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Dye-Sensitized TiO2 Solar Cells Using Imidazolium-Type Ionic Liquid Crystal Systems as Effective Electrolytes† Noriyo Yamanaka,‡ Ryuji Kawano,§ Wataru Kubo,‡ Naruhiko Masaki,| Takayuki Kitamura,‡ Yuji Wada,‡ Masayoshi Watanabe,§ and Shozo Yanagida*,| Material and Life Science, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamada-oka, Suita 565-0871, Japan, Department of Chemistry and Biotechnology, Yokohama National UniVersity, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan, and Center for AdVance Science and InnoVation, Osaka UniVersity, 2-1 Yamada-oka, Suita 565-0871, Japan ReceiVed: October 30, 2006
A novel ionic liquid crystal (ILC) system (C12MImI/I2) with a smectic A phase used as an electrolyte for a dye-sensitized solar cell (DSSC) showed the higher short-circuit current density (JSC) and the higher lightto-electricity conversion efficiency than the system using the non-liquid crystalline ionic liquid (C11MImI/I2), due to the higher conductivity of ILC. To investigate charge transport properties of the electrolytes in detail, the exchange reaction-based diffusion coefficients (Dex) were evaluated. The larger Dex value of ILC supported that the higher conductivity of ILC is attributed to the enhancement of the exchange reaction between iodide species. As a result of formation of the two-dimensional electron conductive pathways organized by the localized I3- and I- at SA layers, the concentration of polyiodide species exemplified by Im- (m ) 5, 7, ...) was higher in C12MImI/I2. However, as the increment of the concentration of polyiodide species is less than that of Dex, the contribution of a two-dimensional structure of the conductive pathway through the increase of collision frequency between iodide species was proposed. Furthermore, a quasi-solid-state ionic liquid crystal DSSC was successfully fabricated by employing a low molecular gelator. Addition of the 5.0 g/L gelator to ILC improved light-to-electricity conversion efficiency through the increase of JSC due to the enhancement of the conductivity in C12MImI/I2-gel.
Introduction Dye-sensitized solar cells (DSSC), constructed by using dye molecules, nanocrystalline metal oxides, and liquid electrolytes, have attractive features in terms of the high light-to-electricity conversion efficiency and the cost effectiveness.1,2 The electrolytes, usually composed of an I-/I3- redox couple in organic solvents, are sealed between two electrodes. These organic solvents cause a serious problem of low durability due to the evaporation.3 It has been reported that ionic liquids4 having unique properties such as nonvolatility,5,6 nonflammability,5,6 high ionic conductivity,7 and gel-forming properties with polymers8 are useful as electrolytes of DSSC, achieving hightemperature stability.9,10 Furthermore, quasi-solid-state DSSC using ionic liquids solidified by gelators achieve higher temperature stability than those without gelators.10 However, the light-to-electricity conversion efficiency of DSSC using ionic liquids has been still lower than those using organic solvents, because high viscosity of the ionic liquids retards physical diffusion of I3- and I-. The diffusion of I3and I- in ionic liquids should be promoted for the realization of DSSC showing high light-to-electricity conversion efficiency. However, many attempts to reduce the viscosity of ionic liquids for improvement of the conductivity of ionic liquids have not yet been successful.11 †
Part of the special issue “Physical Chemistry of Ionic Liquids”. * To whom correspondence should be addressed. E-mail: yanagida@ mls.eng.osaka-u.ac.jp ‡ Graduate School of Engineering, Osaka University. E-mail: ywada@ mls.eng.osaka-u.ac.jp. § Department of Chemistry and Biotechnology, Yokohama National University. E-mail:
[email protected]. | Center for Advance Science and Innovation, Osaka University.
We recently introduced a new strategy for enhancing the diffusion by creating a new path for the diffusion of the I3and I-.12 We have found that 1-dodecyl-3-methylimidazolium iodide (C12MImI) has a self-assembled bilayer structure owing to interdigitated alkyl chains of the imidazolium cations, resulting in formation of an imidazolium-type ionic liquid crystal (ILC) with the smectic A phase (SA). Employing this ILC with iodine (C12MImI/I2) as an electrolyte of DSSC led to an improvement of JSC. When C12MImI/I2 as ILC is compared with 1-undecyl-3methylimidazolium iodide containing iodine (C11MImI/I2) as ionic liquid with high viscosity, the diffusion coefficients of I3- (D) of C12MImI/I2 were higher than that of C11MImI/I2, although the viscosity of C12MImI/I2 was larger than that of C11MImI/I2. This large viscosity of C12MImI/I2 should be connected to the low physical diffusion coefficients of I3- and I- compared to C11MImI/I2. Therefore, we have taken into account the contribution of the exchange reactions to the charge transport process.12 We have also shown that the ionic conductivities along with the direction parallel (σi|) to the SA layer plane were higher than those perpendicular (σi⊥).12 This phenomenon would lead to an idea that the electron conductive pathways were organized by the localization of I3- and I- between SA layers. Then we have decided to investigate the system thoroughly to be able to proposing our new strategy to enhance the conductivity of ionic liquids. In this study, the promotion of the exchange reaction has been successfully revealed by the separation of Dex and Dphys from the whole D value. Raman spectroscopy was employed to reveal the localization of I3- and I- between SA layers in ILC, which
10.1021/jp0671446 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/04/2007
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is responsible for the organization of the electron conductive pathways. Furthermore, we have expanded our successful idea for quasi-solid-state DSSC by gelation of an ILC electrolyte, showing the enhancement in the efficiency. Experimental Section Sample Preparation. The imidazolium iodides with long alkyl chains were synthesized by quaternization reactions of 1-methylimidazole with an equimolar amount of the corresponding alkyl iodides (C11, C12, C16, C18) for 72 h under N2 atmosphere at room temperature. The products were washed with n-hexane to remove the remaining starting materials, dried under vacuum at 40 °C for 4 h, and finally identified by 1H NMR in CDCl3, differential scanning calorimetry (DSC), and polarized optical microscopy (POM). Characterizations. The phase transition temperatures were determined by a Shimadzu DSC-60 on heating at a rate of 5 °C min-1. POM studies on the ILC were carried out using an Olympus CBX51-33P-OC under cross-polarized light at 40× magnification. Viscosity was measured with a viscometer (TVE-20H, Toki Sangyo) with a temperature controller (R-134A, Neslab Instruments) at 40 °C. Raman spectra of electrolytes were measured with a Raman spectrometer (Spectrum GX2000R, Perkin-Elmer) equipped with a Nd:YAG laser (10-1600 mW). Diffusion Coefficients of I3-. The diffusion-limited currents (Ilim) corresponding to the reaction of I3- + 2e- f 3I- were measured by using a microelectrode technique14 to simplify determination of the transport property of I3-. A Pt microdisk electrode (12 µm diameter) was used as a working electrode, while a Pt wire was used as a counter electrode. An Ag wire was used as a reference electrode.12 The measurements of Ilim were carried out at 40 °C. The diffusion coefficients of I3(D) were calculated from Ilim taken as an average of two measurements. The D values as a function of the concentrations ([I-] + [I3-]) were examined to separate Dex and Dphys from the whole D value.15 We mixed C12MImI/I2 or C11MImI/I2 with 1-dodecyl3-methylimidazoliumtetrafluoroborate (C12MImBF4), which showed a SA phase ranging from 26 to 45 °C (Sanko),16 to dilute the concentrations ([I-] + [I3-]). The ILC phase of the mixtures was confirmed by DSC and POM. We measured the Ilim of the samples with different concentrations at 40 °C by using a Pt microdisk electrode (10 µm diameter) as a working electrode. A Pt wire was used as a counter electrode, and an Ag wire was used as a reference electrode. We calculated the Dex values from the D values, which were calculated from Ilim of the samples having different concentrations. Anisotropic Ionic Conductivities. Dynamic ion conductivities were recorded on an impedance analyzer (Solarton 1260, Toyo Technica) with a temperature controller (EC-15MHHPS, Hitachi). Conductivities were determined by the value of the real part of the cell impedance at the frequency that gave the minimum value of the imaginary part. Details in the measurements were described in the literature.20 The anisotropic ionic conductivities21,22 of C12MImI/I2 were successfully measured for the samples forming the oriented monodomains in the cell A and cell B shown in Figure 1. Cell A comprised a glass plate with a comb-shaped platinum electrode. Cell B consisted of a pair of indium tin oxide (ITO) electrodes. The thickness between the two electrodes was fixed by a Teflon spacer to be 500 µm.
Figure 1. Schematic illustrations of the cells A and B for the anisotropic ion-conductive measurements.
CHART 1: Chemical Structures of (a) C12MImI and (b) C11MImI Gelators
C12MImI/I2 filled in the cell A and B showed homeotropic alignment in the SA phases, which was confirmed by POM. We measured σi| by using cell A and σi⊥ by using cell B at temperatures ranging between 32.5 and 57.5 °C on heating, which matches the DSC measurements. Photoelectrochemical Cells. Photoelectrochemical cells were fabricated as previously described.17,18 Nanoporous TiO2 electrodes were prepared as follows. A colloidal suspension of TiO2 particles (P25, Nippon Aerosil) was deposited on a transparent conductive substrate (F-doped SnO2, sheet resistance 10 Ω square-1, Nippon Sheet Glass). The electrodes were sintered at 450 °C for 30 min in air. The resulting TiO2 films (thickness ) 10 µm) were immersed in 5.0 × 10-4 M cis-dithiocyanate-N,N′bis(4-carboxylate-4-tetrabutylammoniumcarboxylate-2,2′-bipyridine)ruthenium(II) (N719, Solaronix) in an acetonitrile/2methyl-2-propanol (1/1) solution for 24 h. After drying of the electrode, the porous surface was covered by a platinized conducting glass as a counter electrode. The effective area of the cell electrode was 0.25 cm2. The electrolytes (C12MImI/I2 or C11MImI/I2) composed of the imidazolium iodides (Chart 1a) and iodine were injected into the gap between the electrodes keeping the temperature at 80 °C because the viscosity of the electrolytes at 80 °C was low enough to penetrate the nanoporous TiO2 electrodes. The ionic liquid crystalline gel electrolytes (C12MImI/I2gel) were prepared by mixing the gelator (Chart 1b), which forms thermoreversible physical gels through intermolecular hydrogen bonds,18 with C12MImI/I2 and then stirring them on heating at 130 °C to dissolve the gelator. The ionic liquid gel electrolytes (C11MImI/I2-gel) were also prepared by mixing the gelator with C11MImI/I2 with the same method. Photoelectrochemical cells using the gel electrolytes (C12MImI/I2-gel, C11MImI/I2-gel) were fabricated with the same method of DSSC using C12MImI/I2 and C11MImI/I2. Photoelectrochemical measurements were performed according to the reported procedures.1 Light-to-electricity conversion efficiency was evaluated using a solar simulator (YSS-80, Yamashita Denso) as AM 1.5 light source and a PC-controlled voltage current source/meter (R6246, Advantest) with a temperature controller (EC-15MHHPS, Hitachi) at 40 °C. We adapted the Japanese Industrial Standard for amorphous solar cells in the cell efficiency determination.19 Each value for cell performance was taken as an average of at least 3 samples.
Dye-Sensitized TiO2 Solar Cells
Figure 2. Differential scanning calorimetric traces observed on heating of C12MImI (bold solid curve) and C11MImI (solid curve). The peak labels indicate the type of phase transition occurring: S ) solid f smectic A; I ) smectic A f isotropic; M ) solid f isotropic.
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Figure 3. Textures observed using POM (40× magnification) of C12MImI and C11MImI (insert). Both photographs were taken after cooling from the isotropic phase.
Results and Discussion Electrolytes Characterization. First of all, suitable compounds forming the ILC with the SA phase needed to be selected. The requirements for the compounds were (1) imidazolium salts, (2) containing iodide as an anion, (3) low melting points, and (4) low viscosity. A few examples of the ILC with the SA phase, such as imidazolium salts consisting of cations with alkyl chains of C12-C18 and anions of hexafluorophosphate or bromide, have been reported.16,23-25 Iodide is indispensable in an electrolyte of DSSC, because an I-/I3- redox couple functions as a hole transport agent. We have reported in a previous paper that the imidazolium iodides with alkyl chains of C12, C16, and C18 showed the liquid crystalline phase at temperatures above their melting points.12 Among the three compounds, C12MImI showed the lowest melting point and viscosity and would be suitable for the hole transport layer in DSSC. Then we selected C12MImI as an ILC compound for our work. In Figure 2, for C12MImI, the two endothermic peaks were observed on heating. According to the previous report,23 it was assumed that the larger peak (S) at 27 °C would be attributed to the phase transition from crystalline phase to liquid crystalline phase and the smaller peak (I) at 80 °C would be attributed to the phase transition from liquid crystalline phase to isotropic phase. On the other hand, for C11MImI, one large endothermic peak (M) at 50 °C corresponding to the phase transition from crystalline phase to isotropic phase was observed. Thus, it was confirmed that C11MImI does not show a liquid crystalline phase. For the characterization of the liquid crystalline phase of C12MImI, POM images were observed. Since characteristic focal conic domains as shown in Figure 3 were observed during the cooling process from 85 °C, the liquid crystalline phase of C12MImI would be the SA phase.23 In contrast, for C11MImI, the isotropic phase was observed in the Figure 3 insert. We added 0.65 M iodine to C12MImI and C11MImI for a use of them as the hole transport layer of DSSC. C12MImI/I2 and C11MImI/I2 were also characterized by POM and DSC. The focal conic domains obtained by POM observation of C12MImI/ I2 implied that the SA phase was maintained in C12MImI/I2. In the DSC traces of C12MImI/I2, the two peaks corresponding to the larger peak (S) and the smaller peak (I) were observed. The phase transition temperature from liquid crystalline phase to isotropic phase of C12MImI/I2 was lowered from 80 to 46 °C. In the DSC traces of C11MImI/I2, one peak corresponding to the large peak (M) was observed. The phase transition temperature from the crystalline phase to isotropic phase of C11MImI/ I2 was lowered from 50 to 37 °C. The larger size of I3- formed by the addition of iodine should reduce the van der Waals forces
Figure 4. Steady-state voltammograms (scan rate, V ) 1 mV s-1) at a Pt disk (12 µm in diameter) electrode in C12MImI/I2 (bold solid curve) and C11MImI/I2 (solid curve).
of the alkyl chains of imidazolium cations. This would lower the phase transition temperature. Thus, C12MImI/I2 showed an SA phase ranging from 21 to 46 °C and C11MImI/I2 showed an isotropic phase above 37 °C. In the following, we compared the properties of C12MImI/I2 showing an SA phase with those of C11MImI/I2 showing an isotropic phase at 40 °C. Diffusion Coefficients of I3-. To examine the properties of the charge transport in C12MImI/I2 and C11MImI/I2, the D values were evaluated from the diffusion-limited currents (Ilim) for the reaction of I3- + 2e- f 3I- as previously described.15 The Ilim is expressed as
Ilim ) 8FDrc
(1)
where F is the Faraday constant, r is the microdisk electrode radius, and c is the total concentration of the iodide species (c ) 1/3[I-] + [I3-] in the reaction of I3- + 2e- f 3I-). Figure 4 shows microelectrode cyclic voltammograms at a Pt disk electrode in C12MImI/I2 and C11MImI/I2. Ilim was determined by the steady state of the cathodic current corresponding to the reduction of I3-. The D value of C12MImI/I2 was 4.2 × 10-8 cm2 s-1, and that of C11MImI/I2 was 3.2 × 10-8 cm2 s-1. The deviations of the D values were 0.2 × 10-8 and 0.1 × 10-8 cm2 s-1, respectively. The D value evaluated from Ilim intrinsically includes not only the simple physical diffusion coefficient (Dphys) but also the diffusion coefficient based on the exchange reaction (Dex).13 The exchange reaction here can be expressed by I- + I3- f I3- + I-. The viscosity (η) at 40 °C of C12MImI/I2 (560 mPa s) was larger than that of C11MImI/I2 (230 mPa s). Taking into account that the Dphys value is proportional to 1/η, the D value of C12MImI/I2 should be smaller than that for C11MImI/I2. However, C12MImI/I2 showed a higher D value than that for
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Figure 5. Diffusion coefficients (D) of I3- as a function of concentration ([I-] + [I3-]) for C12MImI/I2 (filled circles) and C11MImI/I2 (open triangles).
TABLE 1: Physical Diffusion Coefficients (Dphys) and Exchange-Reaction-Based Diffusion Coefficients (Dex) of C12MImI/I2 and C11MImI/I2 with 0.65 M Iodine system
(1/6)kexδ2
C12MImI/I2 C11MImI/I2
8.3 × 4.6 × 10-6 10-6
Dphys (cm2 s-1)
Dex (cm2 s-1)
1.2 × 1.4 × 10-8
3.0 × 10-8 1.8 × 10-8
10-8
Figure 6. Conoscopic images from the direction of top view of cells A and B (Figure 1) at 40 °C: (a) C12MImI/I2 filled in cell A; (b) C11MImI/I2 filled in cell A; (c) C12MImI/I2 filled in cell B; (d) C11MImI/ I2 filled in cell B.
C11MImI/I2. These results would lead to an idea that the Dex value of C12MImI/I2 was higher than that for C11MImI/I2. Contribution of Dex to D. When the simple physical diffusion and the exchange reaction are conjugated, D can be obtained from the Dahms-Ruff equation13
D ) Dex + Dphys ) (1/6)kexδ2c′ + Dphys
(2)
where kex is the exchange-reaction rate constant, δ is the centerto-center intersite distance at the exchange reaction, and c′ is the total concentration of the iodide species ()[I-] + [I3-]) in the reaction of I- + I3- f I3- + I-. According to eqs 1 and 2, the evaluation of Dex requires measuring the dependence of D on the concentrations c′.15 We mixed C12MImI/I2 or C11MImI/I2 with C12MImBF4 to control the concentrations c′. POM observations and DSC measurements of the mixtures of C12MImI/I2 with C12MImBF4 indicated an SA phase at 40 °C, although the mixtures of C11MImI/I2 with C12MImBF4 did not show a liquid crystalline phase. Figure 5 shows the D as a function of c′, giving the values of (1/6)kexδ2, Dphys, and Dex in Table 1. The calculated Dex of C12MImI/I2 (3.0 × 10-8 cm2 s-1) was larger than that of C11MImI/I2 (1.8 × 10-8 cm2 s-1), demonstrating that the exchange reaction in C12MImI/I2 occurred faster than in C11MImI/I2. The high Dex value of C12MImI/I2 has led us to an idea that I3- and I- should be located between the SA layers consisting of the imidazolium cations and the conductive pathways should be organized therein.22 The localization of I3- and I- in the pathways would promote the exchange reaction in C12MImI/I2. Anisotropic Ionic Conductivities for ILC. To prove the organization of the electron conductive pathways in ILC, anisotropic ionic conductivity measurements were carried out. For the ionic conductivity measurements along the direction parallel σi| and perpendicular σi⊥ to the SA layer plane, the homeotropic alignment of the imidazolium cations on the glass in cell A and cell B (Figure 1) filled with C12MImI/I2 is required. The conoscopic images at 40 °C for C12MImI/I2 revealed that the smectic molecular assemblies aligned homeotropically on the glass surface in both cells, as demonstrated in Figure 6a,c. In contrast, the conoscopic images at 40 °C for C11MImI/I2 did
Figure 7. Differential scanning calorimetric traces and ionic conductivities for the samples of (a) C12MImI/I2 and (b) C11MImI/I2 along with the direction parallel σi| (filled circle) and perpendicular σi⊥ (open triangle) to the SA layer plane of the homeotropically aligned ionic liquid crystal. Key: Iso ) isotropic; SA ) smectic A; Cr ) crystalline.
not show the homeotropic alignment of imidazolium cations in both cells, as demonstrated in Figure 6b,d. Figure 7 shows ionic conductivities of C12MImI/I2 and C11MImI/I2 as a function of temperature. For C11MImI/I2, the ionic conductivities gradually increased with the increase in temperature as shown in Figure 7b. This result was a normal behavior in isotropic ionic liquids, as previously described.26 In contrast, the discontinuous changes of ionic conductivities were observed in C12MImI/I2, as shown in Figure 7a. The decrease of σi| and the increase of σi⊥ at 45 °C on heating were observed. When the liquid crystalline phase order of C12MImI/
Dye-Sensitized TiO2 Solar Cells
Figure 8. Raman spectra observed for C12MImI/I2 (bold solid curve) and C11MImI/I2 (solid curve) at 40 °C.
Figure 9. Ratio of polyiodides and I3- (Im-/I3-) as a function of total concentration of iodide species for C12MImI/I2 (filled circles) and C11MImI/I2 (open triangles).
I2 disappeared above the phase transition temperature, σi⊥ and σi⊥ were on the same line within the error. Taking into account that any difference of σi⊥ and σi⊥ in isotropic ionic liquid was not observed in the whole temperature range for C11MImI/I2, the higher σi⊥ than σi⊥ in ionic liquid crystalline phase should be attributed to the electron conductive pathways organized between the SA layers for C12MImI/I2. The σi| values in ionic liquid crystalline phase would be enhanced due to the promotion of the exchange reaction at the pathways. Formation of Polyiodide Species in the Electron Conductive Pathways. The promotion of the exchange reaction in C12MImI/I2 would be attributed to the higher concentrations of polyiodide species (Im- ; m ) 5, 7, ...)27 contributing to the exchange reaction.10 We examined the amount of polyiodide species in C12MImI/I2 and C11MImI/I2 by Raman spectroscopic measurements. Figure 8 shows Raman spectra of C12MImI/I2 and C11MImI/ I2. The peaks attributed to I3- and polyiodides (Im-) appeared around 110 and 150 cm-1, respectively.28-30 The ratio of polyiodides and I3- (Im-/I3-) value was calculated from the intensities of the two peaks. The Im-/I3- ratio of C12MImI/I2 ()2.8) was higher than that of C11MImI/I2 ()2.5), indicating that more Im- was formed in C12MImI/I2 than in C11MImI/I2. Figure 9 shows the Im-/I3- ratios in C12MImI/I2 and C11MImI/ I2 as a function of total concentration of iodide species (c′). The Im-/I3- ratios increased with increasing c′ in both C12MImI/ I2 and C11MImI/I2. However, the Im-/I3- ratio in C12MImI/I2 was 1.1-1.2 times as high as that in C11MImI/I2 at the same c′ value over the whole concentration range. Taking into account that the Im-/I3- ratio increased in proportion to c′, the locally high concentration of I3- and I- in
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Figure 10. Image of the two-dimensional electronic conductive pathways formed between the SA layers consisting of the imidazolium cations.
the electron conductive pathways could lead to the higher Im-/ I3- ratio in C12MImI/I2. Effect of the Two-Dimensional Electron Conductive Pathways. If only 1.1- 1.2 times higher ratio of Im-/I3- in C12MImI/I2 than C11MImI/I2 was effective to enhancing Dex, the Dex value of C12MImI/I2 should be 1.1-1.2 times as high as than C11MImI/I2. However, the Dex value of C12MImI/I2 was 1.7 times as high as that for C11MImI/I2. In addition to the effect of the increase of Im-, we would propose another effect for the promotion, the two-dimensions of the electron conductive pathways (schematic structure is shown in Figure 10).31,32 Recently, we reported that the charge transport of an I-/I3- redox couple in ionic liquid and molecular liquid was found to be completely different. The redox couple in the ionic liquids exists in a strong ionic strength field. The negatively changed ions (I-, I3-, and Im-) can collide with each other more easily in the ionic liquids than in the molecular liquids by “the kinetic salt effect”.33 Therefore, each redox couple can also react easily in the tow-dimensional electron conductive pathway (ionic layer) of ILC. In the two-dimensional electron conductive pathways of C12MImI/I2, more collision frequencies between iodide species (I-, I3-, and Im-) could be achieved than that in the threedimensional space of C11MImI/I2, which could lead to the promotion of the exchange reaction between iodide species. In conclusion, as a reason for the high Dex value of C12MImI/ I2-, the two-dimensional electron conductive pathways organized by the localizing of I3- and I- between SA layers should lead to the promotion of the exchange reactions due to the increase of Im- and the collision frequencies. Solidification of ILC. C12MImI/I2 and C11MImI/I2 were solidified by using the gelator having a hydrogen-bonded fiber structure (C12MImI/I2-gel, C11MImI/I2-gel). It has been reported that this kind of gelator gives a micrometer-scale phase segregated structure and finely forms networks of liquid crystalline physical gels.34,35 Therefore, the gelator could maintain the ILC domains at the submicrometer level. Figure 11 shows the POM image of C12MImI/I2-gel. This image of the focal conic domains implied that the C12MImI/ I2-gel maintained the SA phase. The POM observation of the ILC domains in the submicrometer level suggested that the hydrogen-bonded fibers could finely form networks.34 To examine the effect of the concentration of the gelator on the conductivities, we measured conductivities of C12MImI/I2gel and C11MImI/I2-gel as a function of the concentration of a gelator-HB in Figure 12. For C11MImI/I2-gel, the conductivity
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Figure 11. Textures observed using POM (40× magnification) of C12MImI/I2 with 5.0 g L-1 of a gelator. The photograph was taken after cooling from the isotropic phase. Figure 13. Photocurrent-voltage curves of the cells using C12MImI/ I2 (solid curve), C12MImI/I2-gel (bold solid curve), C11MImI/I2 (short dash curve), and C11MImI/I2-gel (thin solid curve) under AM 1.5 irradiation.
Figure 12. Effect of the concentration of gelators on conductivity for C12MImI/I2-gel (filled circle) and C11MImI/I2-gel (open triangle).
hardly decreased with increasing concentration of the gelator. This result indicates that the moderate intermolecular interaction of gelators could keep the charge-transfer channel of the electrolytes even at their high concentration. The same phenomenon was observed in the ionic liquid physical gels as previously described.10 In contrast, for C12MImI/I2-gel, the enhancement of conductivity was observed when a slight amount of the gelator was added. Positive Effect of the Gelator on Conductivity in ILC. The enthalpies (∆H) of the endothermic SA f isotropic transition were determined from DSC thermograms for C12MImI/I2-gel and C12MImI/I2. The ∆H value of C12MImI/I2-gel (0.36 kJ mol-1) was 1.8 times as large as that of C12MImI/I2 (0.20 kJ mol-1). From this result, the interaction between imidazolium cations would be increased by introducing the micrometer-scale phase segregated structure of the gelator to ILC. We assumed that the finely dispersed solid networks of the gelator would effectively suppress the fluctuations of the imidazolium cations and enhance the molecular order of the bilayer structure of SA phase, leading to the positive effect on the formation of the electron conductive pathways. The well-organized electron conductive pathways between the SA layers achieved by addition of the gelator would promote exchange reaction. Therefore, the conductivity of C12MImI/I2gel was higher than that of C12MImI/I2. DSSC Using Ionic Liquid Crystal Systems as Effective Electrolytes. Figure 13 shows photocurrent-voltage curves of the DSSC using C12MImI/I2 (solid curve), C12MImI/I2 with 5.0 g L-1 of the gelator (bold solid curve), C11MImI/I2 (short dash curve), and C11MImI/I2 with 5.0 g L-1 of the gelator (thin solid curve). The light-to-electricity conversion efficiencies of DSSC were evaluated. In comparison of DSSC using C12MImI/I2 with that using C11MImI/I2, a JSC of the DSSC using C12MImI/I2 (7.0 mA cm-2) was higher than that using C11MImI/I2 (6.0 mA cm-2), while the open circuit voltage (VOC) and fill factor (FF) were the almost
same values as those for C11MImI/I2. This result suggested that higher conductivity of C12MImI/I2 than C11MImI/I2 could lead to a high JSC.18,36 In the DSSC using C12MImI/I2, the long-range order of ionic liquid crystal would not be necessarily achieved in the mesoporous TiO2 electrode. However, the promotion of the exchange reactions by the two-dimensional electron conductive pathways in each domain could improve JSC. DSSC using C12MImI/I2 with 5.0 g L-1 of the gelator showed higher JSC (7.7 mA cm-2) and light-to-electricity conversion efficiency than the DSSC using C12MImI/I2. Taking into account that a JSC and a light-to-electricity conversion efficiency of DSSC using C11MImI/I2 with 5.0 g L-1 of the gelator were almost the same values as those for C11MImI/I2, a high JSC and a high light-to-electricity conversion efficiency of DSSC using C12MImI/I2 with 5.0 g L-1 of the gelator were attributed to the high conductivity of the C12MImI/I2-gel. Conclusions A new ILC, C12MImI/I2, with a smectic A phase with the imidazolium cations showed high conductivity in spite of the high viscosity and was applied to DSSC, which showed a higher JSC and a higher light-to-electricity conversion efficiency than that using the non-liquid crystalline ionic liquid, C11MImI/I2, as an electrolyte. In C12MImI/I2, the two-dimensional electron conductive pathways organized by the localizing of I3- and Iat the SA layers induced by self-assembled structure of the imidazolium cations increased polyiodide species among the redox couple, which promote the exchange reactions through their collision frequencies. The enhancement of the conductivity due to the promotion of the exchange reactions would lead to an increase in JSC of DSSC. We solidified ILC by the gelator giving a hydrogen-bonded fiber structure and applied it to a quasi-solid-state DSSC. When a slight amount of the gelator was added to ILC, the electron conductive pathways between the SA layers would be organized more effectively due to the enhancement of the molecular order of imidazolium cations forming SA layers and the exchange reaction would be promoted. This would lead to the enhancement of the conductivity of C12MImI/I2-gel. A quasi-solidstate DSSC using C12MImI/I2-gel showed a higher JSC and a higher light-to-electricity conversion efficiency than that for C12MImI/I2 without gelator. We proposed a new strategy, which was based on the promotion of the exchange reaction between I-, I3-, and Im-, by using a self-assembled structure of the imidazolium cations. This could lead to the improvement of JSC of DSSC using ionic
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