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Electrosyntheses, characterizations and electrochromic properties of a copolymer based on 4,4′-di(N-carbazoyl)biphenyl and 2,2′-bithiophene
Solar Energy Materials & Solar Cells 95 (2011) 1867–1874
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Electrosyntheses, characterizations and electrochromic properties of a copolymer based on 4,40 -di(N-carbazoyl)biphenyl and 2,20 -bithiophene Bin Wang, Jinsheng Zhao n, Renmin Liu, Jifeng Liu, Qingpeng He Department of Chemistry, Liaocheng University, 252059 Liaocheng, PR China
a r t i c l e i n f o
abstract
Article history: Received 13 December 2010 Accepted 8 February 2011 Available online 3 March 2011
Electrochemical copolymerization of 4,40 -di(N-carbazoyl)biphenyl (CBP) with 2,20 -bithiophene (BT) is carried out in acetonitrile (ACN)/dichloromethane (DCM) (1:1, by volume) solution containing sodium perchlorate (NaClO4) as a supporting electrolyte. Characterizations of the resulting copolymer P(CBP-co-BT) are performed by cyclic voltammetry (CV), UV–vis spectroscopy, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and thermogravimetry (TG). The P(CBP-co-BT) film has distinct electrochromic properties and exhibits four different colors (orange yellow, blue, yellowish green and greenish blue) under various potentials. Maximum contrast (DT%) and response time of the copolymer film are measured as 51.6% and 0.94 s at 667 nm. An electrochromic device (ECD) based on P(CBP-co-BT) and poly(3,4-ethylenedioxythiophene) (PEDOT) is constructed and characterized. The optical contrast (DT%) at 700 nm is found to be 28.6% and the response time is measured as 0.47 s. The coloration efficiency (CE) of the device is calculated to be 234 cm2 C 1 at 700 nm. An ECD also has good optical memories and redox stability. & 2011 Elsevier B.V. All rights reserved.
Keywords: Electrochemistry Electrochemical polymerization Conjugated polymers Spectroelectrochemistry Electrochromic devices
1. Introduction An electrochromic material is one that changes color reversibly by an electrochemical reaction and the phenomenon is called electrochromism [1]. Electrochromic materials have received tremendous interest, due to potential applications in displays [2], energy-saving ‘‘smart’’ windows [3] and electrochromic devices [4–6]. Up to now, many different types of organic and inorganic electrochromic materials have been developed, such as inorganic metal oxide, mixed-valence metal complexes, organic small molecules and finally conjugated polymers [1]. Recently, electrochromic conjugated polymers have received much attention due to their fine-tunability of the band gap (and the color) [7], outstanding coloration efficiency [8], fast switching times, excellent processability and low cost [9]. For conjugated polymers, the electrochromism is related to the doping–dedoping process, the doping process modifies the polymer electronic structure, producing new electronic states in the band gap, causing color changes [10]. These conjugated polymers can be synthesized by either chemical or electrochemical polymerization. Compared with the chemical routes, electrochemical polymerization can obtain conjugated polymer films on conductive substrates directly. This not
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0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.02.009
only enlarges the scope of candidate polymers [11], but also omits the procedure of the film coating [12]. Among the many investigations, the electrochemical copolymerization can effectively modify the structures and properties of conjugated polymers [13], such as electrochemical activity, thermal stability and electrochromic properties. Copolymerization is a logical approach for the fine-tuning of the color for electrochromic applications [14]. Among conjugated polymers, carbazole-containing polymers have various useful properties such as easily forming relatively stable polarons (radical cations), high charge carrier mobility, photochemical stability [15] and electrochromic properties [16,17]. Electrochromic properties of poly-N-vinylcarbazole were realized around 1980 by Desbene-Monvernay et al. [18] and also studied by Chevrot and coworkers [19,20]. Recently the electrochromic properties of poly(4,40 -di(N-carbazoyl)biphenyl) (PCBP) are studied by Sermet Koyuncu et al. [17]; PCBP films exhibit reasonable electrochromic characters, but the electropolymerization of the monomer results in oligomer films and the stability of the electrochromic devices are not very well. Meanwhile, polythiophene and its derivatives have become a subject of considerable interest as electrochromic materials, because of their fast switching times, outstanding stability and high contrast ratios in the visible and near infra-red regions [21]. Among them, 2,20 -bithiophene (BT) attracts much attention due to the fact that the oxidation potential is lower than that of an unsubstituted thiophene and can give rise to structurally more ordered polythiophene [22]. It is interesting to note that the oxidation potential of
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the 4,40 -di(N-carbazoyl)biphenyl (CBP) monomer in ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4 is close to that of 2,20 -bithiophene, implying the possibility of the electrochemical copolymerization between CBP and BT [23]. According to the above considerations, in this study, the copolymer of 4,40 -di(N-carbazoyl)biphenyl (CBP) with 2,20 -bithiophene (BT) is successfully synthesized electrochemically. The electrochromic and spectroelectrochemical properties of the copolymer are studied in detail. The copolymer film has four different colors (orange yellow, blue, yellowish green and greenish blue). In addition, we constructed and characterized dual type electrochromic devices based on P(CBP-co-BT) and PEDOT in detail. Neutral state of device shows orange yellow color, while an oxidized state reveals blue color.
2. Experimental
films grown potentiostatically on an ITO is controlled by the total charge passed through the cell. 2.3. Characterizations The obtained copolymer films are studied by cyclic voltammetry. Infrared spectra are recorded on a Nicololet 5700 FT-IR spectrometer, where the samples are dispersed in KBr pellets. Thermal analysis is performed on a Pyris Diamond TG/DTA thermal analyzer (Perkin-Elmer) under a nitrogen (N2) stream in the temperature range 50–800 1C with a heating rate of 10 1C min 1. UV–vis spectra are carried out on a Perkin-Elmer Lambda 900 UV–vis-near-infrared spectrophotometer. Scanning electron microscopy (SEM) measurements are taken by using a JEOL JSM-6380LV SEM. Digital photographs of the polymer films are taken by a Canon Power Shot A3000 IS digital camera. Colorimetry measurements are obtained by a Coloreye XTH Spectrophotometer (GretagMacbeth).
2.1. Materials 2.4. Spectroelectrochemistry 2,20 -Bithiophene (BT, Aldrich, Analytical Grade), 4,40 -di(N-carbazoyl)biphenyl (CBP, Tokyo Chemical Industry CO., Ltd., Japan), dichloromethane (DCM, Sinopharm Chemical Reagent CO., Ltd., China), commercial high-performance liquid chromatography grade acetonitrile (ACN, Tedia Company, INC., USA), poly(methyl methacrylate) (PMMA, Shanghai Chemical Reagent Company), propylene carbonate (PC, Shanghai Chemical Reagent Company), 3,4-ethylenedioxythiophene (EDOT, Aldrich, 98%) and lithium perchlorate (LiClO4, Shanghai Chemical Reagent Company, 99.9%) are used directly without further purification. Sodium perchlorate (NaClO4, Shanghai Chemical Reagent Company, 98%) is dried in vacuum at 60 1C for 24 h before use. Other reagents are all used as received without further treatment. Indium–tin-oxide-coated (ITO) glass (Sheet resistance: o10 O & 1, purchased from Shenzhen CSG Display Technologies, China) is washed with ethanol, acetone and deionized water successively under ultrasonic, and then dried by an N2 flow. 2.2. Electrochemistry Electrochemical synthesis and experiments are performed in a one-compartment cell with a CHI 760 C Electrochemical Analyzer under computer control. The working and counter electrodes for cyclic voltammetric experiments are two platinum wires each with a diameter of 0.5 mm, placed 0.5 cm apart during the experiments, which are cleaned before each test. An Ag wire is used as a pseudo-reference electrode and all the potentials mentioned follow are versus the Ag wire electrode. All of the electrochemical experiments are carried out at room temperature under nitrogen atmosphere. All electrochemical polymerization and CV tests are taken in an ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4 as a supporting electrolyte. To obtain a sufficient amount of polymer for characterization, an ITO glass (the active area: 1.8 cm 2.7 cm) is employed as a working electrode. After electrochemical polymerization at + 1.3 V potentiostatically, electrochemical dedoping is carried out at 0.1 V. Then, the polymers are peeled off and washed with ACN/DCM (1:1, by volume) for three times to remove the electrolyte, oligomers and monomers. For spectral and thermal analysis, these polymers are dried under vacuum at 60 1C for 24 h. The polymer films used for the UV–vis spectral measurements, electrochromism and morphology characterization are deposited on an ITO glass (the active area: 0.9 cm 2.0 cm) at + 1.3 V potentiostatically, dedoped at 0.1 V and washed with ACN/ DCM (1:1, by volume). Moreover the thickness of the polymer
Spectroelectrochemical data are recorded on Perkin-Elmer Lambda 900 UV–vis-near-infrared spectrophotometer connected to a computer. A three-electrode cell assembly is used, where the working electrode is an ITO glass, the counter electrode is a stainless steel wire, and an Ag wire is used as the pseudoreference electrode. The copolymer films for spectroelectrochemistry are prepared by deposition potentiostatically on an ITO electrode (the active area: 0.9 cm 2.0 cm). The measurements are carried out in ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4. 2.5. Preparation of the gel electrolyte A gel electrolyte based on PMMA and LiClO4 is plasticized with PC to form a highly transparent and conductive gel. An ACN is also included as a high vapor pressure solvent to allow an easy mixing of the gel components. The composition of the casting solution by the weight ratio of ACN:PC:PMMA:LiClO4 is 70:20:7:3. The gel electrolyte is used for the construction of the polymer cell [8]. 2.6. Construction of electrochromic devices Electrochromic devices are constructed using two complementary polymers, namely P(CBP-co-BT) as the anodic material and PEDOT as the cathodic material. Both P(CBP-co-BT) and PEDOT films are electrodeposited on two ITO glass (the active area: 1.8 cm 2.7 cm) at + 1.3 and + 1.4 V, respectively. Electrochromic device is built by arranging the two polymer films (one oxidized, the other reduced), facing each other, separated by a gel electrolyte.
3. Results and discussion 3.1. Electrochemical polymerization and characterizations 3.1.1. Electrochemical polymerization The anodic polarization curves of 0.002 M CBP and 0.002 M BT in an ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4 are shown in Fig. 1. The onset oxidation potential (Epa onset) of CBP and BT in the solution is approximately + 1.02 and +1.08 V, respectively. It is well known that successful electrochemical copolymerization of different monomers is due to the fact that the Epa onset of the monomers is close to each
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other [23]. The difference of the onset oxidation potential between CBP and BT is 0.06 V, implying that the electrochemical copolymerization is readily to be achieved [24]. The synthetic route of the copolymer of CBP and BT is illustrated in Scheme 1. Fig. 2 displays the successive cyclic voltammogram (CV) curves of 0.002 M CBP, 0.002 M BT and the CBP/BT mixture at 1:1 (0.002 M CBP and 0.002 M BT) in ACN/DCM (1:1, by volume) containing 0.2 M NaClO4 between 0 and +1.3 V at a potential scan rate of 100 mV s 1. As the CV scan continued, polymer films are formed on the working electrode surface. The increase in the redox wave current densities implies that the amount of conducting polymers deposited on the electrode is increasing [25]. As shown in Fig. 2(a), the polymerization of CBP shows two reversible redox processes, the oxidation and reduction peaks of the first redox process appears at + 0.92 and +0.75 V, respectively. The reduction peak of the second redox process is located at + 1.03 V, and the corresponding oxidation peak is overlapped with the oxidation waves of the CBP monomer. The CV curves of BT show an oxidation peak at + 0.91 V and a reduction peak at + 0.82 V (Fig. 2(c)). However, the CV curve of the CBP/BT mixture at 1:1 exhibits an anodic and a cathodic peaks at +0.97 and + 0.74 V, respectively (Fig. 2(b)), which is different from those of CBP and BT, indicating the formation of a new polymer consisting of both CBP and BT units [26,27].
Fig. 1. Anodic polarization curves of 0.002 M CBP and 0.002 M BT both in ACN/ DCM (1:1, by volume) containing 0.2 M NaClO4. Scanning rates: 100 mV s 1. j Denotes the current density.
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3.1.2. Electrochemistry of the P(CBP-co-BT) film The copolymer film is prepared with the feed ratio of CBP/BT at 1:1 on platinum wire at + 1.3 V potentiostatically for CV tests. The electrochemistry of the copolymer is carried out in a monomerfree solution of an ACN/DCM (1:1, by volume) containing 0.2 M NaClO4. Fig. 3(a) shows the CV curves of the copolymer at different scanning rates between 25 and 250 mV s 1. The copolymer film exhibits a reversible redox process between +0.95 and +0.80 V. The peak current densities (j) are proportional to the potential scan rates (Fig. 3(b)), indicating that the electrochemical processes of the copolymer are reversible and not diffusion limited [27]. 3.1.3. FT-IR spectra of polymers The FT-IR spectra of PCBP, PBT and P(CBP-co-BT) prepared at +1.3 V potentiostatically are shown in Fig. S1 (see Appendix A). In the spectrum of PCBP (Fig. S1A), the peak at 1228 cm 1 indicates the existence of C–N signals in PCBP [17]. According to the spectrum of PBT (Fig. S1C), the strong absorption band at 785 cm 1 is attributed to the out-of-plane C–H bonding in the b position of the 2,5-disubstituted thiophene ring [23]. The above mentioned bands of PCBP and PBT could also be found in the FT-IR spectrum of P(CBP-co-BT) (Fig. S1B). Compared with corresponding homopolymers, the band at 790 cm 1 in the spectrum of the copolymer originates from 2,5-disubstitued thiophene ring of PBT, indicating the presence of BT units in the copolymer. Meanwhile, the band at 1228 cm 1 can also be found, which is ascribed to the C–N bonds in the PCBP (Fig. S1B). All the above features indicate that copolymer contains both CBP and BT units. 3.1.4. Thermal analysis The thermal stability of a conjugated polymer is very important for its potential application [28]. The thermal behaviors of the PCBP, P(CBP-co-BT) and PBT films are examined in the temperature range 50–800 1C with a heating rate of 10 1C min 1 (see Appendix A, Fig. S2). As shown in Fig. S2, a slight weight loss is observed for all polymer films between 50 and 200 1C, which should be assigned to the evaporation of the residual solvent or the trapped water in the polymer films [28,29]. The percentage related to the solvent/water loss is in the range 2–5%. PCBP, PBT and P(CBP-co-BT) show the onset temperature of weight loss at about 497, 470 and 375 1C, with the weight loss rate at 2.62, 2.24 and 0.82% min 1, respectively. When the temperature increases to 800 1C, the residue of PCBP, PBT and P(CBP-co-BT) are 13.6%, 25.2% and 54.5%, respectively. On the basis of the above analysis, the copolymer has a reasonable thermal stability.
Scheme 1. Synthetic route of the copolymer of CBP and BT.
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Fig. 3. (a) CV curves of the copolymer at different scan rates in the monomer-free solution of ACN/DCM (1:1, by volume) containing 0.2 M NaClO4. (b) Scan rate dependence of the anodic and cathodic peak current densities graph. jp.a and jp.c Denote the anodic and cathodic peak current densities, respectively.
characterization. As shown in Fig. 4a, PCBP shows a cohesive structure with clusters of granules. PBT exhibits a porous coral structure with smaller grains (Fig. 4c). However, the morphology of P(CBP-co-BT) film is circular flaky structure with a growth of stack-up arrangement (Fig. 4b). The difference of morphology between copolymer and homopolymers also confirms the occurrence of copolymerization between CBP and BT units.
Fig. 2. Successive CV curves of (a) 0.002 M CBP, (b) 0.002 M CBP and 0.002 M BT and (c) 0.002 M BT in ACN/DCM (1:1, by volume) containing 0.2 M NaClO4. Scanning rates: 100 mV s 1. j Denotes the current density.
3.1.5. Morphology The morphologies of polymer films are investigated by scanning electron microscopy (SEM). Fig. 4 gives the SEM images of PCBP, P(CBP-co-BT) and PBT, which are prepared on ITO electrodes at + 1.3 V potentiostatically and dedoped at 0.1 V before
3.1.6. UV–vis spectra of polymers The UV–vis spectra of dedoped PCBP, P(CBP-co-BT) and PBT films on an ITO electrode (the active area: 0.9 cm 2.0 cm) are shown in Fig. 5. These polymers are all electrosynthesised at +1.3 V potentiostatically with the same polymerization charge (3.7 10–2 C). As can be seen from Fig. 5, the UV–vis spectrum of PCBP film shows the p–pn electron transition peak at about 340 nm (Fig. 5a), while the PBT film exhibits a broad absorption band at 441 nm, due to the p–pn transition (Fig. 5c). On the other hand, the spectrum of the P(CBP-co-BT) shows both the characteristic absorptions of PCBP and PBT, located at 347 and 407 nm, respectively (Fig. 5b). The former has a slight red-shift about 7 nm in contrast to that of pure PCBP and the latter has a blue-shift about 34 nm in comparison with that of pure PBT, further confirming the occurrence of copolymerization [26].
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Fig. 4. SEM images of (a) PCBP, (b) P(CBP-co-BT) and (c) PBT deposited on an ITO electrode at + 1.3 V potentiostatically.
Fig. 5. UV–vis spectra of dedoped (a) PCBP, (b) P(CBP-co-BT) and (c) PBT deposited on an ITO electrode.
Fig. 7. Spectroelectrochemical spectra of P(CBP-co-BT) with applied potentials between 0 and + 1.3 V in monomer-free ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4. Applied potentials are the following: (a) 0 V; (b) +0.4 V; (c) + 0.6 V; (d) + 0.7 V; (e) + 0.8 V; (f) + 0.9 V; (g) + 1.0 V; (h) + 1.1 V; (i) +1.2 V; and (j) + 1.3 V.
electrodeposited on an ITO at + 1.3 V potentiostatically for the same charge (5.8 10 2 C). As can be seen from Fig. 6, PCBP is light yellow in color in the dedoped state and green in the doped state (Fig. 6a and A), and PBT is a brown red or gray polymer in its dedoped or doped state (Fig. 6c and C), respectively. However, P(CBP-co-BT) changes color from orange yellow in the dedoped state (Fig. 6b) to greenish blue in the doped state (Fig. 6B). The electrochromism phenomenon of the P(CBP-co-BT) is significantly different from those of the two individual homopolymers, further confirming the formation of copolymer consisting of both CBP and BT units.
3.2. Electrochromic properties of P(CBP-co-BT) film
Fig. 6. The electrochromism of polymer films: (a) light yellow, (b) orange yellow, (c) brown red are dedoped films, and (A) green, (B) greenish blue and(C) gray are doped films of PCBP, P(CBP-co-BT) and PBT, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
It has been well known that conjugated polymers are promising materials for electrochromic applications. Fig. 6 shows the electrochromism of PCBP, P(CBP-co-BT) and PBT, which are
3.2.1. Spectroelectrochemical properties of P(CBP-co-BT) film Spectroelectrochemistry is a useful method for studying the changes in the absorption spectra and the information about the electronic structures of conjugated polymers as a function of the applied voltage [30]. The P(CBP-co-BT) film coated ITO (prepared potentiostatically at +1.3 V) is switched between 0 and +1.3 V in ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4 in order to obtain the in situ UV–vis spectra (Fig. 7). At the neutral state, the copolymer film exhibits two absorption bands at 344 and 420 nm, due to the p–pn transition. The intensity of the P(CBP-co-BT) p–pn electron transition absorption decreases while a charge carrier absorption band located at about 667 nm increase dramatically upon an oxidation. At higher doping levels, there are a charge carrier absorption band longer than 900 nm and a weak charge carrier absorption peak at 438 nm.
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While there are many methods to quantify and represent color, one of the most widely applicable method to measure the color of materials illuminated by a standard light source is the 1976 CIE LAB (or Lnanbn) with the value of Ln representing how the measured material is light versus dark, the value an representing how red versus green, and bn representing how yellow versus blue the material is [31]. The P(CBP-co-BT) has distinct electrochromic properties. It shows four different colors in neutral and oxidized states. The orange yellow color of the film at neutral state (0 V) turns into blue color ( + 0.9 V) and yellowish green ( +1.1 V) at an intermedia doped state, and then into greenish blue color at full doped state ( + 1.3 V). These colors and corresponding Ln, an, bn values are given in Fig. 8.
3.2.2. Electrochromic switching of P(CBP-co-BT) film in solution It is important that polymers can switch rapidly and exhibit a noteworthy color change for electrochromic applications [32]. For this purpose, double potential step chronoamperometry technique is used to investigate the switching ability of the P(CBP-coBT) film between its neutral and full doped states (Fig. 9) [33]. The dynamic electrochromic experiment for P(CBP-co-BT) film is carried out at 667 nm. The potential is interchanged between
0 V (the neutral state) and + 1.3 V (the oxidized state) at regular intervals of 4 s. One important characteristic of electrochromic materials is the optical contrast (DT%), which can be defined as the transmittance difference between the redox states. The DT% of the P(CBP-co-BT) is found to be 51.6% at 667 nm, as shown in Fig. 9. The coloration efficiency (CE) is also an important characteristic for the electrochromic materials. CE can be calculated using the equations given below [34] T DOD and Z ¼ DOD ¼ log b Tc DQ where Tb and Tc are the transmittances before and after colorations, respectively. DOD is the change of the optical density, which is proportional to the amount of created color centers. Z denotes the coloration efficiency (CE). DQ is the amount of injected charge per unit sample area. CE of P(CBP-co-BT) film is measured as 75 cm2 C 1 (at 667 nm) at full doped state, which had a reasonable coloration efficiency. Response time, one of the most important characteristics of electrochromic materials, is the time needed to perform a switching between the neutral and oxidized states of the materials [35]. The response required to attain 95% of total transmittance difference is found to be 0.94 s from the reduced to the oxidized state and 1.35 s from the oxidized to the reduced state. Compared to PCBP homopolymer film reported by Sermet Koyuncu et al. [17], the copolymer P(CBP-co-BT) film has a high optical contrast and a fast response time, which are ascribed to the introduction of BT units into the polymer backbone. 3.3. Spectroelectrochemistry of electrochromic devices (ECDs)
Fig. 8. Colors and corresponding Ln, an, bn values of the P(CBP-co-BT) film at different applied potentials in ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
3.3.1. Spectroelectrochemical properties of an ECD A dual type ECD consisting of P(CBP-co-BT) and PEDOT constructed and its spectroelectrochemical behaviors are also studied. Before composing the ECD, the anodic polymer film (P(CBPco-BT)) is fully reduced and the cathodic polymer film (PEDOT) is fully oxidized. The P(CBP-co-BT)/PEDOT device is switched between –0.6 and +1.3 V. The spectroelectrochemical results show P(CBP-co-BT) layer is in its neutral state and PEDOT is in an oxidized state at 0.6 V, and the device color is an orange yellow. As the applied potential increased, the P(CBP-co-BT) layer starts to be oxidized, while the PEDOT layer is reduced, which leads to a new absorption at 700 nm (Fig. 10), and the dominated color of the device is blue at + 1.3 V. 3.3.2. Switching of an ECD Kinetic studies are also done to test the response time of P(CBP-co-BT)/PEDOT ECD. Under a potential input of 0.6 and +1.3 V at regular intervals of 3 s, the optical response at 700 nm, as illustrated in Fig. 11. The response time is found to be 0.47 s at 95% of the maximum transmittance difference from the neutral state to an oxidized state and 0.49 s from the oxidized state to the neutral state, and an optical contrast (DT%) is calculated to be 28.6%. The P(CBP-co-BT)/PEDOT device has a high optical contrast and fast response time compared with those of the PCBP/PEDOT device reported in literature [17]. The CE of the device (the active of an area: 1.8 cm 2.5 cm) is calculated to be 234 cm2 C 1 at 700 nm.
Fig. 9. Electrochromic switching response for P(CBP-co-BT) film monitored at 667 nm in an ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4 between 0 and + 1.3 V with a residence time of 4 s.
3.3.3. Open circuit memory of an ECD The optical memory in the electrochromic devices is an important parameter, because it is directly related to its application and energy consumption during the use of ECDs [32]. The optical spectrum for P(CBP-co-BT)/PEDOT device is monitored at
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Fig. 10. Spectroelectrochemical spectra of the P(CBP-co-BT)/PEDOT device at various applied potentials between 0.6 and + 1.3 V. Applied potentials are the following: (a) 0.6 V; (b) 0.2 V; (c) 0 V; (d) + 0.2 V; (e) + 0.3 V; (f) + 0.4 V; (g) +0.5 V; (h) + 0.6 V; (i) + 0.7 V; (j) +0.8 V; (k) + 0.9 V; (l) + 1.0 V; (m) +1.1 V; (n) +1.2 V; and (o) + 1.3 V. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
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Fig. 12. Open circuit stability of the P(CBP-co-BT)/PEDOT device monitored at 700 nm.
Fig. 13. Cyclic voltammogram of P(CBP-co-BT)/PEDOT device as a function repeated with a scan rate of 500 mV/s.
Fig. 11. Electrochromic switching, optical transmittance change monitored at 700 nm for P(CBP-co-BT)/PEDOT device between 0.6 and +1.3 V with a residence time of 3 s.
700 nm as a function of time at 0.6 and +1.3 V by applying the potential for 1 s for each 200 s time interval. As shown in Fig. 12, this device shows good optical memories both in an oxidized (with only 2.7% transmittance change) and reduced states (with almost no transmittance change), indicating this ECD has potential applications. 3.3.4. Stability of the ECD The stability of the devices towards multiple redox switching usually limits the utility of electrochromic materials in ECD applications. Therefore, redox stability is another important parameter for an ECD [32]. For this reason, the P(CBP-co-BT)/ PEDOT device is tested by cyclic voltammetry of the applied potential between 0.6 and + 1.3 V with 500 mV/s to evaluate the stability of the device (Fig. 13). After 500 cycles, 94.5% of its electroactivity is retained and there is no obvious decrease of activity between 500 and 1000 cycles. The optical contrast remains 87.7% of its initial value, after 1000 cycles, at full switching potentials between 0.6 and + 1.3 V with a residence
time of 3 s at 700 nm (the figure of an optical transmittance change was not shown). These results show that this device has a good redox stability.
4. Conclusion A new copolymer based on CBP and BT is successfully synthesized by electrochemical oxidation of the monomers mixture in ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4. The obtained copolymer P(CBP-co-BT) is studied by cyclic voltammetry, UV–vis spectra, FT-IR spectra, scanning electron microscopy and TG analyses. According to the spectroelectrochemical analyses, the copolymer film has distinct electrochromic properties and shows four different colors (orange yellow, blue, yellowish green and greenish blue) under various potentials. Maximum contrast (DT%) and response time of the copolymer film are measured as 51.6% and 0.94 s at 667 nm. The dual type electrochromic device (ECD) based on P(CBP-co-BT) and PEDOT is constructed and its electrochromic properties are also studied. The studies show that the optical contrast (DT%) and response time are 28.6% and 0.47 s at 700 nm. The coloration efficiency (CE) of the device is calculated to be 234 cm2 C 1 at 700 nm. This ECD
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also shows good optical memories and redox stability. Considering these results, this device is a promising candidate for commercial applications.
Acknowledgements Financial supports from the National Natural Science Foundation of China (No. 20906043) and the Taishan Scholarship of Shandong Province are gratefully acknowledged.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2011.02.009.
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Electrosyntheses, characterizations and electrochromic properties of a copolymer based on 4,40 -di(N-carbazoyl)biphenyl and 2,20 -bithiophene Bin Wang, Jinsheng Zhao n, Renmin Liu, Jifeng Liu, Qingpeng He Department of Chemistry, Liaocheng University, 252059 Liaocheng, PR China
a r t i c l e i n f o
abstract
Article history: Received 13 December 2010 Accepted 8 February 2011 Available online 3 March 2011
Electrochemical copolymerization of 4,40 -di(N-carbazoyl)biphenyl (CBP) with 2,20 -bithiophene (BT) is carried out in acetonitrile (ACN)/dichloromethane (DCM) (1:1, by volume) solution containing sodium perchlorate (NaClO4) as a supporting electrolyte. Characterizations of the resulting copolymer P(CBP-co-BT) are performed by cyclic voltammetry (CV), UV–vis spectroscopy, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and thermogravimetry (TG). The P(CBP-co-BT) film has distinct electrochromic properties and exhibits four different colors (orange yellow, blue, yellowish green and greenish blue) under various potentials. Maximum contrast (DT%) and response time of the copolymer film are measured as 51.6% and 0.94 s at 667 nm. An electrochromic device (ECD) based on P(CBP-co-BT) and poly(3,4-ethylenedioxythiophene) (PEDOT) is constructed and characterized. The optical contrast (DT%) at 700 nm is found to be 28.6% and the response time is measured as 0.47 s. The coloration efficiency (CE) of the device is calculated to be 234 cm2 C 1 at 700 nm. An ECD also has good optical memories and redox stability. & 2011 Elsevier B.V. All rights reserved.
Keywords: Electrochemistry Electrochemical polymerization Conjugated polymers Spectroelectrochemistry Electrochromic devices
1. Introduction An electrochromic material is one that changes color reversibly by an electrochemical reaction and the phenomenon is called electrochromism [1]. Electrochromic materials have received tremendous interest, due to potential applications in displays [2], energy-saving ‘‘smart’’ windows [3] and electrochromic devices [4–6]. Up to now, many different types of organic and inorganic electrochromic materials have been developed, such as inorganic metal oxide, mixed-valence metal complexes, organic small molecules and finally conjugated polymers [1]. Recently, electrochromic conjugated polymers have received much attention due to their fine-tunability of the band gap (and the color) [7], outstanding coloration efficiency [8], fast switching times, excellent processability and low cost [9]. For conjugated polymers, the electrochromism is related to the doping–dedoping process, the doping process modifies the polymer electronic structure, producing new electronic states in the band gap, causing color changes [10]. These conjugated polymers can be synthesized by either chemical or electrochemical polymerization. Compared with the chemical routes, electrochemical polymerization can obtain conjugated polymer films on conductive substrates directly. This not
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Corresponding author. Tel.: +86 635 8324551; fax: + 86 635 8239001. E-mail address: [email protected] (J. Zhao).
0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.02.009
only enlarges the scope of candidate polymers [11], but also omits the procedure of the film coating [12]. Among the many investigations, the electrochemical copolymerization can effectively modify the structures and properties of conjugated polymers [13], such as electrochemical activity, thermal stability and electrochromic properties. Copolymerization is a logical approach for the fine-tuning of the color for electrochromic applications [14]. Among conjugated polymers, carbazole-containing polymers have various useful properties such as easily forming relatively stable polarons (radical cations), high charge carrier mobility, photochemical stability [15] and electrochromic properties [16,17]. Electrochromic properties of poly-N-vinylcarbazole were realized around 1980 by Desbene-Monvernay et al. [18] and also studied by Chevrot and coworkers [19,20]. Recently the electrochromic properties of poly(4,40 -di(N-carbazoyl)biphenyl) (PCBP) are studied by Sermet Koyuncu et al. [17]; PCBP films exhibit reasonable electrochromic characters, but the electropolymerization of the monomer results in oligomer films and the stability of the electrochromic devices are not very well. Meanwhile, polythiophene and its derivatives have become a subject of considerable interest as electrochromic materials, because of their fast switching times, outstanding stability and high contrast ratios in the visible and near infra-red regions [21]. Among them, 2,20 -bithiophene (BT) attracts much attention due to the fact that the oxidation potential is lower than that of an unsubstituted thiophene and can give rise to structurally more ordered polythiophene [22]. It is interesting to note that the oxidation potential of
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the 4,40 -di(N-carbazoyl)biphenyl (CBP) monomer in ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4 is close to that of 2,20 -bithiophene, implying the possibility of the electrochemical copolymerization between CBP and BT [23]. According to the above considerations, in this study, the copolymer of 4,40 -di(N-carbazoyl)biphenyl (CBP) with 2,20 -bithiophene (BT) is successfully synthesized electrochemically. The electrochromic and spectroelectrochemical properties of the copolymer are studied in detail. The copolymer film has four different colors (orange yellow, blue, yellowish green and greenish blue). In addition, we constructed and characterized dual type electrochromic devices based on P(CBP-co-BT) and PEDOT in detail. Neutral state of device shows orange yellow color, while an oxidized state reveals blue color.
2. Experimental
films grown potentiostatically on an ITO is controlled by the total charge passed through the cell. 2.3. Characterizations The obtained copolymer films are studied by cyclic voltammetry. Infrared spectra are recorded on a Nicololet 5700 FT-IR spectrometer, where the samples are dispersed in KBr pellets. Thermal analysis is performed on a Pyris Diamond TG/DTA thermal analyzer (Perkin-Elmer) under a nitrogen (N2) stream in the temperature range 50–800 1C with a heating rate of 10 1C min 1. UV–vis spectra are carried out on a Perkin-Elmer Lambda 900 UV–vis-near-infrared spectrophotometer. Scanning electron microscopy (SEM) measurements are taken by using a JEOL JSM-6380LV SEM. Digital photographs of the polymer films are taken by a Canon Power Shot A3000 IS digital camera. Colorimetry measurements are obtained by a Coloreye XTH Spectrophotometer (GretagMacbeth).
2.1. Materials 2.4. Spectroelectrochemistry 2,20 -Bithiophene (BT, Aldrich, Analytical Grade), 4,40 -di(N-carbazoyl)biphenyl (CBP, Tokyo Chemical Industry CO., Ltd., Japan), dichloromethane (DCM, Sinopharm Chemical Reagent CO., Ltd., China), commercial high-performance liquid chromatography grade acetonitrile (ACN, Tedia Company, INC., USA), poly(methyl methacrylate) (PMMA, Shanghai Chemical Reagent Company), propylene carbonate (PC, Shanghai Chemical Reagent Company), 3,4-ethylenedioxythiophene (EDOT, Aldrich, 98%) and lithium perchlorate (LiClO4, Shanghai Chemical Reagent Company, 99.9%) are used directly without further purification. Sodium perchlorate (NaClO4, Shanghai Chemical Reagent Company, 98%) is dried in vacuum at 60 1C for 24 h before use. Other reagents are all used as received without further treatment. Indium–tin-oxide-coated (ITO) glass (Sheet resistance: o10 O & 1, purchased from Shenzhen CSG Display Technologies, China) is washed with ethanol, acetone and deionized water successively under ultrasonic, and then dried by an N2 flow. 2.2. Electrochemistry Electrochemical synthesis and experiments are performed in a one-compartment cell with a CHI 760 C Electrochemical Analyzer under computer control. The working and counter electrodes for cyclic voltammetric experiments are two platinum wires each with a diameter of 0.5 mm, placed 0.5 cm apart during the experiments, which are cleaned before each test. An Ag wire is used as a pseudo-reference electrode and all the potentials mentioned follow are versus the Ag wire electrode. All of the electrochemical experiments are carried out at room temperature under nitrogen atmosphere. All electrochemical polymerization and CV tests are taken in an ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4 as a supporting electrolyte. To obtain a sufficient amount of polymer for characterization, an ITO glass (the active area: 1.8 cm 2.7 cm) is employed as a working electrode. After electrochemical polymerization at + 1.3 V potentiostatically, electrochemical dedoping is carried out at 0.1 V. Then, the polymers are peeled off and washed with ACN/DCM (1:1, by volume) for three times to remove the electrolyte, oligomers and monomers. For spectral and thermal analysis, these polymers are dried under vacuum at 60 1C for 24 h. The polymer films used for the UV–vis spectral measurements, electrochromism and morphology characterization are deposited on an ITO glass (the active area: 0.9 cm 2.0 cm) at + 1.3 V potentiostatically, dedoped at 0.1 V and washed with ACN/ DCM (1:1, by volume). Moreover the thickness of the polymer
Spectroelectrochemical data are recorded on Perkin-Elmer Lambda 900 UV–vis-near-infrared spectrophotometer connected to a computer. A three-electrode cell assembly is used, where the working electrode is an ITO glass, the counter electrode is a stainless steel wire, and an Ag wire is used as the pseudoreference electrode. The copolymer films for spectroelectrochemistry are prepared by deposition potentiostatically on an ITO electrode (the active area: 0.9 cm 2.0 cm). The measurements are carried out in ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4. 2.5. Preparation of the gel electrolyte A gel electrolyte based on PMMA and LiClO4 is plasticized with PC to form a highly transparent and conductive gel. An ACN is also included as a high vapor pressure solvent to allow an easy mixing of the gel components. The composition of the casting solution by the weight ratio of ACN:PC:PMMA:LiClO4 is 70:20:7:3. The gel electrolyte is used for the construction of the polymer cell [8]. 2.6. Construction of electrochromic devices Electrochromic devices are constructed using two complementary polymers, namely P(CBP-co-BT) as the anodic material and PEDOT as the cathodic material. Both P(CBP-co-BT) and PEDOT films are electrodeposited on two ITO glass (the active area: 1.8 cm 2.7 cm) at + 1.3 and + 1.4 V, respectively. Electrochromic device is built by arranging the two polymer films (one oxidized, the other reduced), facing each other, separated by a gel electrolyte.
3. Results and discussion 3.1. Electrochemical polymerization and characterizations 3.1.1. Electrochemical polymerization The anodic polarization curves of 0.002 M CBP and 0.002 M BT in an ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4 are shown in Fig. 1. The onset oxidation potential (Epa onset) of CBP and BT in the solution is approximately + 1.02 and +1.08 V, respectively. It is well known that successful electrochemical copolymerization of different monomers is due to the fact that the Epa onset of the monomers is close to each
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other [23]. The difference of the onset oxidation potential between CBP and BT is 0.06 V, implying that the electrochemical copolymerization is readily to be achieved [24]. The synthetic route of the copolymer of CBP and BT is illustrated in Scheme 1. Fig. 2 displays the successive cyclic voltammogram (CV) curves of 0.002 M CBP, 0.002 M BT and the CBP/BT mixture at 1:1 (0.002 M CBP and 0.002 M BT) in ACN/DCM (1:1, by volume) containing 0.2 M NaClO4 between 0 and +1.3 V at a potential scan rate of 100 mV s 1. As the CV scan continued, polymer films are formed on the working electrode surface. The increase in the redox wave current densities implies that the amount of conducting polymers deposited on the electrode is increasing [25]. As shown in Fig. 2(a), the polymerization of CBP shows two reversible redox processes, the oxidation and reduction peaks of the first redox process appears at + 0.92 and +0.75 V, respectively. The reduction peak of the second redox process is located at + 1.03 V, and the corresponding oxidation peak is overlapped with the oxidation waves of the CBP monomer. The CV curves of BT show an oxidation peak at + 0.91 V and a reduction peak at + 0.82 V (Fig. 2(c)). However, the CV curve of the CBP/BT mixture at 1:1 exhibits an anodic and a cathodic peaks at +0.97 and + 0.74 V, respectively (Fig. 2(b)), which is different from those of CBP and BT, indicating the formation of a new polymer consisting of both CBP and BT units [26,27].
Fig. 1. Anodic polarization curves of 0.002 M CBP and 0.002 M BT both in ACN/ DCM (1:1, by volume) containing 0.2 M NaClO4. Scanning rates: 100 mV s 1. j Denotes the current density.
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3.1.2. Electrochemistry of the P(CBP-co-BT) film The copolymer film is prepared with the feed ratio of CBP/BT at 1:1 on platinum wire at + 1.3 V potentiostatically for CV tests. The electrochemistry of the copolymer is carried out in a monomerfree solution of an ACN/DCM (1:1, by volume) containing 0.2 M NaClO4. Fig. 3(a) shows the CV curves of the copolymer at different scanning rates between 25 and 250 mV s 1. The copolymer film exhibits a reversible redox process between +0.95 and +0.80 V. The peak current densities (j) are proportional to the potential scan rates (Fig. 3(b)), indicating that the electrochemical processes of the copolymer are reversible and not diffusion limited [27]. 3.1.3. FT-IR spectra of polymers The FT-IR spectra of PCBP, PBT and P(CBP-co-BT) prepared at +1.3 V potentiostatically are shown in Fig. S1 (see Appendix A). In the spectrum of PCBP (Fig. S1A), the peak at 1228 cm 1 indicates the existence of C–N signals in PCBP [17]. According to the spectrum of PBT (Fig. S1C), the strong absorption band at 785 cm 1 is attributed to the out-of-plane C–H bonding in the b position of the 2,5-disubstituted thiophene ring [23]. The above mentioned bands of PCBP and PBT could also be found in the FT-IR spectrum of P(CBP-co-BT) (Fig. S1B). Compared with corresponding homopolymers, the band at 790 cm 1 in the spectrum of the copolymer originates from 2,5-disubstitued thiophene ring of PBT, indicating the presence of BT units in the copolymer. Meanwhile, the band at 1228 cm 1 can also be found, which is ascribed to the C–N bonds in the PCBP (Fig. S1B). All the above features indicate that copolymer contains both CBP and BT units. 3.1.4. Thermal analysis The thermal stability of a conjugated polymer is very important for its potential application [28]. The thermal behaviors of the PCBP, P(CBP-co-BT) and PBT films are examined in the temperature range 50–800 1C with a heating rate of 10 1C min 1 (see Appendix A, Fig. S2). As shown in Fig. S2, a slight weight loss is observed for all polymer films between 50 and 200 1C, which should be assigned to the evaporation of the residual solvent or the trapped water in the polymer films [28,29]. The percentage related to the solvent/water loss is in the range 2–5%. PCBP, PBT and P(CBP-co-BT) show the onset temperature of weight loss at about 497, 470 and 375 1C, with the weight loss rate at 2.62, 2.24 and 0.82% min 1, respectively. When the temperature increases to 800 1C, the residue of PCBP, PBT and P(CBP-co-BT) are 13.6%, 25.2% and 54.5%, respectively. On the basis of the above analysis, the copolymer has a reasonable thermal stability.
Scheme 1. Synthetic route of the copolymer of CBP and BT.
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Fig. 3. (a) CV curves of the copolymer at different scan rates in the monomer-free solution of ACN/DCM (1:1, by volume) containing 0.2 M NaClO4. (b) Scan rate dependence of the anodic and cathodic peak current densities graph. jp.a and jp.c Denote the anodic and cathodic peak current densities, respectively.
characterization. As shown in Fig. 4a, PCBP shows a cohesive structure with clusters of granules. PBT exhibits a porous coral structure with smaller grains (Fig. 4c). However, the morphology of P(CBP-co-BT) film is circular flaky structure with a growth of stack-up arrangement (Fig. 4b). The difference of morphology between copolymer and homopolymers also confirms the occurrence of copolymerization between CBP and BT units.
Fig. 2. Successive CV curves of (a) 0.002 M CBP, (b) 0.002 M CBP and 0.002 M BT and (c) 0.002 M BT in ACN/DCM (1:1, by volume) containing 0.2 M NaClO4. Scanning rates: 100 mV s 1. j Denotes the current density.
3.1.5. Morphology The morphologies of polymer films are investigated by scanning electron microscopy (SEM). Fig. 4 gives the SEM images of PCBP, P(CBP-co-BT) and PBT, which are prepared on ITO electrodes at + 1.3 V potentiostatically and dedoped at 0.1 V before
3.1.6. UV–vis spectra of polymers The UV–vis spectra of dedoped PCBP, P(CBP-co-BT) and PBT films on an ITO electrode (the active area: 0.9 cm 2.0 cm) are shown in Fig. 5. These polymers are all electrosynthesised at +1.3 V potentiostatically with the same polymerization charge (3.7 10–2 C). As can be seen from Fig. 5, the UV–vis spectrum of PCBP film shows the p–pn electron transition peak at about 340 nm (Fig. 5a), while the PBT film exhibits a broad absorption band at 441 nm, due to the p–pn transition (Fig. 5c). On the other hand, the spectrum of the P(CBP-co-BT) shows both the characteristic absorptions of PCBP and PBT, located at 347 and 407 nm, respectively (Fig. 5b). The former has a slight red-shift about 7 nm in contrast to that of pure PCBP and the latter has a blue-shift about 34 nm in comparison with that of pure PBT, further confirming the occurrence of copolymerization [26].
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Fig. 4. SEM images of (a) PCBP, (b) P(CBP-co-BT) and (c) PBT deposited on an ITO electrode at + 1.3 V potentiostatically.
Fig. 5. UV–vis spectra of dedoped (a) PCBP, (b) P(CBP-co-BT) and (c) PBT deposited on an ITO electrode.
Fig. 7. Spectroelectrochemical spectra of P(CBP-co-BT) with applied potentials between 0 and + 1.3 V in monomer-free ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4. Applied potentials are the following: (a) 0 V; (b) +0.4 V; (c) + 0.6 V; (d) + 0.7 V; (e) + 0.8 V; (f) + 0.9 V; (g) + 1.0 V; (h) + 1.1 V; (i) +1.2 V; and (j) + 1.3 V.
electrodeposited on an ITO at + 1.3 V potentiostatically for the same charge (5.8 10 2 C). As can be seen from Fig. 6, PCBP is light yellow in color in the dedoped state and green in the doped state (Fig. 6a and A), and PBT is a brown red or gray polymer in its dedoped or doped state (Fig. 6c and C), respectively. However, P(CBP-co-BT) changes color from orange yellow in the dedoped state (Fig. 6b) to greenish blue in the doped state (Fig. 6B). The electrochromism phenomenon of the P(CBP-co-BT) is significantly different from those of the two individual homopolymers, further confirming the formation of copolymer consisting of both CBP and BT units.
3.2. Electrochromic properties of P(CBP-co-BT) film
Fig. 6. The electrochromism of polymer films: (a) light yellow, (b) orange yellow, (c) brown red are dedoped films, and (A) green, (B) greenish blue and(C) gray are doped films of PCBP, P(CBP-co-BT) and PBT, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
It has been well known that conjugated polymers are promising materials for electrochromic applications. Fig. 6 shows the electrochromism of PCBP, P(CBP-co-BT) and PBT, which are
3.2.1. Spectroelectrochemical properties of P(CBP-co-BT) film Spectroelectrochemistry is a useful method for studying the changes in the absorption spectra and the information about the electronic structures of conjugated polymers as a function of the applied voltage [30]. The P(CBP-co-BT) film coated ITO (prepared potentiostatically at +1.3 V) is switched between 0 and +1.3 V in ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4 in order to obtain the in situ UV–vis spectra (Fig. 7). At the neutral state, the copolymer film exhibits two absorption bands at 344 and 420 nm, due to the p–pn transition. The intensity of the P(CBP-co-BT) p–pn electron transition absorption decreases while a charge carrier absorption band located at about 667 nm increase dramatically upon an oxidation. At higher doping levels, there are a charge carrier absorption band longer than 900 nm and a weak charge carrier absorption peak at 438 nm.
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While there are many methods to quantify and represent color, one of the most widely applicable method to measure the color of materials illuminated by a standard light source is the 1976 CIE LAB (or Lnanbn) with the value of Ln representing how the measured material is light versus dark, the value an representing how red versus green, and bn representing how yellow versus blue the material is [31]. The P(CBP-co-BT) has distinct electrochromic properties. It shows four different colors in neutral and oxidized states. The orange yellow color of the film at neutral state (0 V) turns into blue color ( + 0.9 V) and yellowish green ( +1.1 V) at an intermedia doped state, and then into greenish blue color at full doped state ( + 1.3 V). These colors and corresponding Ln, an, bn values are given in Fig. 8.
3.2.2. Electrochromic switching of P(CBP-co-BT) film in solution It is important that polymers can switch rapidly and exhibit a noteworthy color change for electrochromic applications [32]. For this purpose, double potential step chronoamperometry technique is used to investigate the switching ability of the P(CBP-coBT) film between its neutral and full doped states (Fig. 9) [33]. The dynamic electrochromic experiment for P(CBP-co-BT) film is carried out at 667 nm. The potential is interchanged between
0 V (the neutral state) and + 1.3 V (the oxidized state) at regular intervals of 4 s. One important characteristic of electrochromic materials is the optical contrast (DT%), which can be defined as the transmittance difference between the redox states. The DT% of the P(CBP-co-BT) is found to be 51.6% at 667 nm, as shown in Fig. 9. The coloration efficiency (CE) is also an important characteristic for the electrochromic materials. CE can be calculated using the equations given below [34] T DOD and Z ¼ DOD ¼ log b Tc DQ where Tb and Tc are the transmittances before and after colorations, respectively. DOD is the change of the optical density, which is proportional to the amount of created color centers. Z denotes the coloration efficiency (CE). DQ is the amount of injected charge per unit sample area. CE of P(CBP-co-BT) film is measured as 75 cm2 C 1 (at 667 nm) at full doped state, which had a reasonable coloration efficiency. Response time, one of the most important characteristics of electrochromic materials, is the time needed to perform a switching between the neutral and oxidized states of the materials [35]. The response required to attain 95% of total transmittance difference is found to be 0.94 s from the reduced to the oxidized state and 1.35 s from the oxidized to the reduced state. Compared to PCBP homopolymer film reported by Sermet Koyuncu et al. [17], the copolymer P(CBP-co-BT) film has a high optical contrast and a fast response time, which are ascribed to the introduction of BT units into the polymer backbone. 3.3. Spectroelectrochemistry of electrochromic devices (ECDs)
Fig. 8. Colors and corresponding Ln, an, bn values of the P(CBP-co-BT) film at different applied potentials in ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
3.3.1. Spectroelectrochemical properties of an ECD A dual type ECD consisting of P(CBP-co-BT) and PEDOT constructed and its spectroelectrochemical behaviors are also studied. Before composing the ECD, the anodic polymer film (P(CBPco-BT)) is fully reduced and the cathodic polymer film (PEDOT) is fully oxidized. The P(CBP-co-BT)/PEDOT device is switched between –0.6 and +1.3 V. The spectroelectrochemical results show P(CBP-co-BT) layer is in its neutral state and PEDOT is in an oxidized state at 0.6 V, and the device color is an orange yellow. As the applied potential increased, the P(CBP-co-BT) layer starts to be oxidized, while the PEDOT layer is reduced, which leads to a new absorption at 700 nm (Fig. 10), and the dominated color of the device is blue at + 1.3 V. 3.3.2. Switching of an ECD Kinetic studies are also done to test the response time of P(CBP-co-BT)/PEDOT ECD. Under a potential input of 0.6 and +1.3 V at regular intervals of 3 s, the optical response at 700 nm, as illustrated in Fig. 11. The response time is found to be 0.47 s at 95% of the maximum transmittance difference from the neutral state to an oxidized state and 0.49 s from the oxidized state to the neutral state, and an optical contrast (DT%) is calculated to be 28.6%. The P(CBP-co-BT)/PEDOT device has a high optical contrast and fast response time compared with those of the PCBP/PEDOT device reported in literature [17]. The CE of the device (the active of an area: 1.8 cm 2.5 cm) is calculated to be 234 cm2 C 1 at 700 nm.
Fig. 9. Electrochromic switching response for P(CBP-co-BT) film monitored at 667 nm in an ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4 between 0 and + 1.3 V with a residence time of 4 s.
3.3.3. Open circuit memory of an ECD The optical memory in the electrochromic devices is an important parameter, because it is directly related to its application and energy consumption during the use of ECDs [32]. The optical spectrum for P(CBP-co-BT)/PEDOT device is monitored at
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Fig. 10. Spectroelectrochemical spectra of the P(CBP-co-BT)/PEDOT device at various applied potentials between 0.6 and + 1.3 V. Applied potentials are the following: (a) 0.6 V; (b) 0.2 V; (c) 0 V; (d) + 0.2 V; (e) + 0.3 V; (f) + 0.4 V; (g) +0.5 V; (h) + 0.6 V; (i) + 0.7 V; (j) +0.8 V; (k) + 0.9 V; (l) + 1.0 V; (m) +1.1 V; (n) +1.2 V; and (o) + 1.3 V. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
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Fig. 12. Open circuit stability of the P(CBP-co-BT)/PEDOT device monitored at 700 nm.
Fig. 13. Cyclic voltammogram of P(CBP-co-BT)/PEDOT device as a function repeated with a scan rate of 500 mV/s.
Fig. 11. Electrochromic switching, optical transmittance change monitored at 700 nm for P(CBP-co-BT)/PEDOT device between 0.6 and +1.3 V with a residence time of 3 s.
700 nm as a function of time at 0.6 and +1.3 V by applying the potential for 1 s for each 200 s time interval. As shown in Fig. 12, this device shows good optical memories both in an oxidized (with only 2.7% transmittance change) and reduced states (with almost no transmittance change), indicating this ECD has potential applications. 3.3.4. Stability of the ECD The stability of the devices towards multiple redox switching usually limits the utility of electrochromic materials in ECD applications. Therefore, redox stability is another important parameter for an ECD [32]. For this reason, the P(CBP-co-BT)/ PEDOT device is tested by cyclic voltammetry of the applied potential between 0.6 and + 1.3 V with 500 mV/s to evaluate the stability of the device (Fig. 13). After 500 cycles, 94.5% of its electroactivity is retained and there is no obvious decrease of activity between 500 and 1000 cycles. The optical contrast remains 87.7% of its initial value, after 1000 cycles, at full switching potentials between 0.6 and + 1.3 V with a residence
time of 3 s at 700 nm (the figure of an optical transmittance change was not shown). These results show that this device has a good redox stability.
4. Conclusion A new copolymer based on CBP and BT is successfully synthesized by electrochemical oxidation of the monomers mixture in ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4. The obtained copolymer P(CBP-co-BT) is studied by cyclic voltammetry, UV–vis spectra, FT-IR spectra, scanning electron microscopy and TG analyses. According to the spectroelectrochemical analyses, the copolymer film has distinct electrochromic properties and shows four different colors (orange yellow, blue, yellowish green and greenish blue) under various potentials. Maximum contrast (DT%) and response time of the copolymer film are measured as 51.6% and 0.94 s at 667 nm. The dual type electrochromic device (ECD) based on P(CBP-co-BT) and PEDOT is constructed and its electrochromic properties are also studied. The studies show that the optical contrast (DT%) and response time are 28.6% and 0.47 s at 700 nm. The coloration efficiency (CE) of the device is calculated to be 234 cm2 C 1 at 700 nm. This ECD
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also shows good optical memories and redox stability. Considering these results, this device is a promising candidate for commercial applications.
Acknowledgements Financial supports from the National Natural Science Foundation of China (No. 20906043) and the Taishan Scholarship of Shandong Province are gratefully acknowledged.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2011.02.009.
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