Macromolecular Research, Vol. 21, No. 3, pp 315-320 (2013) DOI 10.1007/s13233-013-1097-3
www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673
Ionic Liquid Crystals: Synthesis, Structure and Applications to I2-Free Solid-State Dye-Sensitized Solar Cells Won Seok Chi, Harim Jeon, Sang Jin Kim, Dong Jun Kim, and Jong Hak Kim* Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 120-749, Korea Received October 20, 2012; Revised January 2, 2013; Accepted January 11, 2013 Abstract: A novel type of ionic liquid crystal (ILC) was synthesized and used as a solid electrolyte in I2-free solidstate dye-sensitized solar cells (ssDSSCs). In particular, the properties of two ILCs, 1-[(4-ethenylphenyl)methyl]-3butyl-imidazolium iodide (EBII) with a single aliphatic C=C bond and 1-[(4-ethenylphenyl)methyl]-3-vinyl-imidazolium iodide (EVII) with two aliphatic C=C bonds, were evaluated. The structures and morphologies of the ILCs were characterized using Fourier transform infrared spectroscopy (FTIR) and polarized optical microscopy (POM). Ultraviolet (UV)-visible spectroscopy, X-ray diffraction (XRD), and differential scanning calorimetry (DSC) analyses revealed that EBII exhibited weaker π-π stacking interactions, longer d-spacing, and a lower melting temperature. The energy conversion efficiency of I2-free ssDSSC with EBII (4.7% at 100 mW/cm2) was higher than with EVII (3.8%) due to facile charge transport and lower electron recombination in the former, as supported by electrochemical impedance spectroscopy (EIS). Keywords: dye-sensitized solar cell (DSSC), ionic liquid crystal, solid electrolyte, I2-free; efficiency.
Introduction
more, iodine (I2)-free ssDSSCs have been extensively investigated with the aim of preventing metal corrosion in cells, absorption of visible light, or sublimation of iodine.17-24 One of the key considerations in fabricating ssDSSCs is to enable thorough infiltration of high molecular weight electrolytes into the photoanode, which tends to depend on the size of pores in the electrode.25 Here, we report the fabrication of I2-free ssDSSCs based on a new ionic liquid crystal (ILC) that is non-flammable and has a large electrochemical window and broad redox stability. In particular, two types of ILC, 1-[(4-ethenylphenyl)methyl]-3butyl-imidazolium iodide (EBII) and 1-[(4-ethenylphenyl)methyl]3-vinyl-imidazolium iodide (EVII), were synthesized and used as a solid electrolyte for I2-free ssDSSCs. The detailed structures and properties of the ILCs were characterized using Fourier transform infrared spectroscopy (FTIR), ultraviolet (UV)-visible spectroscopy, polarized optical microscopy (POM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC).
Dye-sensitized solar cells (DSSCs) have attracted much attention as an alternative to conventional silicon solar cells due to their high efficiency and low production cost.1 DSSCs consist of a sensitizing dye, mesoporous TiO2 photoelectrode, Pt-coated counter electrode and an electrolyte containing a redox couple, i.e. I-/I3-. Much attention has been directed toward developing novel materials such as sensitizing dyes, TiO2 photoelectrodes, electrolytes, and counter electrodes with the aim of improving cell performances. DSSCs based on liquid electrolytes with a I-/I3- redox couple have shown energy conversion efficiencies of up to 11% in one sun conditions.2 However, liquid electrolytes have some critical problems such as leakage of electrolyte and evaporation of solvent, which may cause instability, and thus these devices require perfect sealing.3 In addition, DSSCs with liquid electrolytes are limited in cell design due to their inflexibility. The electrolyte is a critical element in determining the photovoltaic performance and stability of DSSCs. Usually, liquid organic solvents such as acetonitrile and methoxy propionitrile have been used as a carrier for I-/I3- redox couples. In order to overcome the drawbacks of liquid solvents based DSSCs, solid-state DSSCs (ssDSSCs) have been developed based on ionic liquids,4,5 polymer electrolytes,6-9 hole transporting materials,10-12 and organogelators.13-16 Further-
Experimental Materials. 1-Butylimidazole, 1-vinylimidazole, lithium iodide (LiI), chloromethylstyrene, acetonitirile (CH3CN), ethyl acetate (EtOAc), and diethyl ether (Et2O) were purchased from Aldrich and were used as received without further purification. Preparation of Electrolytes. The ILCs, 1-[(4-ethenylphenyl)methyl]-3-butyl-imidazolium iodide (EBII) and 1-[(4-
*Corresponding Author. E-mail:
[email protected] The Polymer Society of Korea
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ethenylphenyl)methyl]-3-vinyl-imidazolium iodide (EVII) were synthesized as follows. First, 125 mmol of 1-butylimidazole or 1-vinylimidazole for EBII or EVII, respectively, was dissolved in CH3CN (30 mL) in a 100 mL round-bottom flask equipped with a stir bar. Chloromethylstyrene (19.5 mL, 139 mmol) was added, and the reaction was heated at 50 oC with stirring overnight. The reaction mixture was poured into Et2O (250 mL), causing the ionic product to precipitate, and the mixture was placed in a freezer for several hours. The Et2O phase was decanted, and the product was dissolved in deionized H2O (125 mL). The aqueous phase was washed with EtOAc (300 mL). Lithium iodide (LiI) (125.0 mmol) was added to the aqueous phase, and an oily liquid immediately separated. The mixture was stirred for 1 h, and then the oily phase was extracted into EtOAc (250 mL) and washed with deionized H2O (300 mL). The organic phase was dried with anhydrous MgSO4, and the filtrate was concentrated by rotary evaporation. The remaining solvent was removed under vacuum while stirring at 40 oC overnight. The ILC product was dissolved in acetonitile to use in ssDSSC as a solid electrolyte. Fabrication of DSSCs. ssDSSCs were constructed according to the following procedure. Transparent SnO2/F-layered conductive glass plates (FTO, Pilkington Co. Ltd., 8 Ω/□) were utilized to prepare the photo- and counter-electrodes. For photoelectrode preparation, Ti(IV) bis(ethyl-acetoacetato) diisopropoxide solution (0.15 M) was spin-coated onto the FTO glass plates, and the sample was then calcinated at 450 oC for 30 min. Next, commercial TiO2 paste (D20, Solaronix) was cast onto the FTO glass plate using a doctor-blade technique followed by sintering at 450 oC for 30 min to produce a nanocrystalline TiO2 film approximately 8 µm thick, which is an optimum thickness for pore infiltration of solid electrolytes. The TiO2 film, which had an active area of 0.20 cm2, was sensitized with N719 solution (0.5 mM in ethanol) for 24 h. A dilute electrolyte solution (0.06 M) was cast onto the dyeadsorbed TiO2 electrode and evaporated very slowly to induce deep penetration of the electrolyte into the TiO2 nanopores. Next, a concentrated electrolyte solution (0.3 M) was cast onto the photoelectrode to minimize the time required for solvent evaporation and to prevent the formation of cavities between the two electrodes during solvent evaporation. The counter-electrode was prepared by drop-casting H2PtCl6 solution (0.25 M in isopropanol) onto the FTO glass plate, followed by calcination at 450 oC for 30 min. Cells were placed in a vacuum oven for 24 h to complete the evaporation of solvent, and were then sealed with an epoxy resin. Characterization. FTIR spectra of the samples were collected using an Excalibur Series FTIR (DIGLAB Co.) in the frequency range of 4000-400 cm-1 using the ATR facility. UV-Visible spectra were measured using a spectrophotometer (Hewlett Packard) in the range of 200-800 nm. XRD measurements were performed with a D5005 from Bruker (Germany). Data were collected from 5o to 80o at a rate of 316
2o/min using CuKα radiation (λ=1.5406 Å) operated at 40 kV and 40 mA. DSC analysis using a DSC-Q1000 TA Instrument (UK) was performed from 0 to 160 oC at a rate of 10 oC/min to characterize the melting point of ILC samples. POM pictures were obtained with an Olympus BX51 microscope. For POM measurements, samples were cast onto Si wafers to observe the crystal phase. Photoelectrochemical performance characteristics were measured using an electrochemical workstation (Keithley Model 2400) and a solar simulator (1000 W xenon lamp, Oriel, 91193). The light was homogeneous over an 8 in.×8 in. area and was calibrated with a Si solar cell (Fraunhofer Institute for Solar Energy System, Mono-Si+KG filter, Certificate No. C-ISE269) to a sun light intensity of 1 (100 mW cm-2). This calibration was confirmed with a NREL-calibrated Si solar cell (PV Measurements Inc.).
Results and Discussion Styrene-based room temperature ILCs were synthesized as shown in Scheme I. FTIR spectra for each intermediate were measured to confirm the synthesis of materials, as shown in Figure 1. After ion exchange of chloride [Cl] with iodide [I], the absorption bands (in EBII and EVII) became sharp and were shifted slightly due to altered bonding interactions resulting from the different anion sizes.26 Furthermore, the state of the ionic liquid changed from an oily to a crystalline phase, indicating an ordered state at room temperature. A sharp peak near 1560 cm-1 was attributable to C=N stretching vibrations in the imidazole ring. The weak absorption bands observed at 3052 and 3012 cm-1 resulted from aromatic and alkenyl C-H bonds, respectively. The C=C double bonds in the alkenyl and aromatic groups were also observed at 1650 and 1626 cm-1. The peak intensity of EVII at 1626 cm-1 was stronger than that of EBII, indicating a higher C=C content. The synthesis of ILCs was also sup-
Scheme I. Synthesis of the ionic liquid crystals (ILCs) EBII and EVII. Macromol. Res., Vol. 21, No. 3, 2013
Ionic Liquid Crystals: Synthesis, Structure and Applications to I2-Free Solid-State Dye-Sensitized Solar Cells
Figure 3. XRD patterns of EBII and EVII.
Figure 1. FTIR spectra of reactants and products in the syntheses of (a) EBII and (b) EVII.
Figure 2. Room temperature POM images of (a) EBII and (b) EVII.
ported by POM, as shown in Figure 2, where the crystalline structure at room temperature was visually apparent. To confirm the molecular arrangement of ILCs with different vinyl groups, XRD patterns of EBII and EVII were measured, as shown in Figure 3. Both ILCs exhibited a highly-ordered lamellar structure with a sharp, strong diffraction peak corresponding to the layer spacing (dcr). A set of intense peaks in the wide angle region represents a high degree of structural order in the materials.27 The d-spacing Macromol. Res., Vol. 21, No. 3, 2013
values of EBII and EVII were determined to be 10.44 and 4.01 Å, respectively, which correspond to the layer thickness for each ILC.27 The lower d-spacing value for EVII is due to the shorter chain length of the vinyl group compared to a butyl group, as illustrated in Scheme II. The interaction of the vinyl substituent with iodide promoted by the positive charge of the imidazolium moiety led to unique crystal packing of the halide salt.28 Thus, EVII, which has a stronger π-π stacking interaction and smaller layer thickness, had a higher crystal packing density than EBII. UV-Visible spectra of ILCs after casting of the ILCs on quartz plates to form crystalline films were measured as shown in Figure 4. Both ILCs showed two absorption bands around 365~370 nm and a weak shoulder band around 565 nm. The weak shoulder band at 565 nm is indicative of a π-π* transition in the crystal matrix from the benzene and imidazole rings.29 The 5 nm red shift of EVII relative to EBII can be attributed to a stronger π-π stacking interaction between the rings.30 The higher packing density and shorter d-spacing of EVII gave rise to a more extended conjugated system in the crystal domain. In order to characterize the crystalline structures of ILCs, DSC traces of EBII and EVII were measured as shown in Figure 5(a). The melting temperatures (Tms) of EBII and EVII were determined to be 89.6 and 115.6 oC, respectively, whereas the changes in the enthalpy of EBII and EVII were 44.5 and 59.5 J/g, respectively. The Tm and the enthalpy change of EVII were greater than those of EBII, indicating a stronger bonding interaction and a more ordered crystalline structure for EVII. The presence of a vinyl group provides additional bonding sites for C-H···I- interactions in a second favorable site in the vicinity of the imidazolium cation, producing a more ordered crystalline structure.28 Above Tm, the crystal phase changed to a smectic A phase (SmA) which is between the crystalline and isotropic phases. Upon transformation into the mesophase, the chains melt first, and wiggling of 317
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Scheme II. Schematics of the crystal phase of ILCs with different layer spacing values (dcr).
Figure 4. UV-Visible spectra of EBII and EVII.
the chains disorients the rigid cores, causing the chains to be less stretched.31 The ILC in the SmA could also result in facile ion transport, forming a π-π stacking interaction through connections with the middle crystals.32 Therefore, ILCs could be used as an electrolyte in DSSC below the temperature in which they are in the isotropic phase. The solid-state ILCs (as shown in Figure 5(b)) were used as the electrolyte in I2-free ssDSSCs without any additives or treatment, which is attractive in terms of preventing metal corrosion in cells, absorption of visible light and sublimation of iodine. The current density-voltage (J-V) curves of the cells were measured at 100 mW/cm2, as shown in Figure 6, and the cell parameters are summarized in Table I. The efficiency of DSSC assembled with EBII reached 4.7% with a higher current density (Jsc) compared to the EVIIbased cell due to faster charge transport of iodide ions through formation of a self-assembled structure that promotes the exchange reaction through locally-increased concentrations of I- and I3-. Furthermore, the π-π stacking interaction between the benzene and imidazole groups facilitates electron move318
Figure 5. (a) DSC curves of EBII and EVII and (b) photos of EBII and EVII.
ment and, in the iodine-free system, fixes the imidazolium cations in the crystal structure. The fill factor (FF) of EVII was lower than that of EBII because of higher crystal packing due to the vinyl group as revealed by DSC analysis. Because the strong C-H···I- bonding interactions of the two vinyl groups of EVII limit ion transport, the Jsc value of EVII was lower than that of EBII. The ILCs possess acid/neutral/base properties based on the anion and cation type.33 The pH values of ILC solutions dissolved in acetonitrile (CH3CN) and Macromol. Res., Vol. 21, No. 3, 2013
Ionic Liquid Crystals: Synthesis, Structure and Applications to I2-Free Solid-State Dye-Sensitized Solar Cells
Table I. DSSC Performance and Electrochemical Parameters of I2-Free ssDSSCs Fabricated with ILCs at 100 mW/cm2 Electrolyte
Voc (mV)
Jsc (mA/cm2)
FF
η (%)
Rs (Ω)
R1 (Ω)
R2 (Ω)
ωmin (Hz)
τr (ms)
EBII
0.82
9.1
0.62
4.7
18.6
14.4
88.0
10.3
97
EVII
0.80
8.0
0.59
3.8
20.6
22.0
89.4
25.0
40
Figure 6. J-V curves of I2-free ssDSSCs fabricated with ILCs at 100 mW/cm2. Table II. pH Values of 0.3M EBII and EVII Solutions in CH3CN and Pure CH3CN pH Value
EBII
EVII
CH3CN
6.3
7.2
8
pure CH3CN solution were measured as shown in Table II, which suggests that the alkalinity of the ILC contributes to relaying the charge to the TiO2 surface. Thus, the slightly higher Voc of the EBII-based cell (0.82 V) was due to a higher acidity than with EVII (0.80 V). To further investigate interfacial resistance affecting the electron transfer and recombination rate, electrochemical impedance spectroscopy (EIS) curves were measured as shown in Figure 7. The cell parameters of the EIS plots are summarized in Table I: Rs is the series resistance associated with resistance of the electrolyte and the sheet resistance of the FTO glass; R1 is the charge transfer resistance between the counter electrode and electrolyte; R2 is the charge transfer resistance between the photo-electrode and electrolyte interface; Rd is the Warburg diffusion resistance in the electrolyte; ωmin is the minimum angular frequency; and τr is the lifetime of electrons for recombination. All of the interfacial resistances of the EBII-based cell were smaller than those of the EVII-based cell, indicating facile charge transport in the former. In addition, the electron lifetime of the EBII-based cell was greater than that of the EVII-based cell, indicating lower electron recombination, and resulting in a higher Voc value in the EBII-based cell. Macromol. Res., Vol. 21, No. 3, 2013
Figure 7. EIS data of I2-free ssDSSCs fabricated with ILCs at 100 mW/cm2; (a) Nyquist plots and (b) Bode-phase plots.
Conclusions We synthesized two new types of ILC: EBII with a single aliphatic C=C bond and EVII with an additional aliphatic C=C bond for use as a solid electrolyte in I2-free DSSCs. Chemical bonding and π-π stacking interactions were confirmed by FTIR and UV-visible spectroscopy. XRD analysis showed highly-ordered lamellar structures in the ILCs. The d-spacing of EBII was 10.44 Å, which was larger than that of EVII (4.01 Å) due to the shorter chain length of the vinyl group than the butyl group. The Tm and enthalpy change of EVII were greater than those of EBII, indicating stronger bonding interactions and a more ordered crystalline structure, as characterized by DSC. The efficiency of the I2-free 319
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DSSC with EBII reached 4.7% at 100 mW/cm2, which was greater than that of the EVII-based cell (3.8%). The greater performance of the EBII cell is due to facile charge transport and lower electron recombination, as revealed by EIS analysis. Acknowledgments. This work was supported by a National Research Foundation (NRF) grant funded by the Korean government (MEST) through the Active Polymer Center for Pattern Integration (R11-2007-050-00000-0), the Korea CCS R&D Center and Core Research Program (2012R1A2A2A02011268). It was also supported by the Low Observable Technology Research Center program of Defense Acquisition Program Administration and Agency for Defense Development.
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