Solution Processable, Electrochromic Ion Gels for Sub-1 V, Flexible

Article pubs.acs.org/cm

Solution Processable, Electrochromic Ion Gels for Sub‑1 V, Flexible Displays on Plastic Hong Chul Moon,† Timothy P. Lodge,*,†,‡ and C. Daniel Frisbie*,† †

Department of Chemical Engineering & Materials Science, University of Minnesota, 421 Washington Avenue Southeast, Minneapolis, Minnesota 55455, United States ‡ Department of Chemistry, University of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: The functionality of ion gels can be enhanced by incorporating different types of redox-active species. Here, we have expanded the functionality of ion gels composed of polystyrene-block-poly(methyl methacrylate)-block-polystyrene and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide to include electrochromism by adding an electrochromic (EC) redox molecule, methyl viologen. Ferrocene was also added to the EC gel as an anodic species. The EC gel was inserted between two indium−tin oxide-coated glass slides (or plastic sheets) to make a simple two-terminal electrochromic device (ECD). The ECD changed color upon application of 0.7 V. The coloration efficiency (η) was 105 cm2/C, and the ECD exhibited good operational stability over 24 h even in air. Because the EC gel is processed from common solvents (acetone) at room temperature, it can be coated onto plastic straightforwardly, and we demonstrated a patterned flexible ECD. Overall, the results demonstrate that sub-1 V, flexible ECDs based on EC ion gels can be prepared by simple solution processing, and thus, they are potentially attractive components for printed electronics.

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low-voltage, flexible electrochemiluminescent (ECL) devices based on ion gels containing ECL luminophores.25,26 In short, ion gels provide an attractive multifunctional electrolyte platform for device applications. In this work, we have expanded the functionality of ion gels consisting of polystyrene-block-poly(methyl methacrylate)block-polystyrene (SMS) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) to include electrochromism by incorporating EC materials. We have selected the well-established methyl viologen dication (MV2+) as the active EC molecule. MV2+ is colorless, but the reduced MV radical cation (MV+•) displays an intense deep blue color because of optical charge transfer between the +1 and zerocharge nitrogens, in which delocalization of the radical in the bipyridine moiety contributes to good stability.1,27−30 To prepare a homogeneous EC gel, we exchanged the anions (chlorides) of MV2+ for hexafluorophosphates (PF6−) to improve the solubility of MV2+ in the gel. To precisely define the operating voltage, in addition to MV2+, we added an anodic species in the gel as a chemical source of electrons. Without such an anodic species, unwanted electrochemical decomposition reactions of the electrolyte or solvents can occur during device operation, and the operating voltage may be higher than desirable. For example, a flexible ECD based on graphene quantum dot (GQD)−MV nanocomposites, where the

INTRODUCTION Electrochromic (EC) materials reversibly alter their optical properties such as absorbance and transmittance upon application of an external electrical stimulus. Electrochromic devices (ECDs) utilizing EC materials have been employed for numerous applications, including car mirrors, smart windows, optical displays, and sensors.1−6 The simplest ECD configuration involves an EC material containing an electrolyte inserted between two transparent electrodes. For practical applications, the development of low-voltage ECDs is important, because operation at higher voltages leads to rapid degradation.7 Thus, the electrolyte layer should have high ionic conductivity to minimize the decrease in voltage across the device. While conventional solid-state electrolytes, e.g., polyelectrolytes or composites of poly(ethylene oxide) (PEO) with lithium perchlorate (LiClO4), have modest ionic conductivity (10−3 to 10−1 mS/cm), ion gels consisting of room-temperature ionic liquids (ILs) and ABA triblock copolymers with IL-insoluble A blocks and an IL-soluble B block can exhibit relatively high ionic conductivity of 1−10 mS/cm even at room temperature.8 In addition, ion gels have significant advantages such as high capacitance, negligible vapor pressure, tunable mechanical strength, and good solution processability for device fabrication.9−11 Thus, ion gels have been extensively employed in diverse applications such as electrolyte-gated transistors,12−22 gas separation membranes,23,24 and electrochemiluminescent devices.25,26 Interestingly, the functionality of ion gels can be enhanced by incorporating various types of electrochemically active species. For instance, we recently demonstrated © XXXX American Chemical Society

Received: January 4, 2015 Revised: January 25, 2015

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DOI: 10.1021/acs.chemmater.5b00026 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 2. (a) Variation of UV−vis absorption spectra for the ECD at various applied voltages. (b) Photographs for the device in the bleached (left) and colored (right) states.

Figure 1. (a) Schematic illustration of electrochromic devices (ECDs) based on the EC gel consisting of MV(PF6)2, Fc, SMS, and [EMI][TFSI] in a 3:1:4:36 weight ratio. (b) Diagram of electrochemical reactions occurring in the ECD in this study. All reduction potentials are given vs Fc+/Fc.

MV2+ and Fc do not react in the gel because the electrontransfer reaction (reaction 3) is thermodynamically unfavorable; i.e., E°rxn = −0.7 V < 0 and ΔGrxn = −nFE°rxn > 0. The E° difference (0.7 V) between reactions 1 and 2 corresponds to the voltage required for device operation. The device exhibited a good coloration efficiency of 105 cm2/C and operational stability over 24 h in air. Also, we fabricated patterned flexible ECDs on plastic using a straightforward solution-based process to take advantage of the rubbery character of the gel.

only active redox species was MV2+, has been reported.27 Although the standard reduction potential (E°) of MV2+/MV+• is −0.7 V (vs Fc+/Fc), distinct coloration was observed only beyond −2.0 V, which is due to the absence of both an anodic redox species and a supporting electrolyte. On the other hand, Chidichimo et al. reported solid thermoplastic EC films containing ethyl viologen (EV2+) and hydroquinone (HQ) as an anodic species.30 The EC response appeared around 1.4 V, which is still higher than the difference (0.48 V) in redox potentials between EV2+ [−0.21 V vs the saturated calomel electrode (SCE)]28 and HQ (0.27 V vs the SCE).31 These results imply the importance of both an anodic species and high ionic conductivity of the electrolyte layer for obtaining a device with a low operational voltage. Therefore, with this consideration, we added ferrocene (Fc) as the anodic species to the highly ion conductive gel, because Fc has good solubility in the ion gel and the E° of Fc+/Fc is more positive than (but close to) that of MV2+/MV+•. The overall electrochemistry of the device is MV2 + + e− → MV +•

Fc+ + e− → Fc

E°red = −0.7 V vs Fc+ /Fc 32

E°red = 0.0 V vs Fc+ /Fc 32

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RESULTS AND DISCUSSION Figure 1a depicts the chemical structures of components in the EC gel and an illustration of the ECD. The homogeneous EC gel was composed of a 3:1:4:36 weight ratio blend of MV(PF6)2, Fc, SMS, and [EMI][TFSI], and the ECD was simply fabricated by sandwiching a 60 μm thick EC gel between two pieces of indium−tin oxide (ITO)-coated glass (or plastic). In the absence of an applied voltage, the EC gel exhibited a slightly yellowish color because of the dissolved Fc. When a voltage higher than 0.7 V was applied, it became a deep blue color as reaction 1 proceeded and MV+• was produced. The concentration gradients of MV+• and Fc+ induced by reactions 1 and 2 cause diffusion from the respective electrodes to the bulk. In contrast to reaction 3, when both species meet, the electron-transfer reaction between MV+• and Fc+ (i.e., reverse of reaction 3) is spontaneous because of the positive E°rxn (0.7 V) and regenerates MV2+ and Fc (Figure 1b). This reaction leads to a diffusion-controlled current that will be discussed later. Figure 2a displays the variation of the UV−vis spectrum of the ECD at different applied voltages. Below −0.7 V, the ECD

(1) (2)

MV2 +(colorless) + Fc → MV +•(deep blue) + Fc+ E°rxn = −0.7 V

(3) B


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Figure 4. (a) Profile of the applied asymmetric square wave between 0.0 and −0.9 V (black) and change of the transmittance at 550 nm (blue). (b) Variation of the transmittance at 550 nm as a function of operation time.

side reactions such as the degradation of [EMI][TFSI]. This result underscores the importance of the presence of an anodic redox species (ferrocene) in the ECD. Transient profiles of the absorbance at 550 nm and the device current at −0.9 V for the ECD are given in Figure 3a. The absorbance at 550 nm increased and eventually was saturated after ∼40 s. On the other hand, the device current peaked as soon as −0.9 V was applied. As the device approached the steady state, the induced current level was ∼1 mA/cm2. Because the device is a type I ECD in which all redox species (MV2+/MV+• and Fc+/Fc) are dissolved in the electrolyte layer,1 mass transport of redox species is one of the most important parameters for device operation. In general, migration, diffusion, and convection contribute to mass transport. In this system, however, we may ignore the effect of migration and convection due to the presence of excess inert ions (i.e., [EMI][TFSI]) in the gel and the absence of stirring. In diffusion-controlled systems, the time dependence of the Faradaic current [I(t)] by the reaction of species i near the electrode is expressed by the Cottrell equation:34

Figure 3. (a) Transient profiles of the device current density and absorbance at 550 nm upon application of −0.9 V. (b) Change in the absorbance at 550 nm under open and short circuits for bleaching. (c) Variation of the optical density (OD) of the ECD at 550 nm as a function of charge density.

exhibited a broad and weak absorption peak around 440 nm. At −0.7 V, the production of MV+• commenced, and the characteristic absorption spectrum for MV+• was clearly observed. The ECD with a deep blue color is shown in the right image of Figure 2b. This result is in good agreement with the potential difference between E° Fc + /Fc and E° MV 2+ /MV +• estimated by cyclic voltammograms (see Figure S1 of the Supporting Information). Although the ECD with the EC gel functioned without added Fc, a much higher voltage (approximately −2.6 V) was required (see Figure S2 of the Supporting Information). On the basis of the electrochemical window of [EMI][TFSI] from −2.4 V (cathodic limit) to 2.2 V (anodic limit),33 such a device would likely encounter unwanted

I(t ) = nFSci Di /πt

(4)

where ci and Di are the initial concentration and diffusion coefficient of species i, respectively, and n, F, and S are the number of electrons for the reaction, the Faraday constant, and the electrode area, respectively. Accordingly, I(t) ∝ t−1/2. As the C


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Figure 5. (a) Schematic illustration of the fabrication process for a flexible ECD on plastic. (b) Photographs of the ECDs in the bleached and colored states: before bending (top) and after bending (bottom). The reduction and oxidation process was conducted by applying −0.9 and 0.0 V, respectively.

The coloration efficiency (η) is an important characteristic parameter for evaluating ECD performance, as defined by1

absorbance (A) is proportional to the injected charge (Q) and dQ/dt = I(t), the relationship dA/dt ∝ t−1/2 (thus A ∝ t1/2) is anticipated.1 When we plotted I versus t−1/2 and A versus t1/2 (see Figure S3 of the Supporting Information), both showed good linearity, implying diffusion-controlled behavior of the ECD. According to the current density of ∼1 mA/cm2 at steady state with an applied bias of −0.9 V, only ∼9 μW/mm2 is consumed to keep the color, indicating that this ECD has the important characteristic of being a low-power consumption display. To investigate the bleaching behavior of the ECD, we employed two different experimental conditions: open circuit and shorted. Under the open-circuit condition, the origin of decoloration is the spontaneous electron-transfer reaction between MV+• and Fc+ (reverse of reaction 3). In addition to this electron-transfer reaction, shorting the device boosted bleaching by the direct oxidation of MV+• to MV2+ (reverse of reaction 1) near the electrode. Consequently, when we shorted the device, the deep blue color of the device disappeared ∼3 times faster than under open-circuit conditions (Figure 3b).

η = ΔOD/ΔQ = log(Tb/Tc)/ΔQ

(5)

where ΔOD is the change in optical density, Tb and Tc are the transmittance at a given wavelength for bleached and colored states, respectively, and ΔQ is the required charge density for the corresponding ΔOD. Figure 3c shows plots of OD at 550 nm as a function of injected charge. The η (105 cm2/C) for the gel-based ECD was estimated from the slope of the line fit to the linear regime, which compares favorably with a previous value of 11 cm2/C for an ionic liquid-based ECD.35 Because the current low-voltage ECD operates within the electrochemical window of the ion gel (or ionic liquid, [EMI][TFSI]), the coloration occurred more efficiently without a loss of charge arising from side reactions such as electrolyte decomposition. The operational stability of the ECD was also investigated. We applied an asymmetric square wave between 0.0 and −0.9 V for bleached and colored states, respectively, because the device showed faster coloration than bleaching (see Figure 3a,b). D


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ECD Fabrication and Characterization. The ECDs were fabricated in ambient air as follows. A MV(PF6)2/Fc/SMS/[EMI][TFSI]/acetone solution (3:1:4:36:40 weight ratio) was deposited by solution casting on the bare ITO-coated glass (or plastic). For gel patterning, we employed a patterned negative photoresist (SU-8 2010, Micro Chem) on an ITO-coated substrate using a fabrication process that is described elsewhere.25 Then, a second ITO-coated glass (or plastic) electrode was placed on the gel and held in place using 60 μm thick double-sided tape. All absorption spectra were recorded on a spectrometer (Spectronic Genesys 5, Thermo Scientific). The dc voltages and square waves were supplied from a source meter (6517A, Keithley) and an arbitrary waveform generator (33220A, Agilent).

The detailed profile of the wave and variation of the transmittance at 550 nm are given in Figure 4a. Recently, Palenzuela et al. reported a flexible ECD utilizing ethyl viologen and reduced graphene oxide (RGO)-coated electrodes.36 Although a relatively lower operating voltage was achieved with RGOcoated electrodes by partial dissolution of the RGO film into the solution (electrochromic) phase, the ECD still required >1.5 V for 30 s to attain a transmittance change (ΔT) of ∼35%. On the other hand, at only −0.9 V for 3 s, the ECD in this study displayed a large ΔT of ∼48% (Figure 4a). Moreover, the transmittance at 550 nm was reversibly varied between the bleached and colored state over a 24 h period in air. Although a decrease in ΔT was observed, the ECD still exhibited ∼29% transmittance contrast even after operating for 24 h (Figure 4b). As the EC gel employed here is rubbery, we were able to fabricate a flexible ECD on plastic. The fabrication processes are illustrated in Figure 5a. First, a 10 μm thick negative photoresist (SU-8) deposited on the working electrode of an ITO-coated polyethylene terephthalate (PET) film was exposed to UV irradiation through a photomask. After unexposed areas had been washed with a developer, a patterned photoresist layer was obtained. Then, the EC gel was deposited by solution-casting on the patterned photoresist, and then the gel was covered with a counter electrode (i.e., another ITO-coated PET film). This sandwiching procedure was conducted by hand. The ECD with patterns of the Arabic numerals composed of small squares (1 mm × 1 mm) are shown in Figure 5b. While the patterns could not be perceived in the bleached state, the numbers appeared in a deep blue color at −0.9 V. When we applied 0.0 V, the ECD returned to the bleached state and the patterns disappeared. Moreover, such reversible electrochromic behavior was also observed after bending.

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Cyclic voltammograms for MV(PF6)2 and ferrocene (Fc) in the EC gel, electrochromic behavior of the ECD without anodic redox species, and diffusion-controlled behavior of the ECD based on the EC gel. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Sipei Zhang for providing the SMS block copolymer. T.P.L. and C.D.F. acknowledge financial support from the Air Force Office of Scientific Research under Grant FA9550-12-1-0067.

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SUMMARY We have demonstrated a simple, solid-state, flexible electrochromic device (ECD) on plastic, based on an electrochromic (EC) ion gel. Highly conductive EC gels including ferrocenes as the anodic species provide an opportunity to fabricate lowvoltage ECDs. The device showed an excellent coloration efficiency of 105 cm2/C and good operational stability in air. Also, by utilizing the rubbery characteristic of the gel, we successfully demonstrated patterned flexible ECDs. Moreover, because the EC gel is fully compatible with straightforward solution processing or printing and the required power to maintain the colored state is reasonably low (∼9 μW/mm2), the ECD in this study has significant potential for printed electronics applications with power supplied from thin film batteries.

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ASSOCIATED CONTENT

S Supporting Information *

REFERENCES

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EXPERIMENTAL SECTION

Materials. All chemicals were purchased from Sigma-Aldrich and used as received. Methyl viologen hexafluorophosphate [MV(PF6)2] was prepared by an anion exchange reaction between methyl viologen chloride hydrate [MV(Cl)2·H2O] and excess ammonium hexafluorophosphate (NH4PF6). Polystyrene-block-poly(methyl methacrylate)-blockpolystyrene (SMS, 17K-86K-17K) triblock copolymer and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) were synthesized previously by a two-step atomtransfer radical polymerization (ATRP)9 and two-step anion exchange reaction,37 respectively. The ITO-coated glass (sheet resistance of 8−12 Ω/sq, Delta Technologies Ltd.) or ITO-coated PET (sheet resistance of 60 Ω/sq, Sigma-Aldrich) was sequentially rinsed with acetone (5 min), methanol (5 min), and 2-propanol (5 min) under sonication, followed by UV/ozone treatment for 10 min before use. E


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Solution Processable, Electrochromic Ion Gels for Sub-1 V, Flexible

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