Three Carbazole-Based Polymers as Potential Anodically Coloring Materials for High-Contrast Electrochromic Devices

Three carbazole-based conjugated polymers (poly(3,6-di(2-thienyl)carbazole) (PDTC), poly(2,7-bis(carbazol-9-yl)-9,9-spirobifluorene) (PS2CBP), and poly(3,6-bis(N-carbazole)-N-ethylcarbazole) (PCEC)) are synthesized using electrochemical polymerization. The spectroelectrochemical studies indicate that the PDTC, PS2CBP, and PCEC films show reversible electrochromic behaviors in their redox states, and the PS2CBP film shows a distinct color transition with four various colors (gray at 0 V, grayish-green at 1.0 V, moss green at 1.2 V, and foliage green at 1.4 V). The maximum optical contrast of the PS2CBP and PCEC films is 39.83% at 428 nm and 32.41% at 420 nm, respectively, in an ionic liquid solution. Dual-type electrochromic devices (ECDs) that employ PDTC, PS2CBP, or PCEC film as an anodic layer, and PProDOT-Et2 film as a cathodic layer, were constructed. The as-prepared PCEC/PProDOT-Et2 ECD shows high optical contrast (38.25% at 586 nm) and high coloration efficiency (369.85 cm2 C−1 at 586 nm), and the PS2CBP/PProDOT-Et2 ECD shows high optical contrast (34.45% at 590 nm), good optical memory, and good long-term cycling stability.


Instrumentation and Measurements
The electrochemical behaviors of the PDTC, PS2CBP, and PCEC films coated on the ITO electrodes were characterized using a CHI6081E electrochemical analyzer (CH Instruments, Austin, TX, USA). The sheet resistance of ITO glass (AimCore Technology Co., Ltd., Hsinchu, Taiwan) is below 15 Ω/sq. The spectroelectrochemical properties of the PDTC film, the PS2CBP film, the PCEC film, a PDTC/PProDOT-Et 2 ECD, a PS2CBP/PProDOT-Et 2 ECD, and a PCEC/PProDOT-Et 2 ECD were characterized using a V-670 JASCO UV-Visible spectrophotometer set to record in situ UV-Visible spectra photometer (JASCO International Co., Ltd., Tokyo, Japan). The chromaticity values of the polymer films and ECDs were calculated according to previous procedures [28].

Construction of ECDs
An electrochromic electrolyte was prepared according to the method described in our previous work [29]. The electrochromic electrolyte is an ionic liquid/polymer composite electrolyte. The ionic liquid and polymer are [EPI + ][TFSI − ] and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), respectively. The anodic coloring PDTC, PS2CBP, and PCEC films were electrodeposited onto ITO glasses potentiostatically at 0.9 V, 1.2 V, and 1.1 V, respectively, whereas cathodic coloring PProDOT-Et 2 film was electrodeposited onto ITO-coated glasses potentiostatically at 1.4 V. The anodic polymer film and PProDOT-Et 2 film were separated by an ionic liquid/polymer composite electrolyte.

Electrochemical Polymerization of Polymer Films
The polymer films can be prepared by a potentiodynamic method. The cyclic voltammograms (CVs) of neat DTC, S2CBP, and CEC monomers in an acetonitrile/dichloromethane (ACN/DCM) solution (1:1, v/v) containing 0.1 M LiClO 4 are shown in Figure 2. As the cyclic voltammetric scan continued, the peak current intensity of Figure 2a-c increased gradually, demonstrating that the PDTC, PS2CBP, and PCEC films were electropolymerized on the surface of the ITO working electrode. The schemes for the electrochemical polymerization of PDTC, PS2CBP, and PCEC are shown in Figure S4 (in supplementary information).

Instrumentation and Measurements
The electrochemical behaviors of the PDTC, PS2CBP, and PCEC films coated on the ITO electrodes were characterized using a CHI6081E electrochemical analyzer (CH Instruments, Austin, TX, USA). The sheet resistance of ITO glass (AimCore Technology Co., Ltd., Hsinchu, Taiwan) is below 15 Ω/sq. The spectroelectrochemical properties of the PDTC film, the PS2CBP film, the PCEC film, a PDTC/PProDOT-Et2 ECD, a PS2CBP/PProDOT-Et2 ECD, and a PCEC/PProDOT-Et2 ECD were characterized using a V-670 JASCO UV-Visible spectrophotometer set to record in situ UV-Visible spectra photometer (JASCO International Co., Ltd., Tokyo, Japan). The chromaticity values of the polymer films and ECDs were calculated according to previous procedures [28].

Construction of ECDs
An electrochromic electrolyte was prepared according to the method described in our previous work [29]. The electrochromic electrolyte is an ionic liquid/polymer composite electrolyte. The ionic liquid and polymer are [EPI + ][TFSI − ] and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), respectively. The anodic coloring PDTC, PS2CBP, and PCEC films were electrodeposited onto ITO glasses potentiostatically at 0.9 V, 1.2 V, and 1.1 V, respectively, whereas cathodic coloring PProDOT-Et2 film was electrodeposited onto ITO-coated glasses potentiostatically at 1.4 V. The anodic polymer film and PProDOT-Et2 film were separated by an ionic liquid/polymer composite electrolyte.

Electrochemical Polymerization of Polymer Films
The polymer films can be prepared by a potentiodynamic method. The cyclic voltammograms (CVs) of neat DTC, S2CBP, and CEC monomers in an acetonitrile/dichloromethane (ACN/DCM) solution (1:1, v/v) containing 0.1 M LiClO4 are shown in Figure 2. As the cyclic voltammetric scan continued, the peak current intensity of Figure 2a-c increased gradually, demonstrating that the PDTC, PS2CBP, and PCEC films were electropolymerized on the surface of the ITO working electrode. The schemes for the electrochemical polymerization of PDTC, PS2CBP, and PCEC are shown in Figure S4 (in supplementary information). The onset potentials of the PDTC, PS2CBP, and PCEC films are 0.82, 0.99, and 0.94 V, respectively. The PDTC film shows a lower onset potential than those of the PS2CBP and PCEC films, which can be attributed to the incorporation of two thiophene units at the 3,6-positions of the carbazole unit that gives rise to an obvious aromatic conjugation after electrochemical polymerization. Consequently, the onset potential of the PDTC film slightly shifts to low potential [30]. Moreover, the PCEC film shows a lower onset potential than that of the PS2CBP film, which can be ascribed to the fact that CEC contains one additional carbazole unit compared to the S2CBP unit [24]. The oxidation peaks of PDTC, PS2CBP, and PCEC as displayed in Figure 2 appear at 1.02, 1.21, and 1.32 V, respectively, whereas the reduction peaks of PDTC, PS2CBP, and PCEC are located at 0.31, 0.73, and 0.87 V, respectively. Figure 3a-c shows the CV plots of the PDTC, PS2CBP, and PCEC films, respectively, at different scan rates in [EPI + ][TFSI − ] solution, and the relationships between peak current density and the scan rate of the PDTC, PS2CBP, and PCEC films are shown in Figure 3d-f, respectively. The anodic and cathodic peak current density values increase linearly with increasing scan rate as displayed in Figure  3d-f, indicating that the oxidation and reduction processes are non-diffusion limited [31]. The onset potentials of the PDTC, PS2CBP, and PCEC films are 0.82, 0.99, and 0.94 V, respectively. The PDTC film shows a lower onset potential than those of the PS2CBP and PCEC films, which can be attributed to the incorporation of two thiophene units at the 3,6-positions of the carbazole unit that gives rise to an obvious aromatic conjugation after electrochemical polymerization. Consequently, the onset potential of the PDTC film slightly shifts to low potential [30]. Moreover, the PCEC film shows a lower onset potential than that of the PS2CBP film, which can be ascribed to the fact that CEC contains one additional carbazole unit compared to the S2CBP unit [24]. The oxidation peaks of PDTC, PS2CBP, and PCEC as displayed in Figure 2 appear at 1.02, 1.21, and 1.32 V, respectively, whereas the reduction peaks of PDTC, PS2CBP, and PCEC are located at 0.31, 0.73, and 0.87 V, respectively. Figure 3a-c shows the CV plots of the PDTC, PS2CBP, and PCEC films, respectively, at different scan rates in [EPI + ][TFSI − ] solution, and the relationships between peak current density and the scan rate of the PDTC, PS2CBP, and PCEC films are shown in Figure 3d-f, respectively. The anodic and cathodic peak current density values increase linearly with increasing scan rate as displayed in Figure 3d-f, indicating that the oxidation and reduction processes are non-diffusion limited [31]. The onset potentials of the PDTC, PS2CBP, and PCEC films are 0.82, 0.99, and 0.94 V, respectively. The PDTC film shows a lower onset potential than those of the PS2CBP and PCEC films, which can be attributed to the incorporation of two thiophene units at the 3,6-positions of the carbazole unit that gives rise to an obvious aromatic conjugation after electrochemical polymerization. Consequently, the onset potential of the PDTC film slightly shifts to low potential [30]. Moreover, the PCEC film shows a lower onset potential than that of the PS2CBP film, which can be ascribed to the fact that CEC contains one additional carbazole unit compared to the S2CBP unit [24]. The oxidation peaks of PDTC, PS2CBP, and PCEC as displayed in Figure 2 appear at 1.02, 1.21, and 1.32 V, respectively, whereas the reduction peaks of PDTC, PS2CBP, and PCEC are located at 0.31, 0.73, and 0.87 V, respectively. Figure 3a-c shows the CV plots of the PDTC, PS2CBP, and PCEC films, respectively, at different scan rates in [EPI + ][TFSI − ] solution, and the relationships between peak current density and the scan rate of the PDTC, PS2CBP, and PCEC films are shown in Figure 3d-f, respectively. The anodic and cathodic peak current density values increase linearly with increasing scan rate as displayed in Figure  3d-f, indicating that the oxidation and reduction processes are non-diffusion limited [31].  (e) (f)

Electrochromic Properties of the PDTC, PS2CBP, and PCEC Films
The absorption spectra of the PDTC, PS2CBP, and PCEC films coated on ITO glass electrodes were investigated in [EPI + ][TFSI − ] solution at various potentials. As shown in Figure 4, the peaks of the PDTC and PS2CBP films in the neutral state were located at 406 nm and 354 nm, respectively, and the shoulder of the PCEC film in the neutral state was located at 415 nm. These peaks could be assigned to the π-π* transition of the PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution. In the PDTC film, the oxidation begins at about 0.5 V, the peak at around 406 nm decreases gradually, and the charge carrier bands appear at around 600 nm and 900 nm. The formation of the charge carrier bands can be ascribed to the evolution of polaron and bipolaron bands [32].

Electrochromic Properties of the PDTC, PS2CBP, and PCEC Films
The absorption spectra of the PDTC, PS2CBP, and PCEC films coated on ITO glass electrodes were investigated in [EPI + ][TFSI − ] solution at various potentials. As shown in Figure 4, the peaks of the PDTC and PS2CBP films in the neutral state were located at 406 nm and 354 nm, respectively, and the shoulder of the PCEC film in the neutral state was located at 415 nm. These peaks could be assigned to the π-π* transition of the PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution. In the PDTC film, the oxidation begins at about 0.5 V, the peak at around 406 nm decreases gradually, and the charge carrier bands appear at around 600 nm and 900 nm. The formation of the charge carrier bands can be ascribed to the evolution of polaron and bipolaron bands [32]. (e) (f)

Electrochromic Properties of the PDTC, PS2CBP, and PCEC Films
The absorption spectra of the PDTC, PS2CBP, and PCEC films coated on ITO glass electrodes were investigated in [EPI + ][TFSI − ] solution at various potentials. As shown in Figure 4, the peaks of the PDTC and PS2CBP films in the neutral state were located at 406 nm and 354 nm, respectively, and the shoulder of the PCEC film in the neutral state was located at 415 nm. These peaks could be assigned to the π-π* transition of the PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution. In the PDTC film, the oxidation begins at about 0.5 V, the peak at around 406 nm decreases gradually, and the charge carrier bands appear at around 600 nm and 900 nm. The formation of the charge carrier bands can be ascribed to the evolution of polaron and bipolaron bands [32].    Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 Table 2, and the CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8-1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.  [33][34][35], and they are 2.45, 3.06, and 3.00 eV, respectively. The Eg of the PS2CBP film is comparable to that of the PCEC film, implying that the incorporation of a spirofluorene (or carbazole) unit between two carbazole groups does not influence Eg significantly. However, the PDTC film shows a lower Eg value than those of the PS2CBP and PCEC films, which can be ascribed to the fact that the incorporation of two thiophene units at the 3,6positions of the carbazole group diminishes the Eg value significantly. The highest occupied molecular orbital energy levels (EHOMO) of the PDTC, PS2CBP, and PCEC films were calculated using the formula [36]: where Eonset is the onset oxidation potentials corrected using an internal standard redox ferrocene/ferrocinium couple. The lowest unoccupied molecular orbital energy levels (ELUMO) of the PDTC, PS2CBP, and PCEC films were calculated by subtracting the optical band gap from EHOMO [37]. The EHOMO of PDTC, PS2CBP, and PCEC are −5.14, −5.31, and −5.26 eV, respectively, and the ELUMO of PDTC, PS2CBP, and PCEC are −2.69, −2.25, and −2.26 eV, respectively. A cyclic potential-step method was used to determine the electrochromic switching of conducting polymer films [38]. The electrochromic switching of PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution was stepped by repeated potential between reduction and oxidation states with a time interval of 5 s. Figure 5 exhibits the transmittance-time profiles of the PDTC film at 578 and 856 nm, the PS2CBP film at 428 and 1208 nm, and the PCEC film at 420 and 1220 nm.
The coloration switching time (τc) and bleaching switching time (τb) of the PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution are listed in Table 3; the τc and τb are determined at 90% of full-  [33][34][35], and they are 2.45, 3.06, and 3.00 eV, respectively. The Eg of the PS2CBP film is comparable to that of the PCEC film, implying that the incorporation of a spirofluorene (or carbazole) unit between two carbazole groups does not influence Eg significantly. However, the PDTC film shows a lower Eg value than those of the PS2CBP and PCEC films, which can be ascribed to the fact that the incorporation of two thiophene units at the 3,6positions of the carbazole group diminishes the Eg value significantly. The highest occupied molecular orbital energy levels (EHOMO) of the PDTC, PS2CBP, and PCEC films were calculated using the formula [36]: where Eonset is the onset oxidation potentials corrected using an internal standard redox ferrocene/ferrocinium couple. The lowest unoccupied molecular orbital energy levels (ELUMO) of the PDTC, PS2CBP, and PCEC films were calculated by subtracting the optical band gap from EHOMO [37]. The EHOMO of PDTC, PS2CBP, and PCEC are −5.14, −5.31, and −5.26 eV, respectively, and the ELUMO of PDTC, PS2CBP, and PCEC are −2.69, −2.25, and −2.26 eV, respectively. A cyclic potential-step method was used to determine the electrochromic switching of conducting polymer films [38]. The electrochromic switching of PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution was stepped by repeated potential between reduction and oxidation states with a time interval of 5 s. Figure 5 exhibits the transmittance-time profiles of the PDTC film at 578 and 856 nm, the PS2CBP film at 428 and 1208 nm, and the PCEC film at 420 and 1220 nm.
The coloration switching time (τc) and bleaching switching time (τb) of the PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution are listed in Table 3; the τc and τb are determined at 90% of full-  [33][34][35], and they are 2.45, 3.06, and 3.00 eV, respectively. The Eg of the PS2CBP film is comparable to that of the PCEC film, implying that the incorporation of a spirofluorene (or carbazole) unit between two carbazole groups does not influence Eg significantly. However, the PDTC film shows a lower Eg value than those of the PS2CBP and PCEC films, which can be ascribed to the fact that the incorporation of two thiophene units at the 3,6positions of the carbazole group diminishes the Eg value significantly. The highest occupied molecular orbital energy levels (EHOMO) of the PDTC, PS2CBP, and PCEC films were calculated using the formula [36]: where Eonset is the onset oxidation potentials corrected using an internal standard redox ferrocene/ferrocinium couple. The lowest unoccupied molecular orbital energy levels (ELUMO) of the PDTC, PS2CBP, and PCEC films were calculated by subtracting the optical band gap from EHOMO [37]. The EHOMO of PDTC, PS2CBP, and PCEC are −5.14, −5.31, and −5.26 eV, respectively, and the ELUMO of PDTC, PS2CBP, and PCEC are −2.69, −2.25, and −2.26 eV, respectively. A cyclic potential-step method was used to determine the electrochromic switching of conducting polymer films [38]. The electrochromic switching of PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution was stepped by repeated potential between reduction and oxidation states with a time interval of 5 s. Figure 5 exhibits the transmittance-time profiles of the PDTC film at 578 and 856 nm, the PS2CBP film at 428 and 1208 nm, and the PCEC film at 420 and 1220 nm.
The coloration switching time (τc) and bleaching switching time (τb) of the PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution are listed in Table 3; the τc and τb are determined at 90% of full-  [33][34][35], and they are 2.45, 3.06, and 3.00 eV, respectively. The Eg of the PS2CBP film is comparable to that of the PCEC film, implying that the incorporation of a spirofluorene (or carbazole) unit between two carbazole groups does not influence Eg significantly. However, the PDTC film shows a lower Eg value than those of the PS2CBP and PCEC films, which can be ascribed to the fact that the incorporation of two thiophene units at the 3,6positions of the carbazole group diminishes the Eg value significantly. The highest occupied molecular orbital energy levels (EHOMO) of the PDTC, PS2CBP, and PCEC films were calculated using the formula [36]: where Eonset is the onset oxidation potentials corrected using an internal standard redox ferrocene/ferrocinium couple. The lowest unoccupied molecular orbital energy levels (ELUMO) of the PDTC, PS2CBP, and PCEC films were calculated by subtracting the optical band gap from EHOMO [37]. The EHOMO of PDTC, PS2CBP, and PCEC are −5.14, −5.31, and −5.26 eV, respectively, and the ELUMO of PDTC, PS2CBP, and PCEC are −2.69, −2.25, and −2.26 eV, respectively. A cyclic potential-step method was used to determine the electrochromic switching of conducting polymer films [38]. The electrochromic switching of PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution was stepped by repeated potential between reduction and oxidation states with a time interval of 5 s. Figure 5 exhibits the transmittance-time profiles of the PDTC film at 578 and 856 nm, the PS2CBP film at 428 and 1208 nm, and the PCEC film at 420 and 1220 nm.
The coloration switching time (τc) and bleaching switching time (τb) of the PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution are listed in Table 3; the τc and τb are determined at 90% of full-  [33][34][35], and they are 2.45, 3.06, and 3.00 eV, respectively. The Eg of the PS2CBP film is comparable to that of the PCEC film, implying that the incorporation of a spirofluorene (or carbazole) unit between two carbazole groups does not influence Eg significantly. However, the PDTC film shows a lower Eg value than those of the PS2CBP and PCEC films, which can be ascribed to the fact that the incorporation of two thiophene units at the 3,6positions of the carbazole group diminishes the Eg value significantly. The highest occupied molecular orbital energy levels (EHOMO) of the PDTC, PS2CBP, and PCEC films were calculated using the formula [36]: where Eonset is the onset oxidation potentials corrected using an internal standard redox ferrocene/ferrocinium couple. The lowest unoccupied molecular orbital energy levels (ELUMO) of the PDTC, PS2CBP, and PCEC films were calculated by subtracting the optical band gap from EHOMO [37]. The EHOMO of PDTC, PS2CBP, and PCEC are −5.14, −5.31, and −5.26 eV, respectively, and the ELUMO of PDTC, PS2CBP, and PCEC are −2.69, −2.25, and −2.26 eV, respectively. A cyclic potential-step method was used to determine the electrochromic switching of conducting polymer films [38]. The electrochromic switching of PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution was stepped by repeated potential between reduction and oxidation states with a time interval of 5 s. Figure 5 exhibits the transmittance-time profiles of the PDTC film at 578 and 856 nm, the PS2CBP film at 428 and 1208 nm, and the PCEC film at 420 and 1220 nm.
The coloration switching time (τc) and bleaching switching time (τb) of the PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution are listed in Table 3; the τc and τb are determined at 90% of full-  [33][34][35], and they are 2.45, 3.06, and 3.00 eV, respectively. The Eg of the PS2CBP film is comparable to that of the PCEC film, implying that the incorporation of a spirofluorene (or carbazole) unit between two carbazole groups does not influence Eg significantly. However, the PDTC film shows a lower Eg value than those of the PS2CBP and PCEC films, which can be ascribed to the fact that the incorporation of two thiophene units at the 3,6positions of the carbazole group diminishes the Eg value significantly. The highest occupied molecular orbital energy levels (EHOMO) of the PDTC, PS2CBP, and PCEC films were calculated using the formula [36]: where Eonset is the onset oxidation potentials corrected using an internal standard redox ferrocene/ferrocinium couple. The lowest unoccupied molecular orbital energy levels (ELUMO) of the PDTC, PS2CBP, and PCEC films were calculated by subtracting the optical band gap from EHOMO [37]. The EHOMO of PDTC, PS2CBP, and PCEC are −5.14, −5.31, and −5.26 eV, respectively, and the ELUMO of PDTC, PS2CBP, and PCEC are −2.69, −2.25, and −2.26 eV, respectively. A cyclic potential-step method was used to determine the electrochromic switching of conducting polymer films [38]. The electrochromic switching of PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution was stepped by repeated potential between reduction and oxidation states with a time interval of 5 s. Figure 5 exhibits the transmittance-time profiles of the PDTC film at 578 and 856 nm, the PS2CBP film at 428 and 1208 nm, and the PCEC film at 420 and 1220 nm.
The coloration switching time (τc) and bleaching switching time (τb) of the PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution are listed in Table 3; the τc and τb are determined at 90% of full-  [33][34][35], and they are 2.45, 3.06, and 3.00 eV, respectively. The Eg of the PS2CBP film is comparable to that of the PCEC film, implying that the incorporation of a spirofluorene (or carbazole) unit between two carbazole groups does not influence Eg significantly. However, the PDTC film shows a lower Eg value than those of the PS2CBP and PCEC films, which can be ascribed to the fact that the incorporation of two thiophene units at the 3,6positions of the carbazole group diminishes the Eg value significantly. The highest occupied molecular orbital energy levels (EHOMO) of the PDTC, PS2CBP, and PCEC films were calculated using the formula [36]: where Eonset is the onset oxidation potentials corrected using an internal standard redox ferrocene/ferrocinium couple. The lowest unoccupied molecular orbital energy levels (ELUMO) of the PDTC, PS2CBP, and PCEC films were calculated by subtracting the optical band gap from EHOMO [37]. The EHOMO of PDTC, PS2CBP, and PCEC are −5.14, −5.31, and −5.26 eV, respectively, and the ELUMO of PDTC, PS2CBP, and PCEC are −2.69, −2.25, and −2.26 eV, respectively. A cyclic potential-step method was used to determine the electrochromic switching of conducting polymer films [38]. The electrochromic switching of PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution was stepped by repeated potential between reduction and oxidation states with a time interval of 5 s. Figure 5 exhibits the transmittance-time profiles of the PDTC film at 578 and 856 nm, the PS2CBP film at 428 and 1208 nm, and the PCEC film at 420 and 1220 nm.
The coloration switching time (τc) and bleaching switching time (τb) of the PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution are listed in Table 3; the τc and τb are determined at 90% of full-  [33][34][35], and they are 2.45, 3.06, and 3.00 eV, respectively. The Eg of the PS2CBP film is comparable to that of the PCEC film, implying that the incorporation of a spirofluorene (or carbazole) unit between two carbazole groups does not influence Eg significantly. However, the PDTC film shows a lower Eg value than those of the PS2CBP and PCEC films, which can be ascribed to the fact that the incorporation of two thiophene units at the 3,6positions of the carbazole group diminishes the Eg value significantly. The highest occupied molecular orbital energy levels (EHOMO) of the PDTC, PS2CBP, and PCEC films were calculated using the formula [36]: where Eonset is the onset oxidation potentials corrected using an internal standard redox ferrocene/ferrocinium couple. The lowest unoccupied molecular orbital energy levels (ELUMO) of the PDTC, PS2CBP, and PCEC films were calculated by subtracting the optical band gap from EHOMO [37]. The EHOMO of PDTC, PS2CBP, and PCEC are −5.14, −5.31, and −5.26 eV, respectively, and the ELUMO of PDTC, PS2CBP, and PCEC are −2.69, −2.25, and −2.26 eV, respectively. A cyclic potential-step method was used to determine the electrochromic switching of conducting polymer films [38]. The electrochromic switching of PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution was stepped by repeated potential between reduction and oxidation states with a time interval of 5 s. Figure 5 exhibits the transmittance-time profiles of the PDTC film at 578 and 856 nm, the PS2CBP film at 428 and 1208 nm, and the PCEC film at 420 and 1220 nm.
The coloration switching time (τc) and bleaching switching time (τb) of the PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution are listed in Table 3; the τc and τb are determined at 90% of full- The optical band gap (E g ) of the PDTC, PS2CBP, and PCEC films can be calculated according to the Planck equation (E g = 1241 (eV·nm)/λ onset (nm)) [33][34][35], and they are 2.45, 3.06, and 3.00 eV, respectively. The E g of the PS2CBP film is comparable to that of the PCEC film, implying that the incorporation of a spirofluorene (or carbazole) unit between two carbazole groups does not influence E g significantly. However, the PDTC film shows a lower E g value than those of the PS2CBP and PCEC films, which can be ascribed to the fact that the incorporation of two thiophene units at the 3,6-positions of the carbazole group diminishes the E g value significantly. The highest occupied molecular orbital energy levels (E HOMO ) of the PDTC, PS2CBP, and PCEC films were calculated using the formula [36]: where E onset is the onset oxidation potentials corrected using an internal standard redox ferrocene/ferrocinium couple. The lowest unoccupied molecular orbital energy levels (E LUMO ) of the PDTC, PS2CBP, and PCEC films were calculated by subtracting the optical band gap from E HOMO [37]. The E HOMO of PDTC, PS2CBP, and PCEC are −5.14, −5.31, and −5.26 eV, respectively, and the E LUMO of PDTC, PS2CBP, and PCEC are −2.69, −2.25, and −2.26 eV, respectively. A cyclic potential-step method was used to determine the electrochromic switching of conducting polymer films [38]. The electrochromic switching of PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution was stepped by repeated potential between reduction and oxidation states with a time interval of 5 s. Figure 5 exhibits the transmittance-time profiles of the PDTC film at 578 and 856 nm, the PS2CBP film at 428 and 1208 nm, and the PCEC film at 420 and 1220 nm.  Table 3; the τ c and τ b are determined at 90% of full-transmittance change (T 90% ). The optical switching time of the PDTC film in [EPI + ][TFSI − ] solution was found to be 2.04 and 1.64 s at 578 and 856 nm, respectively, from the bleaching to coloring state at the 50th cycle, and 2.01 and 1.69 s at 578 and 856 nm, respectively, from the coloring to bleaching state at the 50th cycle. The τ c and τ b of the PS2CBP film in [EPI + ][TFSI − ] solution at 1208 nm were found to be 2.16 and 1.83 s, respectively, at the 100th cycle, and the τ c and τ b of the PCEC film in [EPI + ][TFSI − ] solution at 1220 nm were found to be 1.90 and 1.63 s, respectively, at the 100th cycle.     The optical contrast (∆T%) is the most important parameter for electrochromic applications [39]. The ∆T max of the PDTC film at 578 and 856 nm is 58.79% and 66.04%, respectively, in [EPI + ][TFSI − ] solution, and the ∆T max of the PS2CBP film at 428 nm and 1208 nm is 39.83% and 63.56%, respectively, in [EPI + ][TFSI − ] solution, implying that the incorporation of the DTC unit gives rise to a higher ∆T max than that of the S2CBP unit. The ∆T max of the PCEC film at 420 nm and 1220 nm is 32.41% and 42.36%, respectively, in [EPI + ][TFSI − ] solution. The ∆T max of the PCEC film is lower than that of the PS2CBP film, indicating that two carbazole units linked by a spirobifluorene group leads to a higher ∆T max than that of two carbazole units linked by an N-ethylcarbazole group. Among these polymer films, the PDTC film shows the highest ∆T max (66.04%), at 856 nm in [EPI + ][TFSI − ] solution. The ∆T max of the PS2CBP film is higher than that reported for poly(4-(3,6-di(thiophen-2-yl)-9H-carbazol-9-yl)-phenyl-methanone) (PTCPM) (∆T max = 41% at 1100 nm) [29], and is comparable to that reported for poly(2,8-di(carbazol-9-yl)dibenzothiophene)(PSCZ) (∆T max = 61% at 762 nm) [40]. The ∆T max of the PCEC film is higher than those reported for PTCPM (∆T max = 41% at 1100 nm) and poly(3,6-di(carbazol-9-yl)-N-(4-methoxyphenyl) carbazole)(PPhCz-2Cz) (∆T max = 37% at 741 nm) [29,41], whereas the PCEC film shows a lower ∆T max than that reported for PSCZ (∆T max = 61% at 762 nm) [40].
The coloration efficiency (CE, η) is defined as the change in the optical absorbance per unit of inserted charge (Q d ) in electrochromic materials and ECDs. CE can be calculated using the following equation at a given wavelength [42]: where ∆OD indicates the change of the optical density at a specific wavelength. T b and T c are defined as the transmittance of the bleaching state and coloring state, respectively. The calculated η max of the PDTC film is 201.61 cm 2 C −1 at 578 nm and 167.83 cm 2 C −1 at 856 nm; the η max of the PS2CBP film is 138.09 cm 2 C −1 at 428 nm and 151.70 cm 2 C −1 at 1208 nm; and the η max of the PCEC film is 293.91 cm 2 C −1 at 420 nm and 214.07 cm 2 C −1 at 1220 nm. The PDTC, PS2CBP, and PCEC films in [EPI + ][TFSI − ] solution show higher η than those reported for PTCPM (η = 110.48 cm 2 C −1 at 1100 nm) [29], PSCZ (η = 45 cm 2 C −1 at 762 nm) [40], and PPhCz-2Cz (η = 56 cm 2 C −1 at 741 nm) [41]. This may be attributed to that fact that an ionic liquid solution is employed as an electrochromic electrolyte in this study.

Spectroelectrochemistry of Electrochromic Devices
Dual-type ECDs were constructed using two complementary electrochromically active layers. Dual-type ECDs sometimes exhibit a higher electrochromic (EC) contrast in a wider visible range than those of single-type ECDs. Dual-type ECDs comprise anodically coloring material (PDTC, PS2CBP, or PCEC), cathodically coloring material (PProDOT-Et 2 ), and ionic liquid-PVdF-HFP. Composite electrolytes were fabricated and their spectroelectrochemical properties were characterized. The spectroelectrochemical spectra of the PDTC/PProDOT-Et 2 , PS2CBP/PProDOT-Et 2 , and PCEC/PProDOT-Et 2 ECDs are displayed in Figure 6a-c, respectively. As displayed in Figure 6a, the PDTC/PProDOT-Et 2 ECD shows a shoulder at around 415 nm at 0 V, which can be attributed to the π-π* transition of the PDTC film in the reduction state. In this circumstance, the PProDOT-Et 2 film was light blue in its oxidation state, and the PDTC/PProDOT-Et 2 ECD was olive green at 0 V. However, the π-π* transition absorption of the PDTC film diminished and a new absorption band at 590 nm emerged gradually with increasing potential. The PDTC/PProDOT-Et 2 ECD was dark gray at 1.0 V, prussian blue at 1.2 V, and midnight blue at 1.4 V. In a similar situation, the PS2CBP/PProDOT-Et 2 ECD was silver gray at 0 V, cornflower blue at 1.0 V, and salvia blue at 1.2 V and 1.4 V, and the PCEC/PProDOT-Et 2 ECD was gray at 0 V, dark mineral blue at 1.0 V, and slate blue at 1.2 V and 1.4 V. The colorimetric values and CIE chromaticity values of the PDTC/PProDOT-Et 2 , PS2CBP/PProDOT-Et 2 , and PCEC/PProDOT-Et 2 ECDs are listed in Table 4. Moreover, the CIE chromaticity diagrams of the PDTC/PProDOT-Et 2 , PS2CBP/PProDOT-Et 2 , and PCEC/PProDOT-Et 2 ECDs at bleaching and coloring states are shown in Figure S6 (in supplementary information). The coloration efficiency (CE, η) is defined as the change in the optical absorbance per unit of inserted charge (Qd) in electrochromic materials and ECDs. CE can be calculated using the following equation at a given wavelength [42]: where △OD indicates the change of the optical density at a specific wavelength.  [29], PSCZ (η = 45 cm 2 C −1 at 762 nm) [40], and PPhCz-2Cz (η = 56 cm 2 C −1 at 741 nm) [41]. This may be attributed to that fact that an ionic liquid solution is employed as an electrochromic electrolyte in this study.
The open circuit memory for the PDTC/PProDOT-Et 2 , PS2CBP/PProDOT-Et 2 , and PCEC/PProDOT-Et 2 ECDs were evaluated at ca. 590 nm with a function of time by applying potentials at a bleaching state (0 or −0.4 V) and at a coloring state (+1.4 V) for 1 s for each 200 s time interval. As shown in Figure 9, the PDTC/PProDOT-Et 2 , PS2CBP/PProDOT-Et 2 and PCEC/PProDOT-Et 2 ECDs exhibit a satisfactory optical memory effect at a coloring state (<5% transmittance change) and a bleaching state (transmittance variation is insignificant), demonstrating that the PDTC/PProDOT-Et 2 , PS2CBP/PProDOT-Et 2 and PCEC/PProDOT-Et 2 ECDs display good optical memory at both bleaching and coloring states.