Electrochromic devices based on poly(3-methylthiophene) and various secondary electrochromic materials

Electrochromic devices based on poly(3-methylthiophene) and various secondary electrochromic materials

ARTICLE IN PRESS Solar Energy Materials & Solar Cells 94 (2010) 1338–1345 Contents lists available at ScienceDirect Solar Energy Materials & Solar C...

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ARTICLE IN PRESS Solar Energy Materials & Solar Cells 94 (2010) 1338–1345

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Electrochromic devices based on poly(3-methylthiophene) and various secondary electrochromic materials Emerson da Costa Rios a, Adriane Viana Rosario a,n, Ana Fla´via Nogueira b, Liliana Micaroni c a ´rio Interdisciplinar de Eletroquı´mica e Cerˆ Laborato amica – Centro Multidisciplinar para o Desenvolvimento de Materiais Cerˆ amicos – Departamento de Quı´mica, Universidade ~ Carlos, CP 676, 13565-905 Sa~ o Carlos, SP, Brazil Federal de Sao b ´rio de Nanotecnologia e Energia Solar, Instituto de Quı´mica, Universidade Estadual de Campinas, CP 6154, 13084-971 Campinas, SP, Brazil Laborato c ´, CP 19081, 81531-990 Curitiba, PR, Brazil ´rio de Eletroquı´mica Aplicada e Polı´meros – Departamento de Quı´mica, Universidade Federal do Parana Laborato

a r t i c l e in f o

a b s t r a c t

Article history: Received 12 August 2009 Accepted 22 March 2010

In this paper, the electrochemical properties of poly(3-methylthiophene) (PMT), poly(3,4-ethylenedioxythiophene) (PEDOT) and niobium pentoxide (Nb2O5) films deposited on ITO are described. PMT films were made by galvanostatic electrodeposition. PEDOT and Nb2O5 were deposited by spin-coating from a suspension, in the case of PEDOT, and from a precursor solution obtained by the Pechini method, for Nb2O5. Three electrochromic devices were assembled from these films, with different arrangements of electrodes, using poly(epichlorohydrin-co-ethylene oxide) (P(EPI-EO)) + LiClO4 as the polymer electrolyte. The PMT films were employed as the working electrode (E1) and PEDOT, Nb2O5 or ITO as counter-electrodes (E2). The devices showed color changes from red to blue in response to the applied potential from  1.5 to + 1.5 V (PMT vs. E2), respectively. The transmittance variation was measured in the visible region (l ¼ 650 nm) during the polarization. The systems were also characterized with respect to their coulombic efficiency (CE), electrochromic efficiency (Z) and response time (t). The three devices gave similar results, as follows: CE of 107%, Z from 92 to 126 cm2 C  1 and t  2 s. The cycle life and optical memory were also analyzed and the devices showed good durability for 1000 cycles and good optical memory, demonstrating the potential applicability of the electrochromic devices presented here. & 2010 Elsevier B.V. All rights reserved.

Keywords: Poly(3-methylthiophene) Poly(3,4-ethylenedioxythiophene) Niobium pentoxide Electrochromic device

1. Introduction Electrochromism is of great interest in technology, and a variety of applications have been proposed for electrochromic devices, some of the most promising in the field of architecture, the automotive industry and other sectors such as the construction of optical displays and glasses [1]. Among the most studied electrochromic materials we can include transition metal oxides, prussian blue films, some cyanine derivatives, viologens and conducting polymers [2]. A material is considered suitable to be applied in electrochromic devices when it displays a good optical contrast between its various color states and a fast color response to the polarity change [3]. In an electrochromic device, the counter-electrode can be optically passive or electrochromically complementary. In the latter case, when one of the electrodes is oxidized and the other is reduced their states, colored and uncolored, coincide. In these systems the working electrode is called the primary and the counter-electrode the secondary electrode [4].

n

Corresponding author. Tel./fax: +55 16 3351 8214. E-mail address: [email protected] (A. Viana Rosario).

0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.03.025

Normally, an electrochromic device is assembled in a ‘‘sandwich’’ configuration. The electrochromic materials are deposited on optically transparent electrodes and an ionic conducting layer is needed to allow ionic conduction through the system, generally a polymer electrolyte. Wolfenson et al. [5], Silva et al. [6] and Gazotti et al. [7] studied the ionic conductivity of the poly(epichlorhydrin-co-ethylene oxide) P(EPI-EO) copolymer in the presence of LiClO4, which at room temperature exhibited a conductivity in the order of 10  6 Scm  1. Recently, they achieved an increase in this ionic conductivity (to about 10  3 Scm  1) by adding 12-crown-4 ether to a gel polymer electrolyte based on a PEO copolymer [8]. For efficient operation of an electrochromic device, it is necessary to take a number of properties into consideration: electrochromic efficiency, optical memory, response time, stability and durability [9]. The difficulty in achieving satisfactory values for all these parameters at the same time stimulates the development of new methods of preparation of electrochromic films, new materials and components for the devices. Polythiophene and its derivatives, with alkyl groups substituted at position 3 of the aromatic ring, such as poly(3methylthiophene), PMT, change in color from red to blue between the undoped and doped states, respectively [10]. In the case of

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poly(3,4-ethylenedioxythiophene), PEDOT, another polythiophene derivative polymer currently under investigation [11,12], its color changes from dark blue to almost transparent. Conducting polymers are promising materials for use in electrochromic devices [4,13,14] and, specifically, in all polymer electrochromic devices [15–17]. Iridium [18], rhodium [19], ruthenium [20], tungsten [21] and nickel [22] oxide films are the best studied electrochromic oxides. The electrochromic properties of these materials have also been characterized in combination with other electrochromic device materials, such as the conducting polymers. Although several oxides are known with electrochromic properties, most of them are costly or demand expensive preparation techniques. One of the motivations for studying niobium pentoxide, Nb2O5, is the wide availability of this metal in Brazil, where about 87% of the ore is found. In addition, Nb2O5 thin films have excellent electrochromic properties, with rapid and reversible color variation. When Li + ions are inserted into its structure by electrochemical methods, the transparent Nb2O5 film (in its oxidized form) changes to dark blue (the reduced form), characterizing a cathodic electrochromic material. On the other hand, it has been observed that its electro-optical, structural and morphological properties depend greatly on the preparation method, such as the evaporative [23], sputtering [24], anodizing [25] and sol–gel routes [26]. The polymerized complex method, created by Pechini [27], is a modified sol–gel process, widely used for the preparation of films of metal oxides, which can lead to the formation of porous and homogeneous films with a high specific surface area. Faria and ~ [28] used the Pechini method efficiently for the first time Bulhoes as an alternative approach to the preparation of Nb2O5 electrochromic films. In a subsequent report, Rosario and Pereira [29] used the same method, varying the preparation conditions. The authors observed that when these films were prepared at temperatures above 550 1C, the Nb2O5 crystal structure changed from hexagonal to orthorhombic, improving the intercalation process and coloration sites in the electrochromic oxide. Doping of transition metal oxides during their preparation has also been widely used, varying the dopants in order to improve particular characteristics, such as the electrochromic properties [30,31]. Thus, Nb2O5 films doped with Li + during their preparation have shown good electrochromic properties, mainly related to an increase in the speed of the coloration process and also in its reversibility [32,33]. In this study, electrochemical properties of the PMT, PEDOT and Nb2O5 films were investigated. Various electrochromic devices using these materials were built and characterized. PMT was employed as anodic material and PEDOT or Nb2O5 as secondary electrochromic materials.

2. Experimental 2.1. Electrochemical synthesis of PMT films The PMT films were prepared by electrochemical synthesis, with indium-tin oxide (ITO) supported on glass (Delta Technologies, Rs ¼8–12 O/& and 0.7 mm thick) as the working (primary) electrode (E1) of geometric area 2.0 cm2, a platinum wire (0.5 mm diameter) folded circularly as the counter electrode (E2), and a silver wire (1.0 mm diameter) as a pseudo-reference electrode (RE). The electrolytic solution consisted of 0.1 mol L  1 3-methylthiophene monomer (Aldrich) and 0.05 mol L  1 tetramethylammonium tetrafluoroborate ((CH3)4NBF4) electrolyte (Aldrich) in HPLC-quality acetonitrile (CH3CN; Baker). The synthetic method was galvanostatic, with a current density of

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3.7 mA cm  2, and the PMT film thickness was controlled by the charge consumed in the electropolymerization. 2.2. Preparation of PEDOT films For PEDOT film preparation, 15 mL of poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate), (PEDOT/PSS) (Sigma-Aldrich) was deposited onto ITO (0.7 mm). The suspension was spread by spin-coating (730 rpm/5 min) on the ITO electrode of geometric area 2.0 cm2. The film formed was dried at 50 1C for 120 min. 2.3. Nb2O5 synthesis by the polymerized complex method (Pechini method) The Nb2O5 precursor solution was prepared by dissolving citric acid, CA, (Synth, Brazil) in ethylene glycol, EG, (Mallinckrodt) in the molar ratio CA:EG¼1:4, with stirring and at a temperature of 70 1C, and subsequently adding niobium complex (NH4H2[NbO (C2O4)3]  3H2O) (CBMM-Brazil) in the molar proportion of CA:Nb¼10:1. For doping, LiClO4 (Aldrich) was added to the ethylene glycol, to give 0.2 mol% in relation to the niobium precursor (LiClO4:Nb¼1:500). The precursor solution was deposited on ITO (Donnelly, Rs ¼20 O/& and 1.1 mm thick) by spincoating (730 rpm/15 min) over the geometric area of 2.0 cm2. After deposition the samples were heated in a muffle furnace (EDG 18003P) at 110 1C for 60 min and 600 1C for 60 min, with a temperature ramp of 7 1C/min. 2.4. Characterization of the films The electrochromic films were characterized by cyclic voltammetry. 0.1 mol L  1 LiClO4 in acetonitrile was used as electrolyte. A platinum wire and a silver wire were used as counter and pseudo-reference electrodes, respectively, and the electrochromic film on ITO as WE. 2.5. Preparation of polymer electrolyte The gel electrolyte was prepared by dissolving 0.30 g of poly(epichlorhydrin-co-ethylene oxide), P(EPI-EO) 84-16 (supplied by Daiso Co. Ltd., Osaka) and 0.045 g of LiClO4 (0.15:1 w/w) in 5.0 mL of tetrahydrofuran (THF) under constant stirring rate at room temperature. After dissolution of the solids, 270 mL of gamma-butyrolactone (1:1 w/w) was introduced as a plasticizer and the stirring was maintained for 30 min. 2.6. Assembly of the solid devices Before assembling the devices, the electrochromic films were subjected to cyclic voltammetry and then polarized in 0.1 mol L  1 LiClO4 in acetonitrile solution. The PMT was reduced at a constant potential (E vs. Ag) of  0.2 V for 30 s, the PEDOT was oxidized at a potential of + 0.8 V for 30 s, and Nb2O5 was oxidized at + 0.5 V for 60 s. These parameters had been chosen to take into account the coloration changes, and the stabilization of reduction or oxidation currents in minimum values. All the materials were electrochemically undoped at these potentials. After this process, the films were washed in acetonitrile to remove the electrolytic salt and dried at 50 1C. 100 mL of the polymer electrolyte was then spread on the surface of each of electrochromic films. After 2 h the device was assembled with two separators about 130 mm thick. The geometric area of the device was 1.5 cm2. The device, clasped by a clip, was placed in a desiccator with THF vapor for 30 h.

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the extraction of Li + ion from the structure of oxide. The equation that describes the electrocoloration in crystalline Nb2O5 is

The devices were assembled in the following configurations: ITO9PMT9P(EPI-EO) +LiClO49ITO ITO9PMT9P(EPI-EO) +LiClO49PEDOT9ITO ITO9PMT9P(EPI-EO) +LiClO49Nb2O59ITO The electrochemical synthesis of the polymer and the characterization of the films and devices were carried out in a potentiostat/galvanostat (Autolab, model PGSTAT 30). UV–vis spectrophotometer (FEMTO 800XI) was used for the in situ optical measurements.

3. Results and discussion 3.1. Electrochemical characterization of electrochromic films PMT, PEDOT and Nb2O5 films were characterized by cyclic voltammetry in 0.1 mol L  1 LiClO4/CH3CN. The voltammograms are presented in Fig. 1. During the oxidation or polymer doping  (anodic peak at 0.6 V), the ClO4 ions are inserted and the polymer turns blue. In the reduction or polymer dedoping process (cathodic peak at 0.7 V), the anion is excluded and the polymer turns red. The redox process of the PMT can be represented as ðPMTÞy þ þ ðClO4 Þy þ ye 3

ðPMTÞ þ yðClO4 Þ

ðblueÞ

ðredÞ





The cyclic voltammogram of the PEDOT film does not show any well-defined redox peaks in the potential region presented. The film is blue at potentials more negative than  0.3 V and semitransparent at potentials more positive than this value. The rectangular shape of the PEDOT voltammetric profile is characteristic of a capacitive material, and can be related to the insertion of Li + ions into the film to compensate the poly(styrene sulfonate) macromolecular anion, PSSm  [34]. The redox process can be represented as PEDOTm þ =ðPSSm Þ þ ne þnLi þ 3 ðsemi-transparentÞ

PEDOTðmn Þ=ðPSSm ÞLinþ ðblueÞ

In Fig. 1 it can also be observed that the redox process in the niobium pentoxide film occurs at more negative potentials than in the polymers. The Nb2O5 voltammetric profile shows a continuous increase in current density during the cathodic sweep, associated with the reduction/Li + insertion process. With the reversal of the potential sweep, an anodic peak around  1.1 V is observed, which is attributed to the oxidation of Nb4 + to Nb5 + and

Nb2 O5 þ xLi þ þ xe 3

Lix Nb2 O5

ðcolorlessÞ

ðblueÞ

Table 1 presents the charge densities of PMT, PEDOT and Nb2O5 films, corresponding to the cyclic voltammograms of Fig. 1, and their coulombic efficiency (CE), which is the ratio of the anodic (Qo) to cathodic (Qr) charge densities. It is possible that part of the perchlorate anions that enter the PMT film do not leave during the cathodic sweep or that part of the polymer structure is degraded during the oxidation process, since the anodic charge is higher than the cathodic charge. For the PEDOT and the Nb2O5 films, the cathodic charge is higher than the anodic, indicating that some dopant cations are kept in these structures during the reduction process. A CE of 100% is expected in an electroactive material that has ideal behavior, and it is desirable in electrochromic devices constituted of two electroactive electrodes, E1 and E2, that the Qo of E1 and the Qr of E2 are as near as possible to maximize the coulombic efficiency [3].

3.2. Electro-optical characterization of the electrochromic devices In this study, all measurements on the devices were made with PMT as primary electrode and ITO, PEDOT or Nb2O5 as secondary. In order to find the potentials at which a color change occurred in the electrochromic devices, preliminary measurements were carried out over a wide change of potential differences between the electrodes and the potentials chosen for the device tests were  1.5 and + 1.5 V, for the red and blue color, respectively. All the assembled devices showed transitions between blue and red, as the PEDOT and Nb2O5 films go through transitions from colorless to blue when they are reduced, while PMT goes from blue to red and the ITO electrode is optically inactive. Fig. 2 shows the images of a device in operation in its different states of coloration, assembled with PMT and PEDOT. It was observed that when the devices are assembled and dried in the open air, bubbles are formed in the electrolyte because the solvent evaporates too fast. In order to minimize this problem, the drying should be done in a desiccator with THF vapor. The reason for adding the plasticizer, was to increase the conductivity of the polymer electrolyte P(EPIEO) to values of the order of 10  4 Scm  1 [35]. Fig. 3 displays the spectra of the devices in their red and blue color states. It can be seen that among the three electrochromic devices, the one assembled with PEDOT as secondary electrode exhibited the largest variation in transmittance, DT(650 nm)¼50%, while the devices with Nb2O5 and ITO gave DT(650 nm)¼47% and 45%, respectively. We expected a higher DT for the Nb2O5 film. However, during the heating process in the film preparation, the electrode was slightly bent, and this could have given rise to tension in the oxide layer during cooling, possibly causing microcracks to form on the film surface. As a consequence, the light could be scattered on the surface of the film, leading to a reduction in its transparency. Table 1 Anodic and cathodic charge densities of PMT, PEDOT and Nb2O5 films, and their coulombic efficiency, referring to the cyclic voltammograms of Fig. 1.

Fig. 1. Cyclic voltammograms of PMT, PEDOT and Nb2O5 films. n ¼ 50 mV s 0.1 mol L  1 LiClO4/CH3CN electrolyte.

1

,

PMT PEDOT Nb2O5

Qo (mC cm  2)

Qr (mC cm  2)

CE (%)

2.60 2.16 2.72

2.26 2.54 3.32

115 85 82

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Fig. 2. PMT/electrolyte/PEDOT electrochromic device in operation. Potential of PMT relative to PEDOT: (a)  1.5 V (red) and (b) +1.5 V (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

An increase in the transmittance variation was expected in the devices assembled with an electrochromically active secondary electrode, PEDOT or Nb2O5, since these electrodes become blue when reduced and thus should contribute to the darkening of the device during the oxidation of PMT. This is the intention of using complementary active materials in electrochromic devices. Nevertheless, as the potential difference applied to the PMT is related to that on the secondary electrode since each device has different secondary electrode, it was not possible to make a quantitative comparison between the three devices. This is because the doping levels in PMT and in the other materials may be different in each device. With regard to the optical contrasts of the devices assembled in this study, the values are acceptable for most electrochromic device applications where DT430% [3]. However, other important parameters must be taken into account, such as the response time (t) and the electrochromic efficiency (Z). The response time of an electrochromic device is the time taken to change its color after an electric stimulus. It is defined as the time the sample takes to change its transmittance by 2/3 of the total transmittance change, when a potential step is applied [36]. The electrochromic efficiency is defined as the ratio between the optical density variation (DOD) at a given wavelength (l) and the charge density injected into the material or electrochromic device (Q) [10]. The coulombic efficiency is another important parameter and can also be related to the performance of the device. To calculate the electrochromic efficiency of the devices, electro-optical measurements were made at the same potentials used to obtain the transmittance spectra (  1.5 and + 1.5 V). Fig. 4 illustrates the curves of current density and transmittance (l ¼650 nm) plotted against time for the 10th, 500th and 1000th double potential steps for each device. Table 2 shows the darkening and bleaching electrochromic efficiencies of the devices at l ¼ 650 nm, as well as with their response times, for the 10th potential step. For the devices assembled with ITO and Nb2O5, a longer response time is required for the coloration to

Fig. 3. Transmittance spectra of the electrochromic devices in red (- - -) and blue (—) states.

occur and this can be related to the intercalation speed of the ions within the oxide matrix, which is slower than that in the polymers. The electrocoloration mechanisms, in both the conducting polymer and the transition metal oxides, are quite complex and have not yet been described properly in the literature. In the case of the transition metal oxides, controversy persists about the nature of the color change. Regardless of the source, it is well established that the coloration process depends on the intercalation of ionic species in the oxide matrix. The process of electro-insertion of small ions into transition metal oxide structures, including that of Li + in Nb2O5, has been described by several authors [30,37,38]. Although the diffusion coefficients cited in the literature [35,38–42] indicate faster processes in polymeric films than in electrochromic oxide films, it is necessary to take care when comparing these systems, because they are films with very different physical and morphological characteristics, involving different types of measurement. Regarding the results obtained here, we believe that the shorter response times observed for polymers are related mainly to factors such as higher porosity or even the more amorphous structure. Thus, it is expected that the higher the porosity of the film, the easier the electrolyte permeation and, therefore, the larger the area of the material

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Fig. 4. Current density and transmittance (l ¼650 nm) plotted against time for the 10th (—), 500th (- - -) and 1000th ( . . . ) double potential steps of PMT/electrolyte/ITO, PMT/electrolyte/PEDOT and PMT/electrolyte/Nb2O5 electrochromic devices.

Table 2 Anodic and cathodic charge densities, coulombic efficiency, optical density variation, electrochromic efficiencies and response times for the darkening and bleaching processes for the 10th potential step obtained by spectroelectrochemical measurements on the electrochromic devices.

PMT/electrolyte/ITO PMT/electrolyte/PEDOT PMT/electrolyte/Nb2O5 a

Qo (mC cm  2)

Qr (mC cm  2)

CE (%)

DODa

Zdark a (cm2 C  1)

Zble a (cm2 C  1)

tdark (s)

tble (s)

3.16 3.19 2.70

2.92 3.01 2.50

108 106 108

0.29 0.35 0.34

92 110 126

99 116 136

1.6 1.5 2.1

2.5 1.5 2.9

measured at 650 nm.

that goes through the electro-optical processes cited above at the same time. On the other hand, considering the structural factor, an amorphous polymeric matrix should, in principle, facilitate the incorporation of other species into its structure much more readily than the rigid crystal matrix present in oxides. Although the crystal structure of Nb2O5 has not been the focus of this work, it has been reported by Rosario and Pereira [29] that films of

Nb2O5 prepared under similar conditions are fully crystalline and predominantly in the orthorhombic phase. The response times obtained in this work are considered acceptable for such applications. The device assembled with PEDOT gave the best response times around 1.5 s, since the coloring processes of both electrodes (PMT and PEDOT) are quite fast.

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Table 2 also presents the anodic and cathodic charge values and the coulombic efficiencies calculated from them. CE values can be employed to analyze the reversibility of the process in the devices. The reversibility was, as expected, around 100% during the first cycles. We can observe that the electrochromic devices assembled with PEDOT or Nb2O5 as secondary electrodes gave better results than the device assembled with PMT as the only electrochromic material. For the darkening process, the devices with PEDOT and Nb2O5 exhibited Zdark of 110 and 126 cm2 C  1, and for bleaching Zble of 116 and 136 cm2 C  1, respectively. These values confirm again that better performance is obtained with a device assembled with a complementary secondary electrode. De Paoli et al. [15] built electrochromic devices in which only polymers were used as electrochromic materials, polypyrrole doped with dodecylsulphate and indigo carmin (PPy-DS-IC) with PEDOT/PSS and poly(N0 ,N0 -dimethyl-bipyrrole) (poly(NNDMBP)) with poly(40 ,40 -dipentoxy-20 20 -bithiophene) (Poly(ET2)) doped with perchlorate, and obtained electrochromic efficiency values of 360 cm2 C  1 (l ¼640 nm), and 160 cm2 C  1 (l ¼ 620 nm), respectively. Arbizzani et al. [43] also studied devices assembled with PMT and poly(3,30 -methyl-2,20 -bithiophene) and ITO, which presented electrochromic efficiencies of 250 and 150 cm2 C  1, respectively, in the region of l ¼560 nm. Although some of these values are close to those reported here, the conditions used in these studies, such as the electrodes, operating potentials, device assembling method, wavelength of the optical studies, among several others, were different. In order to verify the electrochromic device stability, successive potential steps up to 1000 cycles were also analyzed (Fig. 4).

It can be observed that during the experiment the reduction and oxidation charges tend to decrease in all devices. This may be related to the partial degradation of the polymers or the oxide, in the case of the device with Nb2O5. Due to this possible degradation of the electrochromic material, the DT values also tend to decrease. The results also show an increase in the electrochromic efficiency of all the devices after 1000 cycles, as can be seen in Fig. 5. This apparent efficiency improvement could be related to a relaxation of the structure of the material after many steps. Another possibility may be falls in the values of Qo and Qr during the experiment that are not directly related to the electrochemical processes of bleaching and coloration of the devices, but only to one electrode, in this case, PMT. After 1000 steps, the devices assembled with ITO, PEDOT and Nb2O5 retained 67%, 74% and 79% of their initial transmittance, respectively. Their coulombic efficiency remained near 100%, with little change after 1000 steps, showing an equilibrium of the redox processes. Regarding the response time, it can also be observed in Fig. 5 that for the device assembled with PEDOT as the secondary electrode, tdark is maintained constant, whereas tble practically doubles after 500 cycles. For the device with ITO, both tdark and tble oscillate around their average values. For the device assembled with Nb2O5, the color changes require longer times, and this may be related to the increasing difficulty of inserting Li + ions into the oxide. This, in turn, could be due to modification of

Fig. 5. Electrochromic efficiency and response time of ( ) coloration and ( ) bleaching for each electrochromic device, over 1000 double potential steps.

Fig. 6. Optical memory (l ¼ 650 nm) of the electrochromic devices polarized at  1.5 V (red, - - -) and + 1.5 V (blue, —) (PMT vs. secondary electrode).

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the oxide by the repeated polarizations, so that it retains part of the Li + inserted into its crystal lattice. An important parameter that should be taken into account in electrochromic devices, depending on their application, is the optic memory. In this work, the device was polarized in its red (  1.5 V PMT vs. E2) or blue (+ 1.5 V) color state for a sufficient time (60 s) for it to stabilize and then the electric circuit was opened and the spectrum was monitored at a well-known wavelength (l ¼650 nm) for 120 min. The smaller the transmittance variation in a given time after the circuit is opened, the better the optical memory of the device. Fig. 6 shows the transmittance responses plotted against time. It can be observed that all the devices were more stable in the red state than in the blue. We observe that, in the absence of a secondary electrochromic material in the device, its coloration tends to remain red, more intensely than with the secondary electrode, Nb2O5 or PEDOT. In the case of the electrochromic device assembled with PEDOT, we observe a good optical memory for its blue color. This may be related to the capacitive property exhibited by PEDOT, which is able to store charge in its structure for a longer time. For the device assembled with Nb2O5, the optical memory for the blue color was relatively low, and this may be related to the fact that Nb2O5 is more stable in its oxidized state than in its reduced state.

4. Conclusions The following electrochromic devices, using poly(epichlorhydrin-co-ethylene oxide)/LiClO4 electrolyte, were constructed and characterized: PMT9electrolyte9ITO, PMT9electrolyte9PEDOT and PMT9electrolyte9Nb2O5. PMT was used as primary electrode (E1) and PEDOT, Nb2O5 or ITO as secondary electrode (E2). The devices showed color variations between red and blue, according to the applied potential, 1.5 or + 1.5 V (PMT vs. E2), respectively. The transmittance variation (DT) in the visible region (l ¼650 nm) for the polarized devices at these two potentials was measured and DT values above 45% were obtained for all three devices. We observe that for the devices assembled with electrochromic materials as E2, this value was higher than for the device assembled with only ITO as E2. The systems were also characterized in terms of their coulombic efficiency (CE) and achieved values were around 107%. The electrochromic efficiency (Z) of the devices assembled with PEDOT (Zdark ¼110 cm2 C  1) and Nb2O5 (Zdark ¼126 cm2 C  1) was higher than that of the device assembled with ITO alone (Zdark ¼92 cm2 C  1) as secondary electrode. The response times for the devices were  2 s and they tended to increase after 1000 double potential steps. The devices assembled with ITO and Nb2O5 exhibited better optical memory in their red color state, while in the device with PEDOT, good memory values for both color states were achieved. However, is difficult to compare the three devices in quantitative terms, given that they were fabricated from different materials, but all devices showed good durability, demonstrating the possibility of application of the electrochromic devices presented here.

Acknowledgements The authors would like to thank CNPq (Proc. no. 473299/ 2004-6), CT-Energ/CNPq, for the financial support and Daiso Co. Ltd., Osaka, for supplying the polymer electrolyte. E.C. Rios acknowledges CNPq for the scholarship.

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