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Article

Electrosynthesis of Electrochromic Polymer Membranes Based on 3,6-Di(2-thienyl)carbazole and Thiophene Derivatives

by
Chung-Wen Kuo
1,
Jui-Cheng Chang
2,3,
Jeng-Kuei Chang
4,
Sheng-Wei Huang
1,
Pei-Ying Lee
2 and
Tzi-Yi Wu
2,*
1
Department of Chemical and Materials Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 80778, Taiwan
2
Department of Chemical Engineering and Materials Engineering, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan
3
Bachelor Program in Interdisciplinary Studies, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan
4
Department of Materials Science and Engineering, National Chiao Tung University, No. 1001 University Road, Hsinchu 30010, Taiwan
*
Author to whom correspondence should be addressed.
Membranes 2021, 11(2), 125; https://doi.org/10.3390/membranes11020125
Submission received: 8 January 2021 / Revised: 2 February 2021 / Accepted: 4 February 2021 / Published: 9 February 2021
(This article belongs to the Section Membrane Applications)
Browse Figures
Versions Notes

Abstract

:
Five carbazole-containing polymeric membranes (PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2)) were electrodeposited on transparent conductive electrodes. P(DTC-co-BTP2) shows a high ΔT (68.4%) at 855 nm. The multichromic properties of P(DTC-co-TF2) membrane range between dark yellow, yellowish-green, gunmetal gray, and dark gray in various reduced and oxidized states. Polymer-based organic electrochromic devices are assembled using 2,2′-bithiophene- and 2-(2-thienyl)furan-based copolymers as anodic membranes, and poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid) (PEDOT-PSS) as the cathodic membrane. P(DTC-co-TF)/PEDOT-PSS electrochromic device (ECD) displays a high transmittance change (ΔT%) (43.4%) at 627 nm as well as a rapid switching time (less than 0.6 s) from a colored to a bleached state. Moreover, P(DTC-co-TF2)/PEDOT-PSS ECD shows satisfactory optical memory (the transmittance change is less than 2.9% in the colored state) and high coloration efficiency (512.6 cm2 C−1) at 627 nm.

1. Introduction

Electrochromism refers to when electroactive species undergo a reversible change in optical absorption properties during the electrochemical oxidation/reduction process, and the species are electrochromic materials [1,2,3]. In the past 20 years, electrochromic materials and electrochemical membrane materials have been utilized in several fields such as for architectural windows, helmet visors, rearview mirrors, optical displays [4], fuel cells [5,6,7], and supercapacitors [8,9]. Electrochromic materials are generally classified into two categories: (a) inorganic complexes and transition metal oxides and (b) organic polymers and viologen derivatives [10]. Among these chromic materials, organic electrochromic polymers have gained extensive attention for promising applications in electrochromic electrodes owing to their short response time, facile color-tuning by chemical structures modification, low-power consumption, and good solution processability [11].
The most commonly used conjugated polymeric materials are polycarbazole [12,13], poly(3,4-ethylenedioxythiophene) (PEDOT) [14], polythiophene [15], polyfuran [16], polypyrrole [17], polyaniline [18], polytriphenylamine [19,20,21], and polyindole [22,23]. Polycarbazole and polytriphenylamine have been extensively applied for various electronic and optical devices owing to their high hole transporting mobility and good photoactive and electroactive performances [24]. Hsiao et al. published the electrochromic properties of carbazole- and triphenylamine-based polymers (P(PhCz-2Cz) and P(TPA-2Cz)). P(TPA-2Cz) film was colorless, dark green, and brown in its neutral, semi-oxidized, and fully oxidized states. P(PhCz-2Cz) displayed satisfactory electrochromic switching with a ΔTmax up to 35% at 1052 nm [25]. Oral et al. reported the electrochromic properties of a dual-type ECD, which employed polystyrene functionalized carbazole (PS-Carb) as the anodically coloring polymer. The ΔT of PS-Carb/PEDOT ECD was 38% at 640 nm and the response time was 1.1 s [26]. Polythiophene is a potentially useful electrode material due to its desirable stability to environment and high conductivity [27]. However, the onset oxidation potential of thiophene is higher than 1.5 V [28]. PEDOT is a derivative of polythiophene which introduces alkoxyl substituent groups into a thiophene ring. The onset oxidation potential of PEDOT is lower than polythiophene. PEDOT is light sky-blue and deep blue in doped and reduced states, respectively. Accordingly, PEDOT is a cathodically coloring layer in electrochromic devices (ECDs). Polyfuran possesses some particular properties such as high fluorescence, a high HOMO energy level, and adequate solubility [29]. Polyfurans synthesized using furan, bifuran, and trifuran have different electrochemical activities, polyfuran prepared using trifuran has the highest electrochemical activity [30].
In the present report, a homopolymer (PDTC), two 2,2′-bithiophene (BTP)-based copolymers (P(DTC-co-BTP) and P(DTC-co-BTP2)), and two 2-(2-thienyl) furan (TF)-based copolymers (P(DTC-co-TF) and P(DTC-co-TF2)) with different DTC/BTP and DTC/TF feed molar ratios are synthesized electrochemically to explore their promising applications in electrochromic products. A 3,6-di(2-thienyl)carbazole unit reveals that because two thiophenes are linked by a carbazole, the strong hole-transporting carbazole group raises the HOMO energy level of PDTC. Therefore, PDTC shows a lower onset potential of oxidation than that of polythiophene. The conjugated chain length of 2,2′-bithiophene is longer than that of thiophene. The onset oxidation potential of 2,2′-bithiophene is lower than that of thiophene. Accordingly, 2,2′-bithiophene is incorporated to the polymer backbone and electrochromic properties of 2,2′-bithiophene-based polymers are characterizated in this study. Moreover, 2-(2-thienyl)furan combines the properties of furan and thiophene, while the conjugated chain length of 2-(2-thienyl)furan is longer than those of furan and thiophene. It is interesting to explore the difference of electrochromic behaviors for 2,2′-bithiophene- and 2-(2-thienyl)furan-based polymer membranes. The electrode membranes could be useful for identifying additional transport properties to be associated to the electrochromic behavior. Furthermore, five ECDs consisted of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), or P(DTC-co-TF2) as the anodic membrane, and PEDOT-PSS as the cathodic membrane were built and their spectroelectrochemical behaviors, electrochromic switching kinetics, and redox stability were explored in detail.

2. Experimental

2.1. Materials

2,2′-bithiophene and 2-(2-thienyl)furan were purchased from Alfa Aesar and Sigma-Aldrich, respectively. 3,6-di(2-thienyl)carbazole (DTC) was synthesized according to previous procedures [31]. Electrolytes of ECDs were prepared using a mixture of poly(methyl methacrylate) (PMMA), propylene carbonate (PC), and LiClO4. The weight ratio of PMMA, PC, and LiClO4 is 33:53:14 [32].

2.2. Electrochemical Preparation of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), P(DTC-co-TF2) Films

The electrosynthesis of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) membranes was implemented in a 0.2 M LiClO4/acetonitrile (ACN) solution, and the feed species and feed ratio of species for anodic polymers were displayed in Table 1. The 3,6-di(2-thienyl)carbazole- and thiophene derivatives-based homopolymer and copolymers were prepared potentiostatically at 1.2 V (vs. Ag/AgCl).

2.3. Assembly of Electrochromic Devices

Polymer ECDs were fabricated using PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), or P(DTC-co-TF2) film as the anodic electrochromic layer and PEDOT-PSS film as the cathodic electrochromic layer. The electrodeposited areas of polymer films were 1 × 1.5 cm2. The ECDs were assembled by arranging the anodic and cathodic films to face each other, and they were separated using a PMMA/PC/LiClO4 electrolyte.

2.4. Characterizations of Electrodes and Devices

The electrochemical properties of polymer films were characterized using a CHI627E electrochemical analyzer/workstation. The electrochemical experiments were operated using a three component cell. The working electrode, counter electrode, and reference electrode were an ITO coated glass plate, a platinum wire, and an Ag/AgCl electrode, respectively. The spectroelectrochemical experiments of polymer films and ECDs were measured using an electrochemical workstation and a JASCO V-630 UV-Visible spectrophotometer.

3. Results and Discussion

3.1. Electrochemical Characterization

Figure 1 displays the electrooxidation curves of 2 mM DTC, 2 mM BTP, and 2 mM TF in 0.2 M LiClO4/ACN solution, indicating that the onset potentials (Eonset) of oxidation for DTC, BTP, and TF were 0.88, 1.22, and 1.19 V, respectively. DTC displayed lower Eonset than those of BTP and TF, implying the electron donating carbazole unit of DTC decreased the Eonset significantly. Moreover, Eonset of 2-(2-thienyl)furan is slightly lower than that of 2,2′-bithiophene.
Figure 2 shows the electrosynthesized curves of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), P(DTC-co-TF2), PBTP, and PTF in 0.2 M LiClO4/ACN solution between 0.0 and 1.5 V. When the number of scanning curves increases, the peak current density increases with increasing number of cycles, indicating that the electrodeposition of polymer membranes is present on ITO glasses [33]. The oxidized and reduced peaks of five polymer films were quasi-reversible. The oxidation peaks of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), P(DTC-co-TF2), PBTP, and PTF films were located at ca. 1.27, 1.2, 1.25, 1.23, 1.26, 1.37, and 1.31 V (vs. Ag/AgCl), respectively. The reduction peaks of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), P(DTC-co-TF2), PBTP, and PTF were situated at ca. 0.65, 0.48, 0.52, 0.43, 0.50, 0.63, and 0.60 V (vs. Ag/AgCl), respectively.
The potentials and wave shapes of redox peaks of P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) are different to those of PDTC, PBTP, and PTF, proving the occurrence of copolymerization for P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) films. The general polymerization schemes of PDTC, P(DTC-co-BTP), and P(DTC-co-TF) are summarized in Figure 3. Specific capacitances of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) electrodes are 84, 109, 88, 98, and 92 F/g, respectively.
Figure 4 shows the CV curves of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) films at scan rates of 10, 50, 100, 150, and 200 mV s−1 in 0.2 M LiClO4/ACN. The anodic and cathodic peaks of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) membranes displayed quasi-reversible behaviors and the inset in Figure 4 revealed that the redox peak current density increased with the increasing scan rate linearly, demonstrating that the oxidation and reduction processes of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) films were not diffusion-limited [34].

3.2. Spectroelectrochemical Measurement of Polymers

Figure 5 displays absorption spectra of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) films in 0.2 M LiClO4/ACN. At 0.0 and 0.2 V, PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) films display definite-transition bands at ca. 400 nm. After increasing the applied voltage gradually, new charge carrier bands appeared at a long wavelength region. The charge carrier bands of PDTC were located at 550 nm and 860 nm. For the corresponding copolymers, the latter charge carrier bands of P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) were situated at 875, 855, 870, and 855 nm, respectively.
PDTC displays four types of colors from neutral to oxidation state; the color of PDTC is light yellow at 0.0 V, mustard yellow at 0.4 V, gray at 0.8 V, and dark gray at 1.2 V. The 2,2′-bithiophene- and 2-(2-thienyl)furan-based copolymers show different electrochromic properties with PDTC homopolymer. The colors of P(DTC-co-BTP) electrode are light yellow, yellowish-green, gray, and dark gray at 0.0, 0.4, 0.8, and 1.2 V, respectively, whereas P(DTC-co-BTP2) is dark yellow, khaki, gray, and dark gray at 0.0, 0.4, 0.8 V, and 1.2 V, respectively. For 2-(2-thienyl)furan-based copolymers, P(DTC-co-TF) is mustard yellow, khaki, grey, and dark grey at 0.0, 0.4, 0.8, and 1.2 V, respectively, P(DTC-co-TF2) is dark yellow, yellowish-green, gunmetal gray, and dark gray at 0.0, 0.4, 0.8, and 1.2 V, respectively. The colorimetric values, CIE chromaticity values, and charts of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) at 0.0—1.0 V are summarized in Table 2.

3.3. Electrochromic Switching of Anodic Polymers

Figure 6 shows the transmittance-time plots of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) membranes caused by potential stepping between reduced and oxidized states. The residence time is 5 s. The ΔT of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) were 37.9% at 860 nm, 61.6% at 875 nm, 68.4% at 855 nm, 67.3% at 870 nm, and 56.1% at 855 nm, respectively (Table 3). The ΔT of 2,2′-bithiophene- and 2-(2-thienyl)furan-based copolymers (P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2)) were higher than that of PDTC in 0.2 M LiClO4/ACN, inferring that copolymers containing 2,2′-bithiophene or 2-(2-thienyl)furan groups increased ΔT significantly. Moreover, the ΔT of P(DTC-co-BTP2) was higher than those reported for P(PtCz-co-BTP2) [35], PITID-2 [36], P(HoT-BSe-OF) [37], PDTCZ-2 [38], and PI-6D [39] (Table 4). The bleaching response time (τb) and coloring response time (τc) needed to reach 90% of the maximum transmittance change were determined to be 0.9–3.1 s for these polymers. The coloration efficiency (η) is a significant factor for effective utilization of polymers in electrochromic devices and it can be obtained using the equation below [40]:
η =   Δ OD Q d
where ΔOD and Qd represent the change of optical density and the injected charges divided by active electrode area, respectively. As listed in Table 3, the η of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) were 125.8 cm2 C−1 at 860 nm, 141.5 cm2 C−1 at 875 nm, 159.4 cm2 C−1 at 855 nm, 161.6 cm2 C−1 at 870 nm, and 152.9 cm2 C−1 at 855 nm, respectively. P(DTC-co-BTP2) shows higher η than that of P(HoT-BSe-OF) [37]. However, P(DTC-co-BTP2) shows a lower η than those of PITID-2 [36], PDTCZ-2 [38], and PI-6D [39] (Table 4).

3.4. Spectroelectrochemical Properties of ECDs

Figure 7 showed the absorption spectra of PDTC/PEDOT-PSS, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs at assorted potentials. The five ECDs showed distinct UV-Visible bands at ca. 400 nm at around −0.5 V, which could be ascribed to the UV-Visible bands of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) at low potential zone. The UV-Visible bands of PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) bleached with increasing potentials, and new visible peaks appeared bit by bit at 627–630 nm. This could be assigned to the reduction of PEDOT-PSS bit by bit. The colors of PDTC/PEDOT-PSS ECD are gray, dark gray, and navy blue at −0.5, 0.8, and 2.0 V, respectively. P(DTC-co-BTP)/PEDOT-PSS ECD is bright gray, dark gray, and deep blue at −0.5, 0.6, and 2.0 V, respectively. P(DTC-co-BTP2)/PEDOT-PSS ECD is bright gray, gunmetal grey, and berlin blue at −0.5, 0.8, and 2.0 V, respectively. P(DTC-co-TF)/PEDOT-PSS ECD is bright gray, iron grey, and sapphire at −0.5, 0.8, and 2.0 V, respectively. P(DTC-co-TF2)/PEDOT-PSS ECD is bright gray, gunmetal grey, and berlin blue at −0.5, 0.8, and 2.0 V, respectively. The photos, L*, a*, b*, x, y, and CIE diagrams of PDTC/PEDOT-PSS, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs at different potentials are summarized in Table 5.

3.5. Colorless-to-Colorful Switching of ECDs

Figure 8 showed the potential stepping of PDTC/PEDOT-PSS, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs between colorless and colorful states with a residence time of 5 s. The ΔOD, ΔT, τb, and τc of PDTC/PEDOT-PSS, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs are displayed in Table 6. The ΔTmax of PDTC/PEDOT-PSS, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs are 34.3% at 630 nm, 38.7% at 630 nm, 41.6% at 630 nm, 43.4% at 627 nm, and 41.1% at 627 nm, respectively. P(DTC-co-TF)/PEDOT-PSS ECD showed the highest ΔTmax, and P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs showed higher ΔTmax than that of PDTC/PEDOT-PSS ECD. This implies that the employment of copolymers (P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2)) as the anodic layers results in a higher ΔTmax at ca. 627 nm than that of homopolymers (PDTCs). Table 4 lists ΔT’s comparisons of P(DTC-co-TF)/PEDOT-PSS ECD with reported ECDs, showing that P(DTC-co-2BTP)/PEDOT-PSS ECD displays a higher ΔT than those reported for PETI/PEDOT [41], P(PS-Carb)/PEDOT [26], P(BCO)/PEDOT [42], and P(DiCP-co-CPDTK)/PEDOT-PSS ECDs [43].
The τb and τc of PDTC/PEDOT-PSS, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs in Table 6 are in the range of 0.3–0.9 s. τb and τc of five ECDs were shorter than their corresponding anodes in 0.2 M LiClO4/ACN, revealing the ECDs switched color quicker than the anodes in 0.2 M LiClO4/ACN from the colored to the bleached state [44].
The η of PDTC/PEDOT-PSS, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs are 420.7 cm2 C−1 at 630 nm, 537.4 cm2 C−1 at 630 nm, 398.3 cm2 C−1 at 630 nm, 496.0 cm2 C−1 at 627 nm, and 512.6 cm2 C−1 at 627 nm, respectively. Among these ECDs, P(DTC-co-BTP) with a feed molar ratio of DTC/BTP = 1/1 displays the highest η. Table 4 displays η’s comparisons of P(DTC-co-TF)/PEDOT-PSS ECD with reported ECDs, while P(DTC-co-TF)/PEDOT-PSS ECD shows a greater η than that reported for PETI/PEDOT ECD [41]. However, P(DTC-co-TF)/PEDOT-PSS ECD displays a lower η than that reported for P(DiCP-co-CPDTK)/PEDOT-PSS ECD [43].

3.6. Optical Memory Influences of ECDs

The optical memory influences of PDTC/PEDOT-PSS, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs were monitored in the colored and bleached states by exerting the voltage for 1 sec for each 100 sec interval. Figure 9 displayed that the transmittances of PDTC/PEDOT-PSS, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs experienced almost no change in the bleached state. On the other hand, the transmittance changes of PDTC/PEDOT-PSS, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs in the colored state were less electrochemically stable than those in the bleached state. However, the change of transmittance for ECDs in the colored states was less than 4.3%. P(DTC-co-TF2)/PEDOT-PSS ECD showed the lowest transmittance change (2.9%) in the colored state and P(DTC-co-BTP2)/PEDOT-PSS ECD showed the lowest transmittance change (0.2%) in the bleached state. Considering the aforementioned results, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs display acceptable optical memory.

3.7. Redox Stability of ECDs

The long-term cycling stability measurement of PDTC/PEDOT-PSS, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs was carried out using CV at the first, 500th and 1000th cycles [45]. From the inspection of redox ability in Figure 10, 90.3%, 87.6%, 85.1%, 91.5%, and 89.7% of electroactivity are conserved at the 500th cycle, and 86.7%, 74.4%, 72.1%, 83.7%, and 84.6% of electroactivity are conserved at the 1000th cycle for PDTC/PEDOT-PSS, P(DTC-co-BTP)/PEDOT-PSS, P(DTC-co-BTP2)/PEDOT-PSS, P(DTC-co-TF)/PEDOT-PSS, and P(DTC-co-TF2)/PEDOT-PSS ECDs, respectively. P(DTC-co-TF)/PEDOT-PSS and P(DTC-co-TF2)/PEDOT-PSS ECDs showed better cycling stability than those of P(DTC-co-BTP)/PEDOT-PSS and P(DTC-co-BTP2)/PEDOT-PSS ECDs, displaying ECDs employed 2-(2-thienyl)furan-containing P(DTC-co-TF) (or P(DTC-co-TF2)) as anodic layer led to a better cycling stability than that of 2,2′-bithiophene-containing P(DTC-co-BTP) (or P(DTC-co-BTP2)).

4. Conclusions

A series of ECDs’ anodic materials (PDTC, P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2)) were prepared electrochemically. The 2,2′-bithiophene- and 2-(2-thienyl)furan-based copolymers prepared with various monomer feed ratios displayed various colorimetry, spectroelectrochemical, and electrochromic switching performances. The colors of P(DTC-co-BTP) electrode are light yellow, yellowish-green, gray, and dark gray at 0.0, 0.4, 0.8, and 1.2 V, respectively. The ΔT of P(DTC-co-BTP), P(DTC-co-BTP2), P(DTC-co-TF), and P(DTC-co-TF2) in solutions were 61.6% at 875 nm, 68.4% at 855 nm, 67.3% at 870 nm, and 56.1% at 855 nm, respectively. Dual type polymer ECDs which consisted of 2,2′-bithiophene- and 2-(2-thienyl)furan-based anodically coloring membranes and a PEDOT-PSS cathodically coloring membrane were made. The transmittance changes of ECDs’ optical memory test in the colored and bleached states were less than 4.3% and 0.4%, respectively. P(DTC-co-TF)/PEDOT-PSS ECD displays the highest ΔT (43.4% at 627 nm), whereas P(DTC-co-BTP)/PEDOT-PSS ECD shows the highest η (537.4 cm2 C−1 at 630 nm). In view of the aforementioned performances, 2,2′-bithiophene- and 2-(2-thienyl)furan-based electrochromic membranes are suitable to apply in electrochromic goggles, e-skins, textiles, and wearable display devices.

Author Contributions

Conceptualization, C.-W.K.; formal analysis, C.-W.K. and S.-W.H.; investigation, C.-W.K. and S.-W.H.; resources, J.-K.C., J.-C.C. and P.-Y.L.; data curation, S.-W.H., J.-K.C., J.-C.C., P.-Y.L. and T.-Y.W.; writing—original draft preparation, C.-W.K. and T.-Y.W.; writing—review and editing, C.-W.K. and T.-Y.W.; project administration, C.-W.K. and S.-W.H.; funding acquisition, C.-W.K., T.-Y.W. and J.-K.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Ministry of Science and Technology of Republic of China, Grant No. 109-2221-E-992-006 and 108-2221-E-224-049-MY3.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Membranes 11 00125 g001 550
Figure 1. Electrooxidation curves of (a) 2 mM DTC, (b) 2 mM BTP, and (c) 2 mM TF in 0.2 M LiClO4/acetonitrile (ACN) at a scan rate of 100 mV s−1.
Figure 1. Electrooxidation curves of (a) 2 mM DTC, (b) 2 mM BTP, and (c) 2 mM TF in 0.2 M LiClO4/acetonitrile (ACN) at a scan rate of 100 mV s−1.
Membranes 11 00125 g001
Membranes 11 00125 g002 550
Figure 2. Electrochemical synthesis of (a) a homopolymer (PDTC), (b) P(DTC-co-BTP), (c) P(DTC-co-BTP2), (d) P(DTC-co-TF), (e) P(DTC-co-TF2), (f) PBTP, and (g) PTF in ACN solution at 100 mV s−1 on an ITO electrode.
Figure 2. Electrochemical synthesis of (a) a homopolymer (PDTC), (b) P(DTC-co-BTP), (c) P(DTC-co-BTP2), (d) P(DTC-co-TF), (e) P(DTC-co-TF2), (f) PBTP, and (g) PTF in ACN solution at 100 mV s−1 on an ITO electrode.
Membranes 11 00125 g002
Membranes 11 00125 g003 550
Figure 3. The electrochemical polymerization schemes of (a) PDTC, (b) P(DTC-co-BTP), and (c) P(DTC-co-TF).
Figure 3. The electrochemical polymerization schemes of (a) PDTC, (b) P(DTC-co-BTP), and (c) P(DTC-co-TF).
Membranes 11 00125 g003
Membranes 11 00125 g004 550
Figure 4. Cyclic voltammetry (CV) curves of (a) PDTC, (b) P(DTC-co-BTP), (c) P(DTC-co-BTP2), (d) P(DTC-co-TF), and (e) P(DTC-co-TF2) films at different scan rates between 10 and 200 mV s−1 in the LiClO4 + ACN solution. Inset: Plots of scan rate vs. anodic and cathodic peak current densities.
Figure 4. Cyclic voltammetry (CV) curves of (a) PDTC, (b) P(DTC-co-BTP), (c) P(DTC-co-BTP2), (d) P(DTC-co-TF), and (e) P(DTC-co-TF2) films at different scan rates between 10 and 200 mV s−1 in the LiClO4 + ACN solution. Inset: Plots of scan rate vs. anodic and cathodic peak current densities.
Membranes 11 00125 g004
Membranes 11 00125 g005 550
Figure 5. UV-Visible spectra of (a) PDTC, (b) P(DTC-co-BTP), (c) P(DTC-co-BTP2), (d) P(DTC-co-TF), and (e) P(DTC-co-TF2) in 0.2 M LiClO4/ACN solution.
Figure 5. UV-Visible spectra of (a) PDTC, (b) P(DTC-co-BTP), (c) P(DTC-co-BTP2), (d) P(DTC-co-TF), and (e) P(DTC-co-TF2) in 0.2 M LiClO4/ACN solution.
Membranes 11 00125 g005
Membranes 11 00125 g006 550
Figure 6. Optical contrast of (a) PDTC, (b) P(DTC-co-BTP), (c) P(DTC-co-BTP2), (d) P(DTC-co-TF), and (e) P(DTC-co-TF2) in 0.2 M LiClO4/ACN solution with a residence time of 5 s.
Figure 6. Optical contrast of (a) PDTC, (b) P(DTC-co-BTP), (c) P(DTC-co-BTP2), (d) P(DTC-co-TF), and (e) P(DTC-co-TF2) in 0.2 M LiClO4/ACN solution with a residence time of 5 s.
Membranes 11 00125 g006
Membranes 11 00125 g007 550
Figure 7. UV-Visible spectra of (a) PDTC/PEDOT-PSS, (b) P(DTC-co-BTP)/PEDOT-PSS, (c) P(DTC-co-BTP2)/PEDOT-PSS, (d) P(DTC-co-TF)/PEDOT-PSS, and (e) P(DTC-co-TF2)/PEDOT-PSS ECDs.
Figure 7. UV-Visible spectra of (a) PDTC/PEDOT-PSS, (b) P(DTC-co-BTP)/PEDOT-PSS, (c) P(DTC-co-BTP2)/PEDOT-PSS, (d) P(DTC-co-TF)/PEDOT-PSS, and (e) P(DTC-co-TF2)/PEDOT-PSS ECDs.
Membranes 11 00125 g007
Membranes 11 00125 g008 550
Figure 8. Optical contrast of (a) PDTC/PEDOT-PSS, (b) P(DTC-co-BTP)/PEDOT-PSS, (c) P(DTC-co-BTP2)/PEDOT-PSS, (d) P(DTC-co-TF)/PEDOT-PSS, and (e) P(DTC-co-TF2)/PEDOT-PSS ECDs with a residence time of 5 s.
Figure 8. Optical contrast of (a) PDTC/PEDOT-PSS, (b) P(DTC-co-BTP)/PEDOT-PSS, (c) P(DTC-co-BTP2)/PEDOT-PSS, (d) P(DTC-co-TF)/PEDOT-PSS, and (e) P(DTC-co-TF2)/PEDOT-PSS ECDs with a residence time of 5 s.
Membranes 11 00125 g008
Membranes 11 00125 g009 550
Figure 9. Open circuit stability of (a) PDTC/PEDOT-PSS, (b) P(DTC-co-BTP)/PEDOT-PSS, (c) P(DTC-co-BTP2)/PEDOT-PSS, (d) P(DTC-co-TF)/PEDOT-PSS, and (e) P(DTC-co-TF2)/PEDOT-PSS ECDs.
Figure 9. Open circuit stability of (a) PDTC/PEDOT-PSS, (b) P(DTC-co-BTP)/PEDOT-PSS, (c) P(DTC-co-BTP2)/PEDOT-PSS, (d) P(DTC-co-TF)/PEDOT-PSS, and (e) P(DTC-co-TF2)/PEDOT-PSS ECDs.
Membranes 11 00125 g009
Membranes 11 00125 g010 550
Figure 10. Cyclic voltammograms of (a) PDTC/PEDOT-PSS, (b) P(DTC-co-BTP)/PEDOT-PSS, (c) P(DTC-co-BTP2)/PEDOT-PSS, (d) P(DTC-co-TF)/PEDOT-PSS, and (e) P(DTC-co-TF2)/PEDOT-PSS devices with a scan rate of 500 mV s−1 between the 1st and 1000th cycles.
Figure 10. Cyclic voltammograms of (a) PDTC/PEDOT-PSS, (b) P(DTC-co-BTP)/PEDOT-PSS, (c) P(DTC-co-BTP2)/PEDOT-PSS, (d) P(DTC-co-TF)/PEDOT-PSS, and (e) P(DTC-co-TF2)/PEDOT-PSS devices with a scan rate of 500 mV s−1 between the 1st and 1000th cycles.
Membranes 11 00125 g010
Table
Table 1. Feed species of anodic polymers.
Table 1. Feed species of anodic polymers.
ElectrodesAnodic PolymersFeed Species of Anodic PolymersFeed Molar Ratio of Anodic Polymers
(a)PDTC2 mM DTCNeat PDTC
(b)P(DTC-co-BTP)2 mM DTC + 2 mM BTPDTC:BTP = 1:1
(c)P(DTC-co-BTP2)2 mM DTC + 4 mM BTPDTC:BTP = 1:2
(d)P(DTC-co-TF)2 mM DTC + 2 mM TFDTC:TF = 1:1
(e)P(DTC-co-TF2)2 mM DTC + 4 mM TFDTC:TF = 1:2
Table
Table 2. Colorimetric values (L*, a*, and b*), CIE chromaticity values (x, y), and CIE diagrams of (a) PDTC, (b) P(DTC-co-BTP), (c) P(DTC-co-BTP2), (d) P(DTC-co-TF), and (e) P(DTC-co-TF2) at various applied potentials.
Table 2. Colorimetric values (L*, a*, and b*), CIE chromaticity values (x, y), and CIE diagrams of (a) PDTC, (b) P(DTC-co-BTP), (c) P(DTC-co-BTP2), (d) P(DTC-co-TF), and (e) P(DTC-co-TF2) at various applied potentials.
Films Potential (V) L* a* b* xy Diagrams
(a) 0.0 70.45−3.237.290.32350.3493 Membranes 11 00125 i001
0.4 67.3−1.072.080.31570.3351
0.6 62.341.36−5.180.30240.3144
0.8 55.89−0.11−13.450.27660.2913
1.0 52.01−1.19−17.140.26240.2794
(b) 0.0 75.43−2.8511.330.33210.3571 Membranes 11 00125 i002
0.4 72.02−0.365.820.3250.3429
0.6 66.682.7−2.940.31060.3195
0.8 60.532.92−12.110.28760.2947
1.0 54.061.08−19.740.26140.2721
(c) 0.0 95.82−13.9844.810.36830.4233 Membranes 11 00125 i003
0.4 92.85−11.6740.390.3660.4154
0.6 78.43−1.4614.040.33930.3609
0.8 65.1910.53−12.60.30120.2899
1.0 62.1911.62−15.40.29570.2810
(d) 0.0 93.35−12.6541.740.36650.4184 Membranes 11 00125 i004
0.4 90.01−9.7535.870.36270.4071
0.6 76.910.411.450.33750.354
0.8 65.5710.39−10.810.30530.2944
1.0 64.019.85−11.980.30130.2914
(e) 0.0 93.22−11.139.370.36510.4126 Membranes 11 00125 i005
0.4 90.72−8.6234.390.36160.4027
0.6 80.06−0.6914.720.34150.3611
0.8 67.3810.15−10.480.30590.296
1.0 65.0010.16−12.550.30060.2902
Table
Table 3. Optical properties of electrodes.
Table 3. Optical properties of electrodes.
Electrodesλ (nm) ToxTred ΔTΔODQd (mC cm−2)η (cm2 C−1)τc (s)τb (s)
PDTC86052.890.737.90.241.87125.83.11.2
55031.968.436.50.331.85178.40.91.9
P(DTC-co-BTP)87523.685.261.60.563.94141.53.11
55021.962.140.20.453.41131.91.41.7
P(DTC-co-BTP2)85515.583.968.40.734.6159.41.91
55023.765.641.90.443.36130.91.31.9
P(DTC-co-TF)87021.388.667.30.623.83161.63.41.8
55025.859.8340.363.6598.61.31.8
P(DTC-co-TF2)85526.882.956.10.493.21152.93.11.2
55027.257.930.70.333.789.21.41.6
Table
Table 4. Transmittance changes and coloration efficiencies of polymer films and ECDs.
Table 4. Transmittance changes and coloration efficiencies of polymer films and ECDs.
Polymer Films or ECD Configurationsλ (nm)ΔT (%)η (cm2 C−1)References
P(PtCz-co-BTP2)56534-[35]
PITID-267518172[36]
P(HoT-BSe-OF)86029142[37]
PDTCZ-289830.7169[38]
PI-6D56857250[39]
P(DTC-co-BTP2)85568.4159.4This work
PETI/PEDOT60032290[41]
P(PS-Carb)/PEDOT64038-[26]
P(BCO)/PEDOT62035-[42]
P(DiCP-co-CPDTK)/PEDOT-PSS63538.2633.8[43]
P(DTC-co-TF)/PEDOT-PSS62743.4496.0This work
Table
Table 5. Electrochromic photographs, colorimetric values (L*, a*, and b*) and CIE chromaticity values (x, y) of (a) PDTC/PEDOT-PSS, (b) P(DTC-co-BTP)/PEDOT-PSS, (c) P(DTC-co-BTP2)/PEDOT-PSS, (d) P(DTC-co-TF)/PEDOT-PSS, and (e) P(DTC-co-TF2)/PEDOT-PSS ECDs at various potentials.
Table 5. Electrochromic photographs, colorimetric values (L*, a*, and b*) and CIE chromaticity values (x, y) of (a) PDTC/PEDOT-PSS, (b) P(DTC-co-BTP)/PEDOT-PSS, (c) P(DTC-co-BTP2)/PEDOT-PSS, (d) P(DTC-co-TF)/PEDOT-PSS, and (e) P(DTC-co-TF2)/PEDOT-PSS ECDs at various potentials.
ECDsPotential (V)PhotographsL*a*b*xyDiagrams
(a) −0.5 Membranes 11 00125 i00673.76−5.4813.000.33140.3640 Membranes 11 00125 i007
0.0 Membranes 11 00125 i00870.45−3.237.290.32350.3493
0.8 Membranes 11 00125 i00962.341.36−5.180.30240.3144
1.2 Membranes 11 00125 i01055.89−0.11−13.450.27660.2913
1.6 Membranes 11 00125 i01152.01−1.19−17.140.26240.2794
2.0 Membranes 11 00125 i01248.40−2.02−21.740.24530.2638
(b) −0.5 Membranes 11 00125 i01373.29−0.245.870.32510.3427 Membranes 11 00125 i014
0.0 Membranes 11 00125 i01570.172.45−0.860.31800.3302
0.6 Membranes 11 00125 i01668.053.90−4.330.30950.3154
1.4 Membranes 11 00125 i01759.391.73−14.330.27950.2894
1.8 Membranes 11 00125 i01856.121.21−17.490.26880.2795
2.0 Membranes 11 00125 i01955.160.83−18.710.26430.2759
(c) −0.5 Membranes 11 00125 i02078.63−5.2416.980.33900.3708 Membranes 11 00125 i021
0.0 Membranes 11 00125 i02275.43−2.8511.330.33210.3571
0.8 Membranes 11 00125 i02366.682.70−2.940.31060.3195
1.2 Membranes 11 00125 i02460.532.92−12.110.28760.2947
1.6 Membranes 11 00125 i02554.061.08−19.740.26140.2721
2.0 Membranes 11 00125 i02652.640.45−21.130.25550.2676
(d) −0.5 Membranes 11 00125 i02775.91−4.1914.220.33580.3647 Membranes 11 00125 i028
0.0 Membranes 11 00125 i02973.10−1.978.570.32810.3506
0.8 Membranes 11 00125 i03064.623.02−5.160.30580.3135
1.2 Membranes 11 00125 i03157.521.82−14.930.27720.2867
1.6 Membranes 11 00125 i03252.821.59−19.790.26140.2706
2.0 Membranes 11 00125 i03349.750.79−23.370.24770.2585
(e) −0.5 Membranes 11 00125 i03473.57−2.289.720.33000.3534 Membranes 11 00125 i035
0.0 Membranes 11 00125 i03671.28−0.604.920.32270.3411
0.8 Membranes 11 00125 i03764.123.47−6.60.30300.3094
1.2 Membranes 11 00125 i03858.172.20−14.480.27940.288
1.6 Membranes 11 00125 i03954.371.44−18.420.26570.2756
2.0 Membranes 11 00125 i04051.950.55−20.230.25750.2694
Table
Table 6. Optical properties of devices.
Table 6. Optical properties of devices.
ECDsToxTredΔTΔODQd (mC cm−2)η (cm2∙C−1)τc/sτb/s
PDTC/PEDOT-PSS (630 nm) 15.449.734.30.5091.21420.70.90.7
P(DTC-co-BTP)/PEDOT-PSS (630 nm)18.657.338.70.4890.91537.40.60.4
P(DTC-co-BTP2)/PEDOT-PSS (630 nm)21.963.541.60.4621.16398.30.90.5
P(DTC-co-TF)/PEDOT-PSS (627 nm)13.556.943.40.6251.264960.60.3
P(DTC-co-TF2)/PEDOT-PSS (627 nm)17.358.441.10.5281.03512.60.50.4
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Kuo, C.-W.; Chang, J.-C.; Chang, J.-K.; Huang, S.-W.; Lee, P.-Y.; Wu, T.-Y. Electrosynthesis of Electrochromic Polymer Membranes Based on 3,6-Di(2-thienyl)carbazole and Thiophene Derivatives. Membranes 2021, 11, 125. https://doi.org/10.3390/membranes11020125

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Kuo C-W, Chang J-C, Chang J-K, Huang S-W, Lee P-Y, Wu T-Y. Electrosynthesis of Electrochromic Polymer Membranes Based on 3,6-Di(2-thienyl)carbazole and Thiophene Derivatives. Membranes. 2021; 11(2):125. https://doi.org/10.3390/membranes11020125

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Kuo, Chung-Wen, Jui-Cheng Chang, Jeng-Kuei Chang, Sheng-Wei Huang, Pei-Ying Lee, and Tzi-Yi Wu. 2021. "Electrosynthesis of Electrochromic Polymer Membranes Based on 3,6-Di(2-thienyl)carbazole and Thiophene Derivatives" Membranes 11, no. 2: 125. https://doi.org/10.3390/membranes11020125

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