3.1. Electrochemical Characterization of the Polymer Electrodes
Figure 2 shows the linear sweep voltammetry curves for the electrochemical oxidation of neat PbCmB, P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF) in a 0.2 M LiClO
4 solution. The onset potentials in the cyclic voltammetry of neat bCmB, bTP, dbBT, and TF were 0.75, 0.81, 0.94, and 0.70 V (vs. Ag/AgNO
3), respectively. The incorporation of two electron-withdrawing bromide groups in the bithiophene unit increased the onset potentials. bCmB displayed a lower onset potential than the bithiophene unit; this could be attributed to the fact that biscarbazole-containing bCmB showed a stronger electron-donating ability than the bithiophene unit. The disparities of the onset potentials between neat bCmB and neat bithiophene derivatives were less than 0.2 V, implying the feasibility of copolymerizations using bCmB and bithiophene derivatives.
Figure 3 displays the electrogrowth of neat bCmB and mixtures of bCmB + bithiophene derivatives (or 2-(thiophen-2-yl)furan) in 0.2 M LiClO
4/ACN/DCM). The potentiodynamic polymerization scans from the first to the tenth cycles revealed that the current densities of the redox peaks increased with the number of increasing cycles, implying the growth of polymers on the ITO substrate [
32]. As shown in
Figure 3a, PbCmB displayed two distinct oxidation peaks at 0.70 and 1.12 V as well as two evident reduction peaks at 0.31 and 0.65 V.
The first oxidation and reduction peaks depicted the generation of radical cations in poly(1,4-bis((9H-carbazol-9-yl)methyl)benzene) and the second redox peaks represented the formation of dications. The incorporation of bithiophene, 3,3′-dibromo-2,2′-bithiophene, and 2-(thiophen-2-yl)furan into the polymeric chain slightly shifted the redox peaks. The first oxidation and reduction peaks of P(bCmB-
co-bTP) were located at 0.78 and 0.33 V, respectively (
Figure 3b). In a similar condition, two oxidation peaks of P(bCmB-
co-dbBT) were situated at 0.70 and 1.09 V, respectively, and two reduction peaks of P(bCmB-
co-dbBT) were located at 0.41 and 0.72 V, respectively (
Figure 3c). The first and second oxidation peaks of P(bCmB-
co-TF) were located at 0.79 and 1.26 V, respectively, and the first and second reduction peaks of P(bCmB-
co-TF) were situated at 0.44 and 0.71 V, respectively (
Figure 3d). The peak potentials and CV wave shapes of PbCmB were diverse compared with those of P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF), proving the coating of P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF) membranes onto the ITO glass substrate. The polymerization schemes of PbCmB, P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF) are listed in
Figure 4 [
33].
The electrocoated P(bCmB-
co-bTP) film was studied using various scan rate cyclic voltammetry measurements. As displayed in
Figure 5, the P(bCmB-
co-bTP) film showed two couples of well-defined redox peaks at various scan rates in 0.2 M LiClO
4/ACN/DCM; the peak current densities of the P(bCmB-
co-bTP) film were linearly proportional to the scan velocities, representing that P(bCmB-
co-bTP) tightly adhered to the conductive glass and the redox behaviors of P(bCmB-
co-bTP) were reversible and activation control [
34].
3.2. Absorption Spectra and the Transmittance Changes of the Polymers
Figure 6a–d shows the absorption spectra of the PbCmB, P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF) electrodes in 0.2 M LiClO
4/ACN/DCM. There was no noticeable absorption peak of the PbCmB film between 370 and 1000 nm at 0.0 and 0.5 V, respectively. As the voltage increased stepwise from 0.0 V to 1.2 V, new charge carrier bands appeared at around 420 and 675 nm, signifying the existence of the generation of radical cations and dications [
35] (
Figure 6a). As displayed in
Figure 6b, P(bCmB-
co-bTP) showed a neutral absorption peak at around 420 nm; this could be attributed to the π–π* (or n–π*) transition of the bithiophene heteroaromatic groups. The PbCmB film displayed four types of color variations from the neutral to the oxidation state, which were bright gray, dark gray, dark khaki, and dark olive green at 0.0, 0.7, 1.0, and 1.2 V, respectively. The
L*,
a*, and
b* values of PbCmB are displayed in
Table 2. Under identical situations, the P(bCmB-
co-bTP) film was celadon, earth gray, iron gray, and navy blue at 0.0, 0.6, 0.8, and 1.2 V, respectively.
The P(bCmB-co-dbBT) film was bright gray, slate gray, khaki, and dark greenish grey at 0.0, 0.7, 1.0, and 1.2 V, respectively. The P(bCmB-co-TF) film was light gray, slate gray, dark khaki, and dark greenish grey at −0.3, 0.6, 1.0, and 1.2 V, respectively. The incorporation of bithiophene, 3,3′-dibromo-2,2′-bithiophene, and 2-(thiophen-2-yl)furan in the polymer chain changed the color variations from the reduced state to the oxidized state.
The optical energy gap (
Eg) of PbCmB calculated using the absorption onset wavelength
(λonset) of the π–π* transition peak was 3.20 eV [
36].
Table 3 shows the
Eg of the reported polymers. PbCmB displayed a larger
Eg than PBCPO [
37], PMCP [
38], and PDCP [
39]. This could be attributed to two methylene groups interrupting the conjugated degree of the polymer chains. The
Eonset of PbCmB (vs. Ag/AgNO
3) was 0.80 V, the
EFOC calculated from the potential of ferrocene/ferrocenium vs. Ag/AgNO
3 was 0.69 V, and the
Eonset (vs.
EFOC) was evaluated as 0.11 V. The reference energy for ferrocene is 4.8 eV below the vacuum level [
40].
EHOMO and
ELUMO, corresponding with the energy levels of HOMO and LUMO, were calculated as −4.91 and −1.71 eV, respectively.
Figure 7 displays the electrochromic switching profiles of PbCmB, P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF) in the solutions, which changed between 0.0 and 1.2 V with a time interval of 10 s. The transmittance changes (Δ
T) of PbCmB, P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF) from the neutral state to the oxidized state were determined to be 23.94% at 685 nm, 39.56% at 685 nm, 27.85% at 685 nm, and 29.84% at 690 nm, respectively. P(bCmB-
co-bTP) revealed the highest Δ
T among the four electrodes. The copolymers (P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF)) showed a higher Δ
T than the homopolymer (PbCmB) in the solutions, suggesting that the electrochemical copolymerization of PbCmB with bithiophene, 3,3′-dibromo-2,2′-bithiophene, or 2-(thiophen-2-yl)furan monomer led to an increase in the Δ
T. P(bCmB-
co-bTP) displayed a higher Δ
T than PBCPO [
37], PMCP [
38], PDCP [
39], and PDTCZ-2 [
41]. However, the Δ
T of P(bCmB-
co-bTP) was smaller than P(DTC-
co-BTP2) [
42].
The response time from the colored to the bleached state (
τb) and from the bleached to the colored state (
τc) of PbCmB, P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF) in the solutions is listed in
Table 4. The
τb and
τc were determined at 90% of the maximum Δ
T. The
τc and
τb of the polymeric films were determined to be 1.46–3.51 s and 4.81–5.18 s, respectively.
As shown in the following equation [
48], the ∆OD could be determined by a logarithmic calculation of the transmission (%) at the oxidation state and the reduction state:
The ΔODs of the PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) films in the solutions were 0.126 at 685 nm, 0.374 at 685 nm, 0.186 at 685 nm, and 0.221 at 690 nm, respectively. The P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) films displayed a higher ΔOD than the PbCmB film.
The coloration efficiency (
η) could be obtained from the following equation [
48]:
where
Qd represents the charge injection/extraction of the electrodes per active area. As listed in
Table 4, the
η of PbCmB, P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF) was 80.3 cm
2∙C
−1 at 685 nm, 160.5 cm
2∙C
−1 at 685 nm, 86.5 cm
2∙C
−1 at 685 nm, and 125.8 cm
2∙C
−1 at 690 nm, respectively.
3.3. Absorption Spectra and the Transmittance Changes of the ECDs
Dual-layer organic ECDs were constructed using the configurations of the PbCmB/PEDOT, P(bCmB-
co-bTP)/PEDOT, P(bCmB-
co-dbBT)/PEDOT, and P(bCmB-
co-TF)/PEDOT ECDs. The absorption spectra of the PbCmB/PEDOT, P(bCmB-
co-bTP)/PEDOT, P(bCmB-
co-dbBT)/PEDOT, and P(bCmB-
co-TF)/PEDOT ECDs are displayed in
Figure 8a–d. At −0.4 V, the PbCmB/PEDOT ECD did not reveal a significant absorption peak in the UV-Vis region. The anodic PbCmB film was in its reduced state and the cathodic PEDOT film was in its oxidized state at −0.4 V, exhibiting a limpid color. The PbCmB/PEDOT ECD showed a gainsboro color at −0.4 V. After progressively raising the potential to 2.0 V, the PbCmB film started to oxidize and the PEDOT film started to reduce. As a consequence, new absorption bands at 420 and 620 nm appeared stepwise. The PbCmB/PEDOT ECD showed a dark gray color at 1.0 V and a bright slate gray color at 2.0 V. Under identical situations, the P(bCmB-
co-bTP)/PEDOT ECD was light gray, dark gray, and slate gray at −0.6, 0.8, and 2.0 V, respectively. The P(bCmB-
co-dbBT)/PEDOT ECD was gainsboro, dark gray, and bright slate gray at −0.4, 1.0, and 2.4 V, respectively. The P(bCmB-
co-TF)/PEDOT ECD was gainsboro, light gray, and dark gray at −0.3, 1.6, and 2.4 V, respectively. The colorimetric results of the four ECDs are listed in
Table 5.
Figure 9 shows the color-bleach kinetics of the PbCmB/PEDOT, P(bCmB-
co-bTP)/PEDOT, P(bCmB-
co-dbBT)/PEDOT, and P(bCmB-
co-TF)/PEDOT ECDs; the time interval of the cycle switched between −0.4 V and 1.8 V was 10 s.
Table 6 lists the Δ
T, ΔOD,
η,
τb, and
τc of the four ECDs. The Δ
T of the PbCmB/PEDOT, P(bCmB-
co-bTP)/PEDOT, P(bCmB-
co-dbBT)/PEDOT, and P(bCmB-
co-TF)/PEDOT ECDs was 35.3% at 640 nm, 40.7% at 635 nm, 36.4% at 635 nm, and 35.7% at 640 nm at the second cycle, respectively. The P(bCmB-
co-bTP)/PEDOT ECD showed the highest Δ
T. The P(bCmB-
co-bTP)/PEDOT, P(bCmB-
co-dbBT)/PEDOT, and P(bCmB-
co-TF)/PEDOT ECDs displayed a higher Δ
T than the PbCmB/PEDOT ECD, indicating that the use of bithiophene, 3,3′-dibromo-2,2′-bithiophene, and 2-(thiophen-2-yl)furan-containing copolymers as the anodically coloring layers in the ECDs gave rise to a higher Δ
T than the PbCmB. The P(bCmB-
co-bTP)/PEDOT ECD displayed a higher Δ
T than the PtCz/PProDOT-Me
2 [
43], P(Bmco)/PEDOT [
44], P(dcbp-
co-cpdt)/PEDOT [
45], P(DiCP-
co-CDTK)/PEDOT-PSS [
39], and P(TTPA-
co-EDOT)/PEDOT ECDs [
46]. However, the P(bCmB-
co-bTP)/PEDOT ECD showed a lower Δ
T than the P(BCz-
co-In)/PProDOT-Et
2 ECD [
47].
The
η of the dual-layer organic ECDs is also shown in
Table 6. The
η of the PbCmB/PEDOT, P(bCmB-
co-bTP)/PEDOT, P(bCmB-
co-dbBT)/PEDOT, and P(bCmB-
co-TF)/PEDOT ECDs was 305.8 cm
2∙C
−1 at 640 nm, 428.4 cm
2∙C
−1 at 635 nm, 341.0 cm
2∙C
−1 at 635 nm, and 316.8 cm
2∙C
−1 at 640 nm at the second cycle, respectively. The P(bCmB-
co-bTP)/PEDOT ECD displayed the highest
η. The use of P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF) as the anodically coloring materials in the ECDs led to a higher
η than the PbCmB.
Table 3 also summarizes the
η of the reported ECDs. The P(bCmB-
co-bTP)/PEDOT ECD showed a higher
η than the PtCz/PProDOT-Me
2 [
43], P(dcbp-
co-cpdt)/PEDOT [
45], and P(TTPA-
co-EDOT)/PEDOT ECDs [
46]. However, the P(bCmB-
co-bTP)/PEDOT ECD displayed a lower
η than the P(BCz-
co-In)/PProDOT-Et
2 [
47] and P(DiCP-
co-CDTK)/PEDOT-PSS ECDs [
39].
The response time of the PbCmB/PEDOT, P(bCmB-
co-bTP)/PEDOT, P(bCmB-
co-dbBT)/PEDOT, and P(bCmB-
co-TF)/PEDOT ECDs was shorter than the PbCmB, P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF) electrodes in the solutions, disclosing that the distances between the anode and the cathode in the ECDs were narrower than between the polymeric electrode and the Pt electrode in the solutions [
49].
3.4. Open Circuit Memories of the ECDs
The open circuit memories of the dual-layer organic ECDs were monitored by applying potentials in colored and bleached states for 1 s for each 100 s interval. As displayed in
Figure 10, the PbCmB/PEDOT, P(bCmB-
co-bTP)/PEDOT, P(bCmB-
co-dbBT)/PEDOT, and P(bCmB-
co-TF)/PEDOT ECDs displayed sufficient open circuit memories with ≤1.1% transmittance variation in the bleached state. However, in an oxidized state of the PbCmB, P(bCmB-
co-bTP), P(bCmB-
co-dbBT), and P(bCmB-
co-TF) films and in a reduced state of the PEDOT film, the transmittance changes of the PbCmB/PEDOT, P(bCmB-
co-bTP)/PEDOT, P(bCmB-
co-dbBT)/PEDOT, and P(bCmB-
co-TF)/PEDOT ECDs in a colored state were less stable than the four ECDs in a bleached state. The P(bCmB-
co-dbBT)/PEDOT ECD showed the largest transmittance change in a colored state. However, the transmittance change of the P(bCmB-
co-dbBT)/PEDOT ECD in a colored state was less than 4.9%, implying that the PbCmB/PEDOT, P(bCmB-
co-bTP)/PEDOT, P(bCmB-
co-dbBT)/PEDOT, and P(bCmB-
co-TF)/PEDOT ECDs had ample open circuit memories in both the colored and bleached states.