Poly(tris(4-carbazoyl-9-ylphenyl)amine)/Three Poly(3,4-ethylenedioxythiophene) Derivatives in Complementary High-Contrast Electrochromic Devices

A carbazole-based polymer (poly(tris(4-carbazoyl-9-ylphenyl)amine) (PtCz)) is electrosynthesized on an indium tin oxide (ITO) electrode. PtCz film displays light yellow at 0.0 V, earthy yellow at 1.3 V, grey at 1.5 V, and dark grey at 1.8 V in 0.2 M LiClO4/ACN/DCM (ACN/DCM = 1:3, by volume) solution. The ΔT and coloration efficiency (η) of PtCz film are 30.5% and 54.8 cm2∙C−1, respectively, in a solution state. Three dual-type electrochromic devices (ECDs) are fabricated using the PtCz as the anodic layer, poly(3,4-ethylenedioxythiophene) (PEDOT), poly(3,3-dimethyl-3,4-dihydro-thieno[3,4-b][1,4]dioxepine) (PProDOT-Me2), and poly(3,4-(2,2-diethylpropylenedioxy)thiophene) (PProDOT-Et2) as the cathodic layers. PtCz/PProDOT-Me2 ECD shows high ΔTmax (36%), high ηmax (343.4 cm2·C−1), and fast switching speed (0.2 s) at 572 nm. In addition, PtCz/PEDOT, PtCz/PProDOT-Me2, and PtCz/PProDOT-Et2 ECDs show satisfactory open circuit memory and long-term stability.


Electrodes
Cathodic Polymer Feed Species Deposition Amount of Cathode

Electrochromic Characterization
Electrochromic characterization of the polymer films and electrochromic devices were carried out using a CHI627D electrochemical analyzer (CH Instruments, Austin, TX, USA). Cyclic voltammetry (CV) studies were performed using in a three-component cell, which contained an ITO-coated glass plate (area: 1 cm × 1.5 cm) as the working electrode, a platinum wire as the counter electrode, and an Ag/AgCl as the reference electrode. The in situ spectroelectrochemical spectra were recorded using an Agilent Cary 60 UV-Visible spectrophotometer (Varian Inc., Walnut Creek, CA, USA) in time course mode.

Preparation of Electrochromic Electrolytes
The polymer electrolytes of the ECDs were prepared using the solution-cast method. To prepare the solution, poly(methyl methacrylate) (PMMA), propylene carbonate (PC), and LiClO 4 were dissolved in acetone, and the mixture was stirred magnetically at room temperature for 36 h. The polymer electrolytes were prepared using PMMA:PC:LiClO 4 in a weight ratio of 33:53:14. The final mixture was cast on glass petri dishes. After evaporating the solvent at room temperature for 2 h, the samples were vacuum-dried at 80 • C for 24 h to remove the remaining solvent completely. Finally, the self-standing polymer electrolytes were obtained. The ECDs were fabricated by sandwiching the polymer electrolytes between two electrodes to perform the electrochromic measurements.

Fabrication of the ECDs
The ECDs were constructed using two complementary polymer layers, PtCz as the anodically coloring layer, PEDOT, PProDOT-Me 2 , or PProDOT-Et 2 as the cathodically coloring layer. PtCz, PEDOT, PProDOT-Me 2 , and PProDOT-Et 2 films were deposited on ITO substrates (active area: 1 cm × 1.5 cm). The ECDs were fabricated by arranging the oxidized and reduced films to face each other, and they were separated by an electrolyte. The fabrication procedures of ECDs are shown in Figure 1.

Electrochromic Characterization
Electrochromic characterization of the polymer films and electrochromic devices were carried out using a CHI627D electrochemical analyzer (CH Instruments, Austin, TX, USA). Cyclic voltammetry (CV) studies were performed using in a three-component cell, which contained an ITOcoated glass plate (area: 1 cm × 1.5 cm) as the working electrode, a platinum wire as the counter electrode, and an Ag/AgCl as the reference electrode. The in situ spectroelectrochemical spectra were recorded using an Agilent Cary 60 UV-Visible spectrophotometer (Varian Inc., Walnut Creek, CA, USA) in time course mode.

Preparation of Electrochromic Electrolytes
The polymer electrolytes of the ECDs were prepared using the solution-cast method. To prepare the solution, poly(methyl methacrylate) (PMMA), propylene carbonate (PC), and LiClO4 were dissolved in acetone, and the mixture was stirred magnetically at room temperature for 36 h. The polymer electrolytes were prepared using PMMA:PC:LiClO4 in a weight ratio of 33:53:14. The final mixture was cast on glass petri dishes. After evaporating the solvent at room temperature for 2 h, the samples were vacuum-dried at 80 °C for 24 h to remove the remaining solvent completely. Finally, the self-standing polymer electrolytes were obtained. The ECDs were fabricated by sandwiching the polymer electrolytes between two electrodes to perform the electrochromic measurements.

Fabrication of the ECDs
The ECDs were constructed using two complementary polymer layers, PtCz as the anodically coloring layer, PEDOT, PProDOT-Me2, or PProDOT-Et2 as the cathodically coloring layer. PtCz, PEDOT, PProDOT-Me2, and PProDOT-Et2 films were deposited on ITO substrates (active area: 1 cm × 1.5 cm). The ECDs were fabricated by arranging the oxidized and reduced films to face each other, and they were separated by an electrolyte. The fabrication procedures of ECDs are shown in Figure 1.

Electrochemistry of tCz and Its Electrochemical Polymerization
The electrosynthesis of PtCz film was implemented using CV with a scan rate of 100 mV•s −1 . The electropolymerization scheme and mechanism of PtCz are shown in Figure 2 [29]. The successive cyclic voltammograms of 0.002 M neat tCz taken in an ACN/DCM (1:3, by volume) solution containing 0.2 M LiClO4 as a supporting electrolyte at a scanning rate of 100 mV•s -1 are shown in Figure 3. For the first scan of cyclic voltammogram, the onset potential of tCz is 0.86 V vs. Ag/AgCl, two oxidation peaks located at 0.95 and 1.18 V indicate the polaron and bipolaron formation of tCz, the reduction peaks of tCz locate at 1.1 and 0.7 V. The increase in the oxidation and reduction curves wave current densities indicates that the amount of polymer deposited on the ITO working electrode

Electrochemistry of tCz and Its Electrochemical Polymerization
The electrosynthesis of PtCz film was implemented using CV with a scan rate of 100 mV·s −1 . The electropolymerization scheme and mechanism of PtCz are shown in Figure 2  densities indicates that the amount of polymer deposited on the ITO working electrode increases with increasing cycles. The polymerization of tCz shows two quasi-reversible oxidation and reduction processes in Figure 3. increases with increasing cycles. The polymerization of tCz shows two quasi-reversible oxidation and reduction processes in Figure 3.

Electrochemical Behavior of PtCz Films
The as-prepared PtCz film was swept between 0.0 to 1.8 V at various scan rates between 10 and 200 mV•s −1 in 0.2 M LiClO4/ACN/DCM solution. As shown in Figure 4, the electrochemical behavior of the PtCz film shows a single well-defined redox process, the anodic and cathodic peak current densities are proportional to the scan rates, implying that PtCz film is electroactive and adheres well to the electrode, and the electrochemical processes of PtCz film are reversible and not dominated by diffusion effects [30].

Electrochemical Behavior of PtCz Films
The as-prepared PtCz film was swept between 0.0 to 1.8 V at various scan rates between 10 and 200 mV·s −1 in 0.2 M LiClO 4 /ACN/DCM solution. As shown in Figure 4, the electrochemical behavior of the PtCz film shows a single well-defined redox process, the anodic and cathodic peak current densities are proportional to the scan rates, implying that PtCz film is electroactive and adheres well to the electrode, and the electrochemical processes of PtCz film are reversible and not dominated by diffusion effects [30].

Electrochemical Behavior of PtCz Films
The as-prepared PtCz film was swept between 0.0 to 1.8 V at various scan rates between 10 and 200 mV•s −1 in 0.2 M LiClO4/ACN/DCM solution. As shown in Figure 4, the electrochemical behavior of the PtCz film shows a single well-defined redox process, the anodic and cathodic peak current densities are proportional to the scan rates, implying that PtCz film is electroactive and adheres well to the electrode, and the electrochemical processes of PtCz film are reversible and not dominated by diffusion effects [30].

Spectroelectrochemistry of PtCz and PProDOT-Me 2 Films
Spectroelectrochemistry can be used to analyze the changes in the absorption spectra of ECDs at various potentials [31]. Optoelectrochemical spectra of PtCz and PProDOT-Me 2 films are shown in Figure 5. The PtCz film shows a π-π* transition peak at around 360 nm at 0.0 V, and it is light yellow in undoped state. Upon stepwise oxidation, the peak intensity at 360 nm diminishes gradually and new absorption bands at around 800 nm emerge, the PtCz film displays earthy yellow at 1.3 V, grey at 1.5 V, and dark grey at 1.8 V. On the other hand, the PProDOT-Me 2 film shows two significant peaks at 570 and 625 nm at −1.5 V and presents dark blue in its neutral state. Upon oxidation progressively, the peak intensity at 570 and 625 nm diminish gradually and new absorption bands at more than 1000 nm emerge, the PProDOT-Me 2 film displays grey at −0.8 V and light blue at −1.5 V.
The colorimetric values (L*, a*, and b*), CIE chromaticity values (x, y), and CIE chromaticity diagrams of the PtCz and PProDOT-Me 2 films at various potentials were shown in Table 2.

Spectroelectrochemistry of PtCz and PProDOT-Me2 Films
Spectroelectrochemistry can be used to analyze the changes in the absorption spectra of ECDs at various potentials [31]. Optoelectrochemical spectra of PtCz and PProDOT-Me2 films are shown in Figure 5. The PtCz film shows a π-π* transition peak at around 360 nm at 0.0 V, and it is light yellow in undoped state. Upon stepwise oxidation, the peak intensity at 360 nm diminishes gradually and new absorption bands at around 800 nm emerge, the PtCz film displays earthy yellow at 1.3 V, grey at 1.5 V, and dark grey at 1.8 V. On the other hand, the PProDOT-Me2 film shows two significant peaks at 570 and 625 nm at −1.5 V and presents dark blue in its neutral state. Upon oxidation progressively, the peak intensity at 570 and 625 nm diminish gradually and new absorption bands at more than 1000 nm emerge, the PProDOT-Me2 film displays grey at −0.8 V and light blue at −1.5 V.
The colorimetric values (L*, a*, and b*), CIE chromaticity values (x, y), and CIE chromaticity diagrams of the PtCz and PProDOT-Me2 films at various potentials were shown in Table 2.

Electrochemical Switching of PtCz Film
Double potential step techniques can be used to investigate the response time and stability of polymer films during consecutive scans [32]. The double potential step chronoamperometry coupled with spectrophotometer of PtCz film was performed by stepping potentials between 0.0 and 1.8 V with a residence time of 10 s, and the transmittance-time profile of PtCz film is displayed in Figure 6. The coloration switching time (τc) and bleaching switching time (τb) were defined as the period

Spectroelectrochemistry of PtCz and PProDOT-Me2 Films
Spectroelectrochemistry can be used to analyze the changes in the absorption spectra of ECDs at various potentials [31]. Optoelectrochemical spectra of PtCz and PProDOT-Me2 films are shown in Figure 5. The PtCz film shows a π-π* transition peak at around 360 nm at 0.0 V, and it is light yellow in undoped state. Upon stepwise oxidation, the peak intensity at 360 nm diminishes gradually and new absorption bands at around 800 nm emerge, the PtCz film displays earthy yellow at 1.3 V, grey at 1.5 V, and dark grey at 1.8 V. On the other hand, the PProDOT-Me2 film shows two significant peaks at 570 and 625 nm at −1.5 V and presents dark blue in its neutral state. Upon oxidation progressively, the peak intensity at 570 and 625 nm diminish gradually and new absorption bands at more than 1000 nm emerge, the PProDOT-Me2 film displays grey at −0.8 V and light blue at −1.5 V.
The colorimetric values (L*, a*, and b*), CIE chromaticity values (x, y), and CIE chromaticity diagrams of the PtCz and PProDOT-Me2 films at various potentials were shown in Table 2.

Electrochemical Switching of PtCz Film
Double potential step techniques can be used to investigate the response time and stability of polymer films during consecutive scans [32]. The double potential step chronoamperometry coupled with spectrophotometer of PtCz film was performed by stepping potentials between 0.0 and 1.8 V with a residence time of 10 s, and the transmittance-time profile of PtCz film is displayed in Figure 6. The coloration switching time (τc) and bleaching switching time (τb) were defined as the period

Spectroelectrochemistry of PtCz and PProDOT-Me2 Films
Spectroelectrochemistry can be used to analyze the changes in the absorption spectra of ECDs at various potentials [31]. Optoelectrochemical spectra of PtCz and PProDOT-Me2 films are shown in Figure 5. The PtCz film shows a π-π* transition peak at around 360 nm at 0.0 V, and it is light yellow in undoped state. Upon stepwise oxidation, the peak intensity at 360 nm diminishes gradually and new absorption bands at around 800 nm emerge, the PtCz film displays earthy yellow at 1.3 V, grey at 1.5 V, and dark grey at 1.8 V. On the other hand, the PProDOT-Me2 film shows two significant peaks at 570 and 625 nm at −1.5 V and presents dark blue in its neutral state. Upon oxidation progressively, the peak intensity at 570 and 625 nm diminish gradually and new absorption bands at more than 1000 nm emerge, the PProDOT-Me2 film displays grey at −0.8 V and light blue at −1.5 V.
The colorimetric values (L*, a*, and b*), CIE chromaticity values (x, y), and CIE chromaticity diagrams of the PtCz and PProDOT-Me2 films at various potentials were shown in Table 2.

Electrochemical Switching of PtCz Film
Double potential step techniques can be used to investigate the response time and stability of polymer films during consecutive scans [32]. The double potential step chronoamperometry coupled with spectrophotometer of PtCz film was performed by stepping potentials between 0.0 and 1.8 V with a residence time of 10 s, and the transmittance-time profile of PtCz film is displayed in Figure 6. The coloration switching time (τc) and bleaching switching time (τb) were defined as the period

Electrochemical Switching of PtCz Film
Double potential step techniques can be used to investigate the response time and stability of polymer films during consecutive scans [32]. The double potential step chronoamperometry coupled with spectrophotometer of PtCz film was performed by stepping potentials between 0.0 and 1.8 V with a residence time of 10 s, and the transmittance-time profile of PtCz film is displayed in Figure 6. The coloration switching time (τ c ) and bleaching switching time (τ b ) were defined as the period Polymers 2017, 9, 543 7 of 15 required for achieving 90% of the desired response [33][34][35][36]. The τ c and τ b of PtCz film estimated at the third cycle at 760 nm are 5.5 and 5.0 s, respectively. The optical contrast (∆T%) is an important property of electrochromic polymer films, which denotes as the transmittance difference between bleaching and coloring states of polymer films in solution state. The optical density (∆OD) can be calculated using the following formula: where T ox is the transmittance of anodic material in coloration state and T red is the transmittance of anodic material in bleaching state.

Open Circuit Memory of ECDs
The optical memory of PtCz/PEDOT, PtCz/PProDOT-Me 2 , and PtCz/PProDOT-Et 2 ECDs were monitored at 600, 572, and 591 nm, respectively, as a function of time at 0.0 and 1.8 V by applying the potential for 1 s at each 100 s time interval. As shown in Figure 9a-c, three ECDs show almost no change of transmittance in the bleached state, i.e., a durable memory effect. The transmittances of three ECDs in the colored state are less stable than in the bleached state, but the transmittance loss is less than 3%. Both the bleached and colored states were highly stable, and the ECDs kept their color without loss, demonstrating PtCz/PEDOT, PtCz/PProDOT-Me 2 , and PtCz/PProDOT-Et 2 ECDs reveal satisfied open circuit memory. change of transmittance in the bleached state, i.e., a durable memory effect. The transmittances of three ECDs in the colored state are less stable than in the bleached state, but the transmittance loss is less than 3%. Both the bleached and colored states were highly stable, and the ECDs kept their color without loss, demonstrating PtCz/PEDOT, PtCz/PProDOT-Me2, and PtCz/PProDOT-Et2 ECDs reveal satisfied open circuit memory.

Conclusions
A carbazole-based monomer (tCz) was synthesized, and its corresponding homopolymer (PtCz) was prepared using electrochemical polymerization. The electrochemical processes of PtCz film are reversible, and the PtCz film shows four color variations (light yellow, earthy yellow, grey, and dark grey) from an undoped state to a doped state. Three ECDs based on PtCz as anodic polymer and PEDOT, PProDOT-Me 2 , and PProDOT-Et 2 as the cathodic polymers were constructed, and the spectroelectrochemical properties of ECDs were characterized. The colors of constructed PtCz/PProDOT-Me 2 ECD switched from yellowish-grey, light grey, purple, and dark blue upon the application of potential between −0.8 and +1.5 V. Electrochromic switching studies showed that the ∆T max values of PtCz/PEDOT, PtCz/PProDOT-Me 2 , and PtCz/PProDOT-Et 2 ECDs were 24.0%, 36.0%, and 28.0%, respectively, and the η max values of PtCz/PEDOT, PtCz/PProDOT-Me 2 , and PtCz/PProDOT-Et 2 ECDs were calculated as 256.5, 343.4, and 336.8 cm 2 ·C −1 , respectively. Moreover, PtCz/PEDOT, PtCz/PProDOT-Me 2 , and PtCz/PProDOT-Et 2 ECDs reveal satisfied open circuit memory and long-term switching ability between redox states. The results show that the PtCz film is a potential anodic material for electrochromic applications in rear-view mirrors and motorcycle helmet-visors.