Flexible electrochromic device based on poly (3,4-(2,2-dimethylpropylenedioxy)thiophene)

Electrochimica Acta 54 (2008) 598–605

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Flexible electrochromic device based on poly (3,4-(2,2-dimethylpropylenedioxy)thiophene) Chao Ma, Minoru Taya, Chunye Xu ∗ Center of Intelligent Materials and Systems, University of Washington, Box 352600, Seattle, WA 98195, United States

a r t i c l e

i n f o

Article history: Received 25 April 2008 Received in revised form 8 July 2008 Accepted 9 July 2008 Available online 30 July 2008 Keywords: Electrochromic Flexible device Conducting polymer Coloration efficiency

a b s t r a c t In this study, the design, fabrication and characterization of a flexible electrochromic device based on indium tin oxide (ITO) coated polyethylene terephthalate (PET) plastic is discussed. The working electrochromic material film was poly (3,4-(2,2-dimethylpropylenedioxy)thiophene) (PProDOT-Me2 ), while the counter layer of the device was vanadium oxide titanium oxide (V2 O5 /TiO2 ) composite film, which serves as an ion storage layer. A solution type electrolyte was used as the ionic transport layer and was sandwiched between the working and counter layers. The device exhibited tuneable light transmittance between transparent and deep blue color, with a maximum contrast ratio at 580 nm wavelength. Other important properties, such as switching speed, life time, and coloration efficiency have been improved. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction Electrochromic (EC) materials can change their optical properties reversibly for an applied potential due to electrochemical oxidation and reduction. Electrochromic devices (ECD) which are able to actively control light transmittance/absorbance could be used in the field of architecture, vehicle, aircraft and displays [1–3]. Fig. 1 shows a hard glass based electrochromic device (30 cm × 30 cm) developed in our lab. With an applied potential, the optical states of the ECD can be changed between blue color and transparency [4]. Generally speaking, the electrochromic device has a multilayered structure as shown in Fig. 2. It has two electrodes, one working, one counter, and one ion conductive layer in between. Normally the working performs the color change during operation of the device. Electrochromic materials, first introduced by Deb [5] in 1969, include inorganic oxide, such as tungsten trioxide (WO3 ) and iridium dioxide (IrO2 ). Recently electrochromic organic polymers, such as pyrrole, thiophene and their derivatives have been developed [1,2,6–8]. In the early stage of electrochromic materials research, most of the attention was focused on inorganic oxides. However, these transition metal oxide electrochromics exhibited a slow response time (tens of seconds) and a high processing cost. Compared with these materials, electrochromic polymers have shown

∗ Corresponding author. Tel.: +1 206 685 7920; fax: +1 206 685 7920. E-mail address: [email protected] (C. Xu). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.07.049

several unique merits: they require power only during switching, have a low operation voltage and energy consumption, show a quick response time; possess long open circuit memory, great repeatability, rich color availability; and large scale and flexible devices are easily fabricated. Recently, our laboratory has developed a series of new electrochromic polymer materials with blue, red or green color. In this study, a cathodic blue color EC polymer material, poly (3,4-(2,2-dimethylpropylenedioxy)thiophene) (PProDOT-Me2 , structure shown in Fig. 3) was utilized to fabricate the flexible ECD. PProDOT-Me2 is a thiophene based conducting polymer. As shown in Scheme 1, in the neutral state it has strong absorption around 580 nm wavelength, so that the polymer film will show up deep blue color. When it is oxidized and P-doped, the energy band gap will become smaller and lower energy transition will arise near 580 nm wavelength. Therefore, the absorption will decrease and cause the polymer film to become transparent. Reduction of the oxidized EC polymer will cause the polymer to switch back to the neutral state; returning the energy band gap and absorption back to the colored state, so that the polymer is cathodically colored. Recently, the research attention of ECD has been focused on flexible substrates [9–14], because of their advantages over glass substrates and the potential application in the field of flexible paper-like displays. In Table 1, the differences between plastic and glass substrates are summarized. A plastic substrate has less weight and volume, is more flexible, and easier to pattern. The only problem is that it is more difficult to get a high quality ITO coating on a plastic substrate [15]. The inorganic oxide EC materials 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.

Fig. 2. Multi-layers structure of ECD.

In the early stage of flexible ECD development, the devices were based on WO3 , as reported by Antinucci in 1995 and Yoshimura in 2007 [16,17]. However, the device had a low switch speed and a high operation potential. In 2003, Argun [18] reported the first truly all-polymer ECD, based on poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT-PSS), and Huang et al. [19] reported a flexible ECD based on polyaniline (PANI) and PEDOT-PSS in 2006. Recently reported flexible ECD was developed by Kobayashi et al. [20] in 2007, which was based on phthalate derivative and exhibited a reddish purple color. The flexible ECD developed in this study was based on EC polymer material that is different from those reported ones and performed improved electrochromic properties. The comparison will be shown in the following discussion paragraph.

Fig. 3. Structure of PProDOT-Me2 .

Scheme 1. The redox action of PProDOT-Me2 .

injected into the electrochromic device later using syringe. Special attention must be paid to avoid any contamination from moisture and oxygen. The solution electrolyte needs to be bubbled with argon gas before use, and all the containers should be dried in oven.

2. Experimental 2.1. Materials and reagents ProDOT-Me2 monomer was synthesized in our lab via an improved route. The method was reported by Xu et al. [2,21]. All starting materials were purchased from Aldrich, except lithium perchlorate (LiClO4 , 99% anhydrous, packed under argon), which was purchased from Alfa Aesar. Because the electrochromic polymer film is sensitive to moisture and oxygen, which could affect the performance of EC devices, all the materials were dried before use and stored in glove box filled with argon. A solution of electrolyte was prepared by dissolving 0.1 M of LiClO4 in propylene carbonate (PC). This solution electrolyte was

2.2. Working and counter electrode The PProDOT-Me2 polymer film was deposited from 0.01 M monomer in a 0.1 M LiClO4 /acetonitrile (ACN) solution. Electrochemical deposition method was carried out on an electrochemical analyzer (CHI 605 A, CH Instruments), utilizing the chronoamperometry method [22,23]. A three-electrode cell with Ag wire as a reference, ITO/PET as a working electrode and a stainless steel plate as a counter electrode was used for electrochemical polymerizing the polymer film.

Table 1 comparison of glass and plastic substrate

Glass Plastic

Density (g/cm3 )

Thickness

Flexibility

Easy patterning

Mechanical stability

Cost

ITO coating

∼3 ∼1.4

∼1 mm ∼100 ␮m

No Yes

No Yes

No Yes

High Low

Easy Difficult

600

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

Vanadium pentoxide and titanium oxide (V2 O5 /TiO2 ) composite film was adopted in this study. As the counter film in the EC device, the composite film exhibits improved performance over V2 O5 film used previously [24]. The V2 O5 /TiO2 composite film was deposited on ITO/PET substrate by the chronoamperometry method in a V2 O5 ·TiO2 ·nH2 O sol–gel solution. The sols of V2 O5 ·TiO2 ·nH2 O were synthesized using a method reported in previous literature [24]. Both of the EC film and V2 O5 /TiO2 composite film need to be placed into a 0.1 M LiClO4 /PC electrolyte solution after polymerization. The films were electrochemically conditioned by the chronocoulometry method in order to change the inorganic ions in the films and have them to be familiar with a LiClO4 /PC environment [2]. 2.3. EC device assembly A V2 O5 /TiO2 composite film coated ITO/PET was placed on the top of an EC film coated ITO/PET, and the two were clamped together. A solution type electrolyte was injected into the EC device using a syringe. An UV cured film sealant (IS 90453, obtained from Adhesives Research, Inc.) was used as a hermetic barrier to seal the device. 2.4. Characterization method The electrochromic devices were switched using a potentiostat CHI 605 A, CH Instruments. Optical characterization of the devices was carried out using an UV-Visible-IR spectrophotometer V-570, JASCO. Digital camera was used to record images of samples. 3. Results and discussion 3.1. EC film and counter film deposition Classical electrochromic devices use ITO glass as transparent substrate. However, plastic substrates have the advantages in flexibility and potential applications in the field of flexible display and switchable optical filters [25]. Currently no industrially developed all-plastic electrochromic device exists because of some technical challenges need to be solved. One of the big issues is related to the low quality of transparent conductor layer (normally ITO) on plastic substrate, compared with glass substrate [15]. Requirements of the ITO coating are: high uniformity, high conductivity, and high transparency. Since electrochemical deposition method is used here to form EC and V2 O5 /TiO2 films, the quality of ITO layer will have a sound effect on the uniformity of films and performance of devices. We did survey and there are many kinds of ITO coated plastic products on the market. They have different surface resistance, light transmittance and physical states. In this study, we adopted an ITO coated PET (obtained from Sheldahl) with surface resistance 20 /, and light transmittance is 83% at 580 nm wavelength. PProDOT-Me2 polymer film is fragile and sensitive to experimental conditions. Therefore parameters of electrochemical deposition are carefully controlled. Oxidative electrochemical polymerization method is adopted in this experiment to deposit polymer films on ITO/PET working electrode. The monomer is oxidized and forms radical cation, which undergoes further coupling reaction with other monomers or radical cations forming insoluble polymer chains on the electrode surface. The applied potential is 1.5 V, and deposition time is 15 s. Due to the relatively low quality and conductivity of ITO coating on PET substrate, a copper tape could be applied to minimize the potential drop through the substrate surface. The deposited PProDOT-Me2 polymer films are shown in Fig. 4. It exhibits a layer of thin, uniformly deposited EC polymer film.

Fig. 4. Electrochemical deposited EC film.

High quality ITO coating on the substrate helped to enhance the uniformity of EC polymer film. It is less complicated to deposit V2 O5 /TiO2 composite film onto ITO/PET substrate. The applied potential is controlled at 3.5 V, and deposition time is 10 s. However, excess liquid needs to be removed from the deposited film and baking at over 100 ◦ C is also required. Since PET plastic can experience some deformation during heating, it is necessary to control the baking time and temperature. After coating, the PET substrate is put on flat glass substrate and heated at 104 ◦ C for 4 h. The deposited V2 O5 /TiO2 composite film is shown in Fig. 5. The cyclic voltammetry (CV) curve of the blue EC film deposited on ITO/PET substrate is shown in Fig. 6. The reduction peak and oxidation peak are quite clear in this figure. The reduction of EC film happened at −0.4 V and changed the film to a deep blue color, while the oxidation of EC film happened at +0.4 V and the film switched to the transparent state. 3.2. Device design and assembly ECD developed in our lab based on glass substrate is a multilayer structure electrochromic device. In this design, parafilm acts as the spacer controlling the distance between working and counter electrodes, epoxy (LOCTITE 9460 Epoxy Adhesive) is the barrier protecting polymer from moisture and oxygen, and gel type electrolyte is the ion conductive layer in between [26]. The EC device based on flexible ITO/PET substrate is of similar configuration, but instead a plastic substrate, flexible film sealant, and solution type electrolyte were adopted (Fig. 7). The UV curable film sealant is sandwiched between the working and counter electrodes. It can maintain a 30 ␮m gap between the electrodes and block all the moisture and oxygen. Namely, the film sealant is able to play both roles of spacer and barrier. After laminating the working and counter electrodes together, the solution type electrolyte is injected into the gap between two electrodes through the inlet port. The inlet port is sealed by UV cured glue in the final step. These improve-

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

601

works with the PProDOT-Me2 film as a pair. When the EC film is oxidized with an applied potential and changes to the transparent state, the V2 O5 /TiO2 composite film will absorb Li+ , simultaneously. When the EC film is reduced with an applied potential and changes color to blue, Li+ will diffuse out from the V2 O5 /TiO2 composite film. During switching, the V2 O5 /TiO2 composite film maintains a pale yellow/green color. A transparent solution type electrolyte is sandwiched between the working and counter layers. It is a good conductor for small ions such as ClO4 − and Li+ and an insulator for electrons. It serves as an ion transport layer and ions move quickly inside during switching. 3.3. Device characterizations

Fig. 5. Electrochemical deposited V2 O5 /TiO2 composite film.

ments assure flexibility and high performance of the flexible EC device. In this configuration, the electrochromic working layer, PProDOT-Me2 film, is deposited on ITO coated PET plastic. Since an electrochemical polymerization method is adopted here, a mask could be used to pattern the deposition area. The counter layer of the device is V2 O5 /TiO2 composite film, also deposited on ITO/PET. The V2 O5 /TiO2 composite film serves as an ion storage layer and

The flexible ECD worked properly and had improved electrochromic properties. Fig. 8 is the as-prepared ECD in transparent and colored states, respectively. Since PProDOT-Me2 polymer film is a cathodic EC polymer, when a −1.2 V potential is applied to the working electrode of the lens, the device changes its color to dark blue. The color change is due to the reduction of PProDOT-Me2 polymer film. Once the applied potential is switched to +1.2 V, the lens will be oxidized and change to transparent state. Fig. 9 is typical light transmittance curves of as-prepared flexible ECD in transparent state and color state through out 380–800 nm wavelength. As shown, the flat line is the transmittance of ECD in transparent state, which has high light transmittance in the visible light range. The “U” shape line is the light transmittance of ECD in the colored state. It has the minimum light transmittance in the range of 550–600 nm wavelength, which is most sensitive to human eyes. Thus, a vivid color change could be expected during the switching. Here, the EC device light transmittance contrast ratio (%T) is defined as, %T = T t () − T c ()

(1)

Tt () and Tc () is the light transmittance of certain wave length  on transparent state and colored state, respectively. The asprepared flexible ECD exhibited satisfactory %T through the

Fig. 6. CV curve of EC film deposited on ITO/PET substrate.

602

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

Fig. 7. Design of new flexible ECD.

visible light wave length. The maximum contrast ratio was obtained at 580 nm wavelength, as the arrow shows in Fig. 9. It reaches 56% (2–58%). Cyclic characterization was used to test the response time and repeatability of the device. The flexible ECD could be switched between the transparent and colored state at a fast speed, which is comparable to that of glass based ECD when they are the same size. The switching speed of 2.5 cm × 2.5 cm flexible ECD and glass based ECD was measured and shown by cyclic performance at 580 nm wavelength. In Fig. 10(a), the solid line is the cyclic light transmittance of flexible ECD while the dash line is that of the glass based

ECD. As shown, the switch speeds of flexible ECD and glass based ECD were almost identical to each other. When zooming in on the color switching, as shown in Fig. 10(b), it can be seen that a dramatic color change for both flexible ECD and glass based ECD happened in 0.5 s. It was found that the flexible ECD experienced 99.77% of its full color change within 0.5 s, while the glass based ECD displayed 99.96% of its full color change in the same interval. The electrochromic properties of as-prepared flexible ECD in a bent state were investigated. Fig. 11 shows the photos of the flexible ECD in normal and bent states, respectively. The device could be switched between transparent and color state when it was bent, and the applied potential was ±1.2 V. As shown, the bended ECD exhibited uniform color change without any tint or stain observed. Fig. 12 displays the current and light transmittance (580 nm) of the bent ECD responding to a cyclic potential. The light transmittance was stable and showed good repeatability. Switching speed and contrast ratio of the flexible ECD remain stable even in the bent state. Optical property of the ECD was maintained, which means the reduction and oxidation of PProDOT-Me2 film was not affected by the bending. These results prove that the developed flexible ECD has potential applications in the field of flexible or paper-like display. Long lifetime of ECD is an important factor that needs to be considered in practical applications. Switching the devices between the colored and transparent state multiple times causes repeated reduction and oxidation of EC film, which can cause the properties of the film to deteriorate. Normally, EC polymer materials with lower operation potentials can perform longer, because high oper-

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

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

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

603

Fig. 11. Photos of flexible ECD in (a) normal state and (b) bended state. Fig. 10. Comparison of flexible ECD and glass based ECD in (a) cyclic performance and (b) one step of the color change.

ation potentials can cause degradation to the EC film, ITO layer, polymeric electrolyte, and delamination in the interface of these materials [27]. In the early research stage, short lifetime often limits the real application of ECD. The developed flexible ECD was characterized using the cyclic performance test method. The flexible ECD was subjected to a cyclic repeated ±1.2 V and the time interval was set to 1 s. The light transmittance at 580 nm wavelength was recorded after 100 cycles, 1000 cycles, 5000 cycles, 10,000 cycles, up to 40,000 cycles of switching. The recorded data is shown in Fig. 13, where round shaped spots are the light transmittance of the transparent state and square shaped spots are of the colored state. As it can be seen, the flexible ECD shows good stability over 40,000 cycles of switching. The light transmittance on color state displayed almost no change during the test. The light transmittance on transparent state decreased a little bit in the first 5000 cycles, but become stable after that. At the beginning, the light transmittance was 55.2–2.4%, after 40,000 cycles it became 52.1–2.6%. Fig. 14(a) shows the cyclic light transmittance of the tested ECD. The transmittance after the first five cycles is represented by the solid line and the transmittance after 40,000 cycles is given by the dotted line. The shape of the curves did not change at all, as well as the switching speed. Fig. 14(b) is the cyclic light transmittance of it after 10,000 cycles (solid line) and 40,000 cycles (dot line). The two

curves are almost identical to each other. It is clear that the optical properties of the flexible ECD remained in a stable situation after the breaking-in period. The long-term stability of as-prepared flexible ECD was also characterized. Since a new device configuration and sealant material were utilized, this test is important to verify if the UV cured film sealant could prevent the penetration of oxygen and moisture or not. The ECD was kept at normal lab environmental condition and tested cyclic optical performance every day for continuously 60 days. The results are shown in Fig. 15. In Fig. 15(a) square spots are the light transmittance on the transparent state and round spots are on the color state. Considering the fluctuation of measurement and instrument, the flexible ECD performed relatively stable light transmittance over 60 days. In Fig. 15(b), the solid line is the cyclic light transmittance at 580 nm wavelength tested on the first day and dash line is that on the 60th day. It can be seen that shape of the curves are almost the same, which means the switching speed and contrast ratio of the ECD were stable after 60 days test. The only change was that the light transmittance curve shifted a little upward towards the more transparent direction. On the 1st day, the light transmittance was from 3% to 58%, while on the 60th day it was from 5% to 61%. Coloration efficiency (CE) is a key parameter when making a comparison between electrochromic materials and devices. It is obtained from the relationship between the injected/ejected charges as a function of electrode area (Qd ), and the change in optical density (OD), at a specific wavelength  during a redox step

604

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

Fig. 12. (a) cyclic current and (b) cyclic light transmittance of flexible ECD in bended state.

of the ECD [28]. The following two equations are used to calculate CE: OD() = log CE() =

T

trans ()



Tcolor ()

OD() Qd

(2) (3)

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

where Ttrans () and Tcolor () are the light transmittance of transparent state and color state at certain wavelength . An ideally designed ECD should have large light transmittance change with small amount of charge consumption. Therefore, the larger the CE is, the better the ECD is. Table 2 lists the charge injected, optical density change, as well as the CE calculated for two kinds of flexible ECD developed in our lab. Device I had smaller area (1.5 cm2 ) and II had larger area (8.5 cm2 ). The charge injected per area was almost same for the two devices (1.9 mC/cm2 and 1.8 mC/cm2 ), while a higher CE of 749 cm2 /C was obtained in the smaller ECD I. For the typical inorganic electrochromic material, the CE of WO3 film could reach 115 cm2 /C fabricated by thermal evaporation method [29]. Huang et al. [19] reported flexible ECD based on PANI:PSS and PEDOT:PSS in 2006 and the CE was 183 cm2 /C. Mecerreyes et al. [25] reported a kind of PEDOT based flexible ECD in 2004 and the CE was 80 cm2 /C.

Table 2 Charge injected, optical density change and coloration efficiency of ECD

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

ECD

Q (mC)

Area (cm2 )

Qd (mC/cm2 )

OD

CE (cm2 /C)

ECD I ECD II

2.9 15

1.5 8.5

1.9 1.8

1.42 1.29

749 717

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

605

ITO coated PET flexible substrate, and the EC working part was PProDOT-Me2 polymer film, while the counter part was V2 O5 /TiO2 composite film. Solution type electrolyte was used instead of semisolid type one. The lens was sustained and sealed by a transparent UV cured film sealant. Similar to the glass based ECD, the developed plastic lens exhibited adjustable transmittance of light, fast response time (about 0.5 s), low driving potential (1.2 V), good repeatability and long lifetime (over 40,000 cycles of switch). Meanwhile it offered several advantages over a glass substrate including greater flexibility, less weight and volume, and easier patterning. This flexible plastic based ECD could be widely used in manufacturing smart sunglasses, automobile windows, and paperlike flexible display. Acknowledgements This study was supported by 3M gift grant #1802592 to Prof. Chunye Xu, University of Washington. The authors are thankful to 3M company. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

[11] [12] Fig. 15. Long-term stability test of the flexible ECD (a) light transmittance over 60 days; (b) light transmittance on the 1st day and 60th day.

[13] [14] [15] [16]

In 1998, Sapp et al. [27] reported the dual polymer ECD developed in their lab which had a CE as high as 1413 cm2 /C. However, their device was based on two layers of working materials and the substrate was ITO/glass. Compared with other researcher’s date, the flexible CED developed in this study has relatively high CE and better electrochromic properties, due to the porous structure of EC polymer, high absorbance on reduction state, high transmittance on oxidation state, and low redox switching potential. These results demonstrate that the developed flexible ECD is a good candidate in the applications such as flexible display. 4. Conclusions In this study, an electrochromic device with a plastic substrate was designed, fabricated and characterized. It was based on an

[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

C. Xu, L. Liu, S.E. Legenski, M. Le Guilly, M. Taya, Proc. SPIE 5051 (2003) 404. C. Xu, H. Tamagawa, M. Uchida, M. Taya, Proc. SPIE 4695 (2002) 442. C.M. Lampert, Sol. Energy Mater. Sol. Cells 76 (2003) 489. C. Kaneko, C. Xu, L. Liu, D. Ning, M. Taya, Proc. SPIE 5759 (2005) 518. S.K. Deb, Appl. Opt. Suppl. 3 (1969) 192. T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds, Handbook of Conducting Polymers, second ed., Marcel Dekker Inc., New York, US, 1998. Q. Pei, G. Zuccarello, M. Ahlskog, O. Inganas, Polymer 35 (7) (1994) 1347. O. Turkarslan, A.K. Metin, C. Tanyeli, I. Akhmedov, L. Toppare, J. Polym. Sci. A: Polym. Chem. 45 (2007) 4496. H. Pages, P. Topart, D. Lemordant, Electrochim. Acta 46 (2001) 2137. C. Pozo-Gonzaloa, D. Mecerreyesa, J.A. Pomposoa, M. Salsamendia, H.G. Marcillaa, R. Vergazb, D. Barriosb, J.M. Sanchez-Pena, Solar Energy Mater. Solar Cells 92 (2008) 101. M. Namboothiry, T. Zimmerman, F. Coldren, J. Liu, K. Kim, D. Carroll, Synth. Met. 157 (2007) 580. M.-A. De Paoli, A.F. Nogueira, D.A. Machado, Longo C., Electroch. Acta 46 (2001) 4243. Seung Cho II, D.H. Choi, S.-H. Kim, S.B. Lee, Chem. Mater. 17 (2005) 4564. S. Sindhu, K.N. Rao, S. Ahuja, A. Kumar, E.S.R. Gopal, Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 132 (1–2) (2006) 39. D.S. Ginley, C. Bright, MRS Bull. 25 (8) (2000) 15. M. Antinucci, B. Chevaliar, A. Ferriolo, Solar energy materials and solar cells 39 (1995) 271. H. Yoshimura, T. Sakaguchi, N. Koshida, Jpn J. Appl. Phys. Part 1—Regular Papers Brief Communications & Review Papers 46 (4B) (2007) 2458. A.A. Argun, A. Cirpan, J.R. Reynold, Adv. Mater. 15 (16) (2003) 1338. L.-M. Huang, C.-H. Chen, T.-C. Wen, Electrochim. Acta 51 (2006) 5858. N. Kobayashi, S. Miura, M. Nishimura, G. Yutaka, Electrochim. Acta 53 (2007) 1643. D.M. Welsh, A. Kumar, E.W. Meijer, J.R. Reynolds, Adv. Mater. 11 (16) (1999) 1379. P.T. Kissinger, W.R. Heineman, Laboratory Techniques in Electroanalytical Chemistry, second ed., Marcel Dekker Inc., New York, US, 1996. C. Xu, L. Liu, S.E. Legenski, D. Ning, M. Taya, J. Mater. Res. 19 (7) (2004) 2072. S. Kim, M. Taya, C. Xu, Mater. Res. Soc. Sympos. P. 928 (2006) 160. D. Mecerreyes, R. Marcilla, E. Ocheoteco, H. Grande, J.A. Pomposo, R. Vergaz, J.M. Sanchez Pena, Electrochim. Acta 49 (2004) 3555. D. Ning, C. Xu, L. Liu, C. Kaneko, M. Taya, Proc. of SPIE 5759 (2005) 260. S.A. Sapp, G.A. Sotzing, J.R. Reynolds, Chem. Mater. 10 (8) (1998) 2101. P. Monk, R. Mortimer, D. Rosseinsky, Electrochromism: fundamentals and applications, VCH, Weinheim, New York, 1995. B.W. Faughnan, R.S. Crandall, P.M. Heyman, RCA Rev. 36 (1) (1975) 177.