Introduction

Electrochromism is defined as a reversible change in the color and/or optical properties of a substance induced by the change in electrical conditions [1, 2]. In recent years, because of their high optical contrasts [3], fast switching times [4], and processability [5], together with the ease of fine-tuning their electrochromic performances by structure modification [6], conducting polymers used as active layers in electrochromic devices have received enormous attention.

Since the electrochromic phenomenon of conducting polymers was discovered in 1980s, polythiophene (PT) and its derivatives have played an important role in this field [7, 8]. Polyselenophene (PSe) and its derivatives, as selenium analogues of PT, have also attracted interests because of their newly found exciting optoelectronic properties and relevant promising applications in various fields [9,10,11,12,13,14]. Theoretical prediction and experimental results in the last few years evidenced several advantages of PSe over PT like better interchain charge transfer from intermolecular Se…Se interactions, lower redox potentials, accommodation of more charge upon doping from larger size of selenium atom, and lower band gaps [15]. PSe has also been employed as an effective building block to fine-tune and further enhance the electrochromic performances of conjugated polymers most recently, as summarized in Scheme 1. Our group [16] also synthesized a series of Se-EDOT oligomers previously and obtained the corresponding hybrid polymers with excellent electrochromic performances and outstanding redox stability. Unfortunately, all these improvements in properties need complex synthetic work. As is known, parent PSe polymer suffers from low conductivity, poor stability, and electrochromic performances in terms of electrochromism, mainly resulting from its intrinsic structural instability during the polymerization [16]. Therefore, it is necessary to figure out an easy way to improve the performances and stability of PSe while maintaining its advantages.

Scheme 1
scheme 1

Recently reported electrochromic materials based on polyselenophenes

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As a common method to combine the advantageous properties of different homopolymers, electrochemical copolymerization is one of the simplest and most effective way to obtain conjugated copolymer films. It is feasible for people to realize the electrochemical copolymerization by choosing an appropriate monomer with similar oxidation potential to copolymerize with Se. Poly(3-methylthiophene) (P3MeT), as an important derivative of PT, shows color changes from red to blue in its redox states and has often been used as the primary electrochromic elements in electrochromic devices [17, 18]. Additionally, due to the advantages of P3MeT such as low cost, excellent stability, and high electrical conductivity [19], its monomer 3MeT has always been employed to design novel π-conjugated polymer systems with outstanding electrochromic performances [20, 21]. Most importantly, because of the electron-donating methyl substitution on the thiophene ring, 3MeT has a lowered oxidation potential similar to Se. It is quite expected that the introduction of P3MeT into PSe main chain could probably combine both of the advantages of PSe and P3MeT, and also the copolymers might show some other interesting properties.

In this work, the electrochemical copolymerization between Se and 3MeT was achieved by choosing tetrahydrofuran–boron trifluoride diethyl etherate (THF–BFEE) as the electrolyte [22], and a series of Se-3MeT copolymers with different molecular ratios were obtained in THF–BFEE (1:1, by volume; Scheme 2). Further, the structural properties, surface morphology, electrochemistry, and electrochromic properties of the homopolymers/copolymers were minutely investigated to disclose the effect of different 3MeT feed ratios on the copolymer properties.

Scheme 2
scheme 2

Electrochemical copolymerization of Se and 3MeT

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Experimental

Materials

Selenophene (Se, 99%; Energy Chemical) and 3-methylthiophene (3MeT, 98%; Energy Chemical) were used as received. BFEE (AR; Beijing Changyang Chemical Plant) was distilled before use. Tetrahydrofuran (THF, AR; Xilong Chemical) was refluxed and distilled over sodium wire. Other chemicals and reagents (AR, > 98%) were purchased commercially and used without any further treatment.

Characterization

Infrared spectra (IR) were recorded using a Bruker Vertex 70 Fourier–transform infrared (FT–IR) spectrometer with samples in KBr pellets. The surface and bulk morphology of polymers deposited on the ITO-coated glasses were carried out employing a VEGA II–LSU scanning electron microscope (Tescan).

Copolymerization

Electrochemical copolymerization and electrochemistry tests were performed with a potentiostat–galvanostat (model 263A, EG&G Princeton Applied Research) under computer control. And the working and counter electrodes were both Pt wire with a diameter of 1.0 mm, respectively. An Ag/AgCl electrode was prepared in 6.0 M HCl (aq) and directly used as the reference electrode during the electrochemical tests. BFEE was refined by a dry nitrogen stream and maintained under a slight overpressure through all the experiments.

Electrochromic experiments

The electrochromic performances studies were recorded on a Model 263 potentiostat–galvanostat (EG&G Princeton Applied Research) and using a SPECORD 200 PLUS UV–Vis spectrophotometer under computer control. During the electrochromic experiments, three-electrode system, including an Ag/AgCl wire, a Pt wire, and an ITO glass as transparent working electrode, was employed.

The optical contrast values (∆T%) between dedoped and doped state at the specific wavelength were used to obtained the optical density (∆OD) (λ max), as illustrated by the following equation [23]:

$$ \varDelta OD=\log \left({T}_{ox}/{T}_{red}\right) $$
(1)

The ratio between ∆OD and the injected/ejected charge as a function of electrode area (Q d) was defined the coloration efficiency (CE) at the specific dominant wavelength, as illustrated by the following equation [24]:

$$ CE=\varDelta OD/{Q}_d $$
(2)

Result and discussion

Electropolymerization

High-quality homopolymer films can be facilely synthesized by direct electropolymerization in BFEE. In order to decrease the catalytic effect of BFEE on Se (probably leads to the ring-opening reaction and other side reactions), THF was introduced into BFEE to form a binary solvent of 50% (volume fraction, vol) THF and 50% (vol) BFEE. In this system, the onset oxidation potentials (E onset) of Se, as shown in anodic oxidation curves (Fig. 1), was 1.03 V, while that of 3MeT was 0.94 V. The difference in E onset between Se and 3MeT was just 0.09 V, theoretically indicating that the copolymerization could occur easily [25, 26]. By varying the monomer feeding ratios, the E onset values of monomer mixtures were initiated in the range of 0.95~1.05 V. A trend was also observed that the E onset of monomer mixtures gradually shifted to that of 3MeT with increasing the feed ratio of 3MeT. When the feed ratio of Se/3MeT reached 1:5, the E onset of monomer mixtures was reduced to 0.95 V, approximately the same as 3MeT. Namely, we can tune the oxidation potentials of monomer mixtures through controlling monomer feeding ratios.

Fig. 1
figure 1

Anodic polarization curves of Se (A), 3MeT (E), and their mixtures with different feed ratios of Se/3MeT for 1:1 (B), 1:2 (C), and 1:5 (D) in THF–BFEE. Scan rate: 50 mV s−1

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During the potentiodynamic polymerization, successive cyclic voltammograms (CVs) of monomer mixtures were obtained, as shown in Fig. 2. The cycling behaviors showed characteristic electrosynthesis features of conducting polymers, also in good agreement with 3MeT and Se [26]. From Fig. 2d, e, PSe films were more easily polarized than P3MeT due to easier polarization of Se than S atom. The increase in the reversible redox intensity implied that amount of polymers on the electrode surface was electrodeposited with CV scan proceeding. CV curves of comonomers under different feeding ratios demonstrated difference against Se and 3MeT. The reversible redox couples shifted to lower potentials with the feed ratios of 3MeT increasing, as seen in Fig. 2a–c, which revealed that these copolymers were consisted with different feed ratio comonomers. Compared with Se/3MeT at the feeding ratios of 1:1 and 1:2 (Fig. 2a, b), the 1:5 monomer feeding (Fig. 2c) displayed the broadest redox waves, which could be ascribed to longer chain distribution and higher degree of polarization [27, 28]. Meanwhile, the different broad redox waves in CVs of both homopolymers and copolymers also proved structural changes in the polymer main chain [28,29,30].

Fig. 2
figure 2

Successive cyclic voltammograms of Se (d), 3MeT (e), and their mixtures with feeding ratios of Se/3MeT for 1:1 (a), 1:2 (b), and 1:5 (c). Scan rate: 100 mV s−1

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We employed the potentiostatic electrosynthesis method to prepare copolymer films for characterization and property investigation. After optimizing the electrical conditions, the polymerization potential for all the homopolymers and copolymers was determined to be 1.20 V vs. Ag/AgCl.

Structural characterization

In order to investigate the homopolymers/copolymers structure and interpret the copolymerization mechanism, infrared spectra of all these homopolymers/copolymers are recorded in Fig. 3. From the spectra, all the polymers exhibited broad absorption bands, mainly due to the wide-chain dispersity of the targeted product (absorption bands overlap one another) and similar to other typical conducting polymers reported previously [30]. As seen in Fig. 3, there are mainly five characteristic peaks for all the polymer films. The peaks at about 705 and 620 cm−1 can be attributed to the stretching vibration of C–Se and C–S bonds, respectively. This confirms that selenophene and 3MeT units were present in the copolymers. A strong peak located at about 1055 cm−1 is originated from the C–H in-plane deformation of selenophene and 3MeT units. The peak at 1298 cm−1 (C–C inter ring stretching vibration) and the 1633 cm−1 peak (C=C stretching vibration) were both observed for all the polymers, revealing that the molecular structures of PSe and P3MeT were not destroyed during the copolymerization.

Fig. 3
figure 3

FT–IR spectra of homopolymers/copolymers: PSe (A), P3MeT (F), and P(Se-co-3MeT) films from different feed ratios of Se/3MeT = 2:1 (B), 1:1 (C), 1:2 (D), and 1:5 (E)

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Morphology

The surface morphology of homopolymers/copolymers is closely related to their photoelectric properties [31]. Figure 4 displays the scanning electron microscopy images (SEM) of dedoped polymer films. Among them, the morphological structure of copolymer (Se/3MeT = 1:2) showed smooth and homogeneous surface. The homopolymer/copolymer films (A, C, E, and F) exhibited compact morphology like a stack of granules. An accumulation state of small globules with interlinked holes among the clusters was observed on the surface of the copolymer (Se/3MeT = 1:2), which may promote the reversible ionic transfer and improve the electrochromic properties [32]. All these homopolymers/copolymers exhibited different morphologies from each other, and this also indicated the different aggregation structures of conducting copolymers.

Fig. 4
figure 4

SEM photographs of dedoped PSe (a), P3MeT (f), and P(Se-co-3MeT) films from different feed ratios of Se/3MeT = 2:1 (b), 1:1 (c), 1:2 (d), and 1:5 (e)

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Spectroelectrochemistry

Spectroelectrochemistry can reveal information about the electronic structure of conducting polymers and examine the spectral changes during the redox switching. All the homopolymers/copolymers coated on ITO glass were switched between the dedoped and doped state in CH3CN–Bu4NPF6 (0.1 mol L−1) electrolyte in order to obtain in situ UV–Vis spectra (Fig. 5).

Fig. 5
figure 5

Spectroelectrochemistry for PSe (a), P3MeT (f), and P(Se-co-3MeT) films on the ITO glass in monomer-free CH3CN–Bu4NPF6 (0.10 mol L−1) solution between the potentials indicated (ΔE = 0.1 V). Monomer feed ratios Se/3MeT = 2:1 (b), 1:1 (b), 1:2 (d), and 1:5 (e)

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At the neutral states, all the polymer films exhibited strong absorption in the visible region from 420 to 500 nm due to the valence band–conduction band (π–π*) transition (Fig. 5 and Table 1). With the increase of applied potentials, the intensity of π–π* transition peaks decreased, and the evolution of new absorption bands in the range of 600~900 nm was observed. This represented typical polaron absorption bands (polarons formed in the homopolymer/copolymer backbone). Upon further oxidation, new absorption bands at longer wavelengths (> 900 nm) appeared due to the formation of bipolarons from polarons [33]. Also, the UV–Vis spectra for homopolymers/copolymers displayed well-defined isosbestic points at approximately 600 nm, indicating that the homopolymers/copolymers were interconverted between two distinct forms on both occasions: the neutral form and radical cations [34]. The optical bandgap (E g, opt), defined as the onset of π–π* transition, was calculated for all the homopolymers/copolymers, as shown in Table 1. The E g,opt of PSe was 1.88 eV while 2.10 eV for P3MeT. With increasing the feed ratios of 3MeT, the π–π* transition absorption peaks of copolymers were continuously blue-shifted and the E g,opt of the copolymers increased gradually towards that of P3MeT.

Table 1 Onset and maximal absorption wavelengths and optical bandgap of PSe (A), P3MeT (F), and P(Se-co-3MeT) films from different monomer feed ratios
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CIE 1976 (L*, a*, b*) color space and photographs were used to determine the color change (− a* and + a* correspond to green and red; − b* and + b* correspond to blue and yellow, respectively). The colorimetric data are given in Table 2. Note that the fully oxidized polymer films coated on ITO glass could be facilely obtained. It is found that by increasing the proportion of 3MeT, more saturated blue colors were achieved and interestingly, the 1:5 copolymer film showed a black color. It was worthy to note that PSe exhibited similar red brown color in both the neutral and oxidized states. Overall, the copolymers and P3MeT showed different colors from sandy-brown to deep blue even black.

Table 2 Experimental switching colors and coordinates of PSe, P3MeT, and P(Se-co-3MeT) films of different feed ratios
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Electrochromic switching of homopolymer/copolymer films

The optical contrast (ΔT%) is defined as the transmittance difference between the redox states [35]. Figure 6 illustrates the time–transmittance curves of all the polymer films at indicated wavelengths. Based on these curves, electrochromic parameters including optical contrast, response times, and coloration efficiency for all the homopolymer and copolymer films at different wavelengths are summarized in Table 3. All these copolymer films revealed relatively high optical contrast values, sufficing for various electrochromic applications. Specifically, the copolymer films here showed higher transmittance values in NIR region, indicating promising potential applications in NIR smart windows. It is also found that the optical contrast of the copolymer (Se/3MeT = 1:2) at 1080 nm was remarkably up to 70.2%, higher than all the other polymer films. Overall, a certain proportion of copolymerization between Se and 3MeT could improve the optical contrast of PSe and even exceed P3MeT itself.

Fig. 6
figure 6

Transmittance–time profiles of PSe (a), P3MeT (f), and P(Se-co-3MeT) films from different monomer feed ratios on the ITO-coated glass in monomer-free CH3CN–Bu4NPF6 (0.10 mol L−1) solution. Monomer feed ratios Se/3MeT = 2:1 (b), 1:1 (c), 1:2 (d), and 1:5 (e). Switching time: 10 s

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Table 3 Electrochromic parameters of PSe (A), P3MeT (F), and P(Se-co-3MeT) films from different monomer feed ratios
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Response time is calculated at 95% of the full switching because of the difficulty in perceiving any further color changes with the naked eye beyond this point [36]. These polymers were switched slowly upon redox processes. To be specific, the optical response time of PSe was more than 10 s during the redox states, longer than other copolymers. The fastest optical response time was demonstrated to be 1.0 s at the copolymer under the feeding ratio of 1:5 under 1080 nm from the oxidized to reduced state. Although the response times of the copolymers were comparable, the copolymers (Se/3MeT = 1:2) still had some advantages in response time of the visible region (5.6 s at 750 nm during redox process). In addition, the reduction of the polymer films was inherently faster than oxidation, which can be attributed to the ease of charge transport in the conducting film when it is reduced [35].

The coloration efficiency (CE) of PSe was measured as 14 cm2 C−1 at 510 nm and merely less than 1 cm2 C−1 at 950 nm at the fully doped state. In comparison, the coloration efficiencies of P3MeT were higher than that of PSe at different wavelengths. By introducing 3MeT into the PSe chain, the copolymers displayed improved CE values, typically in the range of 30~60 cm2 C−1. Namely, all the copolymers exhibited intermediate CE values between PSe and P3MeT.

The optical memory is another important parameter since it reveals the energy consumption of electrochromic devices (ECDs) [37]. The optical spectra for all the polymers were monitored at specific wavelength as a function of time under applying potentials for 2 s between 100 s intervals. As seen in Fig. 7, all the films were highly stable in their reduced states and kept their colors without significant loss. However, the oxidized states of polymer films were less stable with the optical contrast loss of 1~7%. Nevertheless, all the copolymers under different feed ratios revealed better optical memory properties than that of homopolymers.

Fig. 7
figure 7

Open circuit stability of PSe (a), P3MeT (f), and P(Se-co-3MeT) films from different monomer feed ratios on the ITO glass in monomer-free CH3CN–Bu4NPF6 (0.10 mol L−1) solution. Monomer feed ratios Se/3MeT = 2:1 (b), 1:1 (c), 1:2 (d), and 1:5 (e). Applied potentials: − 1.0 and 1.4 V. Wavelength: a 950 nm; b–f 1080 nm

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Stability

It is known that the good stability of conducting polymers is a key property for applications in advanced technological devices [1, 3, 4, 6, 37]. Therefore, the long-term redox stability of all the films was investigated in both monomer-free CH3CN–Bu4NPF6 (0.10 mol L−1) and BFEE solution at the potential scan rate of 150 mV s−1, as shown in Figs. 8 and 9 and Table 4. Obvious redox peaks of these polymers can be observed from the CVs. It can be seen that PSe and P3MeT films just retained 47.24 and 89.48% of their electroactivity after 100 cycles, respectively. As expected, the copolymers at different feed ratios revealed intermediate stability between PSe and P3MeT films, typically over 70% electroactivity left after 100 cycles. The copolymers under 1:2 and 1:5 feed ratios maintained their activity of 85.43 and 85.96% after 100 cycles, but unfortunately decreased to 52.74 and 62.18% after 1000 cycles, respectively. Although better than PSe, these parameters are inferior to P3MeT and even many typical conducting polymers reported previously, which is not suitable for applications in advanced technological devices. This is mainly due to the exchange of doping ions from BF4 − to PF6 − in different electrolytes, causing irreversible doping–dedoping process.

Fig. 8
figure 8

Long-term CVs of PSe (a), P3MeT (f), and P(Se-co-3MeT) films from different monomer feed ratios on the Pt in monomer-free CH3CN–Bu4NPF6 (0.10 mol L−1) solution. Monomer feed ratios Se/3MeT = 2:1 (b), 1:1 (c), 1:2 (d), and 1:5 (e). Scan rate: 150 mV s−1

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Fig. 9
figure 9

Long-term CVs of PSe (a), P3MeT (f), and P(Se-co-3MeT) films from different monomer feed ratios on the Pt in monomer-free BFEE solution. Monomer feed ratios Se/3MeT = 2:1 (b), 1:1 (c), 1:2 (d), and 1:5 (e). Scan rate: 150 mV s−1

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Table 4 Long-term stability parameters of PSe (A), P3MeT (F), and P(Se-co-3MeT) films from different monomer feed ratios in different solutions
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In contrast, the redox stability of these films was significantly improved when cycling in monomer-free BFEE solution. From Fig. 9, the redox stability of all the homopolymers and copolymers was significantly improved after 100 cycles (Table 4). Interestingly, most of the polymers showed an increased electroactivity, e.g., with more than 100% of their electroactivity after cycling. To be specific, the redox waves of the polymer films were relatively weak at first due to their low doping degree in BFEE. Upon repetitive cycling, the redox current densities of the polymers increased gradually; thus, the polymers demonstrated enhanced electroactivity. This is mainly because the polymers deposited in BFEE was less doped than they could be, and secondary cycling can be employed to further dope them (improve their doping level). This phenomenon was also found by us for P3MeT [38] and even some other electrosynthesized conducting polymers [39]. Even after 1000 cycles, no significant loss in electroactivity was found for all these films, indicating their outstanding redox activity in BFEE.

Conclusion

A series of Se-3MeT copolymers were prepared under different feed ratios of Se and 3MeT by electrochemical copolymerization in THF–BFEE solution. The influence of different feed ratios on the electrochemical, spectroelectrochemical, and kinetic properties of the resultant copolymers were investigated in detail. The copolymer films presented obvious electrochromic characteristics with color changes from sandy-brown to deep blue even black during the doped and dedoped states, higher optical contrast in comparison with parent homopolymers, good electroactive stability, and satisfied electrochromic properties. More specifically, the copolymer prepared at the feed ratio of 1:2 (Se/3MeT) possessed optical contrast of 70.2% (1080 nm), significantly higher than that of parent PSe and P3MeT, even more than other copolymers. The electrochemical and electrochromic properties of all copolymers were significantly better than PSe and even competitive to P3MeT. Also, it is also found that the doping level of the copolymer films can be further enhanced by secondary cyclic voltammetry in BFEE after electropolymerization.