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Solar Energy Materials & Solar Cells

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Highly robust and flexible WO3·2H2O/PEDOT films for improvedelectrochromic performance in near-infrared region

Huiying Qua, Xiang Zhangb, Hangchuan Zhangb, Yanlong Tianb, Na Lia, Haiming Lvb,Shuai Houc, Xingang Lid, Jiupeng Zhaoa,⁎, Yao Lib,⁎

a MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, HarbinInstitute of Technology, Harbin 150001, Chinab Center for Composite Materials, Harbin Institute of Technology, Harbin 150001, Chinac School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, Chinad Guizhou Construction Science Research & Design Institute Limited Company of China State Construction Engineering Corporation (CSCEC), Guiyang550000, China

A R T I C L E I N F O

Keywords:FlexibleWO3·2H2O/PEDOTElectrochromicsNear-infrared region

A B S T R A C T

Innovative WO3·2H2O/PEDOT films with strong adhesion of the electrochromic layer and the substrate werefabricated. A WO3·2H2O/PEDOT electrochromic layer can be firmly fixed to an ethylene vinyl acetate/polyethylene terephthalate (EVA/PET) electrode using Ag as the conductive layer by a transient heating method.The electrochromic film is mechanically robust and can be twisted, folded and crumpled without performancedegeneration. Improved performance has been realized by WO3·2H2O/PEDOT films compared with WO3·2H2Ofilms due to the excellent electrical conductivity of PEDOT, including a higher of Li-ion diffusion coefficient(1.1×10−9 cm2 s−1), faster response time (2.6 s for bleaching time and 4.4 s for coloration time), higher colorefficiency (180.2 cm2 C−1) and larger transmittance modulation (63.1%) at 956 nm. The results are verypromising for the next generation of deformable or implantable hidden message displays and wearable smartclothes applications.

1. Introduction

Electrochromism is defined as a reversible change of opticalproperties under a low-voltage signal [1]. The electrochromic (EC)process is the modulation of transmittance, reflectance and absorp-tance of a material changed through an electrochemical reaction.Several kinds of materials, such as transition metal oxides, organicmolecules, and conducting polymers have been reported to show ECproperties. Among them, transition metal oxides such as WO3, NiO,TiO2, and MoO3 are desired EC materials due to their good cyclic andenvironmental stability [2–6]. They have been extensively studiedbecause of their wide applications in information displays, rear-viewmirrors and smart windows [7–10]. In addition, since water isbeneficial for the insertion/extraction of Li+, hydrated oxides such asWO3·2H2O have attracted much attention recently [11–13]. WO3·2H2O possesses a layered structure, where interlayer water results inan increase of the distance between adjacent layers [14], facilitatinginsertion/extraction of Li+ during the EC process [15]. Therefore, WO3·2H2O has become a promising candidate for EC applications.

However, to date, the functionality of general EC materials has been

limited to light modulation at non-flexible substrates, like on glass [7–10]. More diversified functionalities, such as flexibility, have to bedeveloped to meet the requirements of future emerging applications,such as potentially flexible hidden message displays and wearablesmart clothes applications [16,17]. WO3 and its hydrated tungstenoxides are the most thoroughly studied EC species [18,19] and remainthe most commercially successful EC technology [10] due to their goodcyclic stability and high coloration efficiency (CE). However, they maycrack upon bending and their EC performance may degrade.Conducting polymers are considered more suitable for bendableapplications due to their intrinsically soft molecular chains [20,21].Flexible EC device fabrication based on EC conducting polymers isrelatively mature due to the solubility of the conducting polymers incommon organic solvents. Furthermore, by incorporating EC conduct-ing polymers into transition metal oxides, redox reactions within thetransition metal oxides are promoted due to the excellent electricalconductivity of the conducting polymers, resulting in lower powerconsumption. Thus, it would be of interest to design flexible films thatincorporate EC conducting polymers into WO3·2H2O in order to get ECfilms with superior EC performance. Among all the EC conducting

http://dx.doi.org/10.1016/j.solmat.2016.12.030Received 15 July 2016; Received in revised form 21 October 2016; Accepted 17 December 2016

⁎ Corresponding author.E-mail addresses: jpzhao@hit.edu.cn (J. Zhao), yaoli@hit.edu.cn (Y. Li).

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Available online 14 January 20170927-0248/ © 2017 Elsevier B.V. All rights reserved.

MARK

polymers, poly(3,4-ethylenedioxythiophene) (PEDOT), which is knownfor its high optical contrast and remarkable CE, has been regarded asone of the best EC conducting polymers available in terms ofconductivity and stability [22]. PEDOT, as WO3·2H2O, is a cathodicEC material, changing from a transparent sky-blue color in the oxidizedstate to deep-blue in the reduced state [23]. Therefore, when a voltageis applied to WO3·2H2O and PEDOT, they exhibit a variation betweencolored and bleached states simultaneously, thus increasing the opticalmodulation of the EC film. As reported by Xuehong Lu et al., CE issignificantly improved due to the efficient charge transfer between WO3

and PEDOT:PSS and their complementary electrical conductivity in theredox process [24]. Besides, electrode conducting materials encountersevere challenges as well. One of the main obstacles for the flexible ECfilms is the replacement of indium tin oxide (ITO) as the transparentelectrode conducting layer [25,26]. Recently, silver has become apromising candidate due to its excellent electrical, thermal, and opticalproperties [27–29]. Lei Huang et al. fabricated the flexible NO2 gassensor by printing the WO3-PEDOT:PSS sensing layer and the Aginterdigitated electrode on polyimide substrate using gravure techni-que [30]. However, so far, the conductive layer of most works wasdeposited on the surface of the substrate directly [24,30,31], whichmay hinder the conductivity and stability of the devices because of theweak adhesion between the conductive layer and the substrate.

In this study, we design and fabricate highly robust and flexibleWO3·2H2O/PEDOT EC films for the first time. Utilizing ethylene vinylacetate/polyethylene terephthalate (EVA/PET) as the substrate and Agas the conductive layer, WO3·2H2O/PEDOT EC layers are coated on theAg/EVA/PET electrodes by dip-coating. In this work transparent EVAis employed as the adhesive layer between the WO3·2H2O/PEDOT/Aglayer and the flexible PET substrate. The WO3·2H2O/PEDOT/Ag layerexhibits strong adhesion to the substrate due to the high adhesive forceof the melted EVA. PEDOT:PSS is incorporated to the WO3·2H2Onanoplates to guarantee the high electrical conductivity of the EC layer.The flexible WO3·2H2O/PEDOT films show excellent performance,with a high Li-ion diffusion coefficient (1.1×10−9 cm2 s−1), fast re-sponse time (2.6 s for bleaching time and 4.4 s for coloration time),high CE (180.2 cm2 C−1) and large transmittance modulation (63.1%)at 956 nm. The results presented here offer a versatile technique forfabricating flexible inorganic/organic films with excellent electrochro-mic performance.

2. Experimental method

2.1. Preparation of WO3·2H2O/PEDOT solutions

Preparation of the WO3·2H2O nanoplates was carried out asfollows: 0.5 mL of H2O2 (Tianli Chemical Reagent Co., Ltd.) was addedto 80 mL of 0.125 mol L−1 Na2WO4 (Bodi Chemical Co., Ltd.), to which1 mol L−1 H2SO4 (Tianli Chemical Reagent Co., Ltd.) was further addedto obtain a total volume of 100 mL, and the mixture was well stirred.The solution remained resting until the yellow precipitate WO3·2H2Onanoplates appeared at the bottom of the solution. The as-preparedWO3·2H2O nanoplates were subjected to three centrifuge-wash cyclesto remove excess ions. Centrifugation was conducted at 8000 rpm for10 min, and deionized water was used for washing in each cycle. Thewashed WO3·2H2O nanoplates were dispersed in deionized water tomake the WO3·2H2O solution with a mass fraction of 20%. WO3·2H2O/PEDOT solutions with different concentration ratios were prepared bymixing a WO3·2H2O solution with a Heraeus Clevios™ PH1000PEDOT:PSS solution. The mass fractions of the commercialPEDOT:PSS solution in the WO3·2H2O solution were 0%, 10% and20%.

2.2. Fabrication of flexible EC films

The EVA/PET substrates were consecutively cleaned in ethanol and

deionized water for 10 min in an ultrasonic bath. An Ag conductivelayer was deposited on the EVA/PET substrate by reactive DCmagnetron sputtering. During deposition, the total pressure wasmaintained at ~30 mTorr, the power to the target was 18 W and thesputtering time was 15 s to obtain the Ag/EVA/PET electrode (sheetresistance of ∼10 Ω sq–1). The electrodes were then dipped into theWO3·2H2O/PEDOT solutions and were withdrawn vertically at the rateof 500 µm s−1 at room temperature. This was followed by drying thesample at room temperature for 5 min. This dipping and dryingprocess was repeated to obtain WO3·2H2O/PEDOT layers with thick-ness of about 2 µm, followed by transient heating at 100 °C by tworollers, providing heat and mechanical pressure simultaneously to meltEVA in order to make the WO3·2H2O/PEDOT/Ag layers adhere firmlyto the PET substrates. Films dip-coated in three WO3·2H2O/PEDOTsolutions with different concentration ratios of PEDOT:PSS wereinvestigated. The mass fractions of the commercial PEDOT:PSS solu-tion in the WO3·2H2O solution is 0%, 10% and 20%, and the films dip-coated in them are denoted as WO3·2H2O/0% PEDOT, WO3·2H2O/10% PEDOT and WO3·2H2O/20% PEDOT, respectively.

2.3. Characterization

Scanning electron microscopy (SEM) images were obtained byusing FEI Helios Nanolab600i. Transmission electron microscopy(TEM) images, high-resolution transmission electron microscopy(HRTEM) images and selected-area electron diffraction (SAED) pat-terns were recorded by using TEM (JEOL JEM-2010). The crystallinestructure of WO3·2H2O was investigated by X-ray diffraction (XRD,Rigaku D/max-rB, Cu Kα radiation, λ=0.1542 nm, 40 kV, 100 mA).Fourier transform infrared spectra (FT-IR) were obtained with aFourier transform infrared spectrometer (Nicolet iS10, ThermoNicolet, America).

In situ visible and near infrared (NIR) EC measurements wereperformed using an experimental setup produced in-house [32] incombination with a CHI660D electrochemical workstation (ShanghaiChenhua Instrument Co. Ltd.). The experimental setup was sealed inan Argon-filled glove box (Vigor Glove Box from Suzhou, China) beforetesting. One side of the setup was connected to a white lamp (DT-mini-2-GS, Ocean Optics) by an optical fiber; the other side was connected toan optic spectrometer (MAYA 2000-Pro, Ocean Optics). The WO3·2H2O/PEDOT film (or the WO3·2H2O film), the Pt sheet, and the Agwire were used as the working electrode, counter electrode andreference electrode, respectively. 1 mol L−1 LiClO4 in propylene carbo-nate was used as the electrolyte when testing the electrochemical andEC performance of the films. The transmittance of the Ag/EVA/PET inthe electrolyte was used as a reference for 100% transmittance. Beforemeasuring the EC performance of the WO3·2H2O/PEDOT films, theywere subjected to three cyclic voltammetry (CV) cycles to ensurestability. CV measurements were performed at room temperaturebetween +1 and −1 V at a scan rate of 5 mV s−1.Chronoamperometric measurements were done by applying +1 V for1000 s to de-intercalate all lithium ions present, then setting thepotential back to −1 V for 20 s followed by +1 V for 20 s, and thecurrent was monitored as a function of time.

3. Results and discussion

3.1. Synthesis and characterization

The growth of nanocrystal shapes is determined by the inherentcrystal structure during the nucleation and the growth stages [33]. Theformation of the WO3·2H2O nanoplates can be divided into three steps.Fig. 1a–c shows the solution change during the preparation process ofthe WO3·2H2O nanoplates. At the beginning of the synthesis process, aclear light yellow solution was formed, containing [WO4]

2− and H+,which could stay transparent for about 5 min after H2O2 was added, as

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shown in Fig. 1a. As the reaction proceeded, the transparent solutiongradually turned into the sol state [34] and WO3·2H2O nanoparticleswith a diameter of about 50 nm were formed, as shown in Fig. 1b andFig. S1 in the ESI. During this stage, acidification of [WO4]

2− occursand [H2WO4]

0 was formed, which would coordinate with two watermolecules to form the six fold coordinated WVI, as shown schematicallyin Fig. 1d. As the preferred coordination of WVI is known to be mono-oxo, the neutral precursor would be [WO(OH)4(OH2)]

0, as shown inFig. 1e. Oxolation occurs along equivalent x and y directions, leading tothe formation of WO3·2H2O nanoparticles, which have the preferentialgrowing direction in the xy plane (Fig. 1f). At this stage, thenanoparticles formed had uniform morphology and size due to theaddition of H2O2, which can slow down the reaction rate. However, thewashed WO3·2H2O nanoparticles may precipitate gradually when they

are dispersed in deionized water (as shown in the inset of Fig. 1b)because of nanoparticle aggregation, as shown in Fig. S2. Last, as thereaction time prolonged, the sol turned into separated layers, includingan upper clear liquid and yellow WO3·2H2O nanoplate precipitates atthe bottom of the solution (Fig. 1c). Fig. 1g and h shows the SEM andTEM images of the as-prepared WO3·2H2O nanoplates, which have anaverage length of 200 nm and a thickness of 50 nm. WO3·2H2Onanoparticles tend to form nanoplates due to their intrinsic lamellarstructure caused by preferential growth in the direction of the xy plane.The washed WO3·2H2O nanoplates can be dispersed uniformly andstably in deionized water for several days when the mass fraction of thesolution is under 20%, as shown in the inset of Fig. 1(c).

Fig. 2a shows the XRD pattern of the WO3·2H2O nanoplates. Thediffraction peaks indicate a monoclinic crystal structure, which

Fig. 1. Digital photos showing the solution changes during the preparation process of WO3·2H2O nanoplates (a) transparent solution, (b) sol state, (c) layered state; (d, e, f) formationsteps of WO3·2H2O nanoplates; (g) SEM image of WO3·2H2O nanoplates; (h) TEM image of WO3·2H2O nanoplates. (For interpretation of the references to color in this figure, the readeris referred to the web version of this article.)

Fig. 2. (a) XRD pattern of WO3·2H2O nanoplates; (b) HRTEM image of WO3·2H2O nanoplates and corresponding SAED pattern.

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matches JCPDS card 18-1420. The data obtained show three majorpeaks at 12.7°, 25.7° and 39.1°, which are indexed to the (010), (020),and (030) planes of WO3·2H2O, showing a highly (010) preferredorientation [11]. The structure of the nanoplates was further investi-gated using HRTEM. As shown in Fig. 2b, the lattice fringes display alattice spacing of 0.35 nm, corresponding to the (020) planes of themonoclinic WO3·2H2O. The SAED pattern in Fig. 2b reveals that theWO3·2H2O nanoplates are of monocrystalline structure in nature.

The SEM and TEM images of the WO3·2H2O/20% PEDOT film aredisplayed in Fig. S3. After mixing with PEDOT:PSS, WO3·2H2O keepsthe shape of a nanoplate and there is no obvious change in size.PEDOT:PSS cannot be observed in the SEM or TEM images of theWO3·2H2O/20% PEDOT film because of its low concentration in theWO3·2H2O/PEDOT solution. However, it can be detected by FT-IR.Fig. S4 shows FT-IR spectra of WO3·2H2O/0% PEDOT and WO3·2H2O/20% PEDOT films. The characteristic absorption of the ethyle-nedioxy group vibration, specifically the C-O-C stretching modes, canbe observed at around 1080 cm−1, which is typically used to identifyPEDOT. For PSS, the S=O vibration appears at 1180 cm−1 [35],indicating that PEDOT:PSS is successfully incorporated.

A schematic illustration of the structure of the flexible WO3·2H2O/PEDOT films and a digital photograph of the flexible WO3·2H2O/20%PEDOT film are shown in Fig. 3a and b. According to papers reportingthe relationship of resistance and optical properties as the function ofthe Ag film thickness [36,37], as well as the resistance (∼10 Ω sq–1)and transmittance (shown in Fig. S5) of our Ag layer, the thickness ofour Ag layer may be around 4–6 nm. Unlike most of the previous work,where the conductive layer was deposited on the surface of thesubstrate [31], in this work EVA was used as the adhesive. It wasmelted at 100 °C to partially embed the Ag conductive layer on thesurface and to firmly adhere the WO3·2H2O/PEDOT/Ag layer to thePET substrate, as shown in Fig. S6a–c. This process guarantees goodcontact and high adhesive force between the WO3·2H2O/PEDOT EClayer and the substrate, forming a WO3·2H2O/PEDOT/Ag/EVA/PETlaminated structure (Fig. S6d). The unique laminated structure ex-hibited excellent adhesion, significantly improving durability against

mechanical scratching and was free of delamination and peeling offupon repeated bending, which is good to the cyclic stability of the film(Fig. S7). The top-view and cross-section SEM images of the WO3·2H2O/20% PEDOT film after folding, crumpling and scratching areshown in Fig. 3c and d. It can be seen that the film still adhered firmlyto the PET substrate by the EVA adhesive, indicating a remarkablemechanical flexibility.

3.2. Electrochemical and electrochromic performance

The electrochemical characteristics are the most important proper-ties of EC materials. CV measurements were performed to test theelectrochemical behavior of the flexible WO3·2H2O/PEDOT films.Fig. 4 shows the CV curves of WO3·2H2O/0% PEDOT, WO3·2H2O/10% PEDOT and WO3·2H2O/20% PEDOT films at various scan ratesbetween 1 V and −1 V. The reduction peaks of all three kinds of filmsshift to lower potentials, while the oxidation peaks move to higherpotentials with an increase in the scan rate. As the concentration ofPEDOT:PSS increased, both the reduction peak and the oxidation peakof PEDOT became obvious, appearing at −0.441 V and 0.058 V at thescan rate of 0.02 V s−1, as shown in Fig. 4d. The films with higherconcentration of PEDOT:PSS exhibited larger current densities andintercalated area due to the good conductivity of the films, making theEC reaction more effective. However, compared with the WO3·2H2O/0% PEDOT film, the position of the oxidation peak of WO3·2H2O inWO3·2H2O/10% PEDOT and WO3·2H2O/20% PEDOT films did notchange significantly.

To further illustrate the benefits of the incorporation of PEDOT:PSSto WO3·2H2O, the Li-ion diffusion coefficients of the WO3·2H2O/0%PEDOT, WO3·2H2O/10% PEDOT and WO3·2H2O/20% PEDOT filmswere calculated. For a simple solid state diffusion controlled process,the effective diffusion coefficient can be estimated from Randles–Sevcik formula,

I zFA zF RT C D v= 0.4463 ( / )p1/2

01/2 1/2

(1)

Fig. 3. (a) Schematic illustration of the structure for the flexible WO3·2H2O/PEDOT films, (b) digital photograph of the as-fabricated flexible WO3·2H2O/20% PEDOT film, (c) top-viewSEM image of the sample WO3·2H2O/20% PEDOT film, (d) the section-view SEM image of the sample WO3·2H2O/20% PEDOT film after folding, crumpling and scratching.

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where Ip is the peak current, z is the number of electrons transferred ina unit reaction (for WO3·2H2O, z is generally taken as 1), F is theFaraday constant, R is the ideal gas constant, T is the experimentaltemperature, A is the effective surface area of the electrode, C0 is theion surface concentration, D is the effective diffusion coefficient and v isthe potential scan rate.

Fig. 5 shows the correlation between the peak current Ip and thesquare root of the scan rate v1/2 for the oxidation peaks of WO3·2H2O.The good linear relationship demonstrates that the oxidation process ofWO3·2H2O is controlled by ion diffusion from the electrolyte to theelectrode surface. As calculated according to Eq. (1), the Li-iondiffusion coefficients of WO3·2H2O/0% PEDOT, WO3·2H2O/10%PEDOT and WO3·2H2O/20% PEDOT films are 2.2×10−10 cm2 s−1,

6.5×10−10 cm2 s−1 and 1.1×10−9 cm2 s−1, respectively, as listed inTable 1. The Li-ion diffusion coefficient becomes larger when theconcentration of PEDOT:PSS increases, indicating that redox reactionswithin WO3·2H2O are promoted due to the excellent electrical con-ductivity of PEDOT.

In situ NIR transmittance measurements were used to investigatethe EC performance of the WO3·2H2O/PEDOT films. Under potentialsof +1 V and −1 V, all the films exhibited obvious transmittance andcolor changes. Among the three films, the WO3·2H2O/10% PEDOT filmhas the maximum transmittance modulation of 83.3% at 956 nm, asshown in Fig. 6a and c. This value is relatively high in the NIR region,making it an excellent candidate for potential applications using spacethermal control. In contrast, the transmittance modulation values ofWO3·2H2O/0% PEDOT and WO3·2H2O/20% PEDOT films obtained atthe wavelength of 956 nm were 41.7% and 63.1%, respectively, asshown in Fig. 6b and d and listed in Table 1. Optical modulationwithout subtraction of Ag/EVA/PET is presented in Fig. S8. We alsomeasured their optical modulation under bending configuration. Thereis no big difference between flat films and bending films at thewavelength of 956 nm, as shown in Fig. S9. The response times ofWO3·2H2O/0% PEDOT, WO3·2H2O/10% PEDOT and WO3·2H2O/20%PEDOT films, defined as the time required for a 90% change in the fulltransmittance modulation, are listed in Table 1. Parameters tb and tcdenote the bleaching time and coloration time, respectively. The filmsincorporating PEDOT:PSS exhibited shorter response time comparedto the WO3·2H2O/0% PEDOT film under an alternating potentialbetween +1 V and −1 V.

CE is another important parameter for EC materials, which isdefined as the change in optical density (OD) per unit of insertedcharge. It can be calculated as

CE OD Q log T T Q= / = ( / )/b c (2)

Fig. 4. CV curves of (a) the WO3·2H2O/0% PEDOT film, (b) the WO3·2H2O/10% PEDOT film, (c) the WO3·2H2O/20% PEDOT film at various scan rates between 1 V to −1 V; (d) CVcurves of WO3·2H2O/0% PEDOT, WO3·2H2O/10% PEDOT and WO3·2H2O/20% PEDOT films at the scan rate of 0.02 V s−1.

Fig. 5. Fitting plots between the peak current Ip and the square root of the scan rate v1/2.

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where Q is the charge intercalated, Tb and Tc represent the transmit-tance of the bleached and colored samples, respectively. A high CEgives a large optical modulation at small charge insertion or extraction,which can desirably induce long-term cycling stability. Fig. 7a showsthe chronoamperometric curves of WO3·2H2O/PEDOT films in thepotential region of +1 V to −1 V. Fig. 7b–d presents the relation of ODwith charge density. The CEs of the WO3·2H2O/PEDOT films can becalculated from the slope of the linear region of these curves. Theresults obtained were 73.3 cm2 C−1, 163.2 cm2 C−1 and 180.2 cm2 C−1

for WO3·2H2O/0% PEDOT, WO3·2H2O/10% PEDOT and WO3·2H2O/20% PEDOT films, respectively, as listed in Table 1. It can be seen thatthe CEs of the WO3·2H2O/10% PEDOT and WO3·2H2O/20% PEDOTfilms are much larger than that of the WO3·2H2O/0% PEDOT film.These results suggest the improvement in charge utilization byincorporating PEDOT:PSS, as it is a cathodic EC material withrelatively high CE. The enhanced transmittance modulation, shortresponse time and excellent CE of the WO3·2H2O/20% PEDOT film areimpressive values when compared to those of many previously reportedWO3 or WO3-based films, as listed in Table 2.

4. Conclusions

Highly robust and flexible WO3·2H2O/PEDOT EC films have been

prepared by a dip-coating method. The EC layers of WO3·2H2O/PEDOT films adhere firmly to the flexible PET substrate due to thehigh adhesive force of the melted EVA. The films are mechanicallyrobust and can be twisted, folded and crumpled without performancedegeneration. The electrochemical and electrochromic performance ofthe WO3·2H2O/PEDOT films with various concentrations ofPEDOT:PSS were investigated. Improved EC performances wererealized due to the incorporation of PEDOT:PSS. The influence of theconducting polymer PEDOT on the Li-ion diffusion behavior and ECperformances was also discussed. Higher concentration of PEDOT:PSSled to better electrical conductivity of the WO3·2H2O/PEDOT films,resulting in the promotion of redox reactions within WO3·2H2O. As aresult, the WO3·2H2O/20% PEDOT film showed a larger intercalatedarea of the CV curve, higher Li-ion diffusion coefficient, faster responsetime and higher color efficiency. Unlike most existing electrochromictechnologies based on rigid substrates, the innovative WO3·2H2O/PEDOT films in their soft form can be broadly used in deformable orimplantable hidden message displays and wearable smart clothesapplications in the future.

Acknowledgements

Huiying Qu and Xiang Zhang contributed equally to this work. We

Table 1EC performances of the WO3·2H2O/PEDOT films.

Sample Li-ion diffusion coefficient (cm2 s−1) Transmittance modulation (%) tb (s) tc (s) CE (cm2 C−1)

WO3·2H2O/0% PEDOT 2.2×10−10 41.7 8.6 3.8 73.3WO3·2H2O/10% PEDOT 6.5×10−10 83.8 3.9 4.7 163.2WO3·2H2O/20% PEDOT 1.1×10−9 63.1 2.6 4.4 180.2

Fig. 6. (a) Transmittance modulation of bleached (+1 V) and colored (−1 V) states of the WO3·2H2O/10% PEDOT film; change in optical transmittance versus time of (b) the WO3·2H2O/0% PEDOT film, (c) the WO3·2H2O/10% PEDOT film, (d) the WO3·2H2O/20% PEDOT film obtained by applying a pulsed potential of ± 11 V for 20 s for each state at 956 nm.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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thank National Natural Science Foundation of China (Nos. 51572058,91216123, 51174063, 51502057), the Natural Science Foundation ofHeilongjiang Province (E201436), the International Science &Technology Cooperation Program of China (2013DFR10630,2015DFE52770) and Specialized Research Fund for the DoctoralProgram of Higher Education (SRFDP 20132302110031).

Appendix A. Supplementary material

Supplementary material associated with this article can be found inthe online version at doi:10.1016/j.solmat.2016.12.030.

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Fig. 7. (a) Chronoamperometric curves of the WO3·2H2O/PEDOT films in the potential region of +1 V to −1 V; plots of OD as a function of charge density monitored at the wavelengthof 956 nm for (b) the WO3·2H2O/0% PEDOT film, (c) the WO3·2H2O/10% PEDOT film and (d) the WO3·2H2O/20% PEDOT film.

Table 2Comparison of electrochromic performance of the reported WO3-based films and the present work.

Electrochromic materials Transmittance tb (s) tc (s) CE (cm2 C−1) Wavelength (nm) Ref.modulation (%)

TiO2-WO3 wire 72.8 / / 67.41 950 [38]WO3/PANI 34.9 0.8 0.2 40.6 633 [39]WO3 nanorod 62 5.8 9.6 98.3 632.8 [40]WO3/rGO 64.2 4.5 9.5 633 [41]WO3 / 4 1 12.6 350 [16]Tungsten oxide nanoparticles 26 1.7 2.0 27 700 [12]WO3/Ag/WO3 53 / / 136 650 [42]WO3·2H2O ultrathin nanosheets 41.7 9.7 5.1 120.9 700 [11]WO3 nanorolls 33 8 9 67 700 [43]WO3·2H2O/20% PEDOT 63.1 2.6 4.4 180.2 956 This work

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