years, room temperature molten salts have been applied as both iodide sources and solvents for redox electrolytes in DSSCs [5,6]. There have been many research works to improve the efficiency of dye-sensitized solar cell (DSSC) using liquid electrolytes [5–7]. Among them, especially 1-methyl-3alkylimidazolium and 1,2-dimethyl-3-alkylimidazolium iodides are mostly used for DSSCs. Recently, highest photoelectric conversion efficiency of 8.2% for the ionic liquid-based electrolyte in DSSCs has been reported [8]. Diffusion of redox and charge carrier is an important component in electrolyte for DSSCs. Among these ionic salts, there are many species of anions, such as I , BF4 , PF6 , (CF3SO2)2N , TFSI (CF3SO2)(CF3CO)N , and TSAC [9–11]. On the cationic side, molten salts incorporating imidazolium [12–16], pyrrolidinium [17], tetraalkylammonium [18,19], and quaternary phosphonium (QP) [20] cations have undergone extensive investigation. The viscosity of the ionic liquid should be as low as possible to avoid

* Corresponding author. Tel.: +82 438403579; fax: +82 438514169. E-mail address: [email protected] (W.G. Kim).

mass transport limitation of the photocurrent and loss of fill factor under cell operation in full sunlight. In recent years, sulfoniumbased supporting electrolytes have been found to exhibit low viscosity and good electrical properties [21–23]. These results give us an encouragement in exploring non-imidazolium ionic liquid as solvent-free electrolytes for high performance devices. One recent paper reported some asymmetric sulfonium-based room temperature molten salts with TFSI anion and research focused on their temperature dependence of viscosity [24]. Dai et al. reported that the length of substituent on sulfonium cations could inhibit the I3 diffusion and the cyclic sulfonium electrolyte was beneficial for fast triiodide ion diffusion [25]. In order to gain further understanding and extend the range of applications of cyclic sulfonium-based salts electrolytes, investigation on preparation method and properties of new class of such salts is necessary. In this study, we synthesized three cyclic sulfonium iodide salts called SiCSI1, SiCSI2, and SiCSI3 in which siloxane groups were incorporated between sulfonium salts or sited on terminal group. In our previous studies, imidazolium iodides containing siloxane group provide low viscosity and excellent cell performance [26]. Additionally, oxygens of siloxane are expected to have a complex with TiO2 and capturing effect of cation. It can enhance the stability of the electrolytes and possibly be advantageous in transport of anionic species in the cell. Systematic studies were performed to understand the influence of these new type ionic liquids on the performance of DSSCs when they were used as a component of liquid electrolytes in organic solvent based electrolyte system. And also, the long-term stability of these ionic liquids was investigated.

1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.08.019

S.H. Lee et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 322–326

2. Experimental

323

2.4. Synthesis of 1,1,3,3,5,5-hexamethy-trisiloxane-1,5biscyclicsulfoniumdiiodide (SiCSI2)

2.1. Materials Allyl alcohol, thionyl chloride, 1,1,3,3,5,5-hexamethyltrisiloxane, 1,1,1,3,3,5,5-heptamethyltrisiloxane, platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex, sodium iodide were purchased from Aldrich and 1,3-bis(chloromethyl)tetramethyldisiloxane from TCI. Other commercially available reagent & solvents, such as pyridine, dichloromethane, toluene, ethyl acetate, chloroform, acetone, ethyl ether, hexane, acetonitrile were also used with or without further purification. 2.2. Synthesis of tetramethyl-disiloxane-1,3biscyclicsulfoniumdiiodide (SiCSI1) A solution of 1,3-bis(chloromethyl)tetramethyl-disiloxane (3 g, 12.97 mmol), sodium iodide (7.78 g, 51.89 mmol) in acetone (50 mL) was refluxed at 60 8C for 24 h. The reaction mixture was allowed to cool and then filtered. The filtrate was evaporated under reduced pressure and then dissolved in ethyl ether. The reaction mixture was filtered. The filtrate was evaporated to remove the ethyl ether. The 1,3-bis(iodomethyl)tetramethyldisiloxane was obtained as a light yellow oil, yielded 97% (5.2 g). A solution of 1,3-bis((1-methylimidazolium iodide)-3-methyl)tetramethyl-disiloxane (3 g, 7.24 mmol) and tetrahydrothiophene (1.53 g, 17.38 mmol) in acetonitrile (30 mL) was refluxed at 85 8C for 24 h. The reaction mixture was evaporated and washed with ethyl ether. After removal of ethyl ether with a rotary evaporator, the 1,1,3,3-tetramethyl-disiloxane-1,3-biscyclicsulfonium diiodide (SiCSI1) was obtained as a light yellow solid; yield: 3.8 g (90%). 2.3. Synthesis of 1,5-bis-(3-iodopropyl)-(1,1,3,3,5,5hexamethyltrisiloxane) A reaction mixture of 1,1,3,3,5,5-hexamethyltrisiloxane (3 g, 14.39 mmol), allyl alcohol (2.01 g, 34.54 mmol) and a catalytic amount of platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex (1 drop) in toluene (30 mL) was refluxed for 24 h with vigorous stirring. The reaction mixture was then passed through a plug of celite. The residual toluene was removed by vacuum evaporation to obtain 1,5-bis-(3-hydroxypropyl)-(1,1,3,3,5,5-hexamethyltrisiloxane) (4.44 g, 95% yield). Thionyl chloride (10.99 g, 92.4 mmol) was carefully added to a mixture of 1,5-bis-(3hydroxypropyl)-(1,1,3,3,5,5-hexamethyltrisiloxane) (3 g, 9.24 mmol) and pyridine (0.07 g, 0.92 mmol) at 0 8C in an ice bath with constant stirring. After addition, the mixture was allowed to warm to room temperature and then refluxed for 12 h at 70 8C. The reaction mixture was then cooled to room temperature and the excess thionyl chloride was evaporated. The residue was treated with 40 mL H2O and the mixture was extracted with ethyl acetate (50 mL  3). The combined organic extracts were washed with 100 mL saturated NaHCO3 solution and H2O (100 mL  2), and then dried over anhydrous Na2SO4. The ethyl acetate was removed by a rotary evaporator to give 1,5-bis(3-chloropropyl)-(1,1,3,3,5,5-hexamethyltrisiloxane) as a brown oil; yield: 3.01 g (90%). A solution of (1,1,3,3,5,5-hexamethyltrisiloxane)-1,5-dipropylchloride (3 g, 8.30 mmol), sodium iodide (4.98 g, 33.19 mmol) in acetone (50 mL) was refluxed at 60 8C for 24 h. The reaction mixture was allowed to cool and filtered. The filtrate was evaporated under reduced pressure and later dissolved in dichloromethane. The reaction mixture was filtered and evaporated to remove dichloromethane on a rotary evaporator to give 1,5-bis-(3-iodo propyl)-(1,1,3,3,5,5-hexamethyltrisiloxane) as a brown oil; yield: 4.38 g (97%).

A solution of 1,5-bis-(3-iodopropyl)-(1,1,3,3,5,5-hexamethyltrisiloxane) (3 g, 5.51 mmol), tetrahydrothiophene (1.17 g, 13.22 mmol) in acetonitrile (30 mL) was refluxed at 85 8C for 24 h. The reaction mixture was evaporated and washed with hexane. The residual hexane was removed by vacuum evaporation at 60 8C for 6 h to give SiCSI2 as a light yellow solid; yield: 3.57 g (90%). 2.5. Synthesis of 1,1,1,3,3,5,5-heptamethyl-trisiloxane-5cylcicsulfoniumiodide (SiCSI3) Preparation of 1,1,1,3,3,5,5-heptamethyltrisiloxane was followed the same as previous procedure of SiCSI2. A solution of 1-(3iodopropyl)-(1,1,3,3,5,5,5-heptamethyltrisiloxane) (3 g, 7.68 mmol), tetrahydrothiophene (0.88 g, 9.99 mmol) in acetonitrile (30 mL) was refluxed at 85 8C for 24 h. The reaction mixture was evaporated and washed with hexane. The residual hexane was removed by vacuum evaporation at 60 8C for 6 h to give SiCSI3 in 85% yield. 2.6. Preparation of electrolytes and DSSC fabrication Organic solvent-based these different solid type electrolytes were prepared by dissolving dimethylimidazoliumiodide (DMII) and siloxane cyclic sulfonium iodides (SiCSI1, SiCSI2, SiCSI3) with 0.1 M GNCS, 0.05 M I2 and 0.5 M TBP in 3-methoxypropionitrile. The concentration of DMII was fixed as 0.6 M in electrolyte. Commercial titanium dioxide paste (Ti-Nanoxide HT, Solaronix SA) was used as the TiO2 source. A thin layer of TiO2 paste with an area of 0.1 cm2 was deposited on fluorine doped tin oxide (TCO30-8, 8 V/cm2, Solaronix, SA) glass substrate by doctor blade technique and successive sintering at 450 8C for 30 min in air. The resulting TiO2 photoelectrode was immersed into the dye solution (N719, Ru 535 bis-TBA, Solaronix, SA) for 24 h in dark. The sandwich-type solar cell was assembled by sealing a Pt coated conducting glass on the photoelectrode. 2.7. Measurements The 1H NMR spectra were recorded on a Brucker DRX (400 MHz) spectrometer using DMSO-d6 as the solvent and tetramethylsilane as the internal standard. And thermo-gravimetric analyses (TGA) were performed with Perkin-Elmer TGA7 at a heating rate of 20 8C/min under an ambient air flow of 50 mL/min. The electrochemical characterization of the electrolytes, such as cyclic voltammetry and steady-state voltammetry were performed with electrochemical workstation, CHI430A (CHI instruments, Inc., USA). A Pt disk with 3 mm in diameter and 5 mm radius Pt ultramicroelectrode (UME) were used as working electrode for cyclic voltammetry and steady-state voltammetry measurement, respectively. Ag/AgCl wire and Pt wire were used as a reference and counter electrode. A symmetric thin-layer impedance cell was constructed by two platinized FTO-glasses sandwiched with a 50 mm surlyn tape as spacer. The electrode area contacting with electrolyte was 0.1 cm2. The incident light intensity was adjusted to mW/cm2 (1 sun) AM 1.5 using a 450 W Xe high power arc lamp housing (LH 151, Spectral Energy Co.). The light intensity was calibrated using a Radiometer Photometer (ILT 1400-A). Ion conductivity was measured with Mettler Toledo S47 Seven MultiTM and measured in 0.6 M MPN. 3. Results and discussion A series of cyclic sulfonium-based electrolytes, SiCSIs (SiCSI1, SiCSI2 and SiCSI3) were synthesized according to the reaction

S.H. Lee et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 322–326

324

Cl

Si

O

Si

H Si

Cl

O

Si

O

Si H

Si

O

Si

Allylalcohol, Pt(0)

O

Si H

Allylalcohol, Pt(0)

NaI

I

Si

O

HO Si

Si

O

Si

O

Si

OH

Si

O

Si

Tetrahydrothiophene I

I

Si

O

Si

Si

OH

I 1. SOCl2, pyridine 2. NaI

S

O

Si

O

Si

S

O

Si

O

Si

Tetrahydrothiophene

I S

Si

O

Si

O

S

Si

I

- SiCSI1 -

I

Si

1. SOCl2, pyridine 2. NaI I

Si

Tetrahydrothiophene Si

O

Si

I

- SiCSI2 -

O

O

S

Si I

- SiCSI3 -

Scheme 1. Synthesis of SiCSI1, SiCSI2 and SiCSI3.

Scheme 1. Hydrosilylation reaction, which is an attractive route to form thermally and hydrolytically stable carbon silicon bond provided convenient formation of new carbon silicon bond between allyl alcohol and hydride terminated dimethyl siloxane. This reaction was catalyzed by Pt-complex (Karstedt’s catalyst) which typically proceeds to high conversions without the formation of

byproducts [27]. The chemical structures of SiCSIs were identified by 1 H NMR as shown in Fig. 1. The tetrahydrothiophene was converted to salt form of cyclic sulfonium iodide, which acts as an electronwithdrawing group to shift down field. To confirm the chemical structure, we compared SiCSIs with the model compound of propyltetrahydrothiophenium iodide. Interestingly, in SiCSI1, the

Fig. 1. 1H NMR spectra of propyltetrahydrothiophenium, SiCSI1, SiCSI2 and SiCSI3.

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Table 1 Photo-voltaic parameters of DSSCs with DMII, SiCSI1, SiCSI2 and SiCSI3. Electrolyte DMII SiCSI1 SiCSI2 SiCSI3

Voc (V) 0.711 0.708 0.703 0.689

Jsc (mA cm2) 14.84 16.10 15.85 18.03

FF (%) 63.5 60.8 61.8 59.1

proton peaks (Ha) nearby sulfonium (S+) appeared two peaks with multiplet at d 3.56–3.49, and 3.46–3.37. It would be explained that lone-pair electrons and cation of sulfonium (S+) are positioned in the upper and under orbitals individually, which indicate geometrically different environmental. The protons (Hc) also showed multiplet peaks as same reason. The model compound also showed same behaviors. The protons of S+–CH2–CH2–CH3 appeared as triplet at 3.22 ppm. In the 1H NMR studies of SiCSI electrolytes, the protons of Si–CH3 are observed at 0.2–0.6 ppm. The protons of cyclic sulfonium showed characteristic peaks same as model compound. The proton (Hb) should be shown one peak because there are no protons nearby, but three peaks appeared at 2.66 ppm. The protons of CH2 also were affected by unsymmetrical geometry of sulfonium orbital, and they would be exist as isomers. Siloxanes are not only thermally very stable, but also chemically stable without decomposition under severe conditions in the presence of strong base or acid. Thermo-oxidative stabilities of SiCSIs were studied using thermogravimetric analysis (TGA) and the results were shown in Fig. 2. The thermooxidative stabilities of the prepared electrolytes are lower than that of DMII because the bond strength of S+–C in sulfonium is lower than that of N+–C in imidazolium. Mono sulfonium SiCSI3 is little more stable than bis sulfonium one. The colors and viscosities of these SiCSI1 and SiCSI2 are shown light yellow with solid state, but SiCSI3 is brown color with gel state as illustrated in Fig. 3. We have studied linear siloxane containing mono- and bis-imidazolium salts that show

h (%) 6.7 6.9 6.9 7.3

DI3 (cm2 s 6.79  10 6.01  10 6.01  10 6.51  10

4 4 4 4

1

)

Ion conductivity (mS/cm) 18.46 15.43 14.88 17.77

gel state with characteristics of excellent flexibility and low viscosity [26,28]. However, bis-sulfonium electrolytes are solid because of high molecular weight and large size of sulfur element even though they have siloxane group. The mono-sulfonium electrolyte maintains gel state owing to low molecular weight and free siloxane functional group. This quality of siloxane salts arises from two properties: one is its flexibility which enables it to acquire conformations that results in close and efficient packing at various interfaces, and the other is its low cohesive energy derived from its greater voluminous and cross-sectional area of the siloxane unit at the interface [29,30]. The diffusion coefficient of triiodide in the electrolytes was determined by the measurement of acyclic voltammogram using a slow scan rate in acetonitrile at room temperature shown in Table 1 [31]. The diffusion coefficients for I3 of the SiCSIs were 6.01– 6.51  10 4 cm2 s 1, close to that of DMII. The mono functionalized SiCSI1 showed high diffusion coefficients because of low viscosity. The apparent diffusion coefficients of triiodide evidently depended on the chemical structure of sulfonium salt, anion, and molecular weight of electrolyte. This suggests that the size of electrolyte does not linearly affect the diffusion coefficient of anion. Ion conductivities of electrolytes were measured in 0.6 M MPN solution and the results were presented in Table 1. Ion conductivity of DMII was higher than those of electrolytes and these behaviors are the same as the tendency of the diffusion coefficient. The low molecular weight electrolyte SiCSI3 was showed high ion conductivity. Photocurrent density–voltage characteristics of the DSSCs based on new electrolytes SiCSI1, SiCSI2, SiCSI3 and DMII as a reference electrolyte are presented in Fig. 4. Open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and photo-conversion efficiency (h %) are listed in Table 1. The DSSCs with conventional electrolytes, DMII, showed the initial efficiency of 6.7% at 1 sun condition with our home-made solar simulation system. Siloxane cyclic sulfonium electrolytes, SiCSI1, SiCSI2 and SiCSI3, showed initial photoconversion similar or little bit high efficiencies of 6.9%, 6.9% and 7.3% compare to that of DMII. However, the resulting siloxane cyclic sulfonium salts showed high efficiency even though they have gel or solid state and big size molecules. These phenomena could be explained with the siloxane group formed on the TiO2 surface, the

Fig. 2. TGA curves of SiCSI1, SiCSI2 and SiCSI3.

Fig. 3. Sample types of SiCSI1, SiCSI2 and SiCSI3.

Fig. 4. J–V characteristic curves of the DSSCs containing DMII, SiCSI1, SiCSI2 and SiCSI3.

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driving force for the approach. Once the penetration occurs, then molecule structures affect the packing densities and thus surface charges. When considering that lone pair electrons of siloxane group are acting as bronsted base, the increase in photocurrent density is thus unusual. Kim et al. reported that this unusual enhancement of photocurrent along with decrease in voltage observed in the presence of thiourea is first thought to be probably due to an improved electron transport rate and/or an accelerated charge recombination rate [31]. These kinds of modified siloxane electrolytes are interesting chemical structure, and can be promising materials for high efficiency. 4. Conclusions We successfully synthesized a series of new siloxane-containing cyclic sulfonium iodides (SiCSIs) with different siloxane chain groups and bis/mono cyclic sulfonium type to develop a novel electrolyte for DSSCs. The synthesized SiCSI1 & 2 electrolytes were light yellow with solid state and SiCSI3 was gel state with brown color. The SiCSI1 & 2 and SiCSI3 electrolyte showed a maximum photo-conversion efficiency of 6.9% and 7.3% which were slightly higher than the 6.7% value of the reference DMII. These results provide valuable information for further studies of cyclic sulfonium iodide salts containing siloxane to develop gel-type electrolytes for the improvement of DSSCs. Acknowledgements This work was supported by the Technology Innovation Program (SA113423) funded by the Ministry of Knowledge Economy (MKE, Korea) and National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2009-0093168). References [1] F.H. Hurley, T.P. Wier, Journal of the Electrochemical Society 98 (1951) 203. [2] J.S. Wilkes, M.J. Zaworotko, Journal of the Chemical Society, Chemical Communications (1992) 965. [3] M. Gratzel, Journal of Photochemistry and Photobiology C 4 (2003) 145.

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