- Email: [email protected]
Electrochromic property of a copolymer based on 5-cyanoindole and 3,4-ethylenedioxythiophene and its application in electrochromic devices
Journal of Electroanalytical Chemistry 700 (2013) 17–23
Contents lists available at SciVerse ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Electrochromic property of a copolymer based on 5-cyanoindole and 3,4-ethylenedioxythiophene and its application in electrochromic devices Wenying Yu a,b, Juan Chen b, Yunlei Fu b, Jingkun Xu a,⇑, Guangming Nie a,b,⇑ a b
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China State Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China
a r t i c l e
i n f o
Article history: Received 31 December 2012 Received in revised form 8 March 2013 Accepted 3 April 2013 Available online 18 April 2013 Keywords: Conducting polymer Electrochemical copolymerization Electrochromic devices 3,4-Ethylenedioxythiophene Indole
a b s t r a c t The spectroelectrochemical and electrochromic properties of a novel copolymer (P(CNIn-co-EDOT)) obtained from 3,4-ethylenedioxythiophene (EDOT) and 5-cyanoindole (CNIn) were discussed. P(CNInco-EDOT) can change between purple in the reduced state and blue in the oxidized state. Electrochromic devices (ECDs) based on P(CNIn-co-EDOT) and poly(3,4-ethylenedioxythiophene) (PEDOT) were also fabricated. This as-prepared ECD showed good optical contrast (48% at 630 nm), high coloration efficiency (680 cm2 C1), fast response time (0.8 s at 630 nm), good optical memory and long-term stability. Clear change of ECD from brown (reduced state) to dark blue color (oxidized state) was demonstrated with robust cycle life. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Electrochromism is a reversible change in optical properties that can occur when a material is electrochemically oxidized (loss of electron) or reduced (gain of electron) [1]. Electrochromic devices (ECDs) have attract great interest due to their potential use in different applications such as display panels, reflectance mirrors, smart windows, vehicle sunroofs, visible/IR camouflage [2]. Recently, more and more studies have been carried out to find out novel and high-quality electrochromic materials to be used in ECD [3,4]. There are many kinds of electrochromic materials including inorganic (such as WO3 and IrO2) and organic (such as conducting polymers (CPs)) materials. However, the inorganic materials usually have poor coloration efficiency (CE) values, high cost and slow response time, limiting their applications in ECD to some degree [5–7]. On the contrary, CPs have attracted a great deal of attention in the electrochromic field because of their outstanding advantages, such as multichromism, the ease of color control through structure modification, fast switching rates, and superior CE [8– 12]. Thus, ECD based on CPs have become a recent focus of research ⇑ Corresponding authors. Address: State Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China (G. Nie). Tel.: +86 791 88537967; fax: +86 791 83823320. E-mail addresses: [email protected] (J. Xu), [email protected] (G. Nie). 1572-6657/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2013.04.007
due to their fast switching time, high contrast ratios, narrow potential windows and long-term optical stability [13]. The color change of ECD based on CPs is directly related to doping–dedoping process of the polymers. The doping process modifies the polymer electronic structure, producing new electronic states in the band gap, causing color changes [14]. And the color contrast between the dedoped and doped states is bound up with the polymer band gap (Eg, the relative energy of HOMO and LUMO) [15]. Thus, the electrochromic properties of CPs can be improved by controlling the band gap via proper choice of heteroaromatic ring and substituents [16]. The ability to tune color is an important goal in the construction of ECDs. There are some strategies to control the color of electrochromic polymers. One of the most important strategies is electrochemical copolymerization, which is a simple way to achieve color change of ECD. In addition, electrochemical copolymerization can be carried out at room temperature and homogeneous copolymer films can be formed directly at the electrode surface by which the film thickness is well controlled. By this useful method, a variety of CPs with different electrical and optical properties can be obtained. The properties of copolymers are usually intermediate between individual homopolymers [17–19]. Poly(3,4-ethylenedioxythiophene) (PEDOT), an important polythiophene derivative with two electron-donating oxygen atoms on 3,4-positions of thiophene, is an important electrochromic material due to its interesting properties such as small band gap, high
18
W. Yu et al. / Journal of Electroanalytical Chemistry 700 (2013) 17–23
conductivity, environmental stability, good chemical and electrochemical properties in comparison with other kinds of polythiophene derivatives [20–25]. In order to tune the colors of ECD based on PEDOT, there are many studies on the electrochromic properties of copolymers obtained from 3,4-ethylenedioxythiophene (EDOT) with different other monomers such as pyrrole [22], benzene [1,26], anthracene [27], carbazole [28] and fluorene [29]. These results demonstrate that fine-tuning electrochromic copolymers can be obtained due to the introduction of EDOT units, which can lead to an interesting combination of the electrochromic properties. In addition, polyindole family is of much interest due to its several advantages, especially fairly good thermal stability, good photoluminescent property and high redox activity [30–34]. Recently, we reported electrochromic properties of poly(indole-6carboxylic acid) [35] and poly(5-formylindole) [36]. Spectroelectrochemical studies show that the dual-type ECD based on poly(indole-6-carboxylic acid) and poly(5-formylindole) have fast response time, high optical contrast and CE, which provide an avenue for applications of the polyindole family to ECD. 5-Cyanoindole (CNIn) is also an important indole derivative because the electronwithdrawing AC„N group can decrease the band gap of polymers [37–39], which will be beneficial to their electrochromic applications. By means of electrochemical polymerization, we have prepared the copolymer (P(CNIn-co-EDOT)) of CNIn and EDOT with good redox activity, good thermal stability and high conductivity [40]. It is expected that this fine-tuning copolymer may possess better electrochromic properties due to the combination of EDOT and CNIn units. In this paper, we investigated the electrochromic properties of P(CNIn-co-EDOT). Its dual-type high-quality ECD was also constructed with PEDOT. The specreoelectrochemistry, electrochromic switching and open circuit memory of the device were also studied.
2. Experimental 2.1. Materials 5-Cyanoindole (CNIn, 98%; Acros Organics) and 3,4-ethylenedioxythiophene (EDOT, 98%; Aldrich) was used as received. Tetrabutylammonium tetrafluoroborate (TBATFB, 98%; Acros Organics) was dried in vacuum at 60 °C for 24 h before use. Commercial HPLC grade acetonitrile (CH3CN, 99.8%; Aldrich) was used directly without further purification. Propylene carbonate (PC) and poly(methyl metacrylate) (PMMA) were used as received. 2.2. Electrosyntheses of P(CNIn-co-EDOT) films Electrochemical syntheses and examinations were performed in a one-compartment cell with the use of a Model 263 potentiostat– galvanostat (EG&G Princeton Applied Research) under computer control. Indium-tin-oxide (ITO) coated glass with a surface area of 5 cm2 and stainless steel sheet with a surface area of 6 cm2 were employed as the working and counter electrodes, respectively. All potentials were referred to a saturated calomel electrode (SCE). The typical electrolytic solution was CH3CN containing 0.1 mol L1 TBATFB and 0.02 mol L1 CNIn and 0.08 mol L1 EDOT at 1.5 V vs. SCE. All solutions were deaerated by a dry argon stream and maintained at a slight argon overpressure during experiments. The amount of polymer deposited on the electrode was controlled by the integrated charge passed through the cell. 2.3. Construction of ECD To demonstrate the performance of P(CNIn-co-EDOT) in a real ECD, a simple transmissive type of ECD was fabricated as shown
Scheme 1. Mechanism of the construction of the ECD and the color change.
W. Yu et al. / Journal of Electroanalytical Chemistry 700 (2013) 17–23
in Scheme 1. P(CNIn-co-EDOT) and PEDOT were coated separately on ITO electrodes (ITO, Delta Tech. 7–10 X, 0.7 6 cm). The device was constructed by using the electrochromic electrodes separated by gel electrolyte (TBATFB:CH3CN:PMMA:PC in the ratio of 3:70:7:20) [41]. 2.4. Characterizations Spectroelectrochemical and kinetic studies were carried out on a Model 263 potentiostat–galvanostat (EG&G Princeton Applied Research) and a Cary 500 UV–Vis–NIR spectrophotometer under computer control. The optical density (DOD) at a specific wavelength (kmax) was determined by using the optical contrast values (DT%) of the electrochemically oxidized and reduced copolymer films, using the following equation:
DOD ¼ logðT ox =T red Þ
ð1Þ
The coloration efficiency (CE) is defined as the relation between the injected/ejected charge as a function of electrode area (Qd) and the change in optical density (DOD) at a specific dominant wavelength (kmax) as illustrated by the following equation [42,43]:
CE ¼ DOD=Q d
ð2Þ
3. Results and discussion 3.1. Electrochemical copolymerization The successive cyclic voltammograms of 0.02 mol L1 CNIn, 0.08 mol L1 EDOT and mixture of these two monomers taken in CH3CN containing 0.1 mol L1 TBATFB at a potential scanning rate of 100 mV s1 were illustrated in Fig. 1. The increase in the redox
Fig. 1. Cyclic voltammograms of (A) 0.02 mol L1 CNIn, (B) 0.02 mol L1 CNlIn + 0.08 mol L1 EDOT, and (C) 0.08 mol L1 EDOT in CH3CN/TBATFB, respectively. Scanning rates: 100 mV s1.
19
wave currents indicated that the amount of polymer on the electrode was increasing. As shown in Fig. 1A, PCNIn can be oxidized and reduced between 1.25 and 0.95 V. On the other hand, PEDOT shows broader redox waves mainly locating between 0.85 and 0.25 V (Fig. 1C). When the cyclic voltammograms are taken in the electrolytic solutions containing these two monomers (0.02 mol L1 CNIn and 0.08 mol L1 EDOT), obvious difference of cyclic voltammograms can be easily observed (Fig. 1B). The evolution of a new redox wave at potentials (between 1.45 and 0.35 V) different from the potentials of both pure CNIn and EDOT indicated the formation of a copolymer. 3.2. Electrochromic properties of the copolymer In order to probe the electronic structure of the copolymer and examine the nature of electrochromism in the electrochromic polymers, spectroelectrochemical analysis was performed [44]. For the investigations, P(CNIn-co-EDOT) films were cast on an ITO-coated glass slide, and an electrochemical cell was built from a commercial ultraviolet UV–Vis cuvette. The cell was placed in the optical path of the sample light beam in a UV–Vis–NIR spectrophotometer, which allowed us to acquire electronic absorption spectra under potential control. The spectroelectrochemical and electrochromic properties of the copolymer were studied by applying potentials between 1.0 V and +1.0 V in monomer free CH3CN containing 0.1 mol L1 TBATFB. Fig. 2 shows the spectroelectrochemistry of P(CNIn-co-EDOT) revealing p–p transition in the reduced state which fades upon increasing the applied voltage. In the reduced state, the main absorption band is observed at 475 nm and tailed to around 700 nm due to the p–p transition. The band gap value of the polymer is 1.65 eV with p–p transitions on p-doping according to the onset of the lower energy p–p transitions. And in this reduced state, the corresponding color of P(CNIn-co-EDOT) film is purple (Fig. 2, inset). It is well-known that the oxidation of an electrochromic material will produce radical cations (polarons) and further oxidation produce dications (bipolarons), allowing new electronic transition thereby changing absorption spectra [45]. Upon stepwise increase of the applied potential, formation of charge carriers leads to new absorption bands at longer wavelength. When the oxidation potential reaches 1.0 V, the intensity of the p–p transition reduces and the simultaneous increase of absorbance at 800 nm can be observed. This change represents the formation of polaron and bipolaron bands. In this oxidation
Fig. 2. Spectroelectrochemistry for the P(CNIn-co-EDOT) on ITO-coated glass in monomer free CH3CN/TBATFB solution at applied potentials (V): (a) 1.0, (b) 0.7, (c) 0.5, (d) 0.3, (e) 0.2, (f) 0.1, (g) 0.0, (h) 0.1(i) 0.2, (j) 0.3, (k) 0.4, (l) 0.5, (m) 0.6, (n) 0.7, (o) 0.8, (p) 0.9, (q) 1.0. The inset was real photo for the films.
20
W. Yu et al. / Journal of Electroanalytical Chemistry 700 (2013) 17–23
Fig. 3. (a) Potential–time, (b) current–time, and (c) transmittance–time profiles of P(CNIn-co-EDOT) films at 475 nm and 800 nm recorded during double step spectrochronoamperometry between 1.0 V and 1.0 V for a switching time of 5 s.
state, the copolymer film shows a blue1 color (Fig. 2, inset). The polaronic states arise essentially in the near-IR, giving color to transmissive electrochromic copolymers.
3.3. Electrochromic switching of P(CNIn-co-EDOT) film It is important that polymers can switch rapidly and exhibit striking color changes, revealing superior results in electrochromic applications. Hence, electrochromic switching studies are performed to examine properties like switching time and optical contrast. The switching study speed was reported as the time for 95% of the full optical switch (after which the naked eye cannot sense 1 For interpretation of color in Figs. 2 and 4, the reader is referred to the web version of this article.
the color change). The dynamic electrochromic experiment for P(CNIn-co-EDOT) was carried out at 475 nm and 800 nm, respectively. For this purpose, square wave potential step methods were coupled with optical spectroscopy, named chronoabsorptometry, to investigate the switching ability of P(CNIn-co-EDOT) between its neutral and doped state (Fig. 3). The applied potential was interchanged between 1.0 V (the neutral state) and 1.0 V (the oxidized state) at regular intervals of 5 s. One important characteristic is the optical contrast (DT%), which can be defined as the transmittance difference between the redox states. The DT% of the P(CNIn-coEDOT) was found to be 32% at 475 nm and 30% at 800 nm, as showed in Fig. 3c. These values were better than copolymer of benzene and EDOT (about 25%) [1]. Coloration efficiency (CE) is another key parameter for electrochromic materials as it describes the change in optical density at the wavelength of interest with per inserted charge [46]. In this study, CE values were measured
W. Yu et al. / Journal of Electroanalytical Chemistry 700 (2013) 17–23
as 84 cm2 C1 at 475 nm and 109 cm2 C1 at 800 nm, which were similar with copolymer of pyrrole and EDOT (74 or 104 cm2 C1) [19]. Response time, one of the most important parameters, is the time needed to perform switching between the neutral state and oxidized state of materials. The response required to attain 95% of total transmittance difference at 475 nm was found to be 2.2 s from the reduced to the oxidized state and 1.2 s from the oxidized to the reduced state, which was better than copolymer of benzene and EDOT (about 4.2 s) [1]. The corresponding response time was found to be 1.7 s from the reduced to the oxidized state and 0.9 s from the oxidized to the reduced state at 800 nm. Thus, copolymer can be rapidly switched to the reduced state, which can be attributed to the ease of charge transport in the conducting film when it is reduced [47]. Reduction of the oxidized copolymer may begin at any site within the film. Based on the discussion, the extreme stability of the DT% in time and the fast switching property make this copolymer a promising electrochromic material [48]. 3.4. Spectroelectrochemical properties of P(CNIn-co-EDOT)/PEDOT ECD It has been demonstrated that a dual-polymer ECD system using two complementary electrochromic electrodes can effectively improve electrochemical and electrochromic performances, such as optical contrast, response time, and optical memory [49]. For this reason, in this work, PEDOT film was used as a complementary cathodic electrode to the P(CNIn-co-EDOT) film during the oxidation and reduction processes. A simple transmissive dual type ECD consisting of P(CNIn-co-EDOT) and PEDOT was fabricated as shown in Scheme 1 and its spectroelectrochemical behaviors were also studied. This real ECD consists of P(CNIn-co-EDOT) and PEDOT electrochromic materials (one anodically coloring, the other cathodically coloring) deposited on transparent ITO, placed in a position to face each other and a gel electrolyte was applied in between. The cathode material PEDOT and anticathode material P(CNIn-co-EDOT) were both obtained in CH3CN containing 0.1 mol L1 TBATFB on ITO. The anodically coloring P(CNIn-coEDOT) film was fully reduced and the cathodically coloring PEDOT film was fully oxidized prior to construction of this ECD. In order to get a balanced number of redox sites for switching, the redox charges of the two complementary polymer films must be matched by chronocoulometry. Spectroelectrochemical measurement was also performed in the study of this ECD upon the increasing poten-
Fig. 4. Spectroelectrochemistry of P(CNIn-co-EDOT)/PEDOT device at applied potentials (V): (a) 1.0, (b) 0.8, (c) 0.6, (d) 0.4, (e) 0.2, (f) 0.0, (g) 0.2, (h) 0.4, (i) 0.6, (j) 0.8, (k) 1.0, (l) 1.2, (m) 1.4, (n) 1.6, (o) 1.8, (p) 2.0. The inset was real photo for the device.
21
tials. As can be seen from Fig. 4, the ECD was switched between 1.0 V and 2.0 V. At 1.0 V, the P(CNIn-co-EDOT) film layer was in its neutral state, where the absorption was at 490 nm; while at 1.0 V, PEDOT was in the oxidized state, and the device color was brown (Fig. 4, inset). When P(CNIn-co-EDOT) film started to be oxidized while PEDOT layer was in its reduced state at 2.0 V, which led to a new absorption at 630 nm due to the reduced state of PEDOT, and the device color was dark blue (Fig. 4, inset). One of the most important characteristics of ECD is the response time. Kinetic studies are also used to test the response time which is needed to perform switching between the oxidized and reduced states of this device. For this purpose, chronoabsorptometry was employed by stepping the potential with a residence time of 5 s. Under a square potential input of 1.0 V and 2.0 V, the optical response at 630 nm and the electrical response of the device were recorded at the same time, which was shown in Fig. 5. At wavelength of 630 nm, the response time was found to be 0.8 s at 95% of the maximum transmittance, and the optical contrast (DT%) were calculated to be 48%, which was a higher than another ECD obtained from the copolymer of EDOT and anthracene (DT% and response time were 23% and 1.2 s, respectively) [50]. The CE was calculated to be 680 cm2 C1, for the device, a high and advantageous value, especially for large area devices. The CE value of this ECD was higher than that of the copolymer film (84 cm2 C1). The high CE value of this ECD can be ascribed to the compensation of the charge bal-
Fig. 5. (a) Potential–time, (b) current–time, and (c) transmittance–time profiles of P(CNIn-co-EDOT)/PEDOT ECD at 630 nm under an applied square voltage signal between 1.0 V (the neutral state) and 2.0 V (the oxidized state).
22
W. Yu et al. / Journal of Electroanalytical Chemistry 700 (2013) 17–23
ance between the active electrodes and faster dopant ion diffusion during the ECD redox process, because the cathodically coloring polymer (PEDOT) have rapid response times and low power requirements. It is well-known that a strongly electron-donating alkylenedioxy bridge attached across the 3- and 4-positions of the EDOT backbone can prevent linkage defects during electrochemical polymerization and add electron density to the aromatic ring. This unique structure not only reduce the oxidation potentials but also alter the stability of the oxidized state and band-gap of PEDOT, which lead to the more-pronounced optical transparency in the visible spectrum, higher efficient color changes in a narrow potential window, and the faster response time of PEDOT between the neutral and doped states [51]. 3.5. Open circuit stability of ECD The optical memory is an important parameter because it is directly related to its application and energy consumption during the use of ECD. This property was studied by polarizing the material between two states and by measuring its spectra under open circuit conditions at regular time intervals, as shown in Fig. 6. This device was polarized with 1.0 V for 5 s, the circuit was held disconnected for 100 s and the transmittance was recorded at the time. Then, the same procedure was repeated with 2.0 V. Fig. 6 illustrated the optical spectrum at 630 nm as a function of time at open circuit conditions. Both green and dark blue states are highly stable and the device keeps its color without significant loss declared by the insignificant variations in optical contrast. The mechanism of switching is based on reversible redox reactions, which offer bistability in each state [52]. Based on this discussion, P(CNIn-co-EDOT)/PEDOT ECD is well performing under open circuit conditions, indicating this ECD has potential applications such as polymeric memory devices with good optical memory. 3.6. Stability of ECD The stability of the polymer is another crucially important feature in the practical use of the electrochromic material. Long-term stability of redox activity of this ECD was investigated to evaluate the stability of the devices. The potential was scanned between 1.0 and 1.5 V (Fig. 7). Form 1st to 10th, there was no obvious decrease of activity. After 1000 cycles, the CV curve of this ECD reveals the film retaining 84% of its original electroactivity and the changes in anodic (jpa) and cathodic peak current densities (jpc) are 18% and 19%, respectively. When this ECD was scanned continuously up to 2000 cycles, there was no obvious lost of electroactiv-
Fig. 7. Cyclic voltammograms of P(CNIn-co-EDOT)/PEDOT ECD as a function of repeated with a scan rate of 100 mV s1.
ity. These results imply that the ECD has a reasonable environmental and redox stability and could be as a promising candidate material for ECD. 4. Conclusions A novel soluble electrochromic copolymer (P(CNIn-co-EDOT)) based on the 5-cyanoindole and EDOT was successfully prepared via electrocopolymerization. The electrochromic properties of the as-prepared copolymer are characterized and investigated by spectroelectrochemsitry. This copolymer film exhibits excellent electrochromism, low band gap and fast switching response. And the copolymer film shows the optical contrast of 32% at 475 nm and 30% at 800 nm, respectively. The ECD constructed by P(CNIn-coEDOT) and PEDOT has good optical contrast (48% at 630 nm), high coloration efficiency (680 cm2 C1), fast response time (0.8 s at 630 nm) and good optical memory and long-term stability. This copolymer film is expected to be as a promising candidate material for ECD. Acknowledgements This work was supported by Postdoctoral Foundation of Jiangxi Province, China Postdoctoral Science Foundation (2012M521288), the Scientific and Technical Development Project of Qingdao (11– 2–4–3–(10)–jch), NSF of Shandong (No. ZR2011BM003), China Doctor Station Foundation of Higher Education (20123719120006). References
Fig. 6. Open circuit memory of the P(CNIn-co-EDOT)/PEDOT ECD monitored at 630 nm. Applied potentials: +2.0 V and 1.0 V.
[1] S. Kiralp, P. Camurlu, G. Gunbas, C. Tanyeli, I. Akhmedov, L. Toppare, J. Appl. Polym. Sci. 112 (2009) 1082–1087. [2] P. Camurlu, C. Gültekin, Sol. Energy Mater. Sol. Cells 107 (2012) 142–147. [3] B. Hu, Y. Zhang, X.J. Lv, M. Ouyang, Z.Y. Fu, C. Zhang, Opt. Mater. 34 (2012) 1529–1534. [4] H. Akpinar, A.G. Nurioglu, L. Toppare, J. Electroanal. Chem. 683 (2012) 62–69. [5] P. Chandrasekhar, B.J. Zay, G.C. Birur, S. Rawal, E.A. Pierson, L. Kauder, T. Swanson, Adv. Funct. Mater. 12 (2002) 95–103. [6] J.L. Boehme, D.S.K. Mudigonda, J.P. Ferraris, Chem. Mater. 13 (2001) 4469– 4472. [7] D. Wei, M.R.J. Scherer, C. Bower, P. Andrew, T. Ryhänen, U. Steiner, Nano Lett. 12 (2012) 857–1862. [8] K. Krishnamoorthy, A.V. Ambade, M. Kanungo, A.Q. Contractor, A. Kumar, J. Mater. Chem. 11 (2001) 471–475. [9] A.L. Dyer, M.R. Craig, J.E. Babiarz, K. Kiyak, J.R. Reynolds, Macromolecules 43 (2010) 4460–4467. [10] C.G. Wu, M.I. Lu, S.J. Chang, C.S. Wei, Adv. Funct. Mater. 17 (2007) 3153–3158. [11] L. Ma, Y. Li, X.Y.Q. Yang, C.H. Noh, Sol. Energy Mater. Sol. Cells 92 (2008) 1253– 1259. [12] Y.J. Tao, Y.J. Zhou, X.Q. Xu, Z.Y. Zhang, H.F. Cheng, W.W. Zheng, Electrochim. Acta 78 (2012) 353–358.
W. Yu et al. / Journal of Electroanalytical Chemistry 700 (2013) 17–23 [13] P.M. Beaujuge, J.R. Reynolds, Chem. Rev. 110 (2010) 268–320. [14] B. Wang, J.S. Zhao, C.S. Cui, R.M. Liu, J.F. Liu, H.S. Wang, H.T. Liu, Electrochim. Acta 56 (2011) 4819–4827. [15] M. Ak, E. Sahmetlioglu, L. Toppare, J. Electroanal. Chem. 621 (2008) 55–61. [16] B. Bingöl, P. Camurlu, L. Toppare, J. Appl. Polym. Sci. 100 (2006) 1988–1994. [17] R.M. Latonen, C. Kvarnström, A. Ivaska, Electrochim. Acta 44 (1999) 1933– 1943. [18] R. Oliver, A. Muñoz, C. Ocampo, C. Alemán, E. Armelin, F. Estrany, Chem. Phys. 328 (2006) 299–306. [19] G. Nie, H. Yang, S. Wang, X. Li, Crit. Rev. Solid State Mater. Sci. 36 (2011) 209– 228. [20] L.B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12 (2000) 481–494. [21] H.W. Heuer, R. Wehrmann, S. Kirchmeyer, Adv. Funct. Mater. 12 (2002) 89–94. [22] Y.J. Tao, H.F. Cheng, W.W. Zheng, Z.Y. Zhang, D.Q. Liu, Synth. Met. 162 (2012) 728–734. [23] S.J. Tu, Y.C. Wang, J.W. Lan, Q.K. Zheng, J. Wei, S. Chen, J. Appl. Polym. Sci. 124 (2012) 2625–2631. [24] F. Wang, M.S. Wilson, R.D. Rauh, P. Schottland, B.C. Thompson, J.R. Reynolds, Macromolecules 33 (2000) 2083–2091. [25] W.J. Doherty III, R.J. Wysocki Jr., N.R. Armstrong, S.S. Saavedra, J. Phys. Chem. B 110 (2006) 4900–4907. [26] C. Zhang, C. Hua, G.H. Wang, M. Ouyang, C.N. Ma, J. Electroanal. Chem. 645 (2010) 50–57. [27] Y.J. Tao, Z.Y. Zhang, X.Q. Xu, Y.J. Zhou, H.F. Cheng, W.W. Zheng, Electrochim. Acta 77 (2012) 157–162. _ Electrochim. Acta 65 (2012) 104–114. [28] A. Aysel, K. Ismet, [29] G. Nie, H. Yang, J. Chen, Z. Bai, Org. Electron. 13 (2012) 2167–2176. [30] J. Xu, G. Nie, S. Zhang, X. Han, J. Hou, S. Pu, J. Polym. Sci. Part A: Polym. Chem. 43 (2005) 1444–1453. [31] G. Nie, Z. Bai, J. Chen, W. Yu, ACS Macro. Lett. 1 (2012) 1304–1307. [32] J. Xu, W. Zhou, J. Hou, S. Pu, L. Yan, J. Wang, J. Polym. Sci. Part A: Polym. Chem. 47 (2005) 3986–3997.
23
[33] G. Nie, T. Cai, S. Zhang, Q. Bao, J. Xu, Electrochim. Acta 52 (2007) 7097–7106. [34] R. Hassanien, A.H. Mariam, S.A.F. Al-Said, R. Little, L. Siller, N.G. Wright, A. Houlton, B.R. Horrocks, ACS Nano 4 (2010) 2149–2159. [35] G. Nie, L. Zhou, Q. Guo, S. Zhang, Electrochem. Commun. 12 (2010) 160–163. [36] G. Nie, L. Zhou, H. Yang, J. Mater. Chem. 21 (2011) 13873–13880. [37] J.G. Mackintosh, C.R. Redpath, A.C. Jones, P.R.R. Langridge-Smith, A.R. Mount, J. Electroanal. Chem. 388 (1995) 179–185. [38] H. Talbi, B. Humbert, D. Billaud, Spectrochim. Acta A 54 (1998) 1879–1893. [39] H. Talbi, D. Billaud, Synth. Met. 93 (1998) 105–110. [40] G. Nie, L. Qu, J. Xu, S. Zhang, Electrochim. Acta 53 (2008) 8351–8358. [41] A. Metin, P. Camurlu, F. Yilmaz, L. Cianga, Y. Yagci, L. Toppare, J. Appl. Polym. Sci. 102 (2006) 4500–4505. [42] C.L. Gaupp, D.M. Welsh, R.D. Rauh, J.R. Reynolds, Chem. Mater. 14 (2002) 3964–3970. [43] B.D. Reeves, C.R.G. Grenier, A.A. Argun, A. Cirpan, T.D. McCarley, J.R. Reynolds, Macromolecules 37 (2004) 7559–7569. [44] E. Unur, P.M. Beaujuge, S. Ellinger, J.H. Jung, J.R. Reynolds, Chem. Mater. 21 (2009) 5145–5153. [45] S. Celebi, A. Balan, B. Epik, D. Baran, L. Toppare, Org. Electron. 10 (2009) 631– 636. [46] L. Zhang, S. Xiong, J. Ma, X. Lu, Sol. Energy Mater. Sol. Cells 93 (2009) 628–631. [47] G.A. Sotzing, J.R. Reynolds, Chem. Mater. 8 (1996) 882–889. [48] G. Sonmez, C.K.F. Shen, Y. Rubin, F. Wudl, Angew. Chem. Int. Ed. 43 (2004) 1498–1502. [49] J.H. Kang, Y.J. Oh, S.M. Paek, S.J. Hwang, J.H. Choy, Sol. Energy Mater. Sol. Cells 93 (2009) 2040–2044. [50] A. Yildirima, S. Tarkuc, M. Ak, L. Toppare, Electrochim. Acta 53 (2008) 4875– 4882. [51] J.H. Kang, Z. Xu, S.M. Paek, F. Wang, S.J. Hwang, J. Yoon, J.H. Choy, Chem. Asian J. 6 (2011) 2123–2129. [52] J.K. Koh, J.H. Kim, B.G. Kim, J.H. Kim, E.Y. Kim, Adv. Mater. 23 (2011) 1641– 1646.
Contents lists available at SciVerse ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Electrochromic property of a copolymer based on 5-cyanoindole and 3,4-ethylenedioxythiophene and its application in electrochromic devices Wenying Yu a,b, Juan Chen b, Yunlei Fu b, Jingkun Xu a,⇑, Guangming Nie a,b,⇑ a b
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China State Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China
a r t i c l e
i n f o
Article history: Received 31 December 2012 Received in revised form 8 March 2013 Accepted 3 April 2013 Available online 18 April 2013 Keywords: Conducting polymer Electrochemical copolymerization Electrochromic devices 3,4-Ethylenedioxythiophene Indole
a b s t r a c t The spectroelectrochemical and electrochromic properties of a novel copolymer (P(CNIn-co-EDOT)) obtained from 3,4-ethylenedioxythiophene (EDOT) and 5-cyanoindole (CNIn) were discussed. P(CNInco-EDOT) can change between purple in the reduced state and blue in the oxidized state. Electrochromic devices (ECDs) based on P(CNIn-co-EDOT) and poly(3,4-ethylenedioxythiophene) (PEDOT) were also fabricated. This as-prepared ECD showed good optical contrast (48% at 630 nm), high coloration efficiency (680 cm2 C1), fast response time (0.8 s at 630 nm), good optical memory and long-term stability. Clear change of ECD from brown (reduced state) to dark blue color (oxidized state) was demonstrated with robust cycle life. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Electrochromism is a reversible change in optical properties that can occur when a material is electrochemically oxidized (loss of electron) or reduced (gain of electron) [1]. Electrochromic devices (ECDs) have attract great interest due to their potential use in different applications such as display panels, reflectance mirrors, smart windows, vehicle sunroofs, visible/IR camouflage [2]. Recently, more and more studies have been carried out to find out novel and high-quality electrochromic materials to be used in ECD [3,4]. There are many kinds of electrochromic materials including inorganic (such as WO3 and IrO2) and organic (such as conducting polymers (CPs)) materials. However, the inorganic materials usually have poor coloration efficiency (CE) values, high cost and slow response time, limiting their applications in ECD to some degree [5–7]. On the contrary, CPs have attracted a great deal of attention in the electrochromic field because of their outstanding advantages, such as multichromism, the ease of color control through structure modification, fast switching rates, and superior CE [8– 12]. Thus, ECD based on CPs have become a recent focus of research ⇑ Corresponding authors. Address: State Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China (G. Nie). Tel.: +86 791 88537967; fax: +86 791 83823320. E-mail addresses: [email protected] (J. Xu), [email protected] (G. Nie). 1572-6657/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2013.04.007
due to their fast switching time, high contrast ratios, narrow potential windows and long-term optical stability [13]. The color change of ECD based on CPs is directly related to doping–dedoping process of the polymers. The doping process modifies the polymer electronic structure, producing new electronic states in the band gap, causing color changes [14]. And the color contrast between the dedoped and doped states is bound up with the polymer band gap (Eg, the relative energy of HOMO and LUMO) [15]. Thus, the electrochromic properties of CPs can be improved by controlling the band gap via proper choice of heteroaromatic ring and substituents [16]. The ability to tune color is an important goal in the construction of ECDs. There are some strategies to control the color of electrochromic polymers. One of the most important strategies is electrochemical copolymerization, which is a simple way to achieve color change of ECD. In addition, electrochemical copolymerization can be carried out at room temperature and homogeneous copolymer films can be formed directly at the electrode surface by which the film thickness is well controlled. By this useful method, a variety of CPs with different electrical and optical properties can be obtained. The properties of copolymers are usually intermediate between individual homopolymers [17–19]. Poly(3,4-ethylenedioxythiophene) (PEDOT), an important polythiophene derivative with two electron-donating oxygen atoms on 3,4-positions of thiophene, is an important electrochromic material due to its interesting properties such as small band gap, high
18
W. Yu et al. / Journal of Electroanalytical Chemistry 700 (2013) 17–23
conductivity, environmental stability, good chemical and electrochemical properties in comparison with other kinds of polythiophene derivatives [20–25]. In order to tune the colors of ECD based on PEDOT, there are many studies on the electrochromic properties of copolymers obtained from 3,4-ethylenedioxythiophene (EDOT) with different other monomers such as pyrrole [22], benzene [1,26], anthracene [27], carbazole [28] and fluorene [29]. These results demonstrate that fine-tuning electrochromic copolymers can be obtained due to the introduction of EDOT units, which can lead to an interesting combination of the electrochromic properties. In addition, polyindole family is of much interest due to its several advantages, especially fairly good thermal stability, good photoluminescent property and high redox activity [30–34]. Recently, we reported electrochromic properties of poly(indole-6carboxylic acid) [35] and poly(5-formylindole) [36]. Spectroelectrochemical studies show that the dual-type ECD based on poly(indole-6-carboxylic acid) and poly(5-formylindole) have fast response time, high optical contrast and CE, which provide an avenue for applications of the polyindole family to ECD. 5-Cyanoindole (CNIn) is also an important indole derivative because the electronwithdrawing AC„N group can decrease the band gap of polymers [37–39], which will be beneficial to their electrochromic applications. By means of electrochemical polymerization, we have prepared the copolymer (P(CNIn-co-EDOT)) of CNIn and EDOT with good redox activity, good thermal stability and high conductivity [40]. It is expected that this fine-tuning copolymer may possess better electrochromic properties due to the combination of EDOT and CNIn units. In this paper, we investigated the electrochromic properties of P(CNIn-co-EDOT). Its dual-type high-quality ECD was also constructed with PEDOT. The specreoelectrochemistry, electrochromic switching and open circuit memory of the device were also studied.
2. Experimental 2.1. Materials 5-Cyanoindole (CNIn, 98%; Acros Organics) and 3,4-ethylenedioxythiophene (EDOT, 98%; Aldrich) was used as received. Tetrabutylammonium tetrafluoroborate (TBATFB, 98%; Acros Organics) was dried in vacuum at 60 °C for 24 h before use. Commercial HPLC grade acetonitrile (CH3CN, 99.8%; Aldrich) was used directly without further purification. Propylene carbonate (PC) and poly(methyl metacrylate) (PMMA) were used as received. 2.2. Electrosyntheses of P(CNIn-co-EDOT) films Electrochemical syntheses and examinations were performed in a one-compartment cell with the use of a Model 263 potentiostat– galvanostat (EG&G Princeton Applied Research) under computer control. Indium-tin-oxide (ITO) coated glass with a surface area of 5 cm2 and stainless steel sheet with a surface area of 6 cm2 were employed as the working and counter electrodes, respectively. All potentials were referred to a saturated calomel electrode (SCE). The typical electrolytic solution was CH3CN containing 0.1 mol L1 TBATFB and 0.02 mol L1 CNIn and 0.08 mol L1 EDOT at 1.5 V vs. SCE. All solutions were deaerated by a dry argon stream and maintained at a slight argon overpressure during experiments. The amount of polymer deposited on the electrode was controlled by the integrated charge passed through the cell. 2.3. Construction of ECD To demonstrate the performance of P(CNIn-co-EDOT) in a real ECD, a simple transmissive type of ECD was fabricated as shown
Scheme 1. Mechanism of the construction of the ECD and the color change.
W. Yu et al. / Journal of Electroanalytical Chemistry 700 (2013) 17–23
in Scheme 1. P(CNIn-co-EDOT) and PEDOT were coated separately on ITO electrodes (ITO, Delta Tech. 7–10 X, 0.7 6 cm). The device was constructed by using the electrochromic electrodes separated by gel electrolyte (TBATFB:CH3CN:PMMA:PC in the ratio of 3:70:7:20) [41]. 2.4. Characterizations Spectroelectrochemical and kinetic studies were carried out on a Model 263 potentiostat–galvanostat (EG&G Princeton Applied Research) and a Cary 500 UV–Vis–NIR spectrophotometer under computer control. The optical density (DOD) at a specific wavelength (kmax) was determined by using the optical contrast values (DT%) of the electrochemically oxidized and reduced copolymer films, using the following equation:
DOD ¼ logðT ox =T red Þ
ð1Þ
The coloration efficiency (CE) is defined as the relation between the injected/ejected charge as a function of electrode area (Qd) and the change in optical density (DOD) at a specific dominant wavelength (kmax) as illustrated by the following equation [42,43]:
CE ¼ DOD=Q d
ð2Þ
3. Results and discussion 3.1. Electrochemical copolymerization The successive cyclic voltammograms of 0.02 mol L1 CNIn, 0.08 mol L1 EDOT and mixture of these two monomers taken in CH3CN containing 0.1 mol L1 TBATFB at a potential scanning rate of 100 mV s1 were illustrated in Fig. 1. The increase in the redox
Fig. 1. Cyclic voltammograms of (A) 0.02 mol L1 CNIn, (B) 0.02 mol L1 CNlIn + 0.08 mol L1 EDOT, and (C) 0.08 mol L1 EDOT in CH3CN/TBATFB, respectively. Scanning rates: 100 mV s1.
19
wave currents indicated that the amount of polymer on the electrode was increasing. As shown in Fig. 1A, PCNIn can be oxidized and reduced between 1.25 and 0.95 V. On the other hand, PEDOT shows broader redox waves mainly locating between 0.85 and 0.25 V (Fig. 1C). When the cyclic voltammograms are taken in the electrolytic solutions containing these two monomers (0.02 mol L1 CNIn and 0.08 mol L1 EDOT), obvious difference of cyclic voltammograms can be easily observed (Fig. 1B). The evolution of a new redox wave at potentials (between 1.45 and 0.35 V) different from the potentials of both pure CNIn and EDOT indicated the formation of a copolymer. 3.2. Electrochromic properties of the copolymer In order to probe the electronic structure of the copolymer and examine the nature of electrochromism in the electrochromic polymers, spectroelectrochemical analysis was performed [44]. For the investigations, P(CNIn-co-EDOT) films were cast on an ITO-coated glass slide, and an electrochemical cell was built from a commercial ultraviolet UV–Vis cuvette. The cell was placed in the optical path of the sample light beam in a UV–Vis–NIR spectrophotometer, which allowed us to acquire electronic absorption spectra under potential control. The spectroelectrochemical and electrochromic properties of the copolymer were studied by applying potentials between 1.0 V and +1.0 V in monomer free CH3CN containing 0.1 mol L1 TBATFB. Fig. 2 shows the spectroelectrochemistry of P(CNIn-co-EDOT) revealing p–p transition in the reduced state which fades upon increasing the applied voltage. In the reduced state, the main absorption band is observed at 475 nm and tailed to around 700 nm due to the p–p transition. The band gap value of the polymer is 1.65 eV with p–p transitions on p-doping according to the onset of the lower energy p–p transitions. And in this reduced state, the corresponding color of P(CNIn-co-EDOT) film is purple (Fig. 2, inset). It is well-known that the oxidation of an electrochromic material will produce radical cations (polarons) and further oxidation produce dications (bipolarons), allowing new electronic transition thereby changing absorption spectra [45]. Upon stepwise increase of the applied potential, formation of charge carriers leads to new absorption bands at longer wavelength. When the oxidation potential reaches 1.0 V, the intensity of the p–p transition reduces and the simultaneous increase of absorbance at 800 nm can be observed. This change represents the formation of polaron and bipolaron bands. In this oxidation
Fig. 2. Spectroelectrochemistry for the P(CNIn-co-EDOT) on ITO-coated glass in monomer free CH3CN/TBATFB solution at applied potentials (V): (a) 1.0, (b) 0.7, (c) 0.5, (d) 0.3, (e) 0.2, (f) 0.1, (g) 0.0, (h) 0.1(i) 0.2, (j) 0.3, (k) 0.4, (l) 0.5, (m) 0.6, (n) 0.7, (o) 0.8, (p) 0.9, (q) 1.0. The inset was real photo for the films.
20
W. Yu et al. / Journal of Electroanalytical Chemistry 700 (2013) 17–23
Fig. 3. (a) Potential–time, (b) current–time, and (c) transmittance–time profiles of P(CNIn-co-EDOT) films at 475 nm and 800 nm recorded during double step spectrochronoamperometry between 1.0 V and 1.0 V for a switching time of 5 s.
state, the copolymer film shows a blue1 color (Fig. 2, inset). The polaronic states arise essentially in the near-IR, giving color to transmissive electrochromic copolymers.
3.3. Electrochromic switching of P(CNIn-co-EDOT) film It is important that polymers can switch rapidly and exhibit striking color changes, revealing superior results in electrochromic applications. Hence, electrochromic switching studies are performed to examine properties like switching time and optical contrast. The switching study speed was reported as the time for 95% of the full optical switch (after which the naked eye cannot sense 1 For interpretation of color in Figs. 2 and 4, the reader is referred to the web version of this article.
the color change). The dynamic electrochromic experiment for P(CNIn-co-EDOT) was carried out at 475 nm and 800 nm, respectively. For this purpose, square wave potential step methods were coupled with optical spectroscopy, named chronoabsorptometry, to investigate the switching ability of P(CNIn-co-EDOT) between its neutral and doped state (Fig. 3). The applied potential was interchanged between 1.0 V (the neutral state) and 1.0 V (the oxidized state) at regular intervals of 5 s. One important characteristic is the optical contrast (DT%), which can be defined as the transmittance difference between the redox states. The DT% of the P(CNIn-coEDOT) was found to be 32% at 475 nm and 30% at 800 nm, as showed in Fig. 3c. These values were better than copolymer of benzene and EDOT (about 25%) [1]. Coloration efficiency (CE) is another key parameter for electrochromic materials as it describes the change in optical density at the wavelength of interest with per inserted charge [46]. In this study, CE values were measured
W. Yu et al. / Journal of Electroanalytical Chemistry 700 (2013) 17–23
as 84 cm2 C1 at 475 nm and 109 cm2 C1 at 800 nm, which were similar with copolymer of pyrrole and EDOT (74 or 104 cm2 C1) [19]. Response time, one of the most important parameters, is the time needed to perform switching between the neutral state and oxidized state of materials. The response required to attain 95% of total transmittance difference at 475 nm was found to be 2.2 s from the reduced to the oxidized state and 1.2 s from the oxidized to the reduced state, which was better than copolymer of benzene and EDOT (about 4.2 s) [1]. The corresponding response time was found to be 1.7 s from the reduced to the oxidized state and 0.9 s from the oxidized to the reduced state at 800 nm. Thus, copolymer can be rapidly switched to the reduced state, which can be attributed to the ease of charge transport in the conducting film when it is reduced [47]. Reduction of the oxidized copolymer may begin at any site within the film. Based on the discussion, the extreme stability of the DT% in time and the fast switching property make this copolymer a promising electrochromic material [48]. 3.4. Spectroelectrochemical properties of P(CNIn-co-EDOT)/PEDOT ECD It has been demonstrated that a dual-polymer ECD system using two complementary electrochromic electrodes can effectively improve electrochemical and electrochromic performances, such as optical contrast, response time, and optical memory [49]. For this reason, in this work, PEDOT film was used as a complementary cathodic electrode to the P(CNIn-co-EDOT) film during the oxidation and reduction processes. A simple transmissive dual type ECD consisting of P(CNIn-co-EDOT) and PEDOT was fabricated as shown in Scheme 1 and its spectroelectrochemical behaviors were also studied. This real ECD consists of P(CNIn-co-EDOT) and PEDOT electrochromic materials (one anodically coloring, the other cathodically coloring) deposited on transparent ITO, placed in a position to face each other and a gel electrolyte was applied in between. The cathode material PEDOT and anticathode material P(CNIn-co-EDOT) were both obtained in CH3CN containing 0.1 mol L1 TBATFB on ITO. The anodically coloring P(CNIn-coEDOT) film was fully reduced and the cathodically coloring PEDOT film was fully oxidized prior to construction of this ECD. In order to get a balanced number of redox sites for switching, the redox charges of the two complementary polymer films must be matched by chronocoulometry. Spectroelectrochemical measurement was also performed in the study of this ECD upon the increasing poten-
Fig. 4. Spectroelectrochemistry of P(CNIn-co-EDOT)/PEDOT device at applied potentials (V): (a) 1.0, (b) 0.8, (c) 0.6, (d) 0.4, (e) 0.2, (f) 0.0, (g) 0.2, (h) 0.4, (i) 0.6, (j) 0.8, (k) 1.0, (l) 1.2, (m) 1.4, (n) 1.6, (o) 1.8, (p) 2.0. The inset was real photo for the device.
21
tials. As can be seen from Fig. 4, the ECD was switched between 1.0 V and 2.0 V. At 1.0 V, the P(CNIn-co-EDOT) film layer was in its neutral state, where the absorption was at 490 nm; while at 1.0 V, PEDOT was in the oxidized state, and the device color was brown (Fig. 4, inset). When P(CNIn-co-EDOT) film started to be oxidized while PEDOT layer was in its reduced state at 2.0 V, which led to a new absorption at 630 nm due to the reduced state of PEDOT, and the device color was dark blue (Fig. 4, inset). One of the most important characteristics of ECD is the response time. Kinetic studies are also used to test the response time which is needed to perform switching between the oxidized and reduced states of this device. For this purpose, chronoabsorptometry was employed by stepping the potential with a residence time of 5 s. Under a square potential input of 1.0 V and 2.0 V, the optical response at 630 nm and the electrical response of the device were recorded at the same time, which was shown in Fig. 5. At wavelength of 630 nm, the response time was found to be 0.8 s at 95% of the maximum transmittance, and the optical contrast (DT%) were calculated to be 48%, which was a higher than another ECD obtained from the copolymer of EDOT and anthracene (DT% and response time were 23% and 1.2 s, respectively) [50]. The CE was calculated to be 680 cm2 C1, for the device, a high and advantageous value, especially for large area devices. The CE value of this ECD was higher than that of the copolymer film (84 cm2 C1). The high CE value of this ECD can be ascribed to the compensation of the charge bal-
Fig. 5. (a) Potential–time, (b) current–time, and (c) transmittance–time profiles of P(CNIn-co-EDOT)/PEDOT ECD at 630 nm under an applied square voltage signal between 1.0 V (the neutral state) and 2.0 V (the oxidized state).
22
W. Yu et al. / Journal of Electroanalytical Chemistry 700 (2013) 17–23
ance between the active electrodes and faster dopant ion diffusion during the ECD redox process, because the cathodically coloring polymer (PEDOT) have rapid response times and low power requirements. It is well-known that a strongly electron-donating alkylenedioxy bridge attached across the 3- and 4-positions of the EDOT backbone can prevent linkage defects during electrochemical polymerization and add electron density to the aromatic ring. This unique structure not only reduce the oxidation potentials but also alter the stability of the oxidized state and band-gap of PEDOT, which lead to the more-pronounced optical transparency in the visible spectrum, higher efficient color changes in a narrow potential window, and the faster response time of PEDOT between the neutral and doped states [51]. 3.5. Open circuit stability of ECD The optical memory is an important parameter because it is directly related to its application and energy consumption during the use of ECD. This property was studied by polarizing the material between two states and by measuring its spectra under open circuit conditions at regular time intervals, as shown in Fig. 6. This device was polarized with 1.0 V for 5 s, the circuit was held disconnected for 100 s and the transmittance was recorded at the time. Then, the same procedure was repeated with 2.0 V. Fig. 6 illustrated the optical spectrum at 630 nm as a function of time at open circuit conditions. Both green and dark blue states are highly stable and the device keeps its color without significant loss declared by the insignificant variations in optical contrast. The mechanism of switching is based on reversible redox reactions, which offer bistability in each state [52]. Based on this discussion, P(CNIn-co-EDOT)/PEDOT ECD is well performing under open circuit conditions, indicating this ECD has potential applications such as polymeric memory devices with good optical memory. 3.6. Stability of ECD The stability of the polymer is another crucially important feature in the practical use of the electrochromic material. Long-term stability of redox activity of this ECD was investigated to evaluate the stability of the devices. The potential was scanned between 1.0 and 1.5 V (Fig. 7). Form 1st to 10th, there was no obvious decrease of activity. After 1000 cycles, the CV curve of this ECD reveals the film retaining 84% of its original electroactivity and the changes in anodic (jpa) and cathodic peak current densities (jpc) are 18% and 19%, respectively. When this ECD was scanned continuously up to 2000 cycles, there was no obvious lost of electroactiv-
Fig. 7. Cyclic voltammograms of P(CNIn-co-EDOT)/PEDOT ECD as a function of repeated with a scan rate of 100 mV s1.
ity. These results imply that the ECD has a reasonable environmental and redox stability and could be as a promising candidate material for ECD. 4. Conclusions A novel soluble electrochromic copolymer (P(CNIn-co-EDOT)) based on the 5-cyanoindole and EDOT was successfully prepared via electrocopolymerization. The electrochromic properties of the as-prepared copolymer are characterized and investigated by spectroelectrochemsitry. This copolymer film exhibits excellent electrochromism, low band gap and fast switching response. And the copolymer film shows the optical contrast of 32% at 475 nm and 30% at 800 nm, respectively. The ECD constructed by P(CNIn-coEDOT) and PEDOT has good optical contrast (48% at 630 nm), high coloration efficiency (680 cm2 C1), fast response time (0.8 s at 630 nm) and good optical memory and long-term stability. This copolymer film is expected to be as a promising candidate material for ECD. Acknowledgements This work was supported by Postdoctoral Foundation of Jiangxi Province, China Postdoctoral Science Foundation (2012M521288), the Scientific and Technical Development Project of Qingdao (11– 2–4–3–(10)–jch), NSF of Shandong (No. ZR2011BM003), China Doctor Station Foundation of Higher Education (20123719120006). References
Fig. 6. Open circuit memory of the P(CNIn-co-EDOT)/PEDOT ECD monitored at 630 nm. Applied potentials: +2.0 V and 1.0 V.
[1] S. Kiralp, P. Camurlu, G. Gunbas, C. Tanyeli, I. Akhmedov, L. Toppare, J. Appl. Polym. Sci. 112 (2009) 1082–1087. [2] P. Camurlu, C. Gültekin, Sol. Energy Mater. Sol. Cells 107 (2012) 142–147. [3] B. Hu, Y. Zhang, X.J. Lv, M. Ouyang, Z.Y. Fu, C. Zhang, Opt. Mater. 34 (2012) 1529–1534. [4] H. Akpinar, A.G. Nurioglu, L. Toppare, J. Electroanal. Chem. 683 (2012) 62–69. [5] P. Chandrasekhar, B.J. Zay, G.C. Birur, S. Rawal, E.A. Pierson, L. Kauder, T. Swanson, Adv. Funct. Mater. 12 (2002) 95–103. [6] J.L. Boehme, D.S.K. Mudigonda, J.P. Ferraris, Chem. Mater. 13 (2001) 4469– 4472. [7] D. Wei, M.R.J. Scherer, C. Bower, P. Andrew, T. Ryhänen, U. Steiner, Nano Lett. 12 (2012) 857–1862. [8] K. Krishnamoorthy, A.V. Ambade, M. Kanungo, A.Q. Contractor, A. Kumar, J. Mater. Chem. 11 (2001) 471–475. [9] A.L. Dyer, M.R. Craig, J.E. Babiarz, K. Kiyak, J.R. Reynolds, Macromolecules 43 (2010) 4460–4467. [10] C.G. Wu, M.I. Lu, S.J. Chang, C.S. Wei, Adv. Funct. Mater. 17 (2007) 3153–3158. [11] L. Ma, Y. Li, X.Y.Q. Yang, C.H. Noh, Sol. Energy Mater. Sol. Cells 92 (2008) 1253– 1259. [12] Y.J. Tao, Y.J. Zhou, X.Q. Xu, Z.Y. Zhang, H.F. Cheng, W.W. Zheng, Electrochim. Acta 78 (2012) 353–358.
W. Yu et al. / Journal of Electroanalytical Chemistry 700 (2013) 17–23 [13] P.M. Beaujuge, J.R. Reynolds, Chem. Rev. 110 (2010) 268–320. [14] B. Wang, J.S. Zhao, C.S. Cui, R.M. Liu, J.F. Liu, H.S. Wang, H.T. Liu, Electrochim. Acta 56 (2011) 4819–4827. [15] M. Ak, E. Sahmetlioglu, L. Toppare, J. Electroanal. Chem. 621 (2008) 55–61. [16] B. Bingöl, P. Camurlu, L. Toppare, J. Appl. Polym. Sci. 100 (2006) 1988–1994. [17] R.M. Latonen, C. Kvarnström, A. Ivaska, Electrochim. Acta 44 (1999) 1933– 1943. [18] R. Oliver, A. Muñoz, C. Ocampo, C. Alemán, E. Armelin, F. Estrany, Chem. Phys. 328 (2006) 299–306. [19] G. Nie, H. Yang, S. Wang, X. Li, Crit. Rev. Solid State Mater. Sci. 36 (2011) 209– 228. [20] L.B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12 (2000) 481–494. [21] H.W. Heuer, R. Wehrmann, S. Kirchmeyer, Adv. Funct. Mater. 12 (2002) 89–94. [22] Y.J. Tao, H.F. Cheng, W.W. Zheng, Z.Y. Zhang, D.Q. Liu, Synth. Met. 162 (2012) 728–734. [23] S.J. Tu, Y.C. Wang, J.W. Lan, Q.K. Zheng, J. Wei, S. Chen, J. Appl. Polym. Sci. 124 (2012) 2625–2631. [24] F. Wang, M.S. Wilson, R.D. Rauh, P. Schottland, B.C. Thompson, J.R. Reynolds, Macromolecules 33 (2000) 2083–2091. [25] W.J. Doherty III, R.J. Wysocki Jr., N.R. Armstrong, S.S. Saavedra, J. Phys. Chem. B 110 (2006) 4900–4907. [26] C. Zhang, C. Hua, G.H. Wang, M. Ouyang, C.N. Ma, J. Electroanal. Chem. 645 (2010) 50–57. [27] Y.J. Tao, Z.Y. Zhang, X.Q. Xu, Y.J. Zhou, H.F. Cheng, W.W. Zheng, Electrochim. Acta 77 (2012) 157–162. _ Electrochim. Acta 65 (2012) 104–114. [28] A. Aysel, K. Ismet, [29] G. Nie, H. Yang, J. Chen, Z. Bai, Org. Electron. 13 (2012) 2167–2176. [30] J. Xu, G. Nie, S. Zhang, X. Han, J. Hou, S. Pu, J. Polym. Sci. Part A: Polym. Chem. 43 (2005) 1444–1453. [31] G. Nie, Z. Bai, J. Chen, W. Yu, ACS Macro. Lett. 1 (2012) 1304–1307. [32] J. Xu, W. Zhou, J. Hou, S. Pu, L. Yan, J. Wang, J. Polym. Sci. Part A: Polym. Chem. 47 (2005) 3986–3997.
23
[33] G. Nie, T. Cai, S. Zhang, Q. Bao, J. Xu, Electrochim. Acta 52 (2007) 7097–7106. [34] R. Hassanien, A.H. Mariam, S.A.F. Al-Said, R. Little, L. Siller, N.G. Wright, A. Houlton, B.R. Horrocks, ACS Nano 4 (2010) 2149–2159. [35] G. Nie, L. Zhou, Q. Guo, S. Zhang, Electrochem. Commun. 12 (2010) 160–163. [36] G. Nie, L. Zhou, H. Yang, J. Mater. Chem. 21 (2011) 13873–13880. [37] J.G. Mackintosh, C.R. Redpath, A.C. Jones, P.R.R. Langridge-Smith, A.R. Mount, J. Electroanal. Chem. 388 (1995) 179–185. [38] H. Talbi, B. Humbert, D. Billaud, Spectrochim. Acta A 54 (1998) 1879–1893. [39] H. Talbi, D. Billaud, Synth. Met. 93 (1998) 105–110. [40] G. Nie, L. Qu, J. Xu, S. Zhang, Electrochim. Acta 53 (2008) 8351–8358. [41] A. Metin, P. Camurlu, F. Yilmaz, L. Cianga, Y. Yagci, L. Toppare, J. Appl. Polym. Sci. 102 (2006) 4500–4505. [42] C.L. Gaupp, D.M. Welsh, R.D. Rauh, J.R. Reynolds, Chem. Mater. 14 (2002) 3964–3970. [43] B.D. Reeves, C.R.G. Grenier, A.A. Argun, A. Cirpan, T.D. McCarley, J.R. Reynolds, Macromolecules 37 (2004) 7559–7569. [44] E. Unur, P.M. Beaujuge, S. Ellinger, J.H. Jung, J.R. Reynolds, Chem. Mater. 21 (2009) 5145–5153. [45] S. Celebi, A. Balan, B. Epik, D. Baran, L. Toppare, Org. Electron. 10 (2009) 631– 636. [46] L. Zhang, S. Xiong, J. Ma, X. Lu, Sol. Energy Mater. Sol. Cells 93 (2009) 628–631. [47] G.A. Sotzing, J.R. Reynolds, Chem. Mater. 8 (1996) 882–889. [48] G. Sonmez, C.K.F. Shen, Y. Rubin, F. Wudl, Angew. Chem. Int. Ed. 43 (2004) 1498–1502. [49] J.H. Kang, Y.J. Oh, S.M. Paek, S.J. Hwang, J.H. Choy, Sol. Energy Mater. Sol. Cells 93 (2009) 2040–2044. [50] A. Yildirima, S. Tarkuc, M. Ak, L. Toppare, Electrochim. Acta 53 (2008) 4875– 4882. [51] J.H. Kang, Z. Xu, S.M. Paek, F. Wang, S.J. Hwang, J. Yoon, J.H. Choy, Chem. Asian J. 6 (2011) 2123–2129. [52] J.K. Koh, J.H. Kim, B.G. Kim, J.H. Kim, E.Y. Kim, Adv. Mater. 23 (2011) 1641– 1646.