Three Carbazole-Based Polymers as Potential Anodically Coloring Materials for High-Contrast Electrochromic Devices.

Three carbazole-based conjugated polymers (poly(3,6-di(2-thienyl)carbazole) (PDTC), poly(2,7-bis(carbazol-9-yl)-9,9-spirobifluorene) (PS2CBP), and poly(3,6-bis(N-carbazole)-N-ethylcarbazole) (PCEC)) are synthesized using electrochemical polymerization. The spectroelectrochemical studies indicate that the PDTC, PS2CBP, and PCEC films show reversible electrochromic behaviors in their redox states, and the PS2CBP film shows a distinct color transition with four various colors (gray at 0 V, grayish-green at 1.0 V, moss green at 1.2 V, and foliage green at 1.4 V). The maximum optical contrast of the PS2CBP and PCEC films is 39.83% at 428 nm and 32.41% at 420 nm, respectively, in an ionic liquid solution. Dual-type electrochromic devices (ECDs) that employ PDTC, PS2CBP, or PCEC film as an anodic layer, and PProDOT-Et₂ film as a cathodic layer, were constructed. The as-prepared PCEC/PProDOT-Et₂ ECD shows high optical contrast (38.25% at 586 nm) and high coloration efficiency (369.85 cm² C-1 at 586 nm), and the PS2CBP/PProDOT-Et₂ ECD shows high optical contrast (34.45% at 590 nm), good optical memory, and good long-term cycling stability.

1. Introduction

In electrochromism, it is widely known that an electrochromic material is able to change its color reversibly at different potentials [1]. Electrochromic material can be divided into two classes: inorganic and organic electrochromic materials. Common inorganic electrochromic materials include WO3 and V2O5 [2], whereas viologens and conjugated polymers are common organic electrochromic materials [3,4]. Electrochromic materials are widely used in smart windows/mirrors, displays [2], electrochromic devices (ECDs) [5,6], and electronic paper [7]. Among these applications, the use of organic electrochromic materials as cathodically coloring material in electrochromic devices has become popular due to their ease of chemical synthesis, high coloration efficiency, short switching time, high optical contrast, low oxidation potential, and good long-term stability [8,9]. Polycarbazole, polypyrrole, polytriphenylamine, polythiophene, polyaniline, poly(3,4-(2,2-diethylpropylenedioxy)thiophene) (PProDOT-Et2), and poly(3,4-ethylenedioxythiophene) are extensively used as conjugated polymers in electrochromic devices [10,11,12]. Among the various conjugated polymers, PProDOT-Et2 has shown great properties as a cathodically coloring material in electrochromic devices [13], and polycarbazoles were reported as anodically coloring materials in electrochromic devices [14] due to their high thermal stability, good optical properties, and interesting electroactive properties. Moreover, polycarbazoles can be easily functionalized at their N-, (2,7-), or (3,6-) positions [14,15,16,17,18]. Accordingly, thiophene- and carbazole-based conjugated polymers are employed to efficiently fabricate electroactive polymeric films for potential applications in electronic and optoelectronic devices. Moreover, ionic liquids (ILs) are salts which are liquid at a temperature of less than 100 °C, and are composed of organic cations and inorganic (or organic) anions. ILs show wide electrochemical potential windows, low or negligible volatility, high conductivity, and high thermal stability [19,20,21]. These properties make IL-based electrolytes ideal candidates for use as non-volatile electrolytes in ECDs.

In the present work, three polycarbazole derivatives (poly(3,6-di(2-thienyl)carbazole) (PDTC), poly(2,7-bis(carbazol-9-yl)-9,9-spirobifluorene) (PS2CBP), and poly(3,6-bis(N-carbazole)-N-ethylcarbazole) (PCEC)) were polymerized electrochemically, and their spectroelectrochemical and electrochromic properties were investigated. The incorporation of two thiophene units in the PDTC backbone results in a red shift of the absorption maxima and narrows the electrochemical band gap [1,4]dioxin-5-ylthienyl) derivatives bearing azobenzene, coumarine and fluorescein dyes: Effect of chromophore groups on electrochromic properties. Electrochim. Acta. 2014 ">22]. The incorporation of a spirobifluorene core in the PS2CBP backbone can provide specific properties, such as an elevated glass transition temperature and a stable decomposition temperature [23], and the spectroelectrochemical characterization of spirobifluorene core-based polycarbazole derivatives have not been reported so far. The ethylcarbazole core in the PCEC backbone is more electron donating than that of the spirobifluorene core in the PS2CBP backbone [24]. Furthermore, electrochromic devices were constructed using PDTC, PS2CBP, and PCEC films as anodic electrodes, poly(3,3-diethyl-3,4-dihydro-2H-thieno[3,4–b][1,4]dioxepine) (PProDOT-Et2) as cathodic electrodes, and an ionic liquid-based electrolyte as an electrochromic electrolyte. The electrochromic behaviors of PDTC film, PS2CBP film, PCEC film, a PDTC/PProDOT-Et2 ECD, a PS2CBP/PProDOT-Et2 ECD, and a PCEC/PProDOT-Et2 ECD were investigated systematically using spectroelectrochemistry, electrochromic switching, coloration efficiency, and colorimetry. The ECDs were also investigated using open circuit memory and long-term redox stability.

2. Materials and Methods

2.1. Materials and Electrochemical Synthesis

The reagents and compounds used in this study were purchased from Acros (Morris Plains, NJ, USA), TCI (Tokyo, Japan), and Aldrich (St. Louis, MO, USA), and used as received. The 3,3-diethyl-3,4-dihydro-2H-thieno [3,4–b][1,4]dioxepine (ProDOT-Et2) was synthesized according to previously published procedures [25]. The 1-ethyl-3-propylimidazolium bromide ([EPI+][Br−]) and 1-ethyl-3-propylimidazolium bis(trifluoromethanesulfonyl)imide ([EPI+][TFSI−]) were also synthesized based on previous procedures [26,27]. The synthetic routes of DTC, S2CBP, and CEC are shown in Figure 1. The PDTC, PS2CBP, and PCEC films were either prepared potentiostatically at 0.9 V, 1.2 V, and 1.1 V, respectively, on Indium Tin Oxide (ITO) glass electrodes with a charge density of 20 mC cm−2, or prepared potentiodynamically at a scan rate of 100 mV s−1 for 20 cycles. The onset potential of oxidation was determined from the first cyclic voltammogram (CV) cycle. The electrochromic behaviors were captured using a standard illuminant D65 light source.

Figure 1

The synthetic routes of the three carbazole derivatives.

2.1.1. Synthesis of 3,6-Di(2-thienyl)carbazole (DTC)

A mixture of 2-(tributylstannyl)thiophene (2.50 g, 6.7 mmol), 3,6-dibromocarbazole (0.98 g, 3 mmol), 20 mL toluene, and 75 mg tetrakis(triphenylphosphine)palladium(0) was stirred at 90 °C under Argon for 48 h. Afterward, the mixture was filtered and the toluene was evaporated. The remaining crude product was purified using column chromatography (silica gel, eluent: dichloromethane/hexane mixture) to give the desired DTC. Yield: 61%. 1H-NMR (400 MHz, DMSO-d6): δ 11.46 (s, 1H, NH), 8.55, (s, 2H, carbazole-H), 7.72–7.70 (m, 2H, carbazole-H), 7.53–7.47 (m, 6H, carbazole-H and Th-H), 7.16–7.13 (m, 2H, Th-H). The 1H-NMR spectrum of DTC is displayed in Figure S1 (in supplementary information). Elemental analysis: Calculated (Elem. Anal. Calcd.) for C20H13NS2: C, 72.47%; H, 3.95%; N, 4.23%. Found: C, 72.35%; H, 3.88%; N, 4.16%.

2.1.2. Synthesis of 2,7-Bis(carbazol-9-yl)-9,9-spirobifluorene (S2CBP)

In a round bottom flask were added 2,7-dibromo-9,9′-spirobifluorene (4.74 g, 10 mmol), carbazole (6.68 g, 40 mmol), potassium carbonate (5.53 g, 40 mmol), Cu powder (1.60 g, 25 mmol), and 20 mL triethylene glycol dimethyl ether (TEGDME), and the mixture was stirred at 180 °C under an Argon atmosphere for 36 h. TEGDME was evaporated using a rotary evaporator under an oil bath, and the reaction mixture was purified using column chromatography (silica gel, eluent: dichloromethane/hexane mixture). The recrystallization of the crude product from dimethylformamide gives S2CBP. Yield: 65%. 1H-NMR (400 MHz, DMSO-d6): δ 8.48 (d, 2H, carbazole-H), 8.19 (d, 4H, carbazole-H), 7.97 (d, 2H, fluorene-H), 7.78 (d, 2H, fluorene-H), 7.43 (d, 1H, carbazole-H), 7.42 (d, 1H, carbazole-H), 7.33 (dd, 4H, carbazole-H and fluorene-H), 7.28 (dd, 2H, fluorene-H), 7.23 (dd, 4H, carbazole-H and fluorene-H), 7.16 (d, 4H, carbazole-H), 6.98 (d, 2H, fluorene-H), 6.79 (s, 2H, fluorene-H). The 1H-NMR spectrum of S2CBP is displayed in Figure S2 (in supplementary information). Elem. Anal. Calcd. for C49H30N2: C, 90.99%; H, 4.68%; N, 4.33%. Found: C, 90.85%; H, 4.68%; N, 4.26%.

2.1.3. Synthesis of 3,6-Bis(N-carbazolyl)-N-ethylcarbazole (CEC)

In a round bottom flask were added 3,6-dibromo-9-ethylcarbazole (4.94 g, 14 mmol), carbazole (9.36 g, 56 mmol), potassium carbonate (7.74 g, 56 mmol), Cu powder (2.30 g, 36 mmol), and 20 mL triethylene glycol dimethyl ether, and the mixture was stirred at 180 °C under an Argon atmosphere for 36 h. TEGDME was evaporated using a rotary evaporator under an oil bath, and the reaction mixture was purified using column chromatography (silica gel, eluent: dichloromethane/hexane mixture). The recrystallization of the crude product using dimethylformamide gives CEC. Yield: 58%. 1H-NMR (400 MHz, DMSO-d6): δ 8.56 (s, 2H, Hi), 8.26 (d, 4H, Hh), 7.99 (d, 2H, Hg), 7.70 (d, 2H, Hf), 7.42–7.40 (m, 4H, He), 7.35–7.34 (m, 4H, Hd), 7.26–7.28 (m, 4H, Hc), 4.69 (q, 2H, Hb), 1.51 (t, 3H, Ha). The 1H-NMR spectrum of CEC is displayed in Figure S3 (in supplementary information). Elem. Anal. Calcd. for C38H27N3: C, 86.83%; H, 5.18%; N, 7.99%. Found: C, 86.75%; H, 5.10%; N, 7.96%.

2.2. Instrumentation and Measurements

The electrochemical behaviors of the PDTC, PS2CBP, and PCEC films coated on the ITO electrodes were characterized using a CHI6081E electrochemical analyzer (CH Instruments, Austin, TX, USA). The sheet resistance of ITO glass (AimCore Technology Co., Ltd., Hsinchu, Taiwan) is below 15 Ω/sq. The spectroelectrochemical properties of the PDTC film, the PS2CBP film, the PCEC film, a PDTC/PProDOT-Et2 ECD, a PS2CBP/PProDOT-Et2 ECD, and a PCEC/PProDOT-Et2 ECD were characterized using a V-670 JASCO UV-Visible spectrophotometer set to record in situ UV-Visible spectra photometer (JASCO International Co., Ltd., Tokyo, Japan). The chromaticity values of the polymer films and ECDs were calculated according to previous procedures [28].

2.3. Construction of ECDs

An electrochromic electrolyte was prepared according to the method described in our previous work [29]. The electrochromic electrolyte is an ionic liquid/polymer composite electrolyte. The ionic liquid and polymer are [EPI+][TFSI−] and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), respectively. The anodic coloring PDTC, PS2CBP, and PCEC films were electrodeposited onto ITO glasses potentiostatically at 0.9 V, 1.2 V, and 1.1 V, respectively, whereas cathodic coloring PProDOT-Et2 film was electrodeposited onto ITO-coated glasses potentiostatically at 1.4 V. The anodic polymer film and PProDOT-Et2 film were separated by an ionic liquid/polymer composite electrolyte.

3. Results and Discussion

3.1. Electrochemical Polymerization of Polymer Films

The polymer films can be prepared by a potentiodynamic method. The cyclic voltammograms (CVs) of neat DTC, S2CBP, and CEC monomers in an acetonitrile/dichloromethane (ACN/DCM) solution (1:1, v/v) containing 0.1 M LiClO4 are shown in Figure 2. As the cyclic voltammetric scan continued, the peak current intensity of Figure 2a–c increased gradually, demonstrating that the PDTC, PS2CBP, and PCEC films were electropolymerized on the surface of the ITO working electrode. The schemes for the electrochemical polymerization of PDTC, PS2CBP, and PCEC are shown in Figure S4 (in supplementary information).

Figure 2

Cyclic voltammograms (CVs) of 2 mM (a) DTC, (b) S2CBP, and (c) CEC in ACN/DCM solution (1:1, v/v) containing 0.1 M LiClO4 at a scan rate of 100 mV s−1 on an ITO working electrode.

The onset potentials of the PDTC, PS2CBP, and PCEC films are 0.82, 0.99, and 0.94 V, respectively. The PDTC film shows a lower onset potential than those of the PS2CBP and PCEC films, which can be attributed to the incorporation of two thiophene units at the 3,6-positions of the carbazole unit that gives rise to an obvious aromatic conjugation after electrochemical polymerization. Consequently, the onset potential of the PDTC film slightly shifts to low potential [30]. Moreover, the PCEC film shows a lower onset potential than that of the PS2CBP film, which can be ascribed to the fact that CEC contains one additional carbazole unit compared to the S2CBP unit [24]. The oxidation peaks of PDTC, PS2CBP, and PCEC as displayed in Figure 2 appear at 1.02, 1.21, and 1.32 V, respectively, whereas the reduction peaks of PDTC, PS2CBP, and PCEC are located at 0.31, 0.73, and 0.87 V, respectively.

Figure 3a–c shows the CV plots of the PDTC, PS2CBP, and PCEC films, respectively, at different scan rates in [EPI+][TFSI−] solution, and the relationships between peak current density and the scan rate of the PDTC, PS2CBP, and PCEC films are shown in Figure 3d–f, respectively. The anodic and cathodic peak current density values increase linearly with increasing scan rate as displayed in Figure 3d–f, indicating that the oxidation and reduction processes are non-diffusion limited [31].

Figure 3

CV curves of (a) PDTC, (b) PS2CBP, and (c) PCEC films at various scan rates between 25 and 250 mV s−1 in [EPI+][TFSI−] solution, and the relationship between peak current density vs. scan rate of (d) PDTC, (e) PS2CBP, and (f) PCEC films in [EPI+][TFSI−] solution.

3.2. Electrochromic Properties of the PDTC, PS2CBP, and PCEC Films

The absorption spectra of the PDTC, PS2CBP, and PCEC films coated on ITO glass electrodes were investigated in [EPI+][TFSI−] solution at various potentials. As shown in Figure 4, the peaks of the PDTC and PS2CBP films in the neutral state were located at 406 nm and 354 nm, respectively, and the shoulder of the PCEC film in the neutral state was located at 415 nm. These peaks could be assigned to the π–π* transition of the PDTC, PS2CBP, and PCEC films in [EPI+][TFSI−] solution. In the PDTC film, the oxidation begins at about 0.5 V, the peak at around 406 nm decreases gradually, and the charge carrier bands appear at around 600 nm and 900 nm. The formation of the charge carrier bands can be ascribed to the evolution of polaron and bipolaron bands [32].

Figure 4

Spectroelectrochemical spectra of (a) PDTC; (b) PS2CBP; and (c) PCEC films on ITO electrodes at different applied potentials in [EPI+][TFSI−] solution.

Under similar conditions, the oxidation of the PS2CBP film occurs at about 0.6 V, the peaks decrease gradually at around 350 nm, and increase gradually at around 430 nm and 1200 nm. Moreover, the oxidation of the PCEC film takes place at around 0.7 V, and the charge carrier bands at around 800 and 1300 nm increase gradually from 0.7 to 1.3 V. Table 1 shows the photos of the PDTC, PS2CBP, and PCEC films in [EPI+][TFSI−] solution at various potentials. The PDTC, PS2CBP, and PCEC films show reversible electrochromic behaviors in their redox states. The PDTC film was yellowish-green in the neutral state (0 V), green in the intermediate state (1.0 V), iron gray in the oxidized state (1.2 V), and bluish-purple in highly oxidized states (1.4 V). Similarly, the PS2CBP film shows a distinct color transition with four various colors (gray at 0 V, grayish-green at 1.0 V, moss green at 1.2 V, and foliage green at 1.4 V), and the PCEC film displays four various colors from neutral to highly oxidized states (silver gray at 0 V, cobalt green at 1.0 V, celadon green at 1.2 V, and peacock green at 1.4 V). The colorimetric values (L*, a*, b*) of the PDTC, PS2CBP, and PCEC films measured at various potentials in [EPI+][TFSI−] solution are listed in Table 2, and the CIE (Commission Internationale de I’Eclairage) chromaticity diagrams of PDTC, PS2CBP, and PCEC films in neutral and oxidation state are displayed in Figure S5 (in supplementary information). The L*, a*, b* of the PS2CBP film at 0 V was 89.71, 0.30, and 7.51, respectively. The a* values of the PS2CBP film convert from positive to negative at 0.8–1.4 V, indicating that the color of the PS2CBP film turns from light gray to green in the oxidation state.

Table 1

Electrochromic behaviors of the PDTC, PS2CBP, and PCEC films in [EPI+][TFSI−] solution and their corresponding electrochromic devices (ECDs) at a potential range from 0 V to +1.4 V.

Polymer Films and ECDs	0 V	1.0 V	1.2 V	1.4 V
PDTC film
PS2CBP film
PCEC film
PDTC/PProDOT-Et2 ECD
PS2CBP/PProDOT-Et2 ECD
PCEC/PProDOT-Et2 ECD

Table 2

The colorimetric values (L*, a*, b*) of the PDTC, PS2CBP, and PCEC films at different applied potentials in [EPI+][TFSI−] solution.

Polymer Films	E/V	L*	a*	b*
PDTC	0	87.23	−9.03	73.27
	0.8	51.93	−1.93	9.15
	1.0	40.99	8.39	−13.85
	1.2	41.62	6.2	−12.64
	1.4	45.89	2.8	−9.75
PS2CBP	0	89.71	0.3	7.51
	0.8	86.31	−0.97	15.64
	1.0	83.25	−6.74	36.93
	1.2	78.29	−11.76	32.53
	1.4	77.82	−1.2	14.99
	1.6	82.04	0.96	17.56
PCEC	0	94.61	1.18	4.37
	0.8	92.32	−3.15	13.78
	1.0	88.73	−8.42	21.07
	1.2	82.58	−11.22	10.04
	1.4	81.23	−8.44	8.55
	1.6	85.25	−4.96	13.79

The optical band gap (Eg) of the PDTC, PS2CBP, and PCEC films can be calculated according to the Planck equation (Eg = 1241 (eV·nm)/λonset (nm)) [33,34,35], and they are 2.45, 3.06, and 3.00 eV, respectively. The Eg of the PS2CBP film is comparable to that of the PCEC film, implying that the incorporation of a spirofluorene (or carbazole) unit between two carbazole groups does not influence Eg significantly. However, the PDTC film shows a lower Eg value than those of the PS2CBP and PCEC films, which can be ascribed to the fact that the incorporation of two thiophene units at the 3,6-positions of the carbazole group diminishes the Eg value significantly.

The highest occupied molecular orbital energy levels (EHOMO) of the PDTC, PS2CBP, and PCEC films were calculated using the formula [36]: where Eonset is the onset oxidation potentials corrected using an internal standard redox ferrocene/ferrocinium couple. The lowest unoccupied molecular orbital energy levels (ELUMO) of the PDTC, PS2CBP, and PCEC films were calculated by subtracting the optical band gap from EHOMO [37]. The EHOMO of PDTC, PS2CBP, and PCEC are −5.14, −5.31, and −5.26 eV, respectively, and the ELUMO of PDTC, PS2CBP, and PCEC are −2.69, −2.25, and −2.26 eV, respectively.

A cyclic potential-step method was used to determine the electrochromic switching of conducting polymer films [38]. The electrochromic switching of PDTC, PS2CBP, and PCEC films in [EPI+][TFSI−] solution was stepped by repeated potential between reduction and oxidation states with a time interval of 5 s. Figure 5 exhibits the transmittance–time profiles of the PDTC film at 578 and 856 nm, the PS2CBP film at 428 and 1208 nm, and the PCEC film at 420 and 1220 nm.

Figure 5

In situ transmittance of (a) the PDTC film at 578 nm under a voltage interval between 0 V and +1.0 V; (b) PS2CBP film at 428 nm under a voltage interval between 0 V and +1.2 V; (c) the PCEC film at 420 nm under a voltage interval between 0 V and +0.9 V; (d) the PDTC film at 856 nm under a voltage interval between 0 V and +1.0 V; (e) the PS2CBP film at 1208 nm under a voltage interval between 0 V and +1.2 V; (f) the PCEC film at 1220 nm under a voltage interval between 0 V and +1.2 V as a function of time in [EPI+][TFSI−] solution, the time interval is 5 s.

The coloration switching time (τc) and bleaching switching time (τb) of the PDTC, PS2CBP, and PCEC films in [EPI+][TFSI−] solution are listed in Table 3; the τc and τb are determined at 90% of full-transmittance change (T90%). The optical switching time of the PDTC film in [EPI+][TFSI−] solution was found to be 2.04 and 1.64 s at 578 and 856 nm, respectively, from the bleaching to coloring state at the 50th cycle, and 2.01 and 1.69 s at 578 and 856 nm, respectively, from the coloring to bleaching state at the 50th cycle. The τc and τb of the PS2CBP film in [EPI+][TFSI−] solution at 1208 nm were found to be 2.16 and 1.83 s, respectively, at the 100th cycle, and the τc and τb of the PCEC film in [EPI+][TFSI−] solution at 1220 nm were found to be 1.90 and 1.63 s, respectively, at the 100th cycle.

Table 3

Color-bleach kinetics of the PDTC, PS2CBP, and PCEC films in [EPI+][TFSI−] solution and the ECDs.

Polymer Films and ECDs	λmax/nm	Cycle No.	ΔT/%	τc/s	τb/s
PDTC films in [EPI+][TFSI−]	578	1	58.79	1.91	1.93
		50	58.11	2.04	2.01
		100	58.02	2.07	2.04
	856	1	66.04	1.77	1.72
		50	61.77	1.64	1.69
		100	56.96	1.68	1.64
PS2CBP films in [EPI+][TFSI−]	428	1	39.83	1.85	1.80
		50	33.29	2.07	1.97
		100	27.95	2.08	1.95
	1208	1	63.56	2.24	1.98
		50	57.94	2.23	1.91
		100	50.64	2.16	1.83
PCEC films in [EPI+][TFSI−]	420	1	32.41	2.21	1.77
		50	28.27	2.24	1.74
		100	26.98	2.22	1.77
	1220	1	42.36	1.99	1.72
		50	40.15	1.96	1.66
		100	38.59	1.90	1.63
PDTC/PProDOT-Et2 ECD	592	1	31.27	0.96	0.99
		50	26.77	0.96	0.96
		100	24.85	0.99	0.97
PS2CBP/PProDOT-Et2 ECD	590	1	34.45	1.06	0.99
		50	32.67	1.04	0.99
		100	31.84	1.06	1.00
PCEC/PProDOT-Et2 ECD	586	1	38.25	1.01	0.96
		50	32.19	1.01	0.93
		100	29.90	0.98	0.90

For the switching times of the PS2CBP and PCEC films at different cycles, the τc and τb of the PS2CBP film in [EPI+][TFSI−] solution at 1208 nm were 2.24 and 1.98 s, respectively, at the first cycle, and 2.16 and 1.83 s, respectively, at the 100th cycles, and the τc and τb of the PCEC film in [EPI+][TFSI−] solution at 1220 nm were 1.99 and 1.72 s, respectively, at the first cycle, and 1.90 and 1.63 s, respectively, at the 100th cycles, indicating that the τc and τb of PS2CBP and PCEC films do not show significant change at high switching cycles when we employ [EPI+][TFSI−] as a supporting electrolyte.

The optical contrast (ΔT%) is the most important parameter for electrochromic applications [39]. The ΔTmax of the PDTC film at 578 and 856 nm is 58.79% and 66.04%, respectively, in [EPI+][TFSI−] solution, and the ΔTmax of the PS2CBP film at 428 nm and 1208 nm is 39.83% and 63.56%, respectively, in [EPI+][TFSI−] solution, implying that the incorporation of the DTC unit gives rise to a higher ΔTmax than that of the S2CBP unit. The ΔTmax of the PCEC film at 420 nm and 1220 nm is 32.41% and 42.36%, respectively, in [EPI+][TFSI−] solution. The ΔTmax of the PCEC film is lower than that of the PS2CBP film, indicating that two carbazole units linked by a spirobifluorene group leads to a higher ΔTmax than that of two carbazole units linked by an N-ethylcarbazole group. Among these polymer films, the PDTC film shows the highest ΔTmax (66.04%), at 856 nm in [EPI+][TFSI−] solution. The ΔTmax of the PS2CBP film is higher than that reported for poly(4-(3,6-di(thiophen-2-yl)-9H-carbazol-9-yl)-phenyl-methanone) (PTCPM) (ΔTmax = 41% at 1100 nm) [29], and is comparable to that reported for poly(2,8-di(carbazol-9-yl)dibenzothiophene)(PSCZ) (ΔTmax = 61% at 762 nm) [40]. The ΔTmax of the PCEC film is higher than those reported for PTCPM (ΔTmax = 41% at 1100 nm) and poly(3,6-di(carbazol-9-yl)-N-(4-methoxyphenyl) carbazole)(PPhCz-2Cz) (ΔTmax = 37% at 741 nm) [29,41], whereas the PCEC film shows a lower ΔTmax than that reported for PSCZ (ΔTmax = 61% at 762 nm) [40].

The coloration efficiency (CE, η) is defined as the change in the optical absorbance per unit of inserted charge (Qd) in electrochromic materials and ECDs. CE can be calculated using the following equation at a given wavelength [42]:ΔOD = log( CE = ΔOD/ where ΔOD indicates the change of the optical density at a specific wavelength. Tb and Tc are defined as the transmittance of the bleaching state and coloring state, respectively. The calculated ηmax of the PDTC film is 201.61 cm2 C−1 at 578 nm and 167.83 cm2 C−1 at 856 nm; the ηmax of the PS2CBP film is 138.09 cm2 C−1 at 428 nm and 151.70 cm2 C−1 at 1208 nm; and the ηmax of the PCEC film is 293.91 cm2 C−1 at 420 nm and 214.07 cm2 C−1 at 1220 nm. The PDTC, PS2CBP, and PCEC films in [EPI+][TFSI−] solution show higher η than those reported for PTCPM (η = 110.48 cm2 C−1 at 1100 nm) [29], PSCZ (η = 45 cm2 C−1 at 762 nm) [40], and PPhCz-2Cz (η = 56 cm2 C−1 at 741 nm) [41]. This may be attributed to that fact that an ionic liquid solution is employed as an electrochromic electrolyte in this study.

3.3. Spectroelectrochemistry of Electrochromic Devices

Dual-type ECDs were constructed using two complementary electrochromically active layers. Dual-type ECDs sometimes exhibit a higher electrochromic (EC) contrast in a wider visible range than those of single-type ECDs. Dual-type ECDs comprise anodically coloring material (PDTC, PS2CBP, or PCEC), cathodically coloring material (PProDOT-Et2), and ionic liquid-PVdF-HFP. Composite electrolytes were fabricated and their spectroelectrochemical properties were characterized. The spectroelectrochemical spectra of the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs are displayed in Figure 6a–c, respectively. As displayed in Figure 6a, the PDTC/PProDOT-Et2 ECD shows a shoulder at around 415 nm at 0 V, which can be attributed to the π–π* transition of the PDTC film in the reduction state. In this circumstance, the PProDOT-Et2 film was light blue in its oxidation state, and the PDTC/PProDOT-Et2 ECD was olive green at 0 V. However, the π–π* transition absorption of the PDTC film diminished and a new absorption band at 590 nm emerged gradually with increasing potential. The PDTC/PProDOT-Et2 ECD was dark gray at 1.0 V, prussian blue at 1.2 V, and midnight blue at 1.4 V. In a similar situation, the PS2CBP/PProDOT-Et2 ECD was silver gray at 0 V, cornflower blue at 1.0 V, and salvia blue at 1.2 V and 1.4 V, and the PCEC/PProDOT-Et2 ECD was gray at 0 V, dark mineral blue at 1.0 V, and slate blue at 1.2 V and 1.4 V. The colorimetric values and CIE chromaticity values of the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs are listed in Table 4. Moreover, the CIE chromaticity diagrams of the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs at bleaching and coloring states are shown in Figure S6 (in supplementary information).

Figure 6

Spectroelectrochemical spectra of (a) PDTC/PProDOT-Et2; (b) PS2CBP/PProDOT-Et2; and (c) PCEC/PProDOT-Et2 ECDs at applied various potentials.

Table 4

The colorimetric values (L*, a*, b*) of the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs at different applied potentials.

ECDs	E/V	L*	a*	b*
PDTC/PProDOT-Et2	0	47.59	0.46	12.09
	0.8	34.78	3.65	−7.51
	1.0	32.48	4.14	−11.21
	1.2	31.68	5.13	−12.89
	1.4	31.46	5.79	−13.69
PS2CBP/PProDOT-Et2	0	70.88	8.23	19.31
	0.8	60.51	0.89	0.57
	1.0	59.73	0.89	−1.1
	1.2	59.45	0.84	−2.04
	1.4	59.34	0.81	−2.84
PCEC/PProDOT−Et2	0	77.37	−1.62	6.39
	0.8	64.15	−0.22	0.56
	1.0	56.84	−2.66	−6.22
	1.2	52.94	−3.92	−8.46
	1.4	51.66	−4.82	−7.92

The transmittance–time profiles of the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs are shown in Figure 7a–c, respectively, and the ΔTmax, ΔOD, and ηmax of the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs are summarized in Table 5. The ΔTmax of the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs are 31.27% at 592 nm, 34.45% at 590 nm, and 38.25% at 586 nm, respectively. On the other hand, the ηmax of the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs are estimated to be 345.19 cm2 C−1 at 592 nm, 256.12 cm2 C−1 at 590 nm, and 369.85 cm2 C−1 at 586 nm, respectively. The ΔTmax and η of the PCEC/PProDOT-Et2 ECD are larger than those of poly(4,4’-di(N-carbazoyl)biphenyl-co-2,2′-bithiophene)/PEDOT (ΔTmax = 28.6%, ηmax = 234 cm2 C−1 at 700 nm) and poly(9H-carbazol-9-ylpyrene)/PEDOT (ΔTmax = 23%, ηmax = 290 cm2 C−1 at 623 nm) [43,44]. However, the PCEC/PProDOT-Et2 ECD shows a lower ΔTmax% and ηmax than those reported for PCBTD/PEDOT (ΔTmax = 49.4% at 620 nm, ηmax = 1728 cm2 C−1) and P(BCz1-co-Inc2)/PProDOT-Et2 ECDs (ΔTmax = 42%, ηmax = 634 cm2 C−1 at 587 nm) [45,46].

Figure 7

In situ transmittance of (a) the PDTC/PProDOT-Et2 ECD at 592 nm as a function of time under a voltage interval between −0.4 V and +1.0 V; (b) the PS2CBP/PProDOT-Et2 ECD at 590 nm under a voltage interval between −0.4 V and 1.4 V; (c) the PCEC/PProDOT-Et2 ECD at 586 nm under a voltage interval between 0 V and 1.4 V.

Table 5

Comparisons of the ΔTmax and ηmax for various polymer films and ECDs.

Polymer Films and ECDs	λ/nm	Eg/eV	ΔTmax/%	ΔODmax/%	ηmax/cm2 C−1
PDTC	856	2.45	66.04	76.46	167.83
PS2CBP	1208	3.06	63.56	51.41	151.70
PCEC	1220	3.00	42.36	45.55	214.07
PTCPM [29]	1100	-	41	-	110.48
PSCz [40]	762	3.26	61	45.90	45
PPhCz-2Cz [41]	741	2.76	37	-	56
PDTC/PProDOT-Et2 ECD	592	-	31.27	35.55	345.19
PS2CBP/PProDOT-Et2 ECD	590	-	34.45	24.19	256.12
PCEC/PProDOT-Et2 ECD	586	-	38.25	34.52	369.85
P(CBP-co-BT)/PEDOT ECD [43]	700	-	28.6	-	234
PMCzP/PEDOT ECD [44]	623	-	23	-	290
PCBTD/PEDOT ECD [45]	620	-	49.4	-	1728
P(BCz1-co-Inc2)/PProDOT-Et2 ECD [46]	587	-	42	-	634

The τc and τb of the ECDs estimated at different double-step potential cycles are summarized in Table 3. The τc of the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs are 0.99, 1.06, and 0.98 s at the 100th cycles, respectively, and the τb of the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs are 0.97, 1.00, and 0.90 s at the 100th cycles, respectively. The τc and τb of the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs are shorter than those of the PDTC, PS2CBP, and PCEC films in an ionic liquid solution, indicating that the ECDs changed color faster from a bleaching to a coloring state than those of the PDTC, PS2CBP, and PCEC films in an ionic liquid solution.

The long-term redox stability between the dedoped and doped states is an important parameter of ECDs [47]. The redox stability of the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs were measured by CV at potential range between −1.0 V and +1.6 V (or −1.0 V and +1.5 V) with a scan rate of 100 mV s−1. As shown in Figure 8, the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs exhibited 92%, 94%, and 95%, respectively, of electrochemical activity maintenance after the 500th cycle, and 89%, 92%, and 93%, respectively, of electrochemical activity maintenance after the 1000th cycle, implying that the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs have a reasonable environmental and redox stability.

Figure 8

Cyclic voltammograms of (a) PDTC/PProDOT-Et2; (b) PS2CBP/PProDOT-Et2; and (c) PCEC/PProDOT-Et2 ECDs as a function of repeated scans at 100 mV s−1.

Open circuit memory is another important effect for ECDs, as the effect denotes energy consumption during the operations of ECDs [48,49]. The open circuit memory for the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2, and PCEC/PProDOT-Et2 ECDs were evaluated at ca. 590 nm with a function of time by applying potentials at a bleaching state (0 or −0.4 V) and at a coloring state (+1.4 V) for 1 s for each 200 s time interval. As shown in Figure 9, the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2 and PCEC/PProDOT-Et2 ECDs exhibit a satisfactory optical memory effect at a coloring state (<5% transmittance change) and a bleaching state (transmittance variation is insignificant), demonstrating that the PDTC/PProDOT-Et2, PS2CBP/PProDOT-Et2 and PCEC/PProDOT-Et2 ECDs display good optical memory at both bleaching and coloring states.

Figure 9

Open circuit stability of (a) PDTC/PProDOT-Et2 ECD monitored at 592 nm; (b) PS2CBP/PProDOT-Et2 ECD monitored at 590 nm; and (c) PCEC/PProDOT-Et2 ECD monitored at 586 nm.

4. Conclusions

DTC, S2CBP, and CEC were synthesized chemically and their corresponding homopolymers (PDTC, PS2CBP, and PCEC) were chemically synthesized using electrochemical polymerization. The PDTC, PS2CBP, and PCEC films show reversible electrochromic behaviors, and display a distinct color transition with four various colors in an ionic liquid solution. The PCEC film displays various colors from neutral to highly oxidized states (light gray at 0 V, cobalt green at 1.0 V, celadon green at 1.2 V, and peacock green at 1.4 V). Electrochromic switching investigations of the PDTC, PS2CBP, and PCEC films indicate that PS2CBP film has a high ΔTmax (ΔTmax = 63.56% at 1208 nm) and PCEC film has high coloration efficiency (ηmax = 293.91 cm2 C−1 at 420 nm and 214.07 cm2 C−1 at 1220 nm) in an ionic liquid solution. ECDs were also fabricated by utilizing PDTC, PS2CBP, and PCEC as anodically coloring electrochromic materials and PProDOT-Et2 as a cathodically coloring electrochromic material. The PS2CBP/PProDOT-Et2 and PCEC/PProDOT-Et2 ECDs exhibit high ΔTmax (ΔTmax = 34.45% at 590 nm for PS2CBP/PProDOT-Et2 ECD and ΔTmax = 38.25% at 586 nm for PCEC/PProDOT-Et2 ECD) and high coloration efficiency (ηmax = 256.12 cm2 C−1 at 590 nm for PS2CBP/PProDOT-Et2 ECD and ηmax = 369.85 cm2 C−1 at 586 nm for PCEC/PProDOT-Et2 ECD). As a result, the PS2CBP/PProDOT-Et2 and PCEC/PProDOT-Et2 ECDs are promising candidates for electrochromic applications.