Liquid-Crystalline Dye-Sensitized Solar Cells: Design of Two

Article pubs.acs.org/cm

Liquid-Crystalline Dye-Sensitized Solar Cells: Design of TwoDimensional Molecular Assemblies for Efficient Ion Transport and Thermal Stability Daniel Högberg,† Bartolome Soberats,† Ryo Yatagai,† Satoshi Uchida,‡ Masafumi Yoshio,† Lars Kloo,§ Hiroshi Segawa,⊥ and Takashi Kato*,† †

Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Komaba Organization for Educational Excellence, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan § Applied Physical Chemistry, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden ⊥ Research Center for Advanced Science and Technology, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8904, Japan S Supporting Information *

ABSTRACT: Nanostructured liquid-crystalline (LC) electrolytes have been developed for efficient and stable quasi-solid-state dyesensitized solar cells (DSSCs). Two types of ionic LC assemblies for electrolytes have been designed: (i) noncovalent assemblies of two-component mixtures consisting of I2-doped imidazolium ionic liquids and carbonate-terminated mesogenic compounds (noncovalent type) and (ii) single-component mesogenic compounds covalently bonding an imidazolium moiety doped with I2 (covalent type). These mesogenic compounds are designed with flexible oligooxyethylene spacers connecting the mesogenic and the polar moieties. The oligooxyethylene-based material design inhibits crystallization and leads to enhanced ion transport as compared to alkyl-linked analogues due to the higher flexibility of the oligooxyethylene spacer. The noncovalent type mixtures exhibit a more than 10 times higher I3− diffusion coefficient compared to the covalent type assemblies. DSSCs containing the noncovalent type liquid crystals show power conversion efficiencies (PCEs) of up to 5.8 ± 0.2% at 30 °C and 0.9 ± 0.1% at 120 °C. In contrast, solar cells containing the covalent type electrolytes show significant increase in PCE up to 2.4 ± 0.1% at 120 °C and show superior performance to the noncovalent type-based devices at temperature above 90 °C. Furthermore, the LC-DSSCs exhibit excellent long-term stability over 1000 h. These novel electrolyte designs open unexplored paths for the development of DSSCs capable of efficient conversion of light to electricity in a wide range of temperatures.

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INTRODUCTION

based on organic solvents for efficient devices has been a hindrance for commercialization due to their volatile nature and risk of leakage.30,31 Quasi-solid-state and solid-state alternatives such as composites,32−35 polymers,36−38 hole-conducting materials,38−40 and liquid crystals28,41−44 have been the center of focus in recent years. Liquid crystals as ion transport layers in DSSCs were first reported by Yanagida, Watanabe, and coworkers by using I−/I3− dialkylimidazolium derivatives.42,43 More recently, Guldi and co-workers showed that LC electrolytes have positive effects on the temperature dependent performance of DSSCs.44 This behavior is in contrast to conventional photovoltaics (PVs).28,44−46

Self-assembly of organic molecules is an attractive approach for the design of functional materials.1−6 In particular, liquid crystals forming well-defined nanostructures show promising function for energy and optoelectronic devices.7−14 Liquid crystals are easy to process, are thermally robust, and show versatility with respect to materials design. Ionic liquid crystals that form ionic channels capable of efficient ion transport has been proposed as a seminal approach to develop electrolytes for energy devices.15−26 These materials have been applied as electrolytes in lithium ion batteries27 and dye-sensitized solar cells (DSSCs).28 Herein we report new molecular designs of functional liquid crystals that significantly improve the performance of quasi-solid-state liquid-crystalline (LC)-DSSCs. Achieving efficient solid-state-DSSCs has been a great challenge since the breakthrough paper on DSSCs by O’Regan and Grätzel in 1991.29 The requirement of liquid electrolytes © 2016 American Chemical Society

Received: April 19, 2016 Revised: July 9, 2016 Published: July 11, 2016 6493

DOI: 10.1021/acs.chemmater.6b01590 Chem. Mater. 2016, 28, 6493−6500

Article

Chemistry of Materials

Figure 1. Schematic illustration of the assembly of LC electrolytes into layered smectic A phases and an LC-based dye-sensitized solar cell. Blue areas represent insulating parts, and red areas show ion conductive parts. FTO: fluoride-doped tin oxide.

Figure 2. Molecular structures and molecular design of noncovalent and covalent type LC electrolytes based on the compounds 1−4 and ionic liquid EMII.

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binding the imidazolium cations. This material design is expected to increase the thermal stability of the LC phases. Liquid-Crystalline Properties. Liquid-crystalline properties of the mesogenic compounds 1−4 were studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and polarized optical microscopy (POM) (see the Supporting Information). The phase transition behavior of the compounds is presented in Table 1. The oligooxyethylene-

Furthermore, it was reported that rodlike cyanobiphenyl mesogens increase the photovoltage of organic solvent-based DSSCs when used as an additive in liquid electrolytes.47 In our previous work we introduced the concept of self-assembly of multiple components to form nanostructured LC electrolytes for DSSCs.28 This innovative design improved the stability of the LC phases and thermal behavior of the solar cell devices. The LC-DSSCs exhibit an increase in power conversion efficiency (PCE) on heating but show overall low cell efficiencies. If higher PV performance can be achieved, electrolytes based on self-assembled liquid crystals may not only solve the problems of leakage that hamper liquid-based devices but will also address the fundamental temperature issues of solar cell technology. Our intention in the present work is to develop quasi-solidstate DSSCs exhibiting efficient performance and thermal stability. Two types of LC electrolytes for DSSCs were designed to achieve these goals (Figure 1). The first design is based on two-component LC assemblies (noncovalent type) composed of mesogenic compounds, functionalized with polar moieties, and ionic liquids (ILs) (Figure 1, left). These twocomponent assemblies are designed for efficient ion transport. We previously showed that LC assemblies based on binary mixtures of mesogenic molecules and ILs are advantageous with respect to ion transport.48 In contrast, the second electrolyte design (covalent type) consists of ionic mesogenic compounds covalently binding the organic cations (Figure 1, right). This design is expected to enhance the thermal stability of the LC phases and to extend the maximum working temperature of the LC-DSSCs. Furthermore, we have introduced a polar and flexible oligooxyethylene spacer in the mesogenic molecular designs which increases mass-transport of the electrolyte and greatly decreases the interfacial resistance at the electrodes in the LC-DSSCs. The new molecular designs lead to LC-DSSCs exhibiting excellent conversion efficiencies and thermal stability.

Table 1. Liquid-Crystalline Properties of Compounds 1−4 and Equimolar Mixtures of 1 or 2 and EMIIa mixture

phase transition behaviorb,c

1 1/EMII 2 2/EMII 3 4

Sm Sm Sm Sm Sm Sm

Sm B Cr

22 (−0.7) 62 (−7.2)

Cr

77 (−13.7)

A A A A A A

70 (−3.7) 78 (−0.8) 96 (−4.6) 119 (−1.2) 185 (−1.2) 240d (−)e

Iso Iso Iso Iso Iso Iso

a

Compound 2 was previously reported.28 bTransition temperatures (°C) and transition enthalpies (kJ mol−1, within parentheses) were determined using differential scanning calorimetry (DSC) on a second heating cycle at a scan rate of 10 K min−1. cIso: isotropic; Sm A: smectic A; Sm B: smectic B; Cr: crystal. dIsotropic liquid was observed by polarized optical microscopy. eNo transition enthalpy was detected in the DSC scans.

based compound 1 shows a smectic A (Sm A) phase until isotropization at 70 °C. In contrast, compound 2 having an alkyl spacer shows a more ordered phase and higher isotropization temperature. Compound 2 exhibits an Sm B phase until 22 °C and an Sm A phase up to 96 °C. The presence of the oligooxyethylene spacer in the mesogenic compound 1 extends the temperature range of the Sm A phase to lower temperature due to the induction of flexibility and suppression of crystallization. Similar effects of the oligooxyethylene spacer are observed for the ionic compounds 3 and 4. Compound 3 shows an Sm A phase until isotropization at 185 °C, while 4 shows a crystal phase until 77 °C and an Sm A phase until 240 °C. The significant difference in isotropization temperature between 1 and 3 is related to the intermolecular interactions. The self-assembled lamellar structure of 1 is stabilized through dipolar interactions between the terminal carbonate moieties. These interactions are weaker than the ionic interactions of the imidazolium moieties that stabilize the layered structured of 3. Sufficient intermolecular interactions are essential to generate stable LC phases in a wide temperature range. Addition of the IL EMII in a 1:1 molar ratio to 1 (1/EMII) increases the temperature range of the Sm A phase to 78 °C. A polarized optical micrograph and X-ray diffractogram of 1/EMII are shown in Figure 3. The interlayer spacing of the Sm A phase is increased with the addition of the IL, indicating that the ions are located in the polar layers. FT-IR spectra of 1/EMII reveal a shift of the C O stretching mode of the carbonate moiety from 1794 to 1790 cm−1 (see the Supporting Information). A shift is also observed for the C−H stretching modes of the imidazolium ring from 3143 to 3148 cm−1. These results suggest that the dipole-ion interactions between the carbonyl group and imidazolium ions stabilize the self-assembled structures of the two components. It should be noted that the single-component 3 shows a markedly higher isotropization temperature as compared to 1/EMII. The

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RESULTS AND DISCUSSION Molecular Design. Our molecular designs of 1−4 (Figure 2) take advantage of rigid rodlike mesogens, which lead to stable LC phases. This design is expected to exert positive effects on the electron lifetime in the TiO2 electrode of the DSSCs.28,47 The mesogen is linked to polar cyclic carbonate or imidazolium moieties to induce nanosegregation and to enhance ion transport properties. The high dipole and dielectric properties of the cyclic carbonate moieties are expected to drive the formation of stable nanosegregated LC structures in the presence of ionic species. We previously reported on mixtures composed of cyclic carbonate functionalized compound 2 and salts, which form stable LC phases.28,49 In our previous design, the polar moieties were linked to the mesogens by alkylene spacers.28,49,50 In the new design, we have introduced a longer and more flexible oligooxyethylene spacer in compounds 1 and 3. Herein we intend to increase the fluidity of the layers in the LC nanostructures and to suppress crystallization of the materials, which is expected to result in an increase in ion mobility. The noncovalent LC assemblies consist of the carbonate-terminated mesogenic compounds 1 and 2 mixed with I2-doped 1-ethyl-3-methylimidazolium iodide (EMII). This material design is expected to have advantageous ion transport properties.48 On the other hand, the singlecomponent liquid crystals consist of mesogens covalently 6495


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EMII exhibits a DI3− of 6.7 × 10−8 cm2 s−1 at 30 °C. This value is 13.6 times higher than that observed for the covalent type electrolyte 3 (4.9 × 10−9 cm2 s−1). The higher I3− diffusion coefficient in 1/EMII can be attributed to the noncovalent dipole-ion interactions between the cyclic carbonate-terminated mesogen and the mobile imidazolium-based IL. The lower DI3− of 3 is due to the imidazolium cations being covalently bound to the mesogenic core through the flexible spacer. The stronger interactions between the immobile imidazolium cations and the counteranions for 3 restrict the anion mobility. Our design of introducing mobile ion pairs in the noncovalent type electrolytes is clearly advantageous for ion transport in the two-dimensional LC ion conductors. As for the alkyl-linked electrolytes 2/EMII and 4, they show lower DI3− than their oligooxyethylene analogues. The lowest DI3− is observed for 4 at 30 °C (9.4 × 10−10 cm2 s−1). The diffusion coefficient of 3 is 5.2 times higher than for 4. A similar behavior is observed for the electrolytes based on the noncovalent type material design. The 2/EMII mixture shows a DI3− of 2.7 × 10−8 cm2 s−1 at 30 °C meaning that the DI3− of 1/EMII is 2.5 times higher. As temperature is increased the difference in DI3− of the 1/EMII and 2/EMII electrolytes decreases. The 1/EMII electrolyte reaches a DI3− of 3.9 × 10−7 cm2 s−1 at 90 °C, which is 1.4 times higher than for 2/EMII (2.7 × 10−7 cm2 s−1). The higher disparity in DI3− at 30 °C is likely due to the crystalline state of the 2/EMII electrolyte. The same trend is observed for the covalent type electrolytes 3 and 4. The enhanced molecular flexibility induced by the longer oligooxyethylene spacer successfully increases the ion transport properties in the 1/EMII- and 3-based electrolytes. Photovoltaic Performance in LC-DSSCs. LC electrolytes 1/EMII, 2/EMII, and 3 were applied in DSSCs. The liquid crystals are assumed to form randomly oriented microdomains in the devices.28 Compound 4 exhibits a very low I3− diffusion rate and has therefore been excluded from the device studies. The LC-DSSCs were evaluated by current−voltage (I−V) measurements (see the Supporting Information) and incident photon-to-current efficiency (IPCE) measurements (see the Supporting Information). The photovoltaic parameters of the 1/EMII-, 2/EMII-, and 3-based DSSCs are shown in Table 2. The integrated current densities obtained from the IPCE spectra amount to 15.1, 11.9, and 9.9 mA cm−2 for the 1/ EMII-, 2/EMII-, and 3-based DSSCs, respectively. This is in good agreement with the short-circuit photocurrent density (Jsc) values obtained from the I−V measurements. The 1/EMII-DSSCs show the best performance, and a maximum PCE of 6.1% (average 5.8%) at 30 °C was recorded. This is significantly higher than what is observed for the 2/ EMII-DSSCs (1.6%) and 3-DSSCs (0.2%). Furthermore, it is to the best of our knowledge the highest efficiency reported for an LC-based DSSC. Moreover, it is not far from state-of the art solid-state devices.30 The excellent performance of the 1/EMIIbased devices can be attributed to the higher Jsc of 14.6 mA cm−2 and fill-factor (FF) of 59%. The self-assembled ion transport pathways in 1/EMII enable sufficiently efficient I3− transport in the highly viscous LC state. Introducing the longer and more flexible oligooxyethylene spacer in the noncovalent molecular design is clearly a successful approach to improve the PCE of LC-DSSCs. As temperature is increased, the device performance of the 1/EMII-DSSCs gradually decreases (Figure 5). This is in contrast to the trend observed for the DI3−, which is expected

Figure 3. a) Polarized optical microscopy image of 1/EMII at 30 °C on cooling from the isotropic state. b) X-ray diffraction pattern of 1/ EMII at 30 °C.

strong covalent bond between the mesogenic core and imidazolium cation stabilizes the nanostructured LC state. Ion Transport in the Liquid Crystals: Triiodide Diffusion. The I3− diffusion coefficients (DI3−) have been determined by cyclic voltammetry (CV) using a symmetric thin-film setup, as previously described,28 for the self-assembled LC materials 1/EMII, 2/EMII, 3, and 4. The DI3− values increase with increasing temperature (Figure 4). The noncovalent type electrolyte based on 1/EMII shows the highest DI3− in the temperature range studied. The mixture based on 1/

Figure 4. I3− diffusion coefficients (DI3−) of 1/EMII (●, red), 2/EMII (●, blue), 3 (▼), and 4 (▼, green) as a function of temperature. 6496


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Table 2. Photovoltaic Parameters for LC-DSSCs Containing the Electrolytes 1/EMII, 2/EMII, or 3 at 30, 90, and 120 °Ca mixture

T (°C)

1/EMII

30 90 120 30 90 120 30 90 120

2/EMII

3

phase Sm Iso Iso Cr Sm Iso Sm Sm Sm

A

A A A A

Vocb (mV) 675 559 511 667 546 448 541 549 529

± ± ± ± ± ± ± ± ±

0 5 1 4 2 1 7 1 1

Jscc (mA cm−2) 14.6 10.9 3.5 5.9 10.1 8.1 1.0 10.9 11.3

± ± ± ± ± ± ± ± ±

0.2 2.1 0.4 0.1 1.4 1.0 0.2 0.3 0.7

FFd (%) 59 52 48 40 43 38 31 35 40

± ± ± ± ± ± ± ± ±

1 5 1 7 4 3 6 4 2

PCEe (%) 5.8 3.0 0.9 1.6 2.4 1.4 0.2 2.1 2.4

± ± ± ± ± ± ± ± ±

0.2 0.3 0.1 0.4 0.3 0.1 0.0 0.3 0.1

a

Average values and standard deviations of the photovoltaic parameters were estimated from three devices of each type. bOpen-circuit voltage. Short-circuit current density. dFill factor. ePower conversion efficiency. All measurements were conducted under AM 1.5 G illumination (1000 W m−2).

c

achieve stable device performance at elevated temperatures. As for the 2/EMII-based DSSCs, it records a PCE of 2.4% at 90 °C. On further heating, a drop in PCE is observed. The gradual destabilization of the LC phase, as 2/EMII approaches its isotropization point, accelerates the electron recombination reactions and results in the observed drop in PCE. This behavior is consistent with our previous report on DSSCs containing LC electrolytes based on 2 and 1-propyl-3methylimidazolium iodide.28 It is worth noting that there is a large difference in PCE between the 1/EMII and 2/EMIIbased DSSCs in temperatures ranging from 30 to 80 °C. The oligooxyethylene spacer in 1 appears to be essential to achieve the excellent PV performance recorded below 80 °C. The long-term chemical stability of the 1/EMII, 2/EMII, and 3 devices at 60, 90, and 120 °C, respectively, is presented in Figure 6. The devices were stored in darkness at room Figure 5. Power conversion efficiency (PCE) as a function of temperature of DSSCs containing 1/EMII (●, red), 2/EMII (●, blue), and 3 (●). All measurements were conducted under AM 1.5 G illumination (1000 W m−2).

to improve the device performance. The effect of temperature on the PCE is dependent on two competitive processes in the DSSCs: the positive effect on I3− transport and the negative effect of increased electron recombination rates. In the case of the 1/EMII-DSSC, the increase in electron recombination rates on heating exceeded the enhanced I3− transport and results in an overall loss of Voc and Jsc.45,46 This is a generally observed phenomenon for PV devices.45 Nonetheless, more than 90% of the initial PCE value (5.2%) is maintained at 60 °C. This is 15% higher than that observed in our previous report on DSSCs containing ILs.28 These results suggest that the liquid crystal stabilizes the temperature dependent performance of the 1/ EMII-based devices.28,44 A rapid decrease in PCE is observed above the isotropiziation temperature of 1/EMII (78 °C). The isotropization of the LC electrolyte leads to a loss of the well-defined nanostructures, and as a consequence a decrease in PCE is observed. In contrast, LC-DSSCs containing the more thermally stable liquid crystal 3 show higher PCEs than 1/EMII-based devices above 90 °C. The maximum PCE (2.4%) of the 3-based DSSCs was recorded at 120 °C. This is an improvement of the maximum operation temperature by 30 °C as compared to our previously reported systems.28 These results can most likely be traced to the thermally stable LC phase of 3. The stability of the self-assembled nanostructures is apparently paramount to

Figure 6. Normalized PCE as a function of time of DSSCs containing 1/EMII (●, red), 2/EMII (●, blue), and 3 (●) at 60 °C, 90 °C, and 120 °C respectively. All measurements were conducted under AM 1.5 G illumination (1000 W m−2). The cells were stored in darkness at room temperature.

temperature between measurements. All devices show excellent stability over 1000 h at their respective operation temperature. This illustrates that LC-based electrolytes are robust enough for long-term application in DSSCs. Effects of Spacers on Interfacial Resistances. The interactions at the interfaces between the electrodes and the 6497


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advances promote LC-based devices as a new class of DSSCs capable of operating efficiently under extreme conditions.

electrolyte are important factors that determine the performance of DSSCs.31 Polar groups and ions in electrolytes can be strongly absorbed to the TiO2 surface and induce positive effects on charge transfer processes, such as electron injection into the TiO2 or electron recombination to the electrolyte.31,47,51,52 We previously reported that compound 2 has a positive effect on the electron lifetime (τ) in TiO2.28 It was assumed that the cyclic carbonate moiety of 2 interacts with the TiO2 surface, forming hydrophobic layers that retards the rate at which the oxidized component of the redox system, I3− (or I2), can approach the electrode surface.28 Information about the processes at the electrode interfaces can be obtained by electrochemical impedance spectroscopy (EIS).53,54 EIS spectra of a DSSC generally consist of a high, a middle, and a low frequency arc that contain information about charge transfer or transport processes at the platinum/electrolyte interface, the TiO2/dye/electrolyte interface, and the electrolyte, respectively.53,54 The EIS measurements for the 2/EMII-DSSCs reveal that the devices suffer from high charge transfer resistance at the Pt/ electrolyte interface (RCE) (see the Supporting Information). As temperature is increased, a significant decrease in RCE is observed for the 2/EMII-DSSCs. Similar behavior is observed for the recombination resistance (Rrec) which is derived from the middle arc in the EIS spectra. RCE and Rrec are directly related to the rate at which I3− (or I2) approaches the Pt and TiO2 electrodes, respectively, where the reduction at the photoelectrode interface represents an unwanted recombination reaction.53,54 The higher interfacial resistance of the 2/ EMII-devices is likely due to a lower activity of the iodinecontaining species at the electrode surfaces. The molecularly more rigid structures of the 2/EMII-based systems may be speculated to retard the interface reaction rates at both electrodes. The increase in mobility of the system, induced by heating, rapidly increases the rate at which ionic species reaches the electrode surfaces. The same behavior is not observed for 1/EMII-based DSSCs. The mobility and polarity induced by the oligooxyethylene spacer appears to allow ions to readily reach the electrode surfaces at lower temperatures. This may be a key to the excellent PV performance of the 1/EMIIDSSCs.

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EXPERIMENTAL SECTION

General. Phase transition behavior was examined by DSC by using a NETZCH DSC 204 Pheonix system. POM observation was conducted with an Olympus BX-53 polarizing optical microscope equipped with a Linkam LTS350 hot-stage. NMR spectra were recorded at 400 MHz for 1H and 100 MHz for 13C in CDCl3 using a JEOL JNM-ECX400 NMR spectrometer. Chemical shifts of 1H and 13 C NMR signals were referenced to Me4Si (delta = 0.00) and CDCl3 (delta = 77.00) as internal standards, respectively. FT-IR measurements were conducted with a JASCO FT/IR-6100 spectrometer. Matrix-associated laser desorption ionization time-of-flight mass spectra (MALDI-TOF MS) were recorded on a Bruker Daltonics Autoflex Speed using dithranol as the matrix. Elemental analysis was conducted with an Exeter Analytical Inc. CE-440 elemental analyzer. X-ray diffractograms were recorded using a Rigaku RINT-2500 diffractometer with Ni-filtered CuKα radiation, and the samples were placed in a heating stage. The TiO2 films were printed using a screen printing technique. The obtained layer thickness was determined using an Accretech Surfcom130A. EIS was measured with a Solartron SI 1287 electrochemical interface connected to an SI 1260 Impedance/Gain-phase analyzer. The CV was recorded with a ALS/CH Instruments Electrochemical Analyzer Model 600B. The temperature was controlled with a Linkam LTS350 hot-stage during the CV measurements. The I−V and incident photon-to-current conversion efficiency (IPCE) measurements were conducted under AM1.5G solar light simulated by a solar simulator (CEP-25TF), calibrated using a certified c-Si solar cell. Electrical data was recorded on a computer controlled digital source meter (Keithley, Model 2400). The temperature of the cells during the I−V measurements was controlled using a Dataplate digital hot plate OMEGA LHS-720 series, and the temperature was verified with an A&D Company Limited AD5611A infrared thermometer. Materials. 1-Ethyl-3-methylimidazolium iodide was purchased from Sigma-Aldrich and was used without further purification. TiO2 pastes were purchased from Solaronix S.A. The dye molecule (5-[[4[4-(2,2-diphenylethenyl)phenyl]-1,2,3,3a,4,8b-hexahydrocyclopent[b]indol-7-yl]methylene]-2-(3-octyl-4-oxo-2-thioxo-5-thiazolidinylidene)4-oxo-3-thiazolidineacetic acid (D205) was purchased from SigmaAldrich. All reagents of the highest quality were purchased from Sigma-Aldrich, Kanto, Tokyo Kasei, or Wako, and were used as received. Unless otherwise noted, all of the reactions were carried out under an argon atmosphere in a dry solvent purchased from Kanto. FTO coated glass (10Ω/cm¯2) substrates were purchased from Nippon Sheet Glass Co. Preparation of Electrolytes 1/EMII and 2/EMII. Electrolytes 1/ EMII and 2/EMII were prepared by mixing the respective compounds 1 and 2 in an equimolar ratio with EMII. EMII was prepared by the addition of 20 mol % of I2 to a previously weighted amount of 1-ethyl3-methylimidazolium iodide. This mixture was diluted in a known volume of acetonitrile (ACN). The appropriate volume of the EMII solution was added to compounds 1 and 2 in a dry microtube using a micropipette. The mixture was completely solubilized by adding the required volume of ACN, and the solvent was removed by rotary evaporation. The samples were dried under vacuum at 40 °C for 12 h prior to their use in the solar cell devices. Fabrication of DSSCs. The Working-Electrodes Were Fabricated As Follows. A light absorbing layer (0.4 × 0.4 cm2) consisting of Solaronix Ti-Nanoxide HT/SP, Solaronix Ti-Nanoxide T/SP, and Solaronix Ti-Nanoxide R/SP paste was screen-printed on the FTO substrates. The electrodes were sintered at 500 °C for 30 min. The final layer thickness was 12 μm. After cooling to r.t. a 3 μm thick layer of Solaronix Ti-Nanoxide D/SP was screen printed, and the electrodes were sintered again at 500 °C for 30 min and cooled to 80 °C. The hot electrodes were directly immersed into a 0.2 mM ethanol solution of the D205 sensitizer for 3 h. The electrodes were then rinsed with ethanol and dried at ambient conditions before use.

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CONCLUSIONS In summary, we have shown two new strategies for development of LC electrolytes that provide different advantages to the performance of DSSCs. We have designed noncovalent type LC assemblies for efficient ion transport and covalent type ionic liquid crystals for thermally stable LC phases. The self-assembly of the noncovalent type electrolytes is driven by dipole-ion interactions. This design enhances ion mobility in the 2D ionic arrays. The DSSCs containing the 1/EMII electrolytes exhibit a PCE of 5.8%, the highest PCE reported for LC-DSSCs. In contrast, the devices based on the covalent type electrolytes consisting of ionic mesogenic compounds show excellent high temperature stability and performance. A maximum PCE value of 2.4% was obtained at 120 °C for DSSCs containing 3, which is 2.5 times higher than that of the 1/EMII-based DSSCs. Furthermore, the introduction of the flexible oligooxyethylene spacer between the terminal group and the mesogenic moiety extends the LC phase temperature range, increases the mass transport properties of the electrolytes, and significantly improves the PCE of LC-DSSCs. In addition, all devices show excellent long-term stability over 1000 h of storage. These 6498


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Chemistry of Materials The Counter-Electrodes Were Fabricated As Follows. A 5 mM solution of H2PtCl6 in ethanol was dropcasted (10 μL per cm2) onto FTO substrates (1.7 × 1.7 cm2). The substrates were then sintered at 390 °C for 30 min.27 The DSSCs Were Assembled As Follows. The appropriate amount of the electrolyte mixture (∼2 mg) was carefully placed on the TiO2 electrode with a spatula. A 30 μm thick Bynel hot melt plastic with a frame size of 5 × 5 mm2 was placed around the electrolyte coated TiO2 electrode. This electrode was then covered with the platinized counter-electrode and pressed together with two clips. The cells were sealed by heating at 150 °C for 3 min (melting point of the Bynel plastic is 150 °C) and then cooled to r.t. prior the PV studies. The active area of each cell is 0.16 cm2.

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(8) Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.; Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess, H. − W.; Hudson, S. D.; Duan, H. Self-organization of supramolecular helical dendrimers into complex electronic materials. Nature 2002, 419, 384−387. (9) Watson, M. D.; Fechtenkötter, A.; Müllen, K. Big Is Beautiful“Aromaticity” Revisited from the Viewpoint of Macromolecular and Supramolecular Benzene Chemistry. Chem. Rev. 2001, 101, 1267− 1300. (10) Figueira-Duarte, T. M.; Müllen, K. Pyrene-Based Materials for Organic Electronics. Chem. Rev. 2011, 111, 7260−7314. (11) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Discotic liquid crystals: a new generation of organic semiconductors. Chem. Soc. Rev. 2007, 36, 1902−1929. (12) Funahashi, M.; Hanna, J.-I. High Carrier Mobility up to 0.1 cm2 V−1 s−1 at Ambient Temperatures in Thiophene-Based Smectic Liquid Crystals. Adv. Mater. 2005, 17, 594−598. (13) Goodby, J. W.; Collings, P. J.; Kato, T.; Tschierske, C.; Gleeson, H.; Raynes, P. Handbook of Liquid Crystals, 2nd ed.; Wiley-VCH: Weinheim, 2014. (14) Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Self-Organized Discotic Liquid Crystals for High-Efficiency Organic Photovoltaics. Science 2001, 293, 1119−1122. (15) Yoshio, M.; Mukai, T.; Ohno, H.; Kato, T. One-Dimensional Ion Transport in Self-Organized Columnar Ionic Liquids. J. Am. Chem. Soc. 2004, 126, 994−995. (16) Yoshio, M.; Kagata, T.; Hoshino, K.; Mukai, T.; Ohno, H.; Kato, T. One-Dimensional Ion-Conductive Polymer Films: Alignment and Fixation of Ionic Channels Formed by Self-Organization of Polymerizable Columnar Liquid Crystals. J. Am. Chem. Soc. 2006, 128, 5570− 5577. (17) Yoshio, M.; Mukai, T.; Kanie, K.; Yoshizawa, M.; Ohno, H.; Kato, T. Layered Ionic Liquids: Anisotropic Ion Conduction in New Self-Organized Liquid-Crystalline Materials. Adv. Mater. 2002, 14, 351−354. (18) Ichikawa, T.; Yoshio, M.; Hamasaki, A.; Taguchi, S.; Liu, F.; Zeng, X. B.; Ungar, G.; Ohno, H.; Kato, T. Induction of Thermotropic Bicontinuous Cubic Phases in Liquid-Crystalline Ammonium and Phosphonium Salts. J. Am. Chem. Soc. 2012, 134, 2634−2643. (19) Kerr, R. L.; Miller, S. A.; Shoemaker, R. K.; Elliott, B. J.; Gin, D. L. New Type of Li Ion Conductor with 3D Interconnected Nanopores via Polymerization of a Liquid Organic Electrolyte-Filled Lyotropic Liquid-Crystal Assembly. J. Am. Chem. Soc. 2009, 131, 15972−15973. (20) Ichikawa, T.; Yoshio, M.; Hamasaki, A.; Mukai, T.; Ohno, H.; Kato, T. Self-Organization of Room-Temperature Ionic Liquids Exhibiting Liquid-Crystalline Bicontinuous Cubic Phases: Formation of Nano-Ion Channel Networks. J. Am. Chem. Soc. 2007, 129, 10662− 10663. (21) Binnemans, K. Ionic Liquid Crystals. Chem. Rev. 2005, 105, 4148−4204. (22) Kato, T.; Mizoshita, N.; Kishimoto, K. Functional LiquidCrystalline Assemblies: Self-Organized Soft Materials. Angew. Chem., Int. Ed. 2006, 45, 38−68. (23) Yoshio, M.; Kato, T. Liquid Crystals as Ion Conductors. In Handbook of Liquid Crystals, 2nd ed.; Goodby, J. W., Collings, P. J., Kato, T., Tschierske, C., Gleeson, H., Raynes, P., Eds.; Wiley-VCH: Weinheim, 2014; Vol. 8, Chapter 23, pp 727−749. (24) Lowe, A. M.; Abbott, N. L. Liquid Crystalline Materials for Biological Applications. Chem. Mater. 2012, 24, 746−758. (25) Kim, A. J.; Kaucher, M. S.; Davis, K. P.; Peterca, M.; Imam, M. R.; Christian, N. A.; Levine, D. H.; Bates, F. S.; Percec, V.; Hammer, D. A. Proton Transport from Dendritic Helical-Pore-Incorporated Polymersomes. Adv. Funct. Mater. 2009, 19, 2930−2936. (26) Soberats, B.; Yoshio, M.; Ichikawa, T.; Taguchi, S.; Ohno, H.; Kato, T. 3D Anhydrous Proton-Transporting Nanochannels Formed by Self-Assembly of Liquid Crystals Composed of a Sulfobetaine and a Sulfonic acid. J. Am. Chem. Soc. 2013, 135, 15286−15289.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01590. Synthesis and characterization of compounds 1, 3, and 4; LC properties of 1, 3, 4, 1/EMII, and 2/EMII including DSC traces, XRD studies, and POM images; I−V curves and IPCE spectra of the 1/EMII, 2/EMII, and 3-based DSSCs; and FT-IR and EIS experiments (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

D.H. and B.S. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partially supported by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) (T.K.) from the Cabinet Office, Government of Japan. This work was also partially supported by a Grant-in-Aid for Scientific Research (No. 22107003) in the Innovative Area of “Fusion Materials” (Area No. 2206) (T.K.) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and CREST, JST (T.K.). D.H. is grateful for financial support from MEXT.

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REFERENCES

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Liquid-Crystalline Dye-Sensitized Solar Cells: Design of Two

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