1. Introduction

Increasing attentions have been taken on the electrochromic conducting polymers (ECPs), due to their tremendous potentials for many practical applications such as smart windows [1–3], optical attenuators [4–7] and adaptive camouflage [8]. Great efforts have been devoted especially for the design and synthesis of novel ECPs in order to achieve desirable properties for commercial applications [9].

Two main strategies are used to synthesize new conducting polymers, one is using new monomers, and the other is regulating the copolymerization process of existed monomers [10]. In the literatures there are a vast amount of ECPs that possess many kinds of colors such as red, orange, blue and green. However, up to date, there are only a few ECPs that report black [11–13]. This is due to the complexity in inducing a polymer which absorbs evenly over the entire visible spectrum. Furthermore, besides the desired color, the ECP is wished to be prepared easily and economically, and it is fascinating while it can exhibits colors as much as possible which is important for potential applications on electrochromic displays and adaptive camouflage [14–16].

Contrary to the laborious and costly synthetic approach, electrochemical copolymerization is an effective method to improve the properties of conducting polymers especially for the multicolor electrochromism. In addition, it is also a facile and economical method to unite the electrochromic properties of the parent polymers.

Polythiophenes, which are interesting ECPs due to their facile Eg tunability through structural modification [16–18], have been studied extensively in the past few years. However, the pure polythiophenes prepared by electrochemical polymerization using conventional solvent have the “polythiophene paradox” [19]. Shi G solved this problem by using boron trifluoride diethyl etherate (BFEE) as the solvent, the prepared polythiophene film shows strong absorption at short visible region and exhibits red color in the neutral state [20–22]. As a polythiophene derivative, the electrochromic properties of poly (3,4-ethylenedioxythiophene) (PEDOT) are exhaustively investigated during the past two decades, due to the rapid response, high transmittance contrast and high stability. Furthermore, the low band allows the polymer to absorb at long visible region and presents blue in the neutral state [23].

Therefore, a polymer containing thiophene and EDOT units is designed to possess the absorption spectrum extended over the entire visible region and exhibit black color. In this communication, we report a polymer prepared by facile and economical electrochemical copolymerization in BFEE and its electrochromism, especially the black color in the polaron state.

2. Experimental

The electrochemical copolymerization was carried out by cyclic voltammetry in an BFEE solution containing 40 mM thiophene and 10 mM EDOT as the comonomer, indium tin oxide (ITO) coated glass as working electrode, Ag/AgCl as reference electrode and stainless steel sheet as courter electrode. The copolymer film for spectroelectrochemical analyses was deposited in the potentiostatic 1.05 V. The electrochemical measurements were performed in 0.2 M lithium perchlorate propylene carbonate solution. The spectra were measured in the range from 350 of 1050 nm with a SHIMADZU UV-1800 spectrophotometer. The optical images of electrochromic film were taken by a canon EOS 500D digital camera.

3. Results and discussions

To ensure successful electrochemical copolymerization, both monomers should be oxidized to form their reactive radical-cations at approximately the same potential. The cyclic voltymmogram curves of the comonomer are shown in Fig. 1 . An irreversible oxidation of the comonomer appears clearly on the first cycle at 0.89 V indicating the forming of radical-cation [24]. The oxidation current increases with the increase of scanning cycles, demonstrating that the electroactive film has been deposited on the electrode [25].

 

Fig. 1 Cyclic voltymmogram curves of the mixture of 20 mM thiophene and 5 mM EDOT in BFEE solution.

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The copolymer film is electrodeposited on the ITO electrode under potentiostatic 1.05 V with the charge density of 100 mC/cm². The electrochemical properties of the copolymer film are investigated and the results are shown in Fig. 2 . The film shows a single and well-defined redox process, and the current density is proportional to the scan rates indicating the film is electroactive and well adhesive to the electrode [26]. As shown in the inset Fig. 2, the potentials of anodic and cathodic peak current densities show positive and negative shift with the increase of scanning rate, respectively, which implies the appearance of quasi-reversible redox processes at high scan rates. The anodic and cathodic peak current densities show a linear proportion to scan rates as expected, which demonstrates that the electrochemical processes are not diffusion limited [10].

 

Fig. 2 Peak current density of the copolymer under various scan rates; inset: CV curves of the copolymer at different scan rates.

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The electrochromic properties of the copolymer are shown in Fig. 3 . In the neutral state, the film shows a well-defined maximum absorption band centered at 522 nm and the calculated band gap (Eg) is 1.73 eV by the onset wavelength (Fig. 3(a)). As the film gets oxidized, the charge carrier bands at longer wavelengths increase in intensity. In the polar state, the film exhibits a broad absorption band with two shoulders at 522 and 782 nm extending virtually over the full visible spectrum, and the absorbance difference between the two shoulders is only 0.05. The absorbance in the whole visible region is over 0.5. Upon further oxidation, the polar band reaches a maximum while the π-π* transition band diminishes completely [13].

 

Fig. 3 (a) Absorption spectra and (b) optical images of the copolymer under various applied potentials.

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Finally, these changes in the electronic absorption spectra of the copolymer film upon doping are nicely reflected by a color change from purple color (L = 29, a = 15, b = −10) in the neutral state to blue in the oxidized state (L = 22, a = −6, b = −12) (Fig. 3(b)). The film in the intermediate doped state presents strong absorption in the whole visible region, and shows black (L = 11, a = −1, b = 1) as the additional color.

The switching time between the neutral state and oxidized state is also a crucial parameter to realize the versatility of the electrochromism. Hence, the transmittance change at 522nm is recorded via square-wave potential method between −0.4 and 1.0V with switching time of 5s and shown in Fig. 4 . The optical contrast of the copolymer is found to be 25% at 522nm between the neutral and oxidized state, which is reasonable in ECPs. The bleaching and coloration time are 4.2 and 3.3s at 95% of the maximum transmittance contrast, respectively.

 

Fig. 4 Chronoamperometric curve and optical response of the copolymer at 522 nm.

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A key parameter used for comparisons between electrochromic materials is the coloration efficiency (η). The value can be calculated using the equations and given below: $\Delta OD=\lg (\raisebox{1ex}{$T_b$}\!\left/ \!\raisebox{-1ex}{$T_c$}\right.)and\eta =(\raisebox{1ex}{$\Delta OD$}\!\left/ \!\raisebox{-1ex}{$\Delta Q$}\right.)$where T_b and T_c are the transmittances before and after coloration, respectively. △OD is the change of optical density, △Q is the amount of injected charge density. The coloration efficiency of the copolymer film is measured as 133 cm²/C at 522 nm (△OD = 0.76, △Q = 5.7 mC/cm²). This value is higher than those obtained from inorganic electrochromic materials (<100 cm²/C) and comparable to that of other ECPs.

The electrochemical stability of the copolymer is typically characterized by cyclic voltammetry in the potential range of −0.4 to 1.0 V, at a sweep rate of 50 mV s⁻¹. The evolution of CVs of the copolymer is shown in Fig. 5 . It can be noticed that there is no large change about the redox process at the beginning and end of the stability test, which indicates the quite good stability of the film. After 1000 cycles, the film retains 92% of its original electroactivity, and reduces only 5% of the electroactivity during the next 1000 cycles [27].

 

Fig. 5 Electrochemical stability of the copolymer.

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The transmittance contrast of the copolymer is not easy to be improved, as the residual absorption in the long visible region will not disappear under doped state which is necessary for the copolymer for black in the polaron state. However, compared with other black ECPs, the copolymer presents tricolor electrochromism and can be easily prepared by two facile and economical monomers which is important and necessary for large yields in commercial applications.

4. Conclusions

A copolymer based on thiophene and 3, 4-ethylenedioxythiophene is successfully prepared by electrochemical method in boron trifluoride diethyl etherate. The film exhibits a tricolor electrochromism with reversible color changes from drab color to blue black, especially black in the polar state, and presents a transmittance contrast of 25% at 522 nm. The corresponding response time for bleaching and coloration are 4.2 and 3.3 s, respectively. Furthermore, the copolymer film shows quite good electrochemical stability which retains 87% of its original electroactivity after 2000 cycles.