Copolymers Based on 1,3-Bis(carbazol-9-yl)benzene and Three 3,4-Ethylenedioxythiophene Derivatives as Potential Anodically Coloring Copolymers in High-Contrast Electrochromic Devices

In this study, copolymers based on 1,3-bis(carbazol-9-yl)benzene (BCz) and three 3,4-ethylenedioxythiophene derivatives (3,4-ethylenedioxythiophene (EDOT), 3,4-(2,2-dimethylpropylenedioxy)thiophene (ProDOT-Me2), and 3,4-ethylenedithiathiophene (EDTT)) were electrochemically synthesized and their electrochemical and electrochromic properties were characterized. The anodic copolymer P(BCz-co-ProDOT) with BCz/ProDOT-Me2 = 1/1 feed molar ratio showed high optical contrast (ΔT%) and coloring efficiency (η), measured as 52.5% and 153.5 cm2∙C−1 at 748 nm, respectively. Electrochromic devices (ECDs) based on P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) as anodic polymer layers, and poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid) (PEDOT-PSS) as cathodic polymer layer were fabricated. P(BCz-co-ProDOT)/triple-layer PEDOT-PSS ECD showed three different colors (light yellow, yellowish-blue, and dark blue) at different applied potentials. In addition, the highest optical contrast (ΔT%) of P(BCz-co-ProDOT)/triple-layer PEDOT-PSS ECD was found to be 41% at 642 nm and the coloration efficiency was calculated to be 416.5 cm2∙C−1 at 642 nm. All ECDs showed satisfactory optical memories and electrochemical cyclic stability.


Introduction
Organic electroactive materials have gained much attention for commercial electronic devices due to their benefits of facile structural modifications, high optical contrast between redox state, and fast photo-switching ability. π-conjugated polymers (CPs) are among the most widely explored organic electroactive materials. CPs have been widely used in advanced technological fields including polymer solar cells [1,2], polymer light-emitting diodes [3,4], catalysts [5][6][7], fluorescent sensors [8], thin film transistors [9], and electrochromic devices (ECDs) [10]. In these fields, researchers have focused enormously on the application of CPs in ECDs due to CPs being able to change their spectroelectrochemical properties after application of an electrical voltage [11].
In the last decade, a class of CPs, known as polypyrroles [12], polythiophenes (PTh) [13], polyanilines [14], polyfurans [15], polycarbazoles (PCz) [16], polyazulenes [17], polyindoles [18], and poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid) [19] have been widely used in electrochromic materials. Among them, PCz is quite bleached in its neutral state and becomes colored in its oxidized state. Carbazole can be substituted or polymerized at the 3,6-or 2,7-positions and a wide variety of alkyl and aryl chains can be incorporated into the nitrogen atom of the carbazole unit, leading it to be a good candidate for a number of opto-electronic applications. PTh and their derivatives have been widely used due to their good redox reversibility, high conductivity value, and the high optical contrast between redox states [20]. PTh can be formed directly on the electrodes using electrochemical polymerization. Poly(3,4-ethylenedioxythiophene)s (PEDOT) and poly(3,4-(2,2-dimethylpropylenedioxy)thiophene) (PProDOT-Me 2 ) contain two electron-donating oxygen atoms on the 3,4-positions of the thiophene unit, which decreases the onset potentials of polymer films, and makes the band gaps of PEDOT and PProDOT-Me 2 films lower than PTh [21]. Moreover, 3,4-ethylenedithiathiophene (EDTT) is an electron-donating heterocyclic unit with two electron-donating sulfur atoms on the 3,4-positions of the thiophene unit, leading to shifts in the onset potential and absorption maximum of the polymer film. Copolymerization of distinct monomers containing several diverse units can give rise to an interesting combination of the electrochromic and electrochemical properties observed in the corresponding homopolymers. For this matter, copolymers based on the carbazole derivative and 3,4-ethylenedioxythiophene derivatives were synthesized electrochemically in this study. Furthermore, 1,3-bis(carbazol-9-yl)benzene contains two carbazole units linked by a phenyl unit, which permits charge carrier transport and eases the formation of stable radical cations (polaron) and dication (bipoloran). Three copolymers (P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT)) were synthesized using electrochemical copolymerizations, and the spectroelectrochemical and electrochromic properties of copolymer films were systematically and comprehensively studied. It was interesting to find that the slight structural variations of the 3,4-ethylenedioxythiophene derivatives brought about distinct electrochromic properties.
2.2. Electrochemical Polymerization of PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) Films PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) films were prepared potentiostatically at 1.0 V (vs. Ag/AgNO 3 ) on ITO electrodes with a charge density of 60 mC·cm −2 , using 0.002 M monomer in a solution containing 0.2 M LiClO 4 in PC/acetonitrile (ACN) solution. The feed species of these films are summarized in Table 1. An Ag/AgNO 3 electrode (calibrated using ferrocene) and a platinum wire were used as the reference and counter electrodes, respectively. The double layers cathodic PEDOT-PSS polymer film was prepared using spin coating techniques; the spin condition for film preparation was 2000 rpm. The active area of polymer film on indium tin oxide (ITO) electrode was 1.0 × 1.5 cm 2 .

Construction of Electrochromic Devices
A gel electrolyte consisting of PMMA, 0.2 M LiClO 4 , PC, and ACN was prepared and the gel polymer electrolyte was coated on anodic PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) films. A double-layer cathodic PEDOT-PSS film was placed onto the electrolyte membrane to construct an electrochromic device. The edges of electrochromic devices were sealed with epoxy resin to prevent the interface from being attacked by moisture and oxygen. The effective area of the prepared electrochromic devices was about 1.5 cm 2 . For comparison, cathodic films were coated with a triple-layer PEDOT-PSS and a quadruple-layer PEDOT-PSS on ITO electrodes. ECDs were also built by arranging P(BCz-co-ProDOT) and a triple-layer PEDOT-PSS (or quadruple-layer PEDOT-PSS) facing each other in order that they could to be separated by a gel electrolyte. The configuration of ECD is shown in Figure 1. The feed species of these films are summarized in Table 1. An Ag/AgNO3 electrode (calibrated using ferrocene) and a platinum wire were used as the reference and counter electrodes, respectively. The double layers cathodic PEDOT-PSS polymer film was prepared using spin coating techniques; the spin condition for film preparation was 2000 rpm. The active area of polymer film on indium tin oxide (ITO) electrode was 1.0 × 1.5 cm 2 .

Construction of Electrochromic Devices
A gel electrolyte consisting of PMMA, 0.2 M LiClO4, PC, and ACN was prepared and the gel polymer electrolyte was coated on anodic PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) films. A double-layer cathodic PEDOT-PSS film was placed onto the electrolyte membrane to construct an electrochromic device. The edges of electrochromic devices were sealed with epoxy resin to prevent the interface from being attacked by moisture and oxygen. The effective area of the prepared electrochromic devices was about 1.5 cm 2 . For comparison, cathodic films were coated with a triple-layer PEDOT-PSS and a quadruple-layer PEDOT-PSS on ITO electrodes. ECDs were also built by arranging P(BCz-co-ProDOT) and a triple-layer PEDOT-PSS (or quadruple-layer PEDOT-PSS) facing each other in order that they could to be separated by a gel electrolyte. The configuration of ECD is shown in Figure 1.

Electrochemical and Spectroelectrochemical Characterization
The electrochemical experiments were executed in a multi-component cell with a CHI627D electrochemical analyzer (CH Instruments, Austin, TX, USA). An ITO coated glass plate (1 × 1.5 cm 2 area), Ag/AgNO3 electrode, and platinum wire were used as working, reference, and counter electrodes, respectively. The spectroelectrochemical experiments were performed with a HITACHI spectrophotometer to record the in situ UV-Visible spectra. The double potential chronoamperometry was carried out with the assembled cell using a CHI627D electrochemical analyzer and a HITACHI spectrophotometer (Hitachi, Tokyo, Japan).

Electrochemical and Spectroelectrochemical Characterization
The electrochemical experiments were executed in a multi-component cell with a CHI627D electrochemical analyzer (CH Instruments, Austin, TX, USA). An ITO coated glass plate (1 × 1.5 cm 2 area), Ag/AgNO 3 electrode, and platinum wire were used as working, reference, and counter electrodes, respectively. The spectroelectrochemical experiments were performed with a HITACHI spectrophotometer to record the in situ UV-Visible spectra. The double potential chronoamperometry was carried out with the assembled cell using a CHI627D electrochemical analyzer and a HITACHI spectrophotometer (Hitachi, Tokyo, Japan).  Figure 2 shows the anodic polarization curves of the neat EDTT, BCz, ProDOT-Me 2 , and EDOT in PC/ACN solution containing 0.2 M LiClO 4 at a scan rate of 100 mV·s −1 . The onset potential of neat EDTT, BCz, ProDOT-Me 2 , and EDOT were 0.86, 0.94, 1.00, and 1.04 V, respectively. The E onset of EDTT is smaller than that of EDOT, indicating the incorporation of dithio group on 3,4-positions of thiophene led to lower oxidation potentials of polymer films than the dioxy group. Moreover, EDTT showed lower E onset than BCz and ProDOT-Me 2 . The discriminations between onset potential of neat BCz vs. neat EDOT, neat BCz vs. ProDOT-Me 2 , and neat BCz vs. neat EDTT were small (<0.1 V), implying the feasibility of copolymerizations of BCz with EDTT (or ProDOT-Me 2 , EDOT).  Figure 2 shows the anodic polarization curves of the neat EDTT, BCz, ProDOT-Me2, and EDOT in PC/ACN solution containing 0.2 M LiClO4 at a scan rate of 100 mV•s −1 . The onset potential of neat EDTT, BCz, ProDOT-Me2, and EDOT were 0.86, 0.94, 1.00, and 1.04 V, respectively. The Eonset of EDTT is smaller than that of EDOT, indicating the incorporation of dithio group on 3,4-positions of thiophene led to lower oxidation potentials of polymer films than the dioxy group. Moreover, EDTT showed lower Eonset than BCz and ProDOT-Me2. The discriminations between onset potential of neat BCz vs. neat EDOT, neat BCz vs. ProDOT-Me2, and neat BCz vs. neat EDTT were small (<0.1 V), implying the feasibility of copolymerizations of BCz with EDTT (or ProDOT-Me2, EDOT).  solution at a scan rate of 100 mV•s −1 . As the CV scan continued, the relative intensities of the oxidation and reduction peaks increased with increasing scanning cycles; this can be attributed to the growth of homopolymers and copolymers on the electrode [23]. As shown in Figure 3a, PBCz had two oxidation peaks at 0.87 and 1.25 V and two reduction peaks at 0.46 and 0.75 V. The oxidation and reduction peaks of P(BCz-co-EDOT) located at 1.34 and 0.47 V (Figure 3b), which are different to the redox potentials of neat PBCz, demonstrating the formation of P(BCz-co-EDOT). In a similar condition, P(BCz-co-ProDOT) had two oxidation peaks at 0.79 and 1.19 V and two reduction peaks at 0.45 and 0.69 V (Figure 3c). The oxidation and reduction peaks of the P(BCz-co-EDTT) film occurred at 1.32 and 0.50 V (Figure 3d). The redox potentials of P(BCz-co-ProDOT) and P(BCz-co-EDTT) films were different to those of PBCz, indicating that the copolymer films were deposited on the ITO electrodes. The electrochemical polymerization routes of PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) are displayed in Figure 4.

Electrochemical Polymerization
The P(BCz-co-ProDOT) films deposited through the electrocopolymerization of BCz and ProDOT-Me2 were studied at various scan rates between 10 and 200 mV•s −1 in order to verify the scan rate dependence of the copolymer films. Figure 5 shows the cyclic voltammograms of the P(BCz-co-ProDOT) film (prepared by scanning the potentials between 0.0 and 1.7 V) at various scan rates in 0.2 M LiClO4/(PC + ACN) solution. The P(BCz-co-ProDOT) film shows a couple of oxidation  solution at a scan rate of 100 mV·s −1 . As the CV scan continued, the relative intensities of the oxidation and reduction peaks increased with increasing scanning cycles; this can be attributed to the growth of homopolymers and copolymers on the electrode [23]. As shown in Figure 3a, PBCz had two oxidation peaks at 0.87 and 1.25 V and two reduction peaks at 0.46 and 0.75 V. The oxidation and reduction peaks of P(BCz-co-EDOT) located at 1.34 and 0.47 V (Figure 3b), which are different to the redox potentials of neat PBCz, demonstrating the formation of P(BCz-co-EDOT). In a similar condition, P(BCz-co-ProDOT) had two oxidation peaks at 0.79 and 1.19 V and two reduction peaks at 0.45 and 0.69 V (Figure 3c). The oxidation and reduction peaks of the P(BCz-co-EDTT) film occurred at 1.32 and 0.50 V (Figure 3d). The redox potentials of P(BCz-co-ProDOT) and P(BCz-co-EDTT) films were different to those of PBCz, indicating that the copolymer films were deposited on the ITO electrodes. The electrochemical polymerization routes of PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) are displayed in Figure 4.
The P(BCz-co-ProDOT) films deposited through the electrocopolymerization of BCz and ProDOT-Me 2 were studied at various scan rates between 10 and 200 mV·s −1 in order to verify the scan rate dependence of the copolymer films. Figure 5 shows the cyclic voltammograms of the P(BCz-co-ProDOT) film (prepared by scanning the potentials between 0.0 and 1.7 V) at various scan rates in 0.2 M LiClO 4 /(PC + ACN) solution. The P(BCz-co-ProDOT) film shows a couple of oxidation and reduction peaks and the current density of redox peaks is linearly proportional to the scan rate, implying the P(BCz-co-ProDOT) film well-adhered onto the ITO electrodes and that the electrochemical processes of P(BCz-co-ProDOT) film were characteristic of a nondiffusional redox process [24]. and reduction peaks and the current density of redox peaks is linearly proportional to the scan rate, implying the P(BCz-co-ProDOT) film well-adhered onto the ITO electrodes and that the electrochemical processes of P(BCz-co-ProDOT) film were characteristic of a nondiffusional redox process [24].   and reduction peaks and the current density of redox peaks is linearly proportional to the scan rate, implying the P(BCz-co-ProDOT) film well-adhered onto the ITO electrodes and that the electrochemical processes of P(BCz-co-ProDOT) film were characteristic of a nondiffusional redox process [24].

Electrochromic Characterizations of Polymer Films
Spectroelectrochemistry of the PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) films coated on ITO electrode was investigated in 0.2 M LiClO 4 /(PC + ACN) solution. Figure 6a-d shows the spectroelectrochemical spectra of PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) There was no conspicuous absorption peaks of PBCz film in the neutral state. Upon applying 0.9 V, new charge carrier bands appeared at 420 and 1050 nm, which can be assigned to the formation of polaron and bipolaron bands [25]. The polaron and bipolaron bands of P(BCz-co-EDOT) film in 0.2 M LiClO 4 /(PC + ACN) solution located at 420, 732, and 1050 nm, and at the middle band (732 nm) can be ascribed to the polaron and bipolaron bands of 3,4-ethylenedioxythiophene unit in an oxidation state. Similarly, the polaron and bipolaron bands of P(BCz-co-ProDOT) film occurred at 420, 748, and 1050 nm in moderate and high oxidized states, whereas those of P(BCz-co-EDTT) film located themselves at 420, 749, and 1050 nm. The absorption peaks of P(BCz-co-ProDOT) and P(BCz-co-EDTT) at 748 and 749 nm, respectively, can be attributed to the polaron and bipolaron bands of ProDOT-Et 2 and EDTT units in an oxidation state.
3,4-ethylenedioxythiophene unit in an oxidation state. Similarly, the polaron and bipolaron bands of P(BCz-co-ProDOT) film occurred at 420, 748, and 1050 nm in moderate and high oxidized states, whereas those of P(BCz-co-EDTT) film located themselves at 420, 749, and 1050 nm. The absorption peaks of P(BCz-co-ProDOT) and P(BCz-co-EDTT) at 748 and 749 nm, respectively, can be attributed to the polaron and bipolaron bands of ProDOT-Et2 and EDTT units in an oxidation state.
The PBCz film showed multicolor electrochromism, which was transparent in the neutral state  The electrochromic switching of PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) films at specific wavelengths were examined by double-potential-step chronoamperometry [26].  The electrochromic switching of PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) films at specific wavelengths were examined by double-potential-step chronoamperometry [26]. Figure 7 shows the transmittance-time profiles of PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) films in 0.2 M LiClO 4 /(PC + ACN) solution, which were stepped by repeated potential between 0.0 and 1.2 V with a residence time of 10 s. The maximum optical contrast (∆T max %) of PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) from the bleaching state to the coloration state in 0.2 M LiClO 4 /(PC + ACN) solution were estimated to be 18.6%, 36.0%, 52.5% and 50.0%, respectively. Among these electrodes, P(BCz-co-ProDOT) film shows the highest ∆T max , and copolymers (P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT)) show higher ∆T max than that of homopolymer (PBCz) in 0.  Figure 7 shows the transmittance-time profiles of PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) films in 0.2 M LiClO4/(PC + ACN) solution, which were stepped by repeated potential between 0.0 and 1.2 V with a residence time of 10 s. The maximum optical contrast (ΔTmax%) of PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) from the bleaching state to the coloration state in 0.2 M LiClO4/(PC + ACN) solution were estimated to be 18.6%, 36.0%, 52.5% and 50.0%, respectively. Among these electrodes, P(BCz-co-ProDOT) film shows the highest ΔTmax, and copolymers (P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT)) show higher ΔTmax than that of homopolymer (PBCz) in 0.   Table 2; the response time was calculated at 90% of the full-transmittance change. For PBCz film in 0.2 M LiClO4/(PC + ACN) solution, the response time at 1050 nm was estimated to be 7.0 s from the bleaching state to the coloring state and 6.0 s from the coloring state to the bleaching state. Copolymer (P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT)) films show a shorter τb than the PBCz film, indicating the incorporation of EDOT, ProDOT-Me2, or EDTT unit into the polymer backbone facilitated color change from the coloring to the bleaching state when we used 0.2 M LiClO4/(PC + ACN) as a supporting electrolyte. ΔOD is the variation of optical density, which can be determined using the formula,  Table 2; the response time was calculated at 90% of the full-transmittance change. For PBCz film in 0.2 M LiClO 4 /(PC + ACN) solution, the response time at 1050 nm was estimated to be 7.0 s from the bleaching state to the coloring state and 6.0 s from the coloring state to the bleaching state. Copolymer (P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT)) films show a shorter τ b than the PBCz film, indicating the incorporation of EDOT, ProDOT-Me 2 , or EDTT unit into the polymer backbone facilitated color change from the coloring to the bleaching state when we used 0.2 M LiClO 4 /(PC + ACN) as a supporting electrolyte. Table 2. Optical and electrochemical properties investigated at selected applied wavelength for the electrodes. ∆OD is the variation of optical density, which can be determined using the formula, where T ox and T red indicate the percentage of transmittance in the oxidized state and the reduced state, respectively. The ∆OD max of PBCz, P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) films in 0.2 M LiClO 4 /(PC + ACN) solution are also summarized in Table 2. Similar to the tendency of ∆T max , P(BCz-co-ProDOT) and P(BCz-co-EDTT) films show a larger ∆OD than PBCz and P(BCz-co-EDOT) films. Coloration efficiency (η) is an efficient tool for the measurement of the power requirements of an electrochromic material, and can be determined using the following formula at a specific wavelength, where Q d is the injected/ejected electronic charge of the electrodes per active area, and ∆OD is the variation of optical density at a specific wavelength. As shown in Table 2, the η of PBCz film at 1050 nm, P(BCz-co-EDOT) film at 732 nm, P(BCz-co-ProDOT) at 748 nm, and P(BCz-co-EDTT) at 749 nm were 180.3, 78.2, 153.5, and 138.5 cm 2 ·C −1 , respectively.

Open Circuit Memory of ECDs
The ability to retain a colored (or bleached) state for an open circuit of the ECDs was monitored at specific wavelength as a function of time in neutral and oxidation states by applying the potential for 1 s for each 200 s time interval. As seen in Figure 10, the P(BCz-co-ProDOT)/triple-layer PEDOT-PSS ECD showed good optical memories in a reduced state of P(BCz-co-ProDOT) film, but almost no transmittance change in a reduced state. In the oxidized state of P(BCz-co-ProDOT) film, the ECD is rather less stable than the reduced state of P(BCz-co-ProDOT) film, but the transmittance change is less than 5% in an oxidized state of P(BCz-co-ProDOT) film, implying the P(BCz-co-ProDOT)/triple-layer PEDOT-PSS ECD has a reasonable optical memory under open circuit conditions.

Open Circuit Memory of ECDs
The ability to retain a colored (or bleached) state for an open circuit of the ECDs was monitored at specific wavelength as a function of time in neutral and oxidation states by applying the potential for 1 s for each 200 s time interval. As seen in Figure 10, the P(BCz-co-ProDOT)/triple-layer PEDOT-PSS ECD showed good optical memories in a reduced state of P(BCz-co-ProDOT) film, but almost no transmittance change in a reduced state. In the oxidized state of P(BCz-co-ProDOT) film, the ECD is rather less stable than the reduced state of P(BCz-co-ProDOT) film, but the transmittance change is less than 5% in an oxidized state of P(BCz-co-ProDOT) film, implying the P(BCz-co-ProDOT)/triple-layer PEDOT-PSS ECD has a reasonable optical memory under open circuit conditions.