Applications of Poly ( indole-6-carboxylic acid-co2 , 2 ′-bithiophene ) Films in High-Contrast Electrochromic Devices

Chung-Wen Kuo 1, Tzi-Yi Wu 2,* ID and Shu-Chien Fan 1 1 Department of Chemical and Materials Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 80778, Taiwan; welly@cc.kuas.edu.tw (C.-W.K.); zjan70255@gmail.com (S.-C.F.) 2 Department of Chemical Engineering and Materials Engineering, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan * Correspondence: wuty@gemail.yuntech.edu.tw; Tel.: +886-5-534-2601 (ext. 4626)


Introduction
Conjugated polymers have sparked enormous attention for applications in solar energy conversion [1,2], electrochromic devices (ECDs) [3][4][5], chemical sensing materials [6,7], catalysts for methanol oxidation in direct methanol fuel cells [8][9][10], and polymer organic light emitting diodes (polymer OLEDs, or PLEDs) [11][12][13] due to their intriguing optical, electrochemical, and structural properties.Among them, the applications of conjugated polymers in developing high contrast electrochromic devices are currently attractive due to their energy-saving and light control properties.During the last decade, the most frequently studied electrochromic polymers are polythiophenes [14,15], polycarbazoles [16,17], polyindoles [18], polyanilines [19], polytriphenylamine [20], and polypyrroles [21].Among them, polyindoles have several benefits such as high conductivity, facile polymerization from organic or aqueous media, and good chemical stability.Moreover, polyindoles show higher thermal stability and redox activity than those of polypyrroles and polyanilines.Polythiophenes and their derivatives also show good stability toward moisture and oxygen in their oxidized and reduced states, they can be functionalized by the incorporations of oxygen and sulfur atoms at 3,4-postions

Fabrication of ECDs
ECDs were assembled using PInc, PbT, or P(Inc-co-bT) film as the anodic layer, PEDOT-PSS film as the cathodic layer and a PMMA-PC-ACN-LiClO 4 composite film as the electrochromic electrolyte.ECDs were fabricated by anodic and cathodic layers facing each other and were separated by a PMMA-PC-ACN-LiClO 4 composite film as shown in Figure 1.

Electrochemical Polymerizations of PInc, PbT and P(Inc-co-bT) Films
The anodic polarization curves of Inc., bT, and their mixture (Inc.+ bT) in PC+ACN solution containing 0.2 M LiClO4 are shown in Figure 2, the onset potentials of Inc., bT, and their mixture (Inc.+ bT) are 0.64, 0.81, and 0.84 V vs. Ag/AgNO3, respectively.The deviation of onset potentials between Inc. and bT is less than 0.2 V, implying the feasibility of copolymerization using the two monomers.Figure 3 shows the repetitive cycling at potentials between 0.0 and 1.4 V for 8 cycles, the increased current density clearly indicated PInc, P(Inc-co-bT), and PbT films were electrodeposited onto ITO glass substrate during the repetitive cycling.

Electrochemical Polymerizations of PInc, PbT and P(Inc-co-bT) Films
The anodic polarization curves of Inc., bT, and their mixture (Inc.+ bT) in PC+ACN solution containing 0.2 M LiClO 4 are shown in Figure 2, the onset potentials of Inc., bT, and their mixture (Inc.+ bT) are 0.64, 0.81, and 0.84 V vs. Ag/AgNO 3 , respectively.The deviation of onset potentials between Inc. and bT is less than 0.2 V, implying the feasibility of copolymerization using the two monomers.

Electrochemical Polymerizations of PInc, PbT and P(Inc-co-bT) Films
The anodic polarization curves of Inc., bT, and their mixture (Inc.+ bT) in PC+ACN solution containing 0.2 M LiClO4 are shown in Figure 2, the onset potentials of Inc., bT, and their mixture (Inc.+ bT) are 0.64, 0.81, and 0.84 V vs. Ag/AgNO3, respectively.The deviation of onset potentials between Inc. and bT is less than 0.2 V, implying the feasibility of copolymerization using the two monomers.Figure 3 shows the repetitive cycling at potentials between 0.0 and 1.4 V for 8 cycles, the increased current density clearly indicated PInc, P(Inc-co-bT), and PbT films were electrodeposited onto ITO glass substrate during the repetitive cycling.Figure 3 shows the repetitive cycling at potentials between 0.0 and 1.4 V for 8 cycles, the increased current density clearly indicated PInc, P(Inc-co-bT), and PbT films were electrodeposited onto ITO glass substrate during the repetitive cycling.Moreover, the oxidation peaks of PInc, P(Inc-co-bT), and PbT films located at 0.92, 1.26, and 1.05 V, respectively, and the reduction peaks of PInc, P(Inc-co-bT), and PbT films located at 0.46, 0.40, and 0.44 V, respectively.The redox peaks and wave shapes of P(Inc-co-bT) film are different to those of PInc and PbT films, demonstrating the formation of P(Inc-co-bT) film on ITO glass substrate.The electrosynthetic scheme of P(Inc-co-bT) is displayed in Figure 4 [24,25].The redox behavior of P(Inc-co-bT) film was studied using cyclic voltammograms at scanning rates between 10 and 200 mV s −1 in 0.2 M LiClO4/(PC+ACN) solution, and the results are shown in Figure 5.
The P(Inc-co-bT) films reveal a quasi-reversible oxidation-reduction couple, indicating the doping and de-doping processes of the P(Inc-co-bT) film, the redox behavior and polaron mechanism of P(Inc-co-bT) film is shown in Figure 6.As shown in Figure 5b, both anodic and cathodic peak current density increase linearly with increasing scan rate, representing P(Inc-co-bT) film was well adhered onto ITO surface and the oxidation and reduction processes of the polymer film was not diffusion controlled [26].Moreover, the oxidation peaks of PInc, P(Inc-co-bT), and PbT films located at 0.92, 1.26, and 1.05 V, respectively, and the reduction peaks of PInc, P(Inc-co-bT), and PbT films located at 0.46, 0.40, and 0.44 V, respectively.The redox peaks and wave shapes of P(Inc-co-bT) film are different to those of PInc and PbT films, demonstrating the formation of P(Inc-co-bT) film on ITO glass substrate.The electrosynthetic scheme of P(Inc-co-bT) is displayed in Figure 4 [24,25].Moreover, the oxidation peaks of PInc, P(Inc-co-bT), and PbT films located at 0.92, 1.26, and 1.05 V, respectively, and the reduction peaks of PInc, P(Inc-co-bT), and PbT films located at 0.46, 0.40, and 0.44 V, respectively.The redox peaks and wave shapes of P(Inc-co-bT) film are different to those of PInc and PbT films, demonstrating the formation of P(Inc-co-bT) film on ITO glass substrate.The electrosynthetic scheme of P(Inc-co-bT) is displayed in Figure 4 [24,25].The redox behavior of P(Inc-co-bT) film was studied using cyclic voltammograms at scanning rates between 10 and 200 mV s −1 in 0.2 M LiClO4/(PC+ACN) solution, and the results are shown in Figure 5.
The P(Inc-co-bT) films reveal a quasi-reversible oxidation-reduction couple, indicating the doping and de-doping processes of the P(Inc-co-bT) film, the redox behavior and polaron mechanism of P(Inc-co-bT) film is shown in Figure 6.As shown in Figure 5b, both anodic and cathodic peak current density increase linearly with increasing scan rate, representing P(Inc-co-bT) film was well adhered onto ITO surface and the oxidation and reduction processes of the polymer film was not diffusion controlled [26].The redox behavior of P(Inc-co-bT) film was studied using cyclic voltammograms at scanning rates between 10 and 200 mV s −1 in 0.2 M LiClO 4 /(PC+ACN) solution, and the results are shown in Figure 5.

Spectroscopic Properties of PInc, P(Inc-co-bT), and PbT Films
Figure 7a-c showed the absorption spectra and photoimages of PInc, P(Inc-co-bT), and PbT films coated on ITO glass substrate at various potentials in 0.2 M LiClO4/(PC + ACN) solution.The π-π * and n-π * transition peak of PInc and PbT films in the neutral state located at around 350 and 450 nm, respectively.However, the corresponding transition peak of P(Inc-co-bT) film located at about 395 nm (Figure 7b), the transition peak of P(Inc-co-bT) film shifted bathochromically compared to that of PInc film but shifted hypsochromically compared to that of PbT film.The absorption peaks of PInc film at 350 nm, PbT film at 700 nm, and P(Inc-co-bT) film at 395 nm decreased gradually when the potentials grew up, meanwhile, new absorption bands of PInc film at 750 nm, PbT film at 450 nm, and P(Inc-co-bT) film at 1020 nm emerged gradually, implying the generation of charge carriers [27].
The PInc film was light yellow (0.0 V) in the neutral state, yellowish grey (+0.8 V) in the intermediate state, and purple (+0.9 V) in the oxidized state.The PbT film was light orange, dark orange, and dark blue at 0.0, +0.7, and +1.4 V, respectively.Their corresponding P(Inc-co-bT) copolymer film revealed light yellow, yellowish green, and bluish grey at 0.0, +0.6, and +0.8 V, The P(Inc-co-bT) films reveal a quasi-reversible oxidation-reduction couple, indicating the doping and de-doping processes of the P(Inc-co-bT) film, the redox behavior and polaron mechanism of P(Inc-co-bT) film is shown in Figure 6.As shown in Figure 5b, both anodic and cathodic peak current density increase linearly with increasing scan rate, representing P(Inc-co-bT) film was well adhered onto ITO surface and the oxidation and reduction processes of the polymer film was not diffusion controlled [26].respectively.However, the corresponding transition peak of P(Inc-co-bT) film located at about 395 nm (Figure 7b), the transition peak of P(Inc-co-bT) film shifted bathochromically compared to that of PInc film but shifted hypsochromically compared to that of PbT film.The absorption peaks of PInc film at 350 nm, PbT film at 700 nm, and P(Inc-co-bT) film at 395 nm decreased gradually when the potentials grew up, meanwhile, new absorption bands of PInc film at 750 nm, PbT film at 450 nm, and P(Inc-co-bT) film at 1020 nm emerged gradually, implying the generation of charge carriers [27].

Electrochemical Switching of PInc, P(Inc-co-bT), and PbT Films
Kinetic studies of PInc, P(Inc-co-bT), and PbT films in 0.2 M LiClO4/(PC+ACN) solution were monitored by increasing and decreasing of transmittance with respect to time between 0.0 and +0.8 V (vs.Ag/AgNO3) with a residence time of 10 s.
As shown in Figure 8 and Table 2, the ∆T of PInc, P(Inc-co-bT), and PbT films at the wavelength of absorption maxima are 17.8% at 510 nm, 30.2% at 890 nm, and 27.0% at 470 nm, respectively, in 0.2 M LiClO4/(PC+ACN) solution.P(Inc-co-bT) film shows higher ∆T than those of PInc and PbT films, implying the copolymer prepared using Inc. and bT monomers gave rise to significant charge carrier band than those of PInc and PbT homopolymer films.Response time was defined as the time required for achieving 90% of the desired response.The coloring response time (τc) and bleaching response time (τb) at the 2nd cycle were measured as 4.0 and 6.5 s for PInc film, 3.5 and 5.0 s for P(Inc-co-bT) film, and 3.0 and 5.2 s for PbT film, respectively.
The coloration efficiency (η) of PInc, P(Inc-co-bT), and PbT films can be calculated using the following formulas [28]: The PInc film was light yellow (0.0 V) in the neutral state, yellowish grey (+0.8 V) in the intermediate state, and purple (+0.9 V) in the oxidized state.The PbT film was light orange, dark orange, and dark blue at 0.0, +0.7, and +1.4 V, respectively.Their corresponding P(Inc-co-bT) copolymer film revealed light yellow, yellowish green, and bluish grey at 0.0, +0.6, and +0.8 V, respectively.The colors of P(Inc-co-bT) film are different to those of PInc and PbT films during the doping process.

Electrochemical Switching of PInc, P(Inc-co-bT), and PbT Films
Kinetic studies of PInc, P(Inc-co-bT), and PbT films in 0.2 M LiClO 4 /(PC+ACN) solution were monitored by increasing and decreasing of transmittance with respect to time between 0.0 and +0.8 V (vs.Ag/AgNO 3 ) with a residence time of 10 s.
As shown in Figure 8 and Table 2, the ∆T of PInc, P(Inc-co-bT), and PbT films at the wavelength of absorption maxima are 17.8% at 510 nm, 30.2% at 890 nm, and 27.0% at 470 nm, respectively, in 0.2 M Coatings 2018, 8, 102 7 of 13 LiClO 4 /(PC+ACN) solution.P(Inc-co-bT) film shows higher ∆T than those of PInc and PbT films, implying the copolymer prepared using Inc. and bT monomers gave rise to significant charge carrier band than those of PInc and PbT homopolymer films.Response time was defined as the time required for achieving 90% of the desired response.The coloring response time (τ c ) and bleaching response time (τ b ) at the 2nd cycle were measured as 4.0 and 6.5 s for PInc film, 3.5 and 5.0 s for P(Inc-co-bT) film, and 3.0 and 5.2 s for PbT film, respectively.
In the situation, PEDOT-PSS was transparent in its oxidation state, and PInc film was light yellow at 0.0 V.Under similar condition, P(Inc-co-bT)/PMMA-PC-ACN-LiClO4/PEDOT-PSS ECD shows a peak at around 380 nm at 0.0 V, the absorption peak for P(Inc-co-bT) film diminished and a new absorption band at 700 nm appeared at +1.4 V, the P(Inc-co-bT)/PMMA-PC-ACN-LiClO4/PEDOT-PSS ECD was indigo at +1.4 V as shown in Figure 1.PInc/PMMA-PC-ACN-  The coloration efficiency (η) of PInc, P(Inc-co-bT), and PbT films can be calculated using the following formulas [28]: coloring states, respectively.The η of PInc, P(Inc-co-bT), and PbT films are 41 cm 2 C −1 at 510 nm, 112 cm 2 C −1 at 890 nm, and 113 cm 2 C −1 at 470 nm, respectively.

Figure 5 .
Figure 5. (a) CV curves of the P(Inc-co-bT) film at different scan rates between 10 and 200 mV s −1 in PC+ACN solution containing 0.2 M LiClO4; (b) Scan rate dependence of the anodic and cathodic peak current densities of P(Inc-co-bT) film.

Figure 5 .
Figure 5. (a) CV curves of the P(Inc-co-bT) film at different scan rates between 10 and 200 mV s −1 in PC+ACN solution containing 0.2 M LiClO 4 ; (b) Scan rate dependence of the anodic and cathodic peak current densities of P(Inc-co-bT) film.

Figure 5 .
Figure 5. (a) CV curves of the P(Inc-co-bT) film at different scan rates between 10 and 200 mV s −1 in PC+ACN solution containing 0.2 M LiClO4; (b) Scan rate dependence of the anodic and cathodic peak current densities of P(Inc-co-bT) film.

FigureFigure 6 .3. 2 .
Figure7a-cshowed the absorption spectra and photoimages of PInc, P(Inc-co-bT), and PbT films coated on ITO glass substrate at various potentials in 0.2 M LiClO4/(PC + ACN) solution.The π-π * and n-π * transition peak of PInc and PbT films in the neutral state located at around 350 and 450 nm, respectively.However, the corresponding transition peak of P(Inc-co-bT) film located at about 395 nm respectively.The colors of P(Inc-co-bT) film are different to those of PInc and PbT films during the doping process.

Figure 8 .
Figure 8. Transmittance variations of (a) PInc, (b) P(Inc-co-bT), and (c) PbT electrodes in PC+ACN solution containing 0.2 M LiClO4 between 0.0 V and +0.8 V with a residence time of 10 s.

Figure 8 .
Figure 8. Transmittance variations of (a) PInc, (b) P(Inc-co-bT), and (c) PbT electrodes in PC+ACN solution containing 0.2 M LiClO 4 between 0.0 V and +0.8 V with a residence time of 10 s.
) where ∆OD indicates the change of the optical density at a specific wavelength, Q d is the charge density of electrochromic electrodes, and T b and T c represent the transmittance of the bleaching and Coatings 2018, 8, 102 8 of 13

Table 2 .
Optical and electrochemical properties investigated at the selected applied wavelength for the electrodes.

Table 2 .
Optical and electrochemical properties investigated at the selected applied wavelength for the electrodes.
a The selected applied wavelength for the electrodes.

Table 3 .
Optical and electrochemical properties investigated at the selected applied wavelength for the devices.

Table 4 .
Electrochemical optical contrast and coloration efficiencies of ECDs.

Table 3 .
Optical and electrochemical properties investigated at the selected applied wavelength for the devices.
a The selected applied wavelength for the devices.

Table 4 .
Electrochemical optical contrast and coloration efficiencies of ECDs.