Multicolored, Low-Power, Flexible Electrochromic Devices Based on

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Multicolored, Low Power, Flexible Electrochromic Devices Based on Ion Gels Hong Chul Moon, Chang-Hyun Kim, Timothy P. Lodge, and C. Daniel Frisbie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01307 • Publication Date (Web): 12 Feb 2016 Downloaded from http://pubs.acs.org on February 17, 2016

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ACS Applied Materials & Interfaces

Multicolored, Low Power, Flexible Electrochromic Devices Based on Ion Gels

Hong Chul Moon,†,§ Chang-Hyun Kim,† Timothy P. Lodge,†,‡,* and C. Daniel Frisbie†,*

†

Department of Chemical Engineering & Materials Science

University of Minnesota, 421 Washington Ave. SE, Minneapolis, Minnesota 55455, USA ‡

Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, USA

§

Department of Chemical Engineering, University of Seoul, Seoul 02504, Republic of Korea

* Corresponding authors. E-mail: [email protected] (T.P.L), [email protected] (C.D.F)

1

ACS Paragon Plus Environment


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Abstract Ion gels composed of a copolymer and a room temperature ionic liquid are versatile solidstate electrolytes with excellent features including high ionic conductivity, non-volatility, easily tunable mechanical properties, good flexibility and solution processability. Ion gels can be functionalized by incorporating redox-active species such as electrochemiluminescent (ECL) luminophores or electrochromic (EC) dyes. Here, we enhance the functionality of EC gels for realizing multicolored EC devices (ECDs), either by controlling the chemical equilibrium between a monomer and dimer of a colored EC species, or by modifying the molecular structures of the EC species. All devices in this work are conveniently fabricated by a “cut-and-stick” strategy, and require very low power for maintaining the colored state [i.e. 90 μW/cm2 (113 μA/cm2 at –0.8 V) for blue, 4 μW/cm2 (10 μA/cm2 at –0.4 V) for green, and 32 μW/cm2 (79 μA/cm2 at –0.4 V) for red ECD]. We also successfully demonstrate a patterned, multicolored, flexible ECD on plastic. Overall, these results suggest that gel-based ECDs have significant potential as low power displays in printed electronics powered by thin film batteries.

Keywords: Electrochromism, Flexible Electronics, Electrochemical Displays, Ion Gels, Copolymers

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Introduction Electrochromic devices (ECDs) are employed in diverse applications including smart windows, antiglare mirrors and information displays, where modulation of optical absorbance and transmittance by means of an external voltage is convenient and desirable.1-5 ECDs are electrochemical devices and therefore include at least two electrodes and an electrolyte layer. In general, the EC material can be dissolved in the electrolyte or coated on one (or both) of the electrodes. The various ECD designs are classified into three categories according to the solubility of the bleached and colored EC species in the electrolyte layer.1,2 If both species are soluble in the electrolyte during operation, the device is classified as Type I. In a Type III device, both species form insoluble solids on the electrodes. In these devices, the electrolyte layer does not contain EC materials, and just provides counter ions to the EC layers for charge neutrality during electrochemical coloration and bleaching. A Type II ECD is intermediate between Type I and III; in these devices only the bleached species is dissolved in the electrolyte; the colored species coats one of the electrodes.1,2 Certainly the design of the electrolyte layer is important for all three types of ECDs. In particular, the electrolyte should have high ionic conductivity to achieve rapid response and low voltage operation, and it must be chemically stable to repeated cycling over a prescribed voltage range. For Type I and Type II ECDs, the electrolyte must also solubilize EC components. Finally, solid or gel electrolytes are clearly more desirable than liquid electrolytes for ECD fabrication. Here our interest is in flexible ECDs for printed electronics on plastic substrates, which means that all materials–electrodes and electrolyte–must be robust to bending and must be processable at temperatures below the softening temperature of the plastic substrate. Polymer gel electrolytes, which have been employed in a variety of applications including high capacitance electronic insulators in electrolyte gated transistors (EGTs)6-13 and supercapacitors,14,15 and as ion 3



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conductive media in batteries,16,17 dye-sensitized solar cells (DSSCs),18 actuators19 and electrochemical displays,20-23 are an attractive option for ECDs. A particular subclass of PGEs known as ion gels have received great attention recently, due to their high ionic conductivity (1– 10 mS/cm) even at room temperature, excellent thermal and chemical stability, negligible vapor pressure and good solution processability.24 An ion gel is a composite of a room temperature ionic liquid (IL) and a network-forming polymer, for example an ABA triblock copolymer where the B block is soluble in the IL but both A blocks are not. Ion gels provide an opportunity to fabricate flexible or stretchable electrochemical devices.20,23,25 Previously, we have shown that the functionality of ion gels can be expanded by incorporating redox-active species. For example, introduction of electrochemiluminescent (ECL) luminophores or EC materials result in production of ECL gels20,21 or EC gels,23 respectively. Recently, we reported solution processable, flexible Type I ECDs based on EC ion gels containing methyl viologen (MV2+, EC material) and ferrocene (Fc, anodic species).23 Although the ECD required low operation voltage (below 1 V) and showed good coloration efficiency and operational stability, only a deep blue color was achieved. The realization of a multicolored device is desirable to make the devices more attractive for practical use. In this paper, we demonstrate multicolored flexible ECDs based on ion gels. While reported Type III ECDs based on polymers or metal oxides (e.g. tungsten oxide) require multilayered structure including two different electrochromic materials on the opposing electrodes and electrolyte,26-31 a Type I ECD just requires an EC electrolyte layer and two electrodes. We prepared a homogeneous EC gel by blending commercially available poly(vinylidene fluorideco-hexafluoropropylene) methylimidazolium

(P(VDF-co-HFP))

and

ionic

bis(trifluoromethylsulfonyl)imide

liquids

such

([BMI][TFSI])

as or

1-butyl-31-butyl-3-

methylimidazolium tetrafluoroborate ([BMI][BF 4 ]). As a result, the device could be readily fabricated by sandwiching the EC gel between two electrodes. In addition to the simple device 4


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structure, mechanically robust EC gels allowed device fabrication by the “cut-and-stick” strategy reported previously.32 We have employed viologen derivatives as EC chromophores in the ion gels as they have well-known reversible EC behavior.1,2 We show here that the colors of these species can be tuned by choice of ionic liquid in the gel, which affects the monomer-dimer equilibrium, or by chemical substitution. In principle, viologen dications are colorless, but viologen cation radicals are colored (reaction (1)) due to optically induced charge transfer between the +1 and zero-charge nitrogens on the bipyridine moiety.1,2 The color of alkyl viologen cation radicals (AV+,•) varies from blue to maroon (hereafter denoted as red) as the length of the alkyl N-substituent group increases, because of dimerization of AV+,• by spinpairing (reaction (2)).1 AV2+ (colorless) + e− 2AV+,• ⇄

→

AV+,• (colored)

(AV+,•) 2

(1) (2)

However, if the alkyl chain is too long, the corresponding alkyl viologens are not soluble in ion gels, resulting in production of inhomogeneous EC gels that do not work well. With this consideration, we selected the heptyl viologen dication (HV2+) as a model EC material, since it is fully compatible with the gel and also shows suitable dimerization behavior. While other groups have introduced additives such as double stranded DNA33 or cucurbit[7]uril (CB7)34 to control the monomer-dimer equilibrium chemistry, we control this equilibrium by simply modifying the ionic liquid in the gel. When we fabricated ECDs based on an EC gel containing [BMI][TFSI], HV+,• (blue color) was always dominant in the colored state regardless of the applied voltage. A simple change in anion of the ionic liquid (i.e. [BMI][BF 4 ]) induced a significant difference in the monomer-dimer equilibrium of the colored EC species (reaction (2)). When we applied the onset coloration voltage (–0.65 V), the device was blue, indicating a shift to the colored state, HV+,•. All applied voltages were negative with respect to 5



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ground. However, as the applied voltage was increased, the blue-colored ECD turned red (second coloration) due to the monomer-dimer equilibrium shift toward red-colored dimer of (HV+,•) 2 . In other words, substituting the ionic liquid in EC gels is a simple but effective way to tune the colored state of ECDs. Nonetheless, the development of a new viologen derivative for stable red ECDs is still required, because the red-colored (HV+,•) 2 is quasi-reversible.35,36 In addition, for full color ECDs, another EC material exhibiting a green color is also needed. To this end, we employed two arylsubstituted

viologens,

p-cyanophenyl

viologen

dication

(CN-PV2+)37-39

and

p-

trifluoromethylphenyl viologen dication (CF 3 -PV2+), for green and red ECDs, respectively. Based on these EC materials, a multicolored flexible ECD on plastic was demonstrated. In addition, all devices showed very low steady-state power consumption: 90 μW/cm2, 4 μW/cm2 and 32 μW/cm2 for blue, green and red ECDs, respectively. This result implies that ECDs based on ion gels compatible with solution process or printing provide an efficient and simple ECD display platform for printed electronics. Results and Discussion Device Fabrication by ‘Cut-and-Stick’ Strategy. To expand the “cut-and-stick” protocol developed for electrolyte gated transistors (EGTs)32 to ECDs, we first prepared free-standing EC gels by blending P(VDF-co-HFP), [BMI][TFSI], HV2+ and dimethyl ferrocene (dmFc). Previously, we found that an anodic species such as ferrocene (Fc) must be included in the ECD to achieve a low working voltage and good operational stability.23 Thus, dmFc, which is more soluble in the gel then Fc, was incorporated into all EC gels described here. The resulting homogeneous EC gel was slightly yellowish in the bleached state due to the dissolved dmFc. Figure 1a depicts the entire “cut-and-stick” process. Since the P(VDF-co-HFP)-based EC gel has sufficient mechanical strength, it could be cut into the desired 6


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shape with a razor blade and easily picked up with tweezers. Then, the EC gel was transferred onto the target substrate (e.g. ITO-coated glass or plastic sheet), and sandwiched with another ITO-coated substrate. This convenient fabrication process was used for all ECDs described in this work.

Figure 1. (a) Photographs of fabrication processes for ECDs based on EC gels by the “cutand stick” strategy, in which the demonstrated EC gel was composed of HV(TFSI) 2 , dmFc, P(VDF-co-HFP) and [BMI][TFSI] in the weight ratio of 3:1:12:60. (b) Diagram for overall electrochemical reactions during bleaching and coloration in the gel. The ECD demonstrated in Figure 1a contains HV2+ as the EC material. Accordingly, HV+,• formed at the cathode by reaction (3) provides the blue colored state upon application of a 7



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voltage of approximately −0.7 V. Meanwhile, dmFc is oxidized at the anode (the reverse of reaction (4)) during coloration. HV2+

+ e−

dmFc+ + e−

→ →

HV+,•

dmFc

HV2+ (colorless) + dmFc →

Eº red = −0.7 V vs dmFc+/dmFc40

(3)

Eº red = 0.0 V

(4)

vs dmFc+/dmFc

HV+,• (blue) + dmFc+

Eº rxn = −0.7 V

(5)

The generated HV+,• and dmFc+ diffuse from the respective electrodes to the bulk due to the induced concentration gradient, and their concentration profiles eventually overlap (see Figure 1b). As a result, HV2+ and dmFc are regenerated by the spontaneous electron transfer reaction between HV+,• and dmFc+ (i.e. reverse of reaction (5), ΔG rxn = −nFEº rxn < 0 ). Dependence of EC Behavior on the Ionic Liquid in the Gel. ECD based on gels containing [BMI][TFSI] The variation of the UV-Vis absorption spectra for the ECD based on the gel consisting of HV2+, dmFc, P(VDF-co-HFP) and [BMI][TFSI] (Figure 2a) is shown in Figure 2b. One broad and weak peak at 450 nm arising from dissolved dmFc was observed in the bleached state. Reduction of HV2+ commenced at –0.65 V, and the spectrum exhibited the characteristic absorption peaks of HV+,• (λ max ≈ 605 nm).41 Application of larger voltages accelerates the production of HV+,•, resulting in a higher concentration of HV+,•. Thus, the absorption becomes stronger, but the λ max is unchanged, indicating the major species in colored state is HV+,• in this ECD. The blue colored state of the ECD at –0.80 V also supports the presence of excess HV+,• (Figure 2c). Moreover, when we detached the two electrodes (ITO-coated glass) and separated the EC gel from the device after coloration, the blue colored gel was observed without any residue on the electrodes (see Figure S2), implying that the major colored species HV+,• is dissolved in the gel and the device corresponds to a Type I ECD.

8


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Figure 2. (a) Key components of the blue EC gel (anodic species, dmFc, is not shown). (b) UVVis spectra at various voltages for the ECD based on the gel shown in Figure 2a. The wavelength (λ max ) at which maximum absorption occurred from the colored state is 605 nm. (c) Photographs for bleached state at 0.00 V and blue colored state at –0.80 V. (d) Transient profiles of device current density and optical properties (absorbance and transmittance at 605 nm) upon application of –0.80 V followed by open-circuit conditions. (e) Plots of optical density difference versus injected charge density. To investigate device dynamics, transient profiles of current density and transmittance and absorbance at 605 nm were recorded (Figure 2d). The device current density peaked as soon as a voltage of –0.80 V was applied, and rapidly decreased as HV2+ and dmFc near the electrodes were consumed. Then, the current density gradually approached a diffusion controlled, steady9



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state value. Similar to the current density profile, the transmittance dropped quickly; the time required for a 90% change in the maximum transmittance contrast (Δt 90% ) at –0.80 V was ~11 s. On the other hand, bleaching under open circuit conditions required a much longer time (Δt 90% ~ 280 s) than coloration (Figure 2d). The only route for bleaching under open circuit conditions was the spontaneous reaction in the bulk (see reverse of reaction (5)), which depends on the relatively slow diffusion of HV+,• and dmFc+. Therefore, bleaching is slower than coloration. In comparison with gel-based Type I ECDs, Type III ECDs seem to show more rapid switching; the response time for 95% of a full switch was 25 s for both coloration and bleaching, whereas the maximum transmittance contrast (∆T max ) was only ~35 %.26 However, in fairness it should be noted that ΔT max in this work was ~ 95 %, which is much larger than Type III ECDs (ΔT max ~ 40 %26). If we set the benchmark of ∆T ~ 40 % to determine switching speed before reaching steady-state, the bleaching time will be cut in half. In addition, bleaching can be boosted under short circuit conditions by direct oxidation of viologen radical cations near the electrode, as demonstrated previously.23 Device performance can also be quantified by estimating the coloration efficiency η defined as:1 η = ∆OD⁄∆Q = log (Tb /Tc )⁄∆Q

(6)

where ∆OD is the change in optical density, T b and T c are the transmittance at a given wavelength for the bleached and colored states, respectively, and ∆Q is the required charge for the corresponding ∆OD. Figure 2e shows the dependence of ∆OD on the injected charge. The value of η can be extracted from the slope of the fitted line in the linear regime of the plot, yielding η = 78 cm2/C, which is ~25 % lower than that for the gel-based ECD containing methyl viologen dication (MV2+) but still ~7 times higher than η = 11 cm2/C for an ionic liquid-based ECD.42 In addition, when we tested the operational stability of the gel-based ECD previously, the device still maintained ∆T ≈ 30 % after operating 24 h even in air.23 The photochemical stability 10


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is also be important for practical use,28 so we are currently evaluating photochemical stability of the device.

Figure 3. (a) Key components of the EC gel exhibiting multi-electrochromism (anodic species, dmFc, is not shown). (b) Dependence of UV-Vis spectra on applied voltage for the ECD based on the gel shown in Figure 3a. The wavelength (λ max ) at which maximum absorption occurred is 605 nm for blue and 540 nm for red. (c) Photographs for three different states: bleached state at 0.00 V, blue colored state at –0.70 V and red colored state at –0.80 V. (d) Transient profiles of device current density and optical properties (absorbance and transmittance) upon application of –0.70 V for blue and –0.80 V for red. (e) Bleaching behavior under open-circuit condition after coloration at –0.70 V for blue and –0.80 V for red.

11



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ECD based on gels containing [BMI][BF 4 ] Interestingly, a simple exchange of anions in the ionic liquid from TFSI− to BF 4 − (Figure 3a) induced significant differences in EC behavior. While there was no change in absorption for the bleached state, the colored state displayed multiple electrochromic states, Figure 3b. For an applied voltage of –0.65 V, coloration began and the absorption spectrum corresponded to that from the monomer HV+,•. This feature was maintained up to –0.70 V, where the ECD exhibited the blue colored state (middle image in Figure 3c). Absorption at wavelengths shorter than 605 nm began upon application of –0.75 V, and two peaks at 505 nm and 540 nm were fully developed at –0.80 V, corresponding to the characteristic peaks of the dimer (HV+,•) 2 .1,33 The red color of the ECD at this voltage (bottom image in Figure 3c) also supports (HV+,•) 2 as the dominant equilibrium species. Moreover, disassembly of the ECD after coloration at –0.80 V revealed a reddish solid product of dimer (HV+,•) 2 on the electrode surface, and a less bluish gel (see Figure S3), indicative of Type II behavior. Therefore, the ECD with the gel based on [BMI][BF 4 ] is switchable from Type I to Type II by adjusting the applied voltage. Figures 3d and e show a distinction between coloration and bleaching dynamics of the ECD at –0.70 V (Type I) and –0.80 V (Type II). Operation at –0.70 V resulted in very similar kinetic behavior to the device with the gel based on [BMI][TFSI] (see Figure 2d). On the other hand, ion gel insoluble red (HV+,•) 2 developed much more quickly on the electrode surface at –0.80 V (Δt 90% ~ 11.5 s), which is attributed to the fact that coloration of Type II ECDs is related to fast electron transfer near the electrode. In contrast, under open circuit conditions, (HV+,•) 2 in the Type II ECD required longer bleaching time (Δt 90% ~ 500 s) than HV+,• (Δt 90% ~ 144 s) (see Figure 3e), because the bleaching reaction is limited by diffusion of dmFc+ through the full thickness of the gel layer. Although the dimer (HV+,•) 2 can serve as the EC material for red devices, quasireversibility of the oxidation of (HV+,•) 2 was previously reported,35,36 which limits device performance over many cycles. Accordingly, the development of a new EC material with a red 12


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colored state is essential for fabricating more stable red ECDs. To address this problem, we introduced aryl-substituted viologen derivatives as EC materials, as discussed in the following section.

Figure 4. (a) Key components of the EC gel for green (anodic species, dmFc, is not shown). (b) Variation of UV-Vis spectra at different applied voltages for the ECD based on the gel shown in Figure 4a. The wavelength (λ max ) at which maximum absorption occurred is 595 nm. (c) Photographs for bleached state at 0.00 V and green colored state –0.50 V. (d) Transient profiles of device current density and optical properties (absorbance and transmittance at 595 nm) upon application of –0.40 V followed by opened. (e) Dependence of optical density difference on injected charge density.

13



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EC Behavior of ECDs Based on Aryl-Substituted Viologen Derivatives. One way to enhance the functionality of ECDs for practical applications is demonstration of a full-color device including red, green and blue (RGB) display elements. In general, arylsubstituted viologens (ArV2+) provide radicals (ArV+,•) having higher stability owing to their extended conjugation system (and thus better delocalization), and a high molar absorptivity.1,2 Moreover, realization of green EC color, which cannot be achieved with alkyl viologens, is possible. For example, when p-cyanophenyl viologen dication (CN-PV2+) a well-known Type II EC material, is reduced in water, insoluble green CN-PV+,• is precipitated on the electrode surface.1,2,37-39 Therefore, we also selected CN-PV2+ for the green ECD, and modified the counter anion to make it soluble in the gel. The homogeneous EC gel was prepared by incorporating CN-PV(TFSI) 2 (2) into the “cutand-stick” gel consisting of P(VDF-co-HFP) and [BMI][TFSI] (Figure 4a). Although dissolved 2 intensifies the yellowish color of the gel in the bleached state (see top image in Figure 4c), no specific absorption by 2 was detected from the UV-Vis spectrum at 0.0 V (Figure 4b). The ECD turned green at –0.30 V, with a strong absorption at 595 nm (Figure 4c). Such a low coloration voltage is one advantage for ECDs with ArV2+. Additional interesting EC behavior of the device is evident in the dynamics (Figure 4d). While coloration occurred quickly (time for Δt 90% ~ 18 s), bleaching proceeded very slowly. After removing the external voltage, there was no significant change in transmittance during the first 300 s, after which there was a gradual transmittance increase, indicating the high stability of CN-PV+,•. Although CN-PV2+ is known as a Type II EC material in aqueous systems,1,2,37-39 the device in this condition operates as a Type I ECD. Indeed, dissassembling the device revealed a green colored gel without any solid products on the electrode (see Figure S4), which may be attributed to enhanced solubility of CN-PV+,• in the gel. The η for this ECD was determined to be as high as 155 cm2/C (Figure 4e).

14


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Figure 5. (a) Key components of the red EC gel (anodic species, dmFc, is not shown). (b) Change of UV-Vis spectra at different applied voltages for the ECD based on the gel shown in Figure 5a. The wavelength (λ max ) at which maximum absorption occurred is 540 nm. (c) Photographs for bleached state at 0.00 V and red colored state –0.50 V. (d) Transient profiles of device current density and optical properties (absorbance and transmittance at 540 nm) upon application of –0.40 V followed by under open-circuit condition. (e) Variation of optical density difference as a function of injected charge density. To achieve stable red ECDs, we functionalized phenyl viologens with trifluoromethyl groups on both para-positions according to the synthetic route given in Scheme S1.43-45 Then, we blended the prepared p-trifluoromethylphenyl viologen bis(trifluoromethylsulfonyl)imide (CF 3 15



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PV(TFSI) 2 , 3) with P(VDF-co-HFP), dmFc and [BMI][BF 4 ] to obtain an EC gel for red devices (Figure 5a). The UV-Vis spectra shown in Figure 5b indicate that coloration commenced at – 0.30 V. The colored state has λ max of 540 nm, as shown in Figure 5c. When we disassembled the ECD after coloration, the red-colored gel was separated from the electrode without leaving any solid products on the electrode (see Figure S5), indicative of Type I ECD behavior as well. We examined the dynamics of this ECD with application of –0.40 V for coloration and opencircuit conditions for bleaching (Figure 5d). The ECD with 3 displayed a longer coloration time (Δt 90% ~ 40 s) and shorter bleaching time (Δt 90% ~ 180 s) than the previous red ECD with (HV+,•) 2 . In addition, it provided the red colored state with a larger transmittance contrast and lower coloration voltage (ΔT max ~ 95% at –0.40 V). A favorable η of 81 cm2/C at –0.40 V was extracted from the slope of the linear fit in Figure 5e. Power Consumption for Gel-based ECDs. We also verified an advantageous feature of gel-based ECDs, namely the low power requirement necessary for maintaining the colored state. To estimate the power consumption, we designed an experiment in which we applied a voltage pulse whenever the transmittance exceeded a threshold value (T ~ 25 %) after the first coloration. Points at which the external voltage was applied or removed are designated by blue inverse triangles and green triangles, respectively, in Figure 6. From the integration of device current density versus time, the quantity of charge required to meet the threshold transmittance during the experimental time (1200 s) was calculated, followed by extraction of the average current density across the device: 113 μA/cm2 for blue, 10 μA/cm2 for green, and 79 μA/cm2 for red. Pairing these current densities with the relevant operating voltages results in power consumption estimates of 90 μW/cm2 (113 μA/cm2 at –0.8 V), 4 μW/cm2 (10 μA/cm2 at –0.4 V) and 32 μW/cm2 (79 μA/cm2 at –0.4 V) for blue, green and red ECDs, respectively. Power consumption potentially puts these ECDs in the same class as

16


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other ultralow power consumption displays such as cholesteric liquid crystal displays (~1 mW/cm2) and e-paper displays (~ 2 mW/cm2).43

Figure 6. Dynamic profiles of device current density and transmittance at λ max for 1200 s, in which external voltage was applied whenever transmittance was higher than the threshold of 25 % to maintain the colored state after coloration for the (a) blue, (b) green and (c) red ECDs. Total amount of required charge to maintain the colored state for 1200 s was estimated by integrating the device current density profile. 17



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Figure 7. (a) Schematic illustration of fabrication processes for a flexible, patterned, multicolored ECD on plastic sheet by using the “cut-and-stick” method. (b) Photographs of bleached (at 0.00 V) and colored (at –0.70 V) flexible ECDs with bending. Demonstration of Multicolored Flexible ECDs on Plastic. To demonstrate the use of rubbery EC gels in flexible devices, we fabricated a patterned multicolored ECD on plastic. A schematic illustration for the fabrication process is depicted in Figure 7a. First, a negative photoresist film (SU-8, thickness ~ 10 μm) deposited on an ITOcoated polyethylene terephthalate (PET) sheet (working electrode) was patterned by photolithography with a photomask. The EC gels for each color were transferred onto the target 18


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positions by the “cut-and-stick” method. Then, the resulting gel layer was covered with another ITO-coated PET film (counter electrode). The left image in Figure 7b displays the bleached state of the ECD with bending, in which only yellowish gels are observed without distinguishable patterns. When we applied –0.70 V, patterns on the bent plastic distinctly appeared in RGB colors. Summary We successfully expanded the functionality of ion gels to produce multicolored ECDs. This was accomplished by adding electrochromic viologen chromophores and a ferrocene electron source to the gels. Different colors were obtained either by controlling the chemical equilibrium between viologen cation radical monomer (e.g. HV+,•) and the dimer ((HV+,•) 2 ) by exchanging anions in the ionic liquids of the gel, or by modifying the chemical structures of the viologen derivatives. A patterned, multicolored, flexible ECD on plastic was demonstrated, and low power requirements characteristic of ECDs were also verified. We anticipate that ECDs based on ion gels will serve as a versatile component in printed electronics applications where power is supplied by thin film batteries. Experimental Section Materials: All chemicals were purchased from Sigma-Aldrich and used as received unless otherwise noted. Heptyl viologen bis(trifluoromethylsulfonyl)imide (HV(TFSI) 2 , 1) was prepared by an anion exchange reaction between heptyl viologen dibromide (HV(Br) 2 ) and excess lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in DI water. Both CN-PV(TFSI) 2 (2) and CF 3 -PV(TFSI) 2 (3) were synthesized by using the Zincke reaction shown in Scheme S1. The detailed synthetic procedure is described elsewhere.44-46 The ITO-coated glass (Delta Technologies Ltd., sheet resistance: 8–12 Ω/sq) and PET sheet (Sigma-Aldrich, sheet resistance: 60 Ω/sq) were rinsed by acetone (5 min), methanol (5 min), and isopropanol (5 min), and further treated with UV/ozone before use. 19



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EC gel preparation, ECD fabrication and characterization: The “cut-and-stick” EC gels in this work were prepared as follows. First, a 3:1:12:60 weight ratio (typical total mass ~ 152 mg) of EC material, dmFc, poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)) and ionic

liquid

(either

1-butyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide

([BMI][TFSI]) or 1-ethyl-3-methylimidazolium tetrafluoroborate ([BMI][BF 4 ])) were fully dissolved in acetone at 50 ºC. The solution was cast onto a glass slide, giving a homogeneous free-standing EC gel (thickness ~ 60 μm). To fabricate an ECD, the EC gel was cut into the desired shape with a razor blade, and transferred on to the target location (ITO-coated substrate). Then, the gel was sandwiched with another ITO-coated substrate, and firmly fixed using doublesided tape. For a patterned flexible ECD, we employed a patterned negative photoresist on ITOcoated PET sheet instead of the bare electrode. To fabricate the patterned photoresist, a negative photoresist (SU-8 2010, Micro Chem) was deposited onto the ITO-coated PET by spin-coating at 3000 rpm for 60 s, and then baking at 95 °C for 3 min. The pattern was created by exposing the film to UV light (MA6, Karl Suss) for 12 s through a mask, baking at 95 °C for 3 min, and rinsing with SU-8 developer. Finally, the film was cured at 150 °C for 15 min. UV-Vis absorption spectra and transient transmittance profiles for the dynamic study were recorded on a spectrometer (Spectronic Genesys 5, Thermo Scientific). Device current-voltage measurements were made using a Keithley 236 source-measure unit controlled by Labview programs. All measurements were carried out in air. Acknowledgment. TPL and CDF acknowledge financial support from the Air Force Office of Scientific Research under FA9550-12-1-0067. Supporting Information Available. Cyclic voltammogram of the EC gel containing HV2+ and dmFc, photograph of the disassembled ECD based on the gel consisting of HV(TFSI) 2 (1), [BMI][TFSI], P(VDF-co-HFP) and dmFc after coloration at –0.8 V, photograph of the disassembled ECD based on the gel consisting of HV(TFSI) 2 (1), [BMI][BF 4 ], P(VDF-co-HFP) 20


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and dmFc after coloration at –0.8 V, photograph of the disassembled ECD based on the gel consisting of CN-PV(TFSI) 2 (2), [BMI][TFSI], P(VDF-co-HFP) and dmFc after coloration at – 0.5 V, photograph of the disassembled ECD based on the gel consisting of CF 3 -PV(TFSI) 2 (3), [BMI][BF 4 ], P(VDF-co-HFP) and dmFc after coloration at –0.5 V, synthetic schematics for CNPV(TFSI) 2 (2) and CF 3 -PV(TFSI) 2 (3). These materials are available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Monk, P. M. S.; Mortimer, R. J.; Rosseinsky, D. R. Electrochromism and Electrochromic Devices, Cambridge University Press, Cambridge, UK 2007. (2) Monk, P. M. S. The Viologens: Physicochemical Properties, Synthesis, and Applications of the Salts of 4,4′-Bipyridine , Wiley, Chichester, England 1998. (3) Rosseinsky, D. R.; Mortimer, R. J. Electrochromic Systems and the Prospects for Devices. Adv. Mater. 2001, 13, 783-793. (4) Thakur, V. K.; Ding, G.; Ma, J.; Lee, P. S.; Lu, X. Hybrid Materials and Polymer Electrolytes for Electrochromic Device Applications. Adv. Mater. 2012, 24, 4071-4096. (5) Jensen, J.; Hösel, M.; Dyer, A. L.; Krebs, F. C. Development and Manufacture of PolymerBased Electrochromic Devices. Adv. Funct. Mater. 2015, 25, 2073-2090. (6) Cho, J. H.; Lee, J.; He, Y.; Kim, B. S.; Lodge, T. P.; Frisbie, C. D. High-Capacitance Ion Gel Gate Dielectrics with Faster Polarization Response Times for Organic Thin Film Transistors. Adv. Mater. 2008, 20, 686-690. (7) Kim, S. H.; Hong, K.; Xie, W.; Lee, K. H.; Zhang, S.; Lodge, T. P.; Frisbie, C. D. Electrolyte-Gated Transistors for Organic and Printed Electronics. Adv. Mater. 2013, 25, 18221846. 21



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(8) Hong, K.; Kim, S. H.; Lee, K. H.; Frisbie, C. D. Printed, Sub-2V ZnO Electrolyte Gated Transistors and Inverters on Plastic. Adv. Mater. 2013, 25, 3413-3418. (9) Bhat, S. N.; Pietro, R. D.; Sirringhaus, H. Electroluminescence in Ion-Gel Gated Conjugated Polymer Field-Effect Transistors. Chem. Mater. 2012, 24, 4060-4067. (10) Lee, J.; Panzer, M. J.; He, Y.; Lodge, T. P.; Frisbie, C. D. Ion Gel Gated Polymer Thin-Film Transistors. J. Am. Chem. Soc. 2007, 129, 4532-4533. (11) Thiemann, S.; Sachnov, S. J.; Pettersson, F.; Bollström, R.; Österbacka, R.; Wasserscheid, P.; Zaumseil, J. Cellulose-Based Ionogels for Paper Electronics. Adv. Funct. Mater. 2014, 24, 625-634. (12) Shin, M.; Song, J. H.; Lim, G. -H.; Lim, B.; Park, J. -J.; Jeong, U. Highly Stretchable Polymer Transistors Consisting Entirely of Stretchable Device Components. Adv. Mater. 2014, 26, 3706-3711. (13) Lee, S. W.; Lee, H. J.; Choi, J. H.; Koh, W. G.; Myoung, J. M.; Hur, J. H.; Park, J. J.; Cho, J. H.; Jeong, U. Periodic Array of Polyelectrolyte-Gated Organic Transistors from Electrospun Poly(3-hexylthiophene) Nanofibers. Nano Lett. 2010, 10, 347-351. (14) Yang, X.; Zhang, F.; Zhang, L.; Zhang, T.; Huang, Y.; Chen, Y. A High-Performance Graphene Oxide-Doped Ion Gel as Gel Polymer Electrolyte for All-Solid-State Supercapacitor Applications. Adv. Funct. Mater. 2013, 23, 3353-3360. (15) Kang, Y. J.; Chun, S. -J.; Lee, S. -S.; Kim, B. -Y.; Kim, J. H.; Chung, H.; Lee, S. -Y.; Kim, W. All-Solid-State Flexible Supercapacitors Fabricated with Bacterial Nanocellulose Papers, Carbon Nanotubes, and Triblock-Copolymer Ion Gels. ACS Nano 2012, 6, 6400-6406. (16) Stephan, A. M. Review on Gel Polymer Electrolytes for Lithium Batteries. Eur. Polym. J. 2006, 42, 21-42. 22


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(17) Kil, E. -H.; Choi, K. -H.; Ha, H. -J.; Xu, S.; Rogers, J. A.; Kim, M. R.; Lee, Y. -G.; Kim, K. M.; Cho, K. Y.; Lee, S. -Y. Imprintable, Bendable, and Shape-Conformable Polymer Electrolytes for Versatile-Shaped Lithium-Ion Batteries. Adv. Mater. 2013, 25, 1395-1400. (18) Dong, R. -X.; Shen, S. -Y.; Chen, H. -W.; Wang, C. -C.; Shih, P. -T.; Liu, C. -T.; Vittal, R.; Lin, J. -J.; Ho, K. -C. A Novel Polymer Gel Electrolyte for Highly Efficient Dye-Sensitized Solar Cells. J. Mater. Chem. A 2013, 1, 8471-8478. (19) Imaizumi, S.; Kokubo, H.; Watanabe, M. Polymer Actuators Using Ion-Gel Electrolytes Prepared by Self-Assembly of ABA-Triblock Copolymers. Macromolecules 2012, 45, 401-409. (20) Moon, H. C.; Lodge, T. P.; Frisbie, C. D. Solution-Processable Electrochemiluminescent Ion Gels for Flexible, Low-Voltage, Emissive Displays on Plastic. J. Am. Chem. Soc. 2014, 136, 3705-3712. (21) Moon, H. C.; Lodge, T. P.; Frisbie, C. D. DC-Driven, Sub-2 V Solid-State Electrochemiluminescent Devices by Incorporating Redox Coreactants into Emissive Ion Gels. Chem. Mater. 2014, 26, 5358-5364. (22)

Deepa,

M.;

Awadhia,

A.;

Bhandari,

S.

Electrochemistry

of

Poly(3,4-

ethylenedioxythiophene)-Polyaniline/Prussian Blue Electrochromic Devices Containing an Ionic Liquid Based Gel Electrolyte Film. Phys. Chem. Chem. Phys. 2009, 11, 5674-5685. (23) Moon, H. C.; Lodge, T. P.; Frisbie, C. D. Solution Processable, Electrochromic Ion Gels for Sub-1 V, Flexible Displays on Plastic. Chem. Mater. 2015, 27, 1420-1425. (24) Lodge, T. P. A Unique Platform for Materials Design. Science 2008, 321, 50-51. (25) Feng, X.; Wu, M. -Y.; Safron, N. S.; Roy, S. S.; Jacobberger, R. M.; Bindl, D. J.; Seo, J. -H.; Chang, T. -H.; Ma, Z.; Arnold, M. S. Highly Stretchable Carbon Nanotube Transistors with Ion Gel Gate Dielectrics. Nano Lett. 2014, 14, 682-686. 23



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(26) Jensen, J.; Krebs, F. C. From the Bottom Up – Flexible Solid State Electrochromic Devices. Adv. Mater. 2014, 26, 7231-7234. (27) Jensen, J.; Dyer, A. L.; Shen, D. E.; Krebs, F. C.; Reynolds, J. R. Direct Photopatterning of Electrochromic Polymers. Adv. Funct. Mater. 2013, 23, 3728-3737. (28) Jensen, J.; Madsen, M. V.; Krebs, F. C. Photochemical Stability of Electrochromic Polymers and Devices. J. Mater. Chem. C 2013, 1, 4826-4835. (29) Gupta, R.; Walia, S.; Hösel, M.; Jensen, J.; Angmo, D.; Krebs, F. C.; Kulkarni, G. U. Solution Processed Large Area Fabrication of Ag Patterns as Electrodes for Flexible Heaters, Electrochromics and Organic Solar Cells. J. Mater. Chem. A 2014, 2, 10930-10937. (30) Jensen, J.; Dam, H. F.; Reynolds, J. R.; Dyer, A. L.; Krebs, F. C. Manufacture and Demonstration of Organic Photovoltaic-Powered Electrochromic Displays Using Roll Coating Methods and Printable Electrolytes. J. Polym. Sci. Part B: Polym. Phys. 2012, 50, 536-545. (31) Jensen, J.; Hösel, M.; Kim, I.; Yu, J. -S.; Jo, J.; Krebs, F. C. Fast Switching ITO Free Electrochromic Devices. Adv. Funct. Mater. 2014, 24, 1228-1233. (32) Lee, K. H.; Kang, M. S.; Zhang, S.; Gu, Y.; Lodge, T. P.; Frisbie, C. D. “Cut and Stick” Rubbery Ion Gels as High Capacitance Gate Dielectrics. Adv. Mater. 2012, 24, 4457-4462. (33) Kakibe, T.; Ohno, H. Immobilization of Heptyl Viologens in DNA Strands Both to Inhibit Dimerization and to Accelerate Quasi-reversible Electron Transfer Reaction. Chem. Commun. 2008, 377-379. (34) Nchimi-Nono, K.; Dalvand, P.; Wadhwa, K.; Nuryyeva, S.; Alneyadi, S.; Prakasam, T.; Fahrenbach, A. C.; Olsen, J. -C.; Asfari, Z.; Platas-Iglesias, C.; Elhabiri, M.; Trabolsi, A. Radical-Cation Dimerization Overwhelms Inclusion in [n]Pseudorotaxanes. Chem. Eur. J. 2014, 20, 7334-7344. 24


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(35) Mortimer, R. J. Organic Electrochromic Materials. Electrochim. Acta 1999, 44, 2971-2981. (36) Monk, P. M. S. The Effect of Ferrocyanide on the Performance of Heptyl Viologen-Based Electrochromic Display Devices. J. Electroanal. Chem.1997, 432, 175-179. (37) Rosseinsky, D. R.; Monk, P. M. S. Electrochromic Cyanophenylparaquat (CPQ: 1,1′-biscyanophenyl-4,4′-bipyridilium) Studied Voltammetrically, Spectroelectrochemically and by ESR. Sol. Energy Mater. Sol. Cells 1992, 25, 201-210. (38) Compton, R. G.; Waller, A. M.; Monk, P. M. S.; Rosseinsky, D. R. Electron Paramagnetic Resonance Spectroscopy of Electrodeposited Species from Solutions of 1,1′-bis-(pcyanophenyl)-4,4′-bipyridilium (Cyanophenyl Paraquat, CPQ). J. Chem. Soc. Faraday Trans. 1990, 86, 2583-2586. (39) Rosseinsky, D. R.; Monk, P. M. S. Kinetics of Comproportionation of the Bipyridilium Salt p-Cyanophenyl Paraquat in Propylene Carbonate Studied by Rotating Ring–Disc Electrodes. J. Chem. Soc. Faraday Trans. 1990, 86, 3597-3601. (40) The Reduction Potential (Eº) of HV2+/HV+,• (vs dmFc+/dmFc) in the EC gel was measured in this work (see Figure S1 in the Supporting Information). (41) Hu, C. -W.; Lee, K. -M.; Vittal, R.; Yang, D. -J.; Ho, K. -C. A High Contrast Hybrid Electrochromic Device Containing PEDOT, Heptyl Viologen, and Radical Provider TEMPO. J. Electrochem. Soc. 2010, 157, P75- P78. (42) Kavanagh, A.; Fraser, K. J.; Byrne, R.; Diamond, D. An Electrochromic Ionic Liquid: Design, Characterization, and Performance in a Solid-State Platform. ACS Appl. Mater. Interfaces 2013, 5, 55-62. (43) Fernández, M. R.; Casanova, E. Z.; Alonso, I. G. Review of Display Technologies Focusing on Power Consumption. Sustainability 2015, 7, 10854-10875. 25



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(44) Yamaguchi, I.; Higashi, H.; Shigesue, S.; Shingai, S.; Sato, M. N-Arylated Pyridinium Salts Having Reactive Groups. Tetrahedron Lett. 2007, 48, 7778-7781. (45) Porter, W. W.; Vaid, T. P. Isolation and Characterization of Phenyl Viologen as a Radical Cation and Neutral Molecule. J. Org. Chem. 2005, 70, 5028-5035. (46) Freitag, M.; Gundlach, L.; Piotrowiak, P.; Galoppini, E. Fluorescence Enhancement of Di-ptolyl Viologen by Complexation in Cucurbit[7]uril. J. Am. Chem. Soc. 2012, 134, 3358-3366.

26


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“Graphical abstract”

27


Multicolored, Low-Power, Flexible Electrochromic Devices Based on

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