Thermally Cured Dual Functional Viologen-Based All-in-One

Research Article www.acsami.org

Thermally Cured Dual Functional Viologen-Based All-in-One Electrochromic Devices with Panchromatic Modulation Sheng-Yuan Kao,† Hsin-Che Lu,† Chung-Wei Kung,† Hsin-Wei Chen,† Ting-Hsiang Chang,† and Kuo-Chuan Ho*,†,‡ †

Department of Chemical Engineering and ‡Institute of Polymer Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: Vinyl benzyl viologen (VBV) was synthesized and utilized to obtain all-in-one thermally cured electrochromic devices (ECDs). The vinyl moiety of VBV monomer could react with methyl methacrylate (MMA) to yield bulky VBV/poly(methyl methacrylate) (PMMA) chains and even cross-linked network without the assistance of additional crosslinker. Both the bulky VBV/PMMA chains and the resulting polymer network can hinder the aggregation of the viologens and reduce the possibility of dimerization, rendering enhanced cycling stability. Large transmittance changes (ΔT) over 60% at both 570 and 615 nm were achieved when the VBV-based ECD was switched from 0 V to a low potential bias of 0.5 V. Ultimately, the dual functional of VBV molecules, serving simultaneously as a promising electrochromic material and a cross-linker, is fully utilized in the proposed electrochromic system, making its fabrication process much easier. Negligible decays in ΔT at both wavelengths were observed for the cured ECD after being subjected to 1000 repetitive cycles, while 17.1% and 22.0% decays were noticed at 570 and 615 nm, respectively, for the noncured ECD. In addition, the low voltage-driven feature of the VBV-based ECD enables it to be incorporated with phenyl viologen (PV), further expanding the absorption range of the ECD. Panchromatic characteristic of the proposed PV/VBV-based ECD was demonstrated while exhibiting ΔT over 60% at both wavelengths. Only 5.3% and 6.9% decays, corresponding at 570 and 615 nm, respectively, were observed in the PV/VBV-based ECD after 10 000 continuous cycles at bleaching/coloring voltages of 0/0.5 V with an interval of 10 s for both bleaching and coloring processes. KEYWORDS: electrochromic device (ECD), low driving voltage, N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), panchromatic, phenyl viologen, thermal curing

■

INTRODUCTION Electrochromic devices (ECDs) are electric gadgets that offer various extents of light attenuation by electrically tuning the shade of electrochromic (EC) materials.1−3 Due to the light control and energy-saving properties of EC materials, the research and development in ECDs has gained lots of attention for the past few decades.4−11 Electrochromic products such as energy-saving smart windows are attractive due to their relatively low driving voltages (0.4−3.7 V);12−18 such driving voltages are much smaller than those of other types of smart windows based on liquid crystals19,20 or suspended particles,21,22 which usually require a higher driving voltage of over 5 V. More importantly, EC materials often exhibit strong light absorption at certain wavelengths depending on their chemical structures; therefore, a variety of colors can be obtained through synthesis and design of EC materials.23−27 Such a feature renders them promising candidates for low-power electrochromic applications.28−32 Meanwhile, black-to-transmissive EC materials are highly desirable from the viewpoint of practical energy-saving products23 especially for displays.33,34 However, only a few examples have been demonstrated since it © 2016 American Chemical Society

is difficult to achieve strong light absorption over the entire visible wavelength. Beaujuge et al. have successfully demonstrated a black-to-transmissive EC thin film through a delicate synthesis process.35 Other approaches, like utilizing multiple EC materials to show panchromatic light absorption, have also been reported.34,36−39 Despite efforts having been devoted to obtaining black-to-transmissive ECDs, it remains a challenging task to simultaneously achieve both black-to-transmissive switching with low-driving voltage and excellent cycling stability. In this study, a viologen-based ECD is proposed to meet all these requirements. Viologens, also named 1-1′-disubstituted-4,4′-bipyridiniums, are attractive EC materials for their remarkable color contrast.3,40 They are usually colorless at the dication state but deeply colored at the radical cation state. The ECDs containing viologens normally are assembled in the form of liquid, thus suffering from the possibility of leakage problems.28,41,42 Besides, some undesirable side reactions Received: December 7, 2015 Accepted: January 25, 2016 Published: January 25, 2016 4175

DOI: 10.1021/acsami.5b11947 ACS Appl. Mater. Interfaces 2016, 8, 4175−4184

Research Article

ACS Applied Materials & Interfaces Scheme 1. Thermal Curing Process of VBV Moiety

Another strategy to achieve better stability is through the incorporation of cross-linked polymer network in viologenbased ECDs, yielding an all-in-one EC gel.56 This polymer network was prepared through in situ thermal curing with all EC materials in a sealed ECD. The network can alleviate the aggregation of the viologen radical cations and acquire better cycling stability. In addition, the prepared polymer network encapsulated all electrolytes and EC materials as a homogeneous all-in-one polymer EC gel, thus solving the common leakage problem of viologen-based ECDs. In this study, a vinyl moiety bearing viologen, named vinyl benzyl viologen (VBV), has been synthesized and served as an electrochromic material which renders high optical contrast. In addition, the vinyl groups enable VBV to be further addressed by methyl methacrylate (MMA) monomers or other VBV moieties to yield bulky VBV/PMMA chains and even crosslinked network during the in situ thermal-curing process. The dual functionality of VBV molecules eliminates the need of additional cross-linker to obtain an all-in-one VBV-based electrochromic gel, thus enabling an easier fabrication process. The gel promoted by cross-linked VBV-based network can homogeneously encapsulate other components such as electrolyte and counter electrode material in the sealed cell while

such as comproportionation, dimerization, or even aggregation may occur during electrochemical operation.43−47 These side reactions could cause incomplete bleaching of the colored viologen radical cations, leading to the poor write−erase ability. To solve these problems in the viologen-based ECDs, various approaches have been proposed. Monk has incorporated the ferrocyanide ions into a viologen-based ECD,48 in which ferrocyanide ions can be bridged between two colored viologen radical cations to prevent their dimerization. Bookbinder et al.49 synthesized viologen-based polymers and anchored them onto conducting substrate to eliminate their aggregation, while Campus et al.50 anchored them onto porous conducting substrates to yield better EC performances. Gadgil et al. also successfully obtained viologen-based polymer thin films by electrosynthesizing cyanopyridinium moieties and yielded enhanced switching behavior.51−54 Recently, Sydam et al. successfully enhanced the write−erase ability of viologen-based ECD by attaching bulky substituents on viologen moieties.55 It is found that these bulky substituents create hindrance among viologen molecules, therefore reducing the possibility of dimerization or aggregation; the cycling stability of the obtained viologen-based ECD can thus be enhanced. 4176


Research Article

ACS Applied Materials & Interfaces

spectrophotometer (Ocean Optics, DH-2000-BAL). CIE (Commission Internationale de l’Eclairage) L*a*b* coordinates of the devices were acquired by using the same spectrophotometer (Ocean Optics, DH-2000-BAL) while the CIE standard illuminant A was used. All electrochemical measurements of ECDs were obtained under twoelectrode system configuration.

eliminating leakage problems. Last but not least, both the bulky VBV/PMMA chains and the VBV-based polymer network can prevent electrochemically active VBV moieties from dimerization and aggregation, resulting in better cycling stability of the obtained ECD. Meanwhile, phenyl viologen (PV), which exhibits an absorbance spectrum very different from most of alkylsubstituted viologens,56 demonstrates great potential to vary its absorbance spectrum when blending with various amounts VBV. Such merit makes it possible to possess the panchromatic absorption feature. It is expected that an ECD featuring high optical contrast, black-to-transmissive switching, low-driving voltage, good cycling stability, and easy fabrication process can be realized by fine-tuning the PV/VBV-based ECD.

■

■

RESULTS AND DISCUSSION Characteristics of EC Gel Utilizing VBV(BF4)2. The thermally initiated curing reaction during the VBV-based ECD Scheme 2. Working Principle of the VBV-Based ECD

EXPERIMENTAL METHODS

Chemicals. Vinyl benzyl chloride (VBC, 90%), methyl methacrylate (MMA, 99%), N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD, 99%), and tetra-n-butylammonium tetrafluoroborate (TBABF4, 99%) were purchased from Sigma-Aldrich, while propylene carbonate (PC, 99%) and acetonitrile (ACN, 99%) was obtained from Alfa Aesar. The thermal curing initiator azobis(isobutyronitrile) (AIBN, 99.7%) was purchased from UniRegion. Phenyl viologen dichloride (PVCl2, >99%) was purchased from Tokyo Chemical Industry (TCI). In this work, VBV was synthesized following the procedure reported in the literature57 with some modification in the reactants’ concentration. Briefly, excess vinyl benzyl chloride (VBC, 12.5 mL) was stirred at 90 °C along with 4,4′-bipridine (4.97 g) in a 15 mL ACN solution for 3 h. Precipitate can be observed during the reflux process, indicating the generation of vinyl benzyl viologen dichloride (VBVCl2). The generated VBVCl2 can be separated from the excess reagents through filtration and extensive washing by acetone. After the separation process, the collected slurry was further recrystallized by DIW and acetone cosolvent. The yellowish VBVCl2 powder can be obtained after filtration and drying at 60 °C for 1 day. In order to increase the compatibility of VBV with PC, the exchange of the anion from chloride to tetrafluoroborate was also performed. This anion exchange process was done based on a previous report in the literature.58 1.54 mmol of VBVCl2 was first completely dissolved in hot DIW, followed by the addition of 2.5 mL NaBF4 (18.85 mmol) aqueous solution and vigorous stirring. Since the ion exchange is a facile reaction, a substantial amount of VBV(BF4)2 precipitation was immediately formed after mixing. The precipitate was then separated by repeated (3 times) centrifugation and extensive wash using DIW/ ethanol 1:1 cosolvent. Ultimately, the VBV(BF4)2 product was collected after 90 °C drying. Fabrication of the Electrochromic Device. Two ITO glasses (Solaronix SA, Rsh = 7 Ω/□) were laminated with a cell gap of 60 μm (controlled by one layer of DuPont 60 μm Surlyn) to obtain the vacant device. The active areas of all ECDs were controlled to be 2 cm × 2 cm. Two holes were drilled on one side of the vacant device for the later injection of electrochromic gel precursor. The device was eventually sealed by epoxy. The electrochromic gel encapsulated with VBV(BF4)2 and TMPD was prepared by an in situ thermal-curing approach. The gel precursor was a 100 μL PC solution containing 10% (v/v) (0.163 M) of VBV(BF4)2, 0.05 M TMPD, 0.5 M TBABF4, and 10 μL MMA. A trace amount of thermal curing initiator AIBN (8.2 mg) was added into the as-prepared gel precursor. This EC gel precursor was then injected into the cell through the drilled holes. Last, the filled device was subjected to 90 °C heating for ca. 40 min to initiate the thermal curing process. Measurements. The electrochemical measurements were performed by a potentiostat/galvanostat (Autolab, model PGSTAT30). All the cyclic voltammograms were obtained at a scan rate of 0.1 V/s, unless mentioned otherwise. To obtain the spectroelectrochemical data, the potentiostat/galvanostat was connected in conjunction with a

fabrication process can be illustrated by Scheme 1. The transforming of the fluid curing precursor into gel (inset image in Scheme 1) verified that sufficient curing extent had been achieved. During the thermal curing process, the vinyl moiety of VBV monomer would be addressed by methyl methacrylate to yield bulky VBV/PMMA chains and even cross-linked network. The interval of thermal treatment was controlled at 40 min since a prolonged curing time would lead to low ionic conductivity of the gel; thus, VBV was unable to reach the electrode surface to complete coloring/bleaching reaction during operation. Since the in situ thermal curing reaction occurs homogeneously in the entire device as long as the temperature reaches 90 °C (curing temperature), it is possible that the cross-linked VBV network can also be obtained for the ECDs of larger size by applying the same curing procedure. In fact, uniform temperature inside the entire device during thermal curing becomes very crucial in determining the device quality and performance. Although the size of the ECD demonstrated in this study is limited to 2 cm × 2 cm, it is envisaged that the cured ECD with enlarged area can be successfully fabricated once the process temperature can be precisely controlled. The working principle of the cured ECD is illustrated in Scheme 2. A part of VBV moieties was immobilized within the network, while the other nonimmobilized VBV/PMMA chains can diffuse easily between the two electrodes. These VBV moieties in VBV/PMMA chains are initially at the oxidized state (VBV2+), rendering a colorless hue. When the potential bias was applied, VBV2+ were reduced by one electron at the electrode surface to give the reduced state (VBV+•) and exhibited a dark purple hue. The coloring process could be illustrated as follows VBV2 + + e− ↔ VBV +•

(1)

On the other hand, TMPD presents the colorless (TMPD) to dark purple (TMPD+) transition at the other electrode 4177


Research Article


Figure 1. (a) Cyclic voltammogram of the VBV-based ECD with the scan rate of 0.1 V/s. (b) Absorbance spectra of VBV-based ECD with the variation of applied cell voltage. (c) CIE L*a*b* coordinates of bleached and colored states in the VBV-based ECD. Images of the VBV-based ECD at (d) bleached state and colored state.

TMPD+. When the applied potential bias reached 0.5 V, a dark blue hue, which was contributed by abundant colored species (VBV+• and TMPD+), was observed. It is worthwhile to note that remarkable optical contrast was obtained even though a low potential bias of 0.5 V was applied, suggesting little hindrance for the diffusion of all reacting species. Since the colored species are not thermodynamically stable, their recombination reaction (which regenerated the colorless species) is quite facile and would go on until all the colored species are consumed. Therefore, no memory effect can be observed in this ECD. The continuous recombination reaction can explain the weak reduction peak locating at ca. 0.25 V when the potential was swept backward. The self-bleaching process in the bulk solution can further be verified by the CV obtained at a slower scan rate (0.001 V/s, Figure S1), from which no reduction peak is observed since almost all of the colored species diffused away from both electrodes and recombined with another colored species in the bulk solution. Eventually, the bleached state of the ECD can be re-obtained as the potential returned to 0 V since all the colored species are consumed. Moreover, no decay in the current density during

surface when subjected to a potential bias. This reaction can be described as follows TMPD ↔ TMPD+ + e−

(2) +•

Simultaneously, the colored VBV moieties would undergo the recombination reaction with TMPD+, converting VBV+• and TMPD+ to colorless VBV2+ and TMPD, respectively. The recombination can be expressed by the following reaction VBV +• + TMPD+ ↔ TMPD + VBV2 +

(3)

Once the applied potential bias was removed, the device would return to colorless since all the colored species were consumed. Figure 1a shows the cyclic voltammograms (CVs) of the thermally cured ECD from the second to the fifth cycle. The voltage is applied to the symmetric cell with two ITO glasses. Initially, the as-prepared ECD is colorless, suggesting that the colorless species (i.e., VBV2+ and TMPD) are thermodynamically stable without potential bias. From the CVs, an onset potential of ca. 0.3 V was noticed, indicating the reduction of VBV2+ to VBV+• moieties and the oxidation of TMPD to 4178


Research Article


Figure 2. Dynamic transmittance curves of the VBV-based ECD collected at (a) 570 and (b) 615 nm. Optical responses at (c) 570 and (d) 615 nm of the VBV-based ECD under the long-term switching test. The ECD was colored and bleached at 0.5 and 0 V, respectively, with an interval of 10 s at both coloring and bleaching processes.

spanned from 500 to 700 nm is noticed. The absorbance peaks located at 570 and 615 nm, suggesting the formation of TMPD+.28,42 In addition, VBV+• moieties also contribute significant light attenuation at 410, 615, and 660 nm,57 giving synergetic light absorption and further enhances the shade of this ECD. Figure 1c presents the bleached and colored states of the ECD on the CIE (Commission Internationale de l’Eclairage) L*a*b* coordinate, which quantitatively describes the colors of the ECDs. The L* dimension represents the lightness while a* and b* dimensions indicate colors from green to red and from blue to yellow, respectively.37 When a potential bias of 0.5 V was applied, blue color is shown in Figure 1d, while the value of b* on the coordinate reached over −50, indicating that its color change is far from black-to-transmissive. When the ECD is short-circuited, the L*a*b* coordinate gradually returned to near the origin while exhibiting an L* value over 88. High optical contrast can also be verified by observing the large variation in L* (ΔL*) of 43.8. In addition to having a large optical contrast, the black-totransmissive property of ECDs has also long been desired. From Figure 1b, a valley in the absorbance spectra at 0.5 V

Table 1. Quantitative EC Switching Performances of the VBV-Based ECD at the 1st and 1000th Cycles cycle number

potential (V)

λ (nm)

τba (s)

τcb (s)

Tbc (%)

Tcd (%)

ΔTe

1 1000 1 1000

0−0.5 0−0.5 0−0.5 0−0.5

570 570 615 615

3.0 3.4 3.1 3.4

3.6 3.8 3.5 3.4

71.4 72.0 71.1 71.7

9.5 9.6 9.0 9.3

61.9 62.4 62.1 62.4

a Bleaching time. bColoring time. cBleached transmittance. dColored transmittance. eTransmittance change.

cycling was observed, which demonstrated the good reversibility of the VBV-based ECD. Figure 1b shows the corresponding spectroelectrochemical measurement done for the ECD. The spectrum of the asprepared ECD is the same as that at 0 V, which again verifies that the bleached state of the ECD is thermodynamically stable. The increase in the absorbance spectra becomes noticeable when the potential bias is higher than 0.3 V; this is in good agreement with the onset potential observed in Figure 1a. As the potential bias of 0.5 V is reached, a strong absorbance 4179


Research Article


Figure 3. (a) Cyclic voltammogram of the PV/VBV-based ECD with the scan rate of 0.1 V/s. (b) Absorbance spectra of the PV/VBV-based ECD under various applied cell voltages. (c) CIE L*a*b* coordinates of the bleached and colored states in the PV/VBV-based ECD. Images of the PV/ VBV-based ECD at both (d) bleached state and colored state.

spanning from ca. 430 to 470 nm is noticed, rendering the VBV-based ECD a bluish hue. Huge deviation from the origin on the CIE L*a*b* coordinate, as shown in Figure 1c, further verifies its lack of black-to-transmissive property. The dynamic transmittance curves of the device, collected at both characteristic absorbance peaks of 570 and 615 nm, are presented respectively in Figure 2a and b. The transmittance change (ΔT) of >60% can be observed at both wavelengths with all the response times of 11 cycles without significant decay peak current density decreased by 18.9% after 1000 cycles ΔT remains of 60.5% after 10 000 cycles

ref 18 56 61 62 63 This work

a

Methyl viologen. bPolystyrene-block-poly(methyl methacrylate)-block-polystyrene. c1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. dFerrocene. ePoly(cyclotriphosphazene-4,4′-bipyridinium)chloride. f1-Ethyl-3-methylimidazolium tetracyanoborate. gPrussian blue. hN,N′Bis(3-sulfonatopropyl)-4,4′-bipyridinium doped poly(3,4-ethylenedioxythiophene). iMixture of poly(vinyl alcohol), 1-butyl-1-methylpyrrolidiniumtrifluoromethanesulfonate in dimethyl sulfoxide. j1,1′-Bis[4-(5,6-dimethyl-1H-benzimidazole-1-yl)butyl]-4,4′-bipyridinium dibromide. kPoly(2hydroxyethyl methacrylate). l1-Ethyl-3-methylimidazolium dicyanamide. mMaximun absorbance wavelength. nBleaching potential (Vb) and coloring potential (Vc).

remarkable transmittance changes of over 60% can be achieved. A long-term cycling test for 10 000 cycles was performed and the results collected at 570 and 615 nm are presented respectively in Figure 4c and d. The performances of the PV/VBV-based ECD are summarized in Table 2, in which the response times of less than 5 s are noticed, suggesting that the incorporation of PV causes no or negligible hindrance for the EC switching. By comparing the performances before and after 10 000 cyclings, the decays in transmittances show only 5.3% and 6.9% (at 570 and 615 nm, respectively) while the response time only slightly increased by less than 1 s. Reasonably stable switching behavior is also noticed at 715 nm (Figure S4), at which the characteristic absorbance peak is attributed to PV.59 The increase in the bleached state transmittance from 1000th to 3000th cycle suggests that some colored species of PV+• exist initially. However, during the long-term cycling, the excessive amount of PV+• could gradually be eliminated due to the drifts in the absolute potentials on both electrodes. Despite the drift in the bleached state transmittance, quite stable behavior is observed after 3000 cycles. Figure S5 provides the spectra of the bleached and colored states of the PV/VBV-based ECD before and after subjecting to long-term cyclings. Only slight deviation of the spectra can be noticed either in the bleached states or in the colored states, further demonstrating that the desired cyclability was realized. These experiment results verify that both PV and VBV aggregations can effectively be eliminated by the thermally cured cross-linked VBV network. Table 3 is a partial list of EC performances of viologen-based ECDs that utilize gel electrolytes.18,56,61−63 The PV/VBV-based ECD in this study gives the most stable cycling stability among all the cited literature while offering very high transmittance change over 60% even after 10 000 switchings. From all the evaluations being performed, the PV/VBV-based ECD simultaneously achieves low-driving voltage, high optical contrast, good cycling stability, and desirable black-to-transmissive properties. The ECDs proposed in this study may suffer from degradation if operated under exceedingly high temperature. In fact, usage like smart windows or magic panoramic roofs are frequently exposed and/or operated at high temperature. In this regard, a cyclability vs temperature study is needed.

VBV-based ECD to obtain absorbance in the entire visible region. After several tests with various concentrations of VBV and PV, an ECD with the best panchromatic feature is obtained when the concentrations of VBV and PV are chosen to be 0.163 and 0.025 M, respectively. Figure 3a presents the CVs of the PV/VBV-based cured ECD. A significant rise in current density was noticed when the applied potential was swept from 0 to 0.5 V, corresponding to the vigorous coloring process of the ECD. A weak reduction peak in the CV suggests that the bleaching reactions on both electrodes can be achieved by the recombination reactions in the bulk solution. Therefore, the reduction peak becomes less significant if a lower scan rate was applied (0.001 V/s, as presented in Figure S3). Reversible redox reactions of all three species, including PV+•, VBV+•, and TMPD+, can be justified under such a narrow potential, as seen in both Figure 3a and its corresponding spectroelectrochemical data presented in Figure 3b. As suggested from Figure 3b, an additional absorption peak located at 430 nm is attributed to the colored state of PV (PV+•),13,59,60 enabling the colored state of the ECD to approach the origin on the CIE L*a*b* coordinate (as shown in Figure 3c). The coordinate in the colored state on the b* axis moved from −50.6 to −19.0 after the addition of a suitable amount of PV. In other words, both values of a* and b* are located within the range of −20.0−20.0 for the PV/VBV-based ECD. Beaujuge et al. synthesized a black-to-transmissive electrochromic polymer through donor− acceptor approach with ΔL* values ranged from 40 to 53 depending on the film thickness.35 In our study, a comparable panchromatic absorption can be achieved by adjusting the concentrations of VBV and PV with the ΔL* value that exceeds 45. The strong light attenuation ability can be visualized by comparing the images of the ECD at the bleached (0 V) and colored (0.5 V) states, as shown in Figure 3d. Shin et al. also successfully achieved panchromatic absorption by combining four different electrochromic materials in a multilayer configuration.37 Nevertheless, the multilayer structure sacrifices the transparency at the bleached state as well as the relatively thicker multilayer composite film required a large potential bias (−2.0 to 2.0 V) to activate. The PV/VBV-based ECD in this study has a comparable black-to-transmissive performance, but with a much smaller driving voltage (0.5 V). The proposed PV/VBV-based ECD was further subjected to repetitive potential switchings between 0 and 0.5 V. The dynamic transmittance curves at both 570 and 615 nm are shown in Figure 4a and b, respectively. In both cases,

■

CONCLUSIONS An all-in-one thermally cured electrochromic device has been fabricated. The substituent of vinyl benzyl group in VBV 4182


Research Article


(6) Niklasson, G. A.; Granqvist, C. G. Electrochromics for Smart Windows: Thin Films of Tungsten Oxide and Nickel Oxide, and Devices Based on These. J. Mater. Chem. 2007, 17, 127−156. (7) Ramadan, R.; Kamal, H.; Hashem, H. M.; Abdel-Hady, K. Gelatin-Based Solid Electrolyte Releasing Li+ for Smart Window Applications. Sol. Energy Mater. Sol. Cells 2014, 127, 147−156. (8) Palenzuela, J.; Viñuales, A.; Odriozola, I.; Cabañero, G.; Grande, H. J.; Ruiz, V. Flexible Viologen Electrochromic Devices with Low Operational Voltages Using Reduced Graphene Oxide Electrodes. ACS Appl. Mater. Interfaces 2014, 6, 14562−14567. (9) Sialvi, M. Z.; Mortimer, R. J.; Wilcox, G. D.; Teridi, A. M.; Varley, T. S.; Wijayantha, K. G. U.; Kirk, C. A. Electrochromic and Colorimetric Properties of Nickel(II) Oxide Thin Films Prepared by Aerosol-Assisted Chemical Vapor Deposition. ACS Appl. Mater. Interfaces 2013, 5, 5675−5682. (10) Chiang, K.-K.; Wu, J.-J. Fuel-Assisted Solution Route to Nanostructured Nickel Oxide Films for Electrochromic Device Application. ACS Appl. Mater. Interfaces 2013, 5, 6502−6507. (11) Xiong, S.; Wei, J.; Jia, P.; Yang, L.; Ma, J.; Lu, X. WaterProcessable Polyaniline with Covalently Bonded Single-Walled Carbon Nanotubes: Enhanced Electrochromic Properties and Impedance Analysis. ACS Appl. Mater. Interfaces 2011, 3, 782−788. (12) Lee, K.; Kim, A. Y.; Kee Lee, J. Fullerene Coated Indium Tin Oxide Counter Electrode of Prussian Blue Electrode for Enhanced Electrochromic Properties. Sol. Energy Mater. Sol. Cells 2015, 139, 44− 50. (13) Kao, S.-Y.; Kawahara, Y.; Nakatsuji, S. i.; Ho, K.-C. Achieving a Large Contrast, low Driving Voltage, and High Stability Electrochromic Device with a Viologen Chromophore. J. Mater. Chem. C 2015, 3, 3266−3272. (14) Sydam, R.; Deepa, M.; Shivaprasad, S. M.; Srivastava, A. K. A WO3−poly(butyl viologen) Layer-By-Layer Film/ruthenium Purple Film Based Electrochromic Device Switching by 1 V Application. Sol. Energy Mater. Sol. Cells 2015, 132, 148−161. (15) Otley, M. T.; Alamer, F. A.; Zhu, Y.; Singhaviranon, A.; Zhang, X.; Li, M.; Kumar, A.; Sotzing, G. A. Acrylated Poly(3,4propylenedioxythiophene) for Enhancement of Lifetime and Optical Properties for Single-Layer Electrochromic Devices. ACS Appl. Mater. Interfaces 2014, 6, 1734−1739. (16) Higuchi, M. Electrochromic Organic-Metallic Hybrid Polymers: Fundamentals and Device Applications. Polym. J. 2009, 41, 511−520. (17) Hwang, E.; Seo, S.; Bak, S.; Lee, H.; Min, M.; Lee, H. An Electrolyte-Free Flexible Electrochromic Device Using Electrostatically Strong Graphene Quantum Dot−Viologen Nanocomposites. Adv. Mater. 2014, 26, 5129−5136. (18) 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. (19) Choi, W.-S.; Kim, M.-K. Effect of Small Liquid Crystal Molecules on The Driving Voltage of Cholesteric Liquid Crystal Displays. Displays 2004, 25, 195−199. (20) Pirš, J.; Ponikvar, D.; Marin, B.; Chambers, M. Low Power Consumption Driving for Liquid Crystal Light Modulators. Displays 2004, 25, 115−121. (21) Barrios, D.; Vergaz, R.; Sánchez-Pena, J. M.; García-Cámara, B.; Granqvist, C. G.; Niklasson, G. A. Simulation of the Thickness Dependence of the Optical Properties of Suspended Particle Devices. Sol. Energy Mater. Sol. Cells 2015, 143, 613−622. (22) Ghosh, A.; Norton, B.; Duffy, A. Measured Overall Heat Transfer Coefficient of a Suspended Particle Device Switchable Glazing. Appl. Energy 2015, 159, 362−369. (23) Bulloch, R. H.; Kerszulis, J. A.; Dyer, A. L.; Reynolds, J. R. Mapping the Broad CMY Subtractive Primary Color Gamut Using a Dual-Active Electrochromic Device. ACS Appl. Mater. Interfaces 2014, 6, 6623−6630. (24) Alamer, F. A.; Otley, M. T.; Ding, Y. J.; Sotzing, G. A. SolidState High-Throughput Screening for Color Tuning of Electrochromic Polymers. Adv. Mater. 2013, 25, 6256−6260.

enables the formation of electrochromic gel without additional cross-linker. The proposed all-in-one ECD can effectively prevent leakage problem while enhancing the cycling stability by avoiding the generation of aggregation. Besides, the bulky substituents of VBV/PMMA chains after in situ thermal curing further reduce the chance of agglomerate formation. With the remarkable color contrast possessed by VBV itself, the VBVbased ECD successfully takes advantage of the dual functionality of VBV molecules, thus offering high ΔT value (>60% at both 570 and 615 nm), low-driving voltage (0.5 V), good cycling stability (negligible decay in ΔT after 1000 continuous cycles), and easier fabrication step. Moreover, the same operating potential windows of VBV and PV enable them to function together and broaden absorbance in the visible region. The PV/VBV-based ECD has successfully offered high ΔT value and good stability (less than 7% decay in ΔT after 10 000 continuous switchings). For all the merits mentioned above, such ECDs possess potential to be applied in the next generation energy-saving devices.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11947. Variations of the transmittance at 570 and 615 nm for the noncured PV/VBV-based ECD, the variation of the transmittance at 715 nm for the cured PV/VBV-based ECD, CVs of both VBV-based and PV/VBV-based ECDs under the scan rate of 0.001 V/s and the spectra of PV/ VBV-based ECD before and after long-term cyclings (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +886-2-2366-0739; Fax: +8862-2362-3040. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank the Ministry of Science and Technology (MOST) of Taiwan under grant number 1042221-E-002−127 -MY2 for the financial support for this research.

■

REFERENCES

(1) Granqvist, C.-G. In Electrochromic Materials and Devices; Mortimer, R. J., Rosseinsky, D. R., Monk, P. M. S., Eds.; WileyVCH Verlag GmbH: Weinheim, 2015; Chapter 1, pp 3−40. (2) Thakur, V. K.; Ding, G. Q.; Ma, J.; Lee, P. S.; Lu, X. H. Hybrid Materials and Polymer Electrolytes for Electrochromic Device Applications. Adv. Mater. 2012, 24, 4071−4096. (3) Mortimer, R. J. Electrochromic Materials. Chem. Soc. Rev. 1997, 26, 147−156. (4) Baetens, R.; Jelle, B. P.; Gustavsen, A. Properties, Requirements and Possibilities of Smart Windows for Dynamic Daylight and Solar Energy Control in Buildings: A State-of-the-Art Review. Sol. Energy Mater. Sol. Cells 2010, 94, 87−105. (5) Jelle, B. P.; Hynd, A.; Gustavsen, A.; Arasteh, D.; Goudey, H.; Hart, R. Fenestration of Today and Tomorrow: A State-of-the-Art Review and Future Research Opportunities. Sol. Energy Mater. Sol. Cells 2012, 96, 1−28. 4183


Research Article

ACS Applied Materials & Interfaces (25) Mortimer, R. J.; Dyer, A. L.; Reynolds, J. R. Electrochromic Organic and Polymeric Materials for Display Applications. Displays 2006, 27, 2−18. (26) Ö sterholm, A. M.; Shen, D. E.; Kerszulis, J. A.; Bulloch, R. H.; Kuepfert, M.; Dyer, A. L.; Reynolds, J. R. Four Shades of Brown: Tuning of Electrochromic Polymer Blends Toward High-Contrast Eyewear. ACS Appl. Mater. Interfaces 2015, 7, 1413−1421. (27) Navarathne, D.; Skene, W. G. Towards Electrochromic Devices Having Visible Color Switching Using Electronic Push−Push and Push−Pull Cinnamaldehyde Derivatives. ACS Appl. Mater. Interfaces 2013, 5, 12646−12653. (28) Leventis, N.; Chen, M. G.; Liapis, A. I.; Johnson, J. W.; Jain, A. J. Characterization of 3 × 3 Matrix Arrays of Solution-Phase Electrochromic Cells. J. Electrochem. Soc. 1998, 145, L55−L58. (29) Andersson Ersman, P.; Kawahara, J.; Berggren, M. Printed Passive Matrix Addressed Electrochromic Displays. Org. Electron. 2013, 14, 3371−3378. (30) Andersson, P.; Forchheimer, R.; Tehrani, P.; Berggren, M. Printable All-Organic Electrochromic Active-Matrix Displays. Adv. Funct. Mater. 2007, 17, 3074−3082. (31) 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. (32) Nakamura, K.; Kanazawa, K.; Kobayashi, N. ElectrochemicallySwitchable Emission and Absorption by Using Luminescent Lanthanide(III) Complex and Electrochromic Molecule Toward Novel Display Device with Dual Emissive and Reflective Mode. Displays 2013, 34, 389−395. (33) Scherer, M.; Muresan, N. M.; Steiner, U.; Reisner, E. RYB TriColour Electrochromism based on a Molecular Cobaloxime. Chem. Commun. 2013, 49, 10453−10455. (34) Ding, G.; Cho, C. M.; Chen, C.; Zhou, D.; Wang, X.; Tan, A. Y. X.; Xu, J.; Lu, X. Black-to-Transmissive Electrochromism of AzuleneBased Donor−Acceptor Copolymers Complemented by Poly(4styrene sulfonic acid)-Doped Poly(3,4-ethylenedioxythiophene). Org. Electron. 2013, 14, 2748−2755. (35) Beaujuge, P. M.; Ellinger, S.; Reynolds, J. R. The DonorAcceptor Approach Allows a Black-to-Transmissive Switching Polymeric Electrochrome. Nat. Mater. 2008, 7, 795−799. (36) Unur, E.; Beaujuge, P. M.; Ellinger, S.; Jung, J. H.; Reynolds, J. R. Black to Transmissive Switching in a Pseudo Three-Electrode Electrochromic Device. Chem. Mater. 2009, 21, 5145−5153. (37) Shin, H.; Kim, Y.; Bhuvana, T.; Lee, J.; Yang, X.; Park, C.; Kim, E. Color Combination of Conductive Polymers for Black Electrochromism. ACS Appl. Mater. Interfaces 2012, 4, 185−191. (38) Lee, K.-R.; Sotzing, G. A. Color Tuning of Black for Electrochromic Polymers Using Precursor Blends. Chem. Commun. 2013, 49, 5192−5194. (39) Vasilyeva, S. V.; Beaujuge, P. M.; Wang, S.; Babiarz, J. E.; Ballarotto, V. W.; Reynolds, J. R. Material Strategies for Black-toTransmissive Window-Type Polymer Electrochromic Devices. ACS Appl. Mater. Interfaces 2011, 3, 1022−1032. (40) Monk, P. M. S. The Viologens: Physicochemical Properties, Synthesis, and Applications of the Salts of 4,4′-bipyridine; John Wiley & Sons: Chichester, 1998. (41) Mortimer, R. J.; Varley, T. S. Novel Color-Reinforcing Electrochromic Device Based on Surface-Confined Ruthenium Purple and Solution-Phase Methyl Viologen. Chem. Mater. 2011, 23, 4077− 4082. (42) Ho, K. C.; Fang, Y. W.; Hsu, Y. C.; Chen, L. C. The Influences of Operating Voltage and Cell Gap on the Performance of a SolutionPhase Electrochromic Device Containing HV and TMPD. Solid State Ionics 2003, 165, 279−287. (43) Jasinski, R. J. The Electrochemistry of Some n-Heptylviologen Salt Solutions. J. Electrochem. Soc. 1977, 124, 637−641. (44) Fletcher, S.; Duff, L.; Barradas, R. G. Nucleation and ChargeTransfer Kinetics at the Viologen/SnO2 Interface in Electrochromic Device Applications. J. Electroanal. Chem. Interfacial Electrochem. 1979, 100, 759−770.

(45) Bewick, A.; Lowe, A. C.; Wederell, C. W. Recrystallisation Process in Viologen-Based Electrochromic Deposits: Voltammetry Coupled with Rapid Time-Resolved Spectroscopy. Electrochim. Acta 1983, 28, 1899−1902. (46) Kim, S. H.; Bae, J. S.; Hwang, S. H.; Gwon, T. S.; Doh, M. K. Dimer Formation of Viologen Derivatives and Their Electrochromic Properties. Dyes Pigm. 1997, 33, 167−172. (47) Monk, P. M. S. Comment on: “Dimer Formation of Viologen Derivatives and Their Electrochromic Properties. Dyes Pigm. 1998, 39, 125−128. (48) 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. (49) Bookbinder, D. C.; Wrighton, M. S. Electrochromic Polymers Covalently Anchored to Electrode Surfaces: Optical and Electrochemical Properties of a Viologen-Based Polymer. J. Electrochem. Soc. 1983, 130, 1080−1087. (50) Campus, F.; Bonhôte, P.; Grätzel, M.; Heinen, S.; Walder, L. Electrochromic Devices Based on Surface-Modified Nanocrystalline TiO2 Thin-Film Electrodes. Sol. Energy Mater. Sol. Cells 1999, 56, 281−297. (51) Gadgil, B.; Damlin, P.; Ä ar̈ italo, T.; Kankare, J.; Kvarnström, C. Electrosynthesis and Characterization of Viologen Cross Linked Thiophene Copolymer. Electrochim. Acta 2013, 97, 378−385. (52) Gadgil, B.; Damlin, P.; Ä ar̈ italo, T.; Kvarnström, C. Electrosynthesis of Viologen Cross-Linked Polythiophene in Ionic Liquid and its Electrochromic Properties. Electrochim. Acta 2014, 133, 268−274. (53) Gadgil, B.; Damlin, P.; Dmitrieva, E.; Aaritalo, T.; Kvarnström, C. ESR/UV-Vis-NIR Spectroelectrochemical Study and Electrochromic Contrast Enhancement of a Polythiophene Derivative Bearing a Pendant Viologen. RSC Adv. 2015, 5, 42242−42249. (54) Gadgil, B.; Damlin, P.; Heinonen, M.; Kvarnström, C. A Facile One Step Electrostatically Driven Electrocodeposition of Polyviologen−Reduced Graphene Oxide Nanocomposite Films for Enhanced Electrochromic Performance. Carbon 2015, 89, 53−62. (55) Sydam, R.; Ghosh, A.; Deepa, M. Enhanced Electrochromic Write-Erase Efficiency of a Device with a Novel Viologen: 1,1′-bis(2(1H-indol-3-yl)ethyl)-4,4]-bipyridinium Diperchlorate. Org. Electron. 2015, 17, 33−43. (56) Kao, S.-Y.; Kung, C.-W.; Chen, H.-W.; Hu, C.-W.; Ho, K.-C. An Electrochromic Device Based on All-in-One Polymer Gel Through InSitu Thermal Polymerization. Sol. Energy Mater. Sol. Cells 2016, 145, 61−68. (57) Ng, S. W.; Neoh, K. G.; Wong, Y. T.; Sampanthar, J. T.; Kang, E. T.; Tan, K. L. Surface Graft Copolymerization of Viologens on Polymeric Substrates. Langmuir 2001, 17, 1766−1772. (58) 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. (59) Mori, H.; Mizuguchi, J. Green Electrochromism in the System of p-Cyanophenylviologen and Potassium Ferrocyanide. Jpn. J. Appl. Phys. 1987, 26, 1356−1360. (60) Kamogawa, H.; Sato, S. Redox Photochromism of Arylviologen Crystals. Bull. Chem. Soc. Jpn. 1991, 64, 321−323. (61) Sydam, R.; Deepa, M. A new Organo-Inorganic Hybrid of Poly(cyclotriphosphazene-4,4[prime or minute]-bipyridinium)chloride with a Large Electrochromic Contrast. J. Mater. Chem. C 2013, 1, 7930−7940. (62) Bhandari, S.; Deepa, M.; Pahal, S.; Joshi, A. G.; Srivastava, A. K.; Kant, R. A Dual Electrochrome of Poly-(3,4-Ethylenedioxythiophene) Doped by N,N′-Bis(3-sulfonatopropyl)-4−4′-bipyridiniumRedox Chemistry and Electrochromism in Flexible Devices. ChemSusChem 2010, 3, 97−105. (63) Sydam, R.; Deepa, M.; Joshi, A. G. A Novel 1,1′-bis[4-(5,6dimethyl-1H-benzimidazole-1-yl)butyl]-4,4′-bipyridinium Dibromide (Viologen) for a High Contrast Electrochromic Device. Org. Electron. 2013, 14, 1027−1036.

4184


Thermally Cured Dual Functional Viologen-Based All-in-One

Recommend Documents