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Electrochimica Acta 54 (2008) 598–605

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

lexible electrochromic device based on poly3,4-(2,2-dimethylpropylenedioxy)thiophene)

hao Ma, Minoru Taya, Chunye Xu ∗

enter of Intelligent Materials and Systems, University of Washington, Box 352600, Seattle, WA 98195, United States

r t i c l e i n f o

rticle history:eceived 25 April 2008eceived in revised form 8 July 2008

a b s t r a c t

In this study, the design, fabrication and characterization of a flexible electrochromic device based onindium tin oxide (ITO) coated polyethylene terephthalate (PET) plastic is discussed. The working elec-trochromic material film was poly (3,4-(2,2-dimethylpropylenedioxy)thiophene) (PProDOT-Me2), while

ccepted 9 July 2008vailable online 30 July 2008

eywords:lectrochromiclexible device

the counter layer of the device was vanadium oxide titanium oxide (V2O5/TiO2) composite film, whichserves as an ion storage layer. A solution type electrolyte was used as the ionic transport layer and wassandwiched between the working and counter layers. The device exhibited tuneable light transmittancebetween transparent and deep blue color, with a maximum contrast ratio at 580 nm wavelength. Otherimportant properties, such as switching speed, life time, and coloration efficiency have been improved.

siardorm(tpawewdtbtc

onducting polymeroloration efficiency

. Introduction

Electrochromic (EC) materials can change their optical prop-rties reversibly for an applied potential due to electrochemicalxidation and reduction. Electrochromic devices (ECD) which areble to actively control light transmittance/absorbance could besed in the field of architecture, vehicle, aircraft and displays1–3]. Fig. 1 shows a hard glass based electrochromic device30 cm × 30 cm) developed in our lab. With an applied potential,he optical states of the ECD can be changed between blue colornd transparency [4].

Generally speaking, the electrochromic device has a multi-ayered structure as shown in Fig. 2. It has two electrodes, one

orking, one counter, and one ion conductive layer in between.ormally the working performs the color change during operationf the device.

Electrochromic materials, first introduced by Deb [5] in 1969,nclude inorganic oxide, such as tungsten trioxide (WO3) and irid-um dioxide (IrO2). Recently electrochromic organic polymers, suchs pyrrole, thiophene and their derivatives have been developed1,2,6–8]. In the early stage of electrochromic materials research,

ost of the attention was focused on inorganic oxides. How-ver, these transition metal oxide electrochromics exhibited a slowesponse time (tens of seconds) and a high processing cost. Com-ared with these materials, electrochromic polymers have shown

∗ Corresponding author. Tel.: +1 206 685 7920; fax: +1 206 685 7920.E-mail address: Chunye@u.washington.edu (C. Xu).

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013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2008.07.049

© 2008 Elsevier Ltd. All rights reserved.

everal unique merits: they require power only during switch-ng, have a low operation voltage and energy consumption, show

quick response time; possess long open circuit memory, greatepeatability, rich color availability; and large scale and flexibleevices are easily fabricated. Recently, our laboratory has devel-ped a series of new electrochromic polymer materials with blue,ed or green color. In this study, a cathodic blue color EC poly-er material, poly (3,4-(2,2-dimethylpropylenedioxy)thiophene)

PProDOT-Me2, structure shown in Fig. 3) was utilized to fabricatehe flexible ECD. PProDOT-Me2 is a thiophene based conductingolymer. As shown in Scheme 1, in the neutral state it has strongbsorption around 580 nm wavelength, so that the polymer filmill show up deep blue color. When it is oxidized and P-doped, the

nergy band gap will become smaller and lower energy transitionill arise near 580 nm wavelength. Therefore, the absorption willecrease and cause the polymer film to become transparent. Reduc-ion of the oxidized EC polymer will cause the polymer to switchack to the neutral state; returning the energy band gap and absorp-ion back to the colored state, so that the polymer is cathodicallyolored.

Recently, the research attention of ECD has been focused onexible substrates [9–14], because of their advantages over glassubstrates and the potential application in the field of flexibleaper-like displays. In Table 1, the differences between plastic

nd glass substrates are summarized. A plastic substrate has lesseight and volume, is more flexible, and easier to pattern. The

nly problem is that it is more difficult to get a high qualityTO coating on a plastic substrate [15]. The inorganic oxide EC

aterials have a longer history than the organic EC materials.

C. Ma et al. / Electrochimica Acta 54 (2008) 598–605 599

Fig. 1. Glass based ECD in transparent and colored states.

IbiatpaR[abop

2

2

iscpfipa

L

Fig. 3. Structure of PProDOT-Me2.

iata

2

mc

Tc

GP

Fig. 2. Multi-layers structure of ECD.

n the early stage of flexible ECD development, the devices wereased on WO3, as reported by Antinucci in 1995 and Yoshimura

n 2007 [16,17]. However, the device had a low switch speed andhigh operation potential. In 2003, Argun [18] reported the first

ruly all-polymer ECD, based on poly(3,4-ethylenedioxythiophene)oly(styrenesulfonate) (PEDOT-PSS), and Huang et al. [19] reportedflexible ECD based on polyaniline (PANI) and PEDOT-PSS in 2006.ecently reported flexible ECD was developed by Kobayashi et al.20] in 2007, which was based on phthalate derivative and exhibitedreddish purple color. The flexible ECD developed in this study wasased on EC polymer material that is different from those reportednes and performed improved electrochromic properties. The com-arison will be shown in the following discussion paragraph.

. Experimental

.1. Materials and reagents

ProDOT-Me2 monomer was synthesized in our lab via anmproved route. The method was reported by Xu et al. [2,21]. Alltarting materials were purchased from Aldrich, except lithium per-hlorate (LiClO4, 99% anhydrous, packed under argon), which wasurchased from Alfa Aesar. Because the electrochromic polymer

lm is sensitive to moisture and oxygen, which could affect theerformance of EC devices, all the materials were dried before usend stored in glove box filled with argon.

A solution of electrolyte was prepared by dissolving 0.1 M ofiClO4 in propylene carbonate (PC). This solution electrolyte was

aorat

able 1omparison of glass and plastic substrate

Density (g/cm3) Thickness Flexibility Eas

lass ∼3 ∼1 mm No Nolastic ∼1.4 ∼100 �m Yes Yes

Scheme 1. The redox action of PProDOT-Me2.

njected into the electrochromic device later using syringe. Specialttention must be paid to avoid any contamination from mois-ure and oxygen. The solution electrolyte needs to be bubbled withrgon gas before use, and all the containers should be dried in oven.

.2. Working and counter electrode

The PProDOT-Me2 polymer film was deposited from 0.01 Monomer in a 0.1 M LiClO4/acetonitrile (ACN) solution. Electro-

hemical deposition method was carried out on an electrochemical

nalyzer (CHI 605 A, CH Instruments), utilizing the chronoamper-metry method [22,23]. A three-electrode cell with Ag wire as aeference, ITO/PET as a working electrode and a stainless steel plates a counter electrode was used for electrochemical polymerizinghe polymer film.

y patterning Mechanical stability Cost ITO coating

No High EasyYes Low Difficult

6 ica Acta 54 (2008) 598–605

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Vanadium pentoxide and titanium oxide (V2O5/TiO2) compositelm was adopted in this study. As the counter film in the EC device,he composite film exhibits improved performance over V2O5 filmsed previously [24]. The V2O5/TiO2 composite film was depositedn ITO/PET substrate by the chronoamperometry method in a2O5·TiO2·nH2O sol–gel solution. The sols of V2O5·TiO2·nH2O wereynthesized using a method reported in previous literature [24].

Both of the EC film and V2O5/TiO2 composite film need toe placed into a 0.1 M LiClO4/PC electrolyte solution after poly-erization. The films were electrochemically conditioned by the

hronocoulometry method in order to change the inorganic ions inhe films and have them to be familiar with a LiClO4/PC environ-

ent [2].

.3. EC device assembly

A V2O5/TiO2 composite film coated ITO/PET was placed on theop of an EC film coated ITO/PET, and the two were clampedogether. A solution type electrolyte was injected into the EC devicesing a syringe. An UV cured film sealant (IS 90453, obtained fromdhesives Research, Inc.) was used as a hermetic barrier to seal theevice.

.4. Characterization method

The electrochromic devices were switched using a potentiostatHI 605 A, CH Instruments. Optical characterization of the devicesas carried out using an UV-Visible-IR spectrophotometer V-570,

ASCO. Digital camera was used to record images of samples.

. Results and discussion

.1. EC film and counter film deposition

Classical electrochromic devices use ITO glass as transparentubstrate. However, plastic substrates have the advantages in flex-bility and potential applications in the field of flexible display andwitchable optical filters [25]. Currently no industrially developedll-plastic electrochromic device exists because of some technicalhallenges need to be solved. One of the big issues is related to theow quality of transparent conductor layer (normally ITO) on plasticubstrate, compared with glass substrate [15]. Requirements of theTO coating are: high uniformity, high conductivity, and high trans-arency. Since electrochemical deposition method is used here toorm EC and V2O5/TiO2 films, the quality of ITO layer will have aound effect on the uniformity of films and performance of devices.

e did survey and there are many kinds of ITO coated plastic prod-cts on the market. They have different surface resistance, lightransmittance and physical states. In this study, we adopted anTO coated PET (obtained from Sheldahl) with surface resistance0 �/�, and light transmittance is 83% at 580 nm wavelength.

PProDOT-Me2 polymer film is fragile and sensitive to exper-mental conditions. Therefore parameters of electrochemicaleposition are carefully controlled. Oxidative electrochemical poly-erization method is adopted in this experiment to deposit

olymer films on ITO/PET working electrode. The monomer is oxi-ized and forms radical cation, which undergoes further couplingeaction with other monomers or radical cations forming insolubleolymer chains on the electrode surface. The applied potential is.5 V, and deposition time is 15 s. Due to the relatively low qual-

ty and conductivity of ITO coating on PET substrate, a copper tapeould be applied to minimize the potential drop through the sub-trate surface.

The deposited PProDOT-Me2 polymer films are shown in Fig. 4.t exhibits a layer of thin, uniformly deposited EC polymer film.

aoetp

Fig. 4. Electrochemical deposited EC film.

igh quality ITO coating on the substrate helped to enhance theniformity of EC polymer film.

It is less complicated to deposit V2O5/TiO2 composite film ontoTO/PET substrate. The applied potential is controlled at 3.5 V, andeposition time is 10 s. However, excess liquid needs to be removedrom the deposited film and baking at over 100 ◦C is also required.ince PET plastic can experience some deformation during heating,t is necessary to control the baking time and temperature. Afteroating, the PET substrate is put on flat glass substrate and heatedt 104 ◦C for 4 h. The deposited V2O5/TiO2 composite film is shownn Fig. 5.

The cyclic voltammetry (CV) curve of the blue EC film depositedn ITO/PET substrate is shown in Fig. 6. The reduction peak andxidation peak are quite clear in this figure. The reduction of EC filmappened at −0.4 V and changed the film to a deep blue color, whilehe oxidation of EC film happened at +0.4 V and the film switchedo the transparent state.

.2. Device design and assembly

ECD developed in our lab based on glass substrate is a multi-ayer structure electrochromic device. In this design, parafilm actss the spacer controlling the distance between working and counterlectrodes, epoxy (LOCTITE 9460 Epoxy Adhesive) is the barrier pro-ecting polymer from moisture and oxygen, and gel type electrolytes the ion conductive layer in between [26]. The EC device based onexible ITO/PET substrate is of similar configuration, but instead alastic substrate, flexible film sealant, and solution type electrolyteere adopted (Fig. 7). The UV curable film sealant is sandwichedetween the working and counter electrodes. It can maintain a0 �m gap between the electrodes and block all the moisture

nd oxygen. Namely, the film sealant is able to play both rolesf spacer and barrier. After laminating the working and counterlectrodes together, the solution type electrolyte is injected intohe gap between two electrodes through the inlet port. The inletort is sealed by UV cured glue in the final step. These improve-

C. Ma et al. / Electrochimica A

md

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wosWcfipscei

3

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d

�%T = T t(�) − Tc(�) (1)

Fig. 5. Electrochemical deposited V2O5/TiO2 composite film.

ents assure flexibility and high performance of the flexible ECevice.

In this configuration, the electrochromic working layer,ProDOT-Me2 film, is deposited on ITO coated PET plastic. Since an

lectrochemical polymerization method is adopted here, a maskould be used to pattern the deposition area. The counter layer ofhe device is V2O5/TiO2 composite film, also deposited on ITO/PET.he V2O5/TiO2 composite film serves as an ion storage layer and

�p

Fig. 6. CV curve of EC film depos

cta 54 (2008) 598–605 601

orks with the PProDOT-Me2 film as a pair. When the EC film isxidized with an applied potential and changes to the transparenttate, the V2O5/TiO2 composite film will absorb Li+, simultaneously.

hen the EC film is reduced with an applied potential and changesolor to blue, Li+ will diffuse out from the V2O5/TiO2 compositelm. During switching, the V2O5/TiO2 composite film maintains aale yellow/green color. A transparent solution type electrolyte isandwiched between the working and counter layers. It is a goodonductor for small ions such as ClO4

− and Li+ and an insulator forlectrons. It serves as an ion transport layer and ions move quicklynside during switching.

.3. Device characterizations

The flexible ECD worked properly and had improved elec-rochromic properties. Fig. 8 is the as-prepared ECD in transparentnd colored states, respectively. Since PProDOT-Me2 polymer films a cathodic EC polymer, when a −1.2 V potential is applied to the

orking electrode of the lens, the device changes its color to darklue. The color change is due to the reduction of PProDOT-Me2 poly-er film. Once the applied potential is switched to +1.2 V, the lensill be oxidized and change to transparent state. Fig. 9 is typical

ight transmittance curves of as-prepared flexible ECD in transpar-nt state and color state through out 380–800 nm wavelength. Ashown, the flat line is the transmittance of ECD in transparent state,hich has high light transmittance in the visible light range. The

U” shape line is the light transmittance of ECD in the colored state.t has the minimum light transmittance in the range of 550–600 nmavelength, which is most sensitive to human eyes. Thus, a vivid

olor change could be expected during the switching.Here, the EC device light transmittance contrast ratio (�%T) is

efined as,

Tt(�) and Tc(�) is the light transmittance of certain wave lengthon transparent state and colored state, respectively. The as-

repared flexible ECD exhibited satisfactory �%T through the

ited on ITO/PET substrate.

602 C. Ma et al. / Electrochimica Acta 54 (2008) 598–605

of new

va(

rbcTEwt

EEccif

Fig. 7. Design

isible light wave length. The maximum contrast ratio was obtainedt 580 nm wavelength, as the arrow shows in Fig. 9. It reaches 56%2–58%).

Cyclic characterization was used to test the response time andepeatability of the device. The flexible ECD could be switchedetween the transparent and colored state at a fast speed, which is

omparable to that of glass based ECD when they are the same size.he switching speed of 2.5 cm × 2.5 cm flexible ECD and glass basedCD was measured and shown by cyclic performance at 580 nmavelength. In Fig. 10(a), the solid line is the cyclic light transmit-

ance of flexible ECD while the dash line is that of the glass based

Fig. 8. Flexible ECD in (a) transparent and (b) color states.

9

bibaeFbwcstbEd

strtl

flexible ECD.

CD. As shown, the switch speeds of flexible ECD and glass basedCD were almost identical to each other. When zooming in on theolor switching, as shown in Fig. 10(b), it can be seen that a dramaticolor change for both flexible ECD and glass based ECD happenedn 0.5 s. It was found that the flexible ECD experienced 99.77% of itsull color change within 0.5 s, while the glass based ECD displayed9.96% of its full color change in the same interval.

The electrochromic properties of as-prepared flexible ECD in aent state were investigated. Fig. 11 shows the photos of the flex-

ble ECD in normal and bent states, respectively. The device coulde switched between transparent and color state when it was bent,nd the applied potential was ±1.2 V. As shown, the bended ECDxhibited uniform color change without any tint or stain observed.ig. 12 displays the current and light transmittance (580 nm) of theent ECD responding to a cyclic potential. The light transmittanceas stable and showed good repeatability. Switching speed and

ontrast ratio of the flexible ECD remain stable even in the benttate. Optical property of the ECD was maintained, which meanshe reduction and oxidation of PProDOT-Me2 film was not affectedy the bending. These results prove that the developed flexibleCD has potential applications in the field of flexible or paper-likeisplay.

Long lifetime of ECD is an important factor that needs to be con-idered in practical applications. Switching the devices between

he colored and transparent state multiple times causes repeatededuction and oxidation of EC film, which can cause the proper-ies of the film to deteriorate. Normally, EC polymer materials withower operation potentials can perform longer, because high oper-

Fig. 9. Light transmittance of flexible ECD in 380–800 nm wavelength.

C. Ma et al. / Electrochimica Acta 54 (2008) 598–605 603

Fa

apmiciil1imtsot5l5

Tsdta

cpt

crfioa6aaatlataoulw

ig. 10. Comparison of flexible ECD and glass based ECD in (a) cyclic performancend (b) one step of the color change.

tion potentials can cause degradation to the EC film, ITO layer,olymeric electrolyte, and delamination in the interface of theseaterials [27]. In the early research stage, short lifetime often lim-

ts the real application of ECD. The developed flexible ECD washaracterized using the cyclic performance test method. The flex-ble ECD was subjected to a cyclic repeated ±1.2 V and the timenterval was set to 1 s. The light transmittance at 580 nm wave-ength was recorded after 100 cycles, 1000 cycles, 5000 cycles,0,000 cycles, up to 40,000 cycles of switching. The recorded datas shown in Fig. 13, where round shaped spots are the light trans-

ittance of the transparent state and square shaped spots are ofhe colored state. As it can be seen, the flexible ECD shows goodtability over 40,000 cycles of switching. The light transmittancen color state displayed almost no change during the test. The lightransmittance on transparent state decreased a little bit in the first000 cycles, but become stable after that. At the beginning, the

ight transmittance was 55.2–2.4%, after 40,000 cycles it became2.1–2.6%.

Fig. 14(a) shows the cyclic light transmittance of the tested ECD.

he transmittance after the first five cycles is represented by theolid line and the transmittance after 40,000 cycles is given by theotted line. The shape of the curves did not change at all, as well ashe switching speed. Fig. 14(b) is the cyclic light transmittance of itfter 10,000 cycles (solid line) and 40,000 cycles (dot line). The two

aicc

Fig. 11. Photos of flexible ECD in (a) normal state and (b) bended state.

urves are almost identical to each other. It is clear that the opticalroperties of the flexible ECD remained in a stable situation afterhe breaking-in period.

The long-term stability of as-prepared flexible ECD was alsoharacterized. Since a new device configuration and sealant mate-ial were utilized, this test is important to verify if the UV curedlm sealant could prevent the penetration of oxygen and moisturer not. The ECD was kept at normal lab environmental conditionnd tested cyclic optical performance every day for continuously0 days. The results are shown in Fig. 15. In Fig. 15(a) square spotsre the light transmittance on the transparent state and round spotsre on the color state. Considering the fluctuation of measurementnd instrument, the flexible ECD performed relatively stable lightransmittance over 60 days. In Fig. 15(b), the solid line is the cyclicight transmittance at 580 nm wavelength tested on the first daynd dash line is that on the 60th day. It can be seen that shape ofhe curves are almost the same, which means the switching speednd contrast ratio of the ECD were stable after 60 days test. Thenly change was that the light transmittance curve shifted a littlepward towards the more transparent direction. On the 1st day, the

ight transmittance was from 3% to 58%, while on the 60th day itas from 5% to 61%.

Coloration efficiency (CE) is a key parameter when makingcomparison between electrochromic materials and devices. It

s obtained from the relationship between the injected/ejectedharges as a function of electrode area (Qd), and the change in opti-al density (�OD), at a specific wavelength � during a redox step

604 C. Ma et al. / Electrochimica Acta 54 (2008) 598–605

Fs

oC

�

C

F

F

ig. 12. (a) cyclic current and (b) cyclic light transmittance of flexible ECD in bendedtate.

f the ECD [28]. The following two equations are used to calculateE:

OD(�) = log[

Ttrans(�)Tcolor(�)

](2)

E(�) = �OD(�)Qd

(3)

ig. 13. Light transmittance of flexible ECD during 40,000 cycles of switching.

c

wpdsisdlfCi1eik

TC

E

EE

ig. 14. (a) cyclic light transmittance of first 5 cycles and after 40,000 cycles; (b)yclic light transmittance after 10,000 cycles and 40,000 cycles.

here Ttrans(�) and Tcolor(�) are the light transmittance of trans-arent state and color state at certain wavelength �. An ideallyesigned ECD should have large light transmittance change withmall amount of charge consumption. Therefore, the larger the CEs, the better the ECD is. Table 2 lists the charge injected, optical den-ity change, as well as the CE calculated for two kinds of flexible ECDeveloped in our lab. Device I had smaller area (1.5 cm2) and II had

arger area (8.5 cm2). The charge injected per area was almost sameor the two devices (1.9 mC/cm2 and 1.8 mC/cm2), while a higherE of 749 cm2/C was obtained in the smaller ECD I. For the typical

norganic electrochromic material, the CE of WO3 film could reach

15 cm2/C fabricated by thermal evaporation method [29]. Huangt al. [19] reported flexible ECD based on PANI:PSS and PEDOT:PSSn 2006 and the CE was 183 cm2/C. Mecerreyes et al. [25] reported aind of PEDOT based flexible ECD in 2004 and the CE was 80 cm2/C.

able 2harge injected, optical density change and coloration efficiency of ECD

CD Q (mC) Area (cm2) Qd (mC/cm2) �OD CE (cm2/C)

CD I 2.9 1.5 1.9 1.42 749CD II 15 8.5 1.8 1.29 717

C. Ma et al. / Electrochimica A

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ig. 15. Long-term stability test of the flexible ECD (a) light transmittance over 60ays; (b) light transmittance on the 1st day and 60th day.

n 1998, Sapp et al. [27] reported the dual polymer ECD developedn their lab which had a CE as high as 1413 cm2/C. However, theirevice was based on two layers of working materials and the sub-trate was ITO/glass. Compared with other researcher’s date, theexible CED developed in this study has relatively high CE and bet-er electrochromic properties, due to the porous structure of EColymer, high absorbance on reduction state, high transmittancen oxidation state, and low redox switching potential. These resultsemonstrate that the developed flexible ECD is a good candidate inhe applications such as flexible display.

. Conclusions

In this study, an electrochromic device with a plastic substrateas designed, fabricated and characterized. It was based on an

[[[

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cta 54 (2008) 598–605 605

TO coated PET flexible substrate, and the EC working part wasProDOT-Me2 polymer film, while the counter part was V2O5/TiO2omposite film. Solution type electrolyte was used instead of semi-olid type one. The lens was sustained and sealed by a transparentV cured film sealant. Similar to the glass based ECD, the devel-ped plastic lens exhibited adjustable transmittance of light, fastesponse time (about 0.5 s), low driving potential (1.2 V), goodepeatability and long lifetime (over 40,000 cycles of switch).eanwhile it offered several advantages over a glass substrate

ncluding greater flexibility, less weight and volume, and easieratterning. This flexible plastic based ECD could be widely used inanufacturing smart sunglasses, automobile windows, and paper-

ike flexible display.

cknowledgements

This study was supported by 3M gift grant #1802592 to Prof.hunye Xu, University of Washington. The authors are thankful toM company.

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