A Cathodic Electrochromic Material Based on Thick Perylene Bisimide Film with High Optical Contrast and High Stability

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2022A Cathodic Electrochromic Material Based on Thick Perylene Bisimide Film with High Optical Contrast and High Stability Jianqiao Wang, Weitao Ma, Hailong Wang, Zengqi Xie, Huanhuan Zhang and Yuguang Ma Jianqiao Wang State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Tianhe District, Guangzhou 510640 Google Scholar More articles by this author , Weitao Ma State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Tianhe District, Guangzhou 510640 Google Scholar More articles by this author , Hailong Wang State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Tianhe District, Guangzhou 510640 Google Scholar More articles by this author , Zengqi Xie State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Tianhe District, Guangzhou 510640 Google Scholar More articles by this author , Huanhuan Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Tianhe District, Guangzhou 510640 Google Scholar More articles by this author and Yuguang Ma *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Tianhe District, Guangzhou 510640 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101021 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Cathodic electrochromic materials realized by n-type doping of conducting polymers are scarce. Even with limited cases reported in the literature, long-term stability is an urgent problem to be solved. Herein, we report a high performance and stable cathodic electrochromic material, poly(Th-Cl-PBI), based on a perylene bisimide function core. Due to its high electropolymerization efficiency and low steric hindrance of thiophene groups, perylene bisimide, which generally does not dissolve and form film easily, can grow to the sufficient thickness of 350 nm for wide-range spectral modulation and large-capacity energy storage. The optical contrast of optimized film with a thickness of 240 nm reaches as high as 69.1% at 520 nm, 94.1% at 760 nm, and 95.7% at 680 nm. Theoretical simulation based on the Lambert–Beer law also verifies this optical contrast dependent on film thickness. Besides, during the transition between the neutral state and radical anion state, it maintains 90.2% of the electrochemical activity after 4000 cycles, and the transmittance spectrum remains stable after 2000 cycles. The cation size effect on cathodic electrochromic process and cyclic stability has been investigated. Download figure Download PowerPoint Introduction Electrochromic (EC) materials, with color changing reversibly under potential bias, have been widely applied in the fields of smart windows,1–4 autodimming mirrors,5 electronic papers,6 EC display7,8 and infrared or thermal modulation9,10 due to their dynamic optical modulation and low energy cost. At present, EC materials can be divided into the following categories: (1) inorganic materials represented by tungsten trioxide, (2) metal chelates represented by phthalocyanine and bis-terpyridyl ruthenium, (3) organic small molecular materials represented by viologen, and (4) conducting polymers represented by polypyrroles, polyanilines, polythiophenes, and their derivates.11,12 Among them, conducting polymers have attracted much attention due to their high contrast ratio, fast response, high coloration efficiency (CE), and easy modification.13–16 However, conducting polymers are mostly utilized as anodic EC materials since most of them can only be positively doped (i.e., p-type doping). As a result, in anode/electrolyte/cathode sandwich EC devices, cathodes are only used for ion storage without electrochromism function. Cathodic EC materials with high performance not only increase the diversity of color, but also take full advantage of energy consumed by the cathode so as to reduce average energy consumption and improve CE. The main reason for the scarcity of such materials is that the negative doping (n-type doping) of most polymers is unstable. The stability of n-type doping is reported to be closely related to the charge trapping effect of trace H2O/O2 complex,17 the lowest unoccupied molecular orbital (LUMO) of which is about −3.6 eV and slightly changes with the specific environment.18 Therefore, a polymer with a sufficiently low LUMO level is desirable to realize stable n-type doping. Perylene bisimide (PBI) is a molecular dye commonly used in organic optoelectronic devices.19–21 It has a high molar absorption coefficient, above 104 M−1 cm−122 with LUMO of −3.8 eV,23 relatively lower than many other organic semiconductors. Chlorine substitution in the bay region will further reduce its LUMO to −4.1 eV,23 which is a good candidate for cathodic EC chromophore. Although PBI has been widely used in n-type materials in the field of organic solar cells, its film thickness can hardly exceed tens of nanometers,20,24 which limits its practical application in electrochromism and energy storage fields. The reason lies in the fact that PBI has a delocalized π-stacks structure, making it easy to aggregate and difficult to dissolve.25,26 Electropolymerization (EP), in which small molecules can easily form polymers by in situ electrocross-linking on conductive substrates, is an effective technique for tackling this problem.27–30 Herein, to obtain EP films with high thickness, thiophene units which have smaller steric hindrance and higher EP efficiency31,32 were used as the cross-linking units. The substitutions of the PBI molecule were generally at the bay region and amino sites. While the bay region can be modified for low LUMO and consequently stable n-type doping, the amino sites were used for film-forming modification. To get EP product with uniform structure, one of the active sites of thiophene was linked to the PBI core, which will only produce thiophene dimers. Moreover, thiophene dimers with low conjugation length will not interfere with the electrochromism of PBI (Scheme 1). Consequently, EP films with thickness as high as 350 nm were prepared. The maximum optical contrast reached 95.7% at 680 nm with a film thickness of 240 nm. Besides, this film had a fast response time of 1 s, high CE of 565 cm2 C−1, and good stability of more than 4000 cycles. Moreover, we studied the effect of cation size on n-doping stability. The result implies that a larger cation size will suppress the overreduction of PBI to higher doping levels that are less stable after repeated cycling. Scheme 1 | Schematic diagram of EP and doping process of Th-Cl-PBI. Download figure Download PowerPoint Experimental Methods Materials Anhydrous acetonitrile and dichloromethane with water content less than 20 ppm were purchased from J&K (Chaoyang District, Beijing, P. R. China). Tetrabutylammonium hexafluorophosphate (98%) was also purchased from J&K. Tetraethyl ammonium tetrafluoroborate (TEABF4; 98%) and tetrabutyl ammonium tetrafluoroborate (TBABF4; 99%) were purchased from Energy Chemical (Pudong New Area, Shanghai, P. R. China). The electrolytes above were recrystallized by ethanol and dried at 120 °C in a vacuum oven for 12 h before use. Tetraoctyl ammonium tetrafluoroborate (TOABF4; 97%) was purchased from Sigma-Aldrich and was recrystallized by ethanol and dried at 80 °C in a vacuum oven for 12 h before use. Other chemicals and reagents were purchased from J&K, Sigma-Aldrich (St. Louis, MO, USA), and TCI (Nihonbashihoncho, Chuo City, Tokyo, Japan) and used without any purification. Methods The NMR measurements were carried out on a Bruker DRX500 (Bruker, Karlsruhe, Germany) at 400 MHz for 1H NMR and 101 MHz for 13C NMR spectra. Autolab M204 (Metrohm, Haidian District, P. R. China) and Ocean Optics QE65PRO (Xi Pu Guang Dian, Huadu District, Guangzhou, P. R. China) were used to in situ record UV–vis spectroelectrochemistry in absorption mode in the range from 250 to 1000 nm. A quartz cuvette with the inner size of 10 mm × 10 mm × 40 mm was used as the electrochemical cell. Indium tin oxide (ITO) (9 mm × 60 mm), platinum wire, and a Ag+ (0.01 M)/Ag nonaqueous electrode were used as working, counter , and reference electrodes, respectively. The thickness was measured by a Bruker DektakXT profilometer. The Fourier transform infrared (FT-IR) spectra were recorded on a Perkins-Elmer Fourier transform infrared spectrometer (Perkins-Elmer, Liwan District, Guangzhou, P. R. China) with KBr as reference at room temperature. All the ITO glass substrates were cleaned in an ultrasonic bath and treated by O3 plasma. Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)PEDOT:PSS (Clevios AI4083) was spun on the top of the substrate with 3000 rpm spin speed for 30 s and then heated on a culture dish at 150 °C for 20 min. The thickness of this PEDOT:PSS layer was 50 nm, confirmed by a Bruker DektakXT profilometer (Bruker, Karlsruhe, Germany). The purpose of these treatments was to improve the film-forming property and stability of Th-Cl-PBI by improving the conductivity, adhesion, and compatibility of ITO.33,34 Synthesis of the monomer Th-Cl-PBI The synthesis routes of the Th-Cl-PBI monomer are shown in detail in Supporting Information Scheme S1. Results and Discussion First of all, the electrochemical behavior of monomer Th-Cl-PBI was investigated to optimize the deposition condition. The monomer solution for EP consisted of 4.0 × 10−3 M Th-Cl-PBI, 0.1 M tetrabutyl hexafluorophosphate (TBAPF6) in CH2Cl2. Figure 1a shows the first cycle of the cyclic voltammetry (CV) curve in three-electrode configuration. The onset potential of thiophene oxidation was 1.29 V. The reduction peaks between 0.3 and −0.4 V were attributed to the charge trapping effect of polythiophene during the n-doping process.35,36 The current rise around −0.5 V was caused by the reduction of PBI core.37 To maintain the independence of the PBI core, the deposition potential range was determined to be 0.23 to 1.38 V, as Figure 1b shows. As the number of cycles increased, the forward oxidation current and the backward reduction current increased continually, indicating the gradual deposition of polymer film on the substrate. The formation of cross-linking structure was also confirmed by the absorption spectra ( Supporting Information Figure S1) and FT-IR spectra ( Supporting Information Figure S2). The relationship between scan number and film thickness is given in Figure 1c. The film grew linearly when the number of cycles was below 80. As the film became thicker, the growth rate slowed down because of reduced conductivity. Nonetheless, a film with thickness as high as 350 nm was obtained. The thickness measurement data are shown in Supporting Information Figure S3. (The thickness of the PEDOT:PSS layer has been deducted.) Figure 1 | (a) Anodic CV curve of Th-Cl-PBI, 4.0 × 10−3 M in CH2Cl2, scan rate 100 mV s−1. (b) EP of Th-Cl-PBI on ITO by multicycle CV scan, scan rate 100 mV s−1. (c) Thickness dependence of poly(Th-Cl-PBI) film on number of cycles. (d) N-doping process of poly(Th-Cl-PBI) EP films in monomer-free CH3CN containing different electrolytes, concentration 0.1 M, scan rate 100 mV s−1. Download figure Download PowerPoint The electrochemical behavior of poly(Th-Cl-PBI) during the n-doping potential range in three monomer-free electrolytes, that is, TEABF4, TBABF4, and TOABF4 in acetonitrile, is shown in Figure 1d. The EC process is an electron transfer process coupled with ion transport, in which cations need to enter the film to maintain electrical neutrality during the reduction of poly(Th-Cl-PBI). Due to the hydrophobicity of the film, TOA+, which had the longest alkyl chain and thus the highest hydrophobicity, easily entered the film, resulting in the lowest reduction onset among the three electrolytes.38,39 Two reduction processes were observed in TEABF4. As the size of cation ion of the electrolyte increased, the current of the second reduction process decreased in the TBABF4 and even disappeared in TOABF4. This phenomenon was also confirmed by their corresponding spectroscopic behaviors (Figures 2a–2c). In the TEABF4 electrolyte, poly(Th-Cl-PBI) exhibited strong absorption in the visible light region with a maximum absorption peak at 520 nm and a shoulder peak at 490 nm, corresponding to the S0–S1 transition of the perylene core in neutral state. The color of such neutral film was orange red. When the applied potential decreased to −0.6 V, absorption in the visible light region greatly decreased and two new strong absorption bands in the near infrared spectral region with absorption maximum at 760 and 920 nm appeared which belonged to the PBI anion radical (Cl-PBI−).37,40 Meanwhile, the film became almost transparent. When a more negative potential −1.0 V was applied, the absorption of the PBI anion radical decreased, and new absorption band with maximum width of 680 nm appeared, indicating the formation of PBI dianion (Cl-PBI2−),37,41 under which condition the film turned turquoise. In the neutral state, the spectra of PBI in the three electrolytes were almost the same. However, the absorption peak of the dianion state at 680 nm decreased in the TBABF4 electrolyte, and almost completely disappeared in the TOABF4 electrolyte at −1.0 V, reflecting the difference of the maximum doping degree among the three electrolytes. This phenomenon was probably caused by steric hindrance of electrolyte ions.42,43 As the cation size in electrolyte increased, only part of the electrolyte could be injected into the film to maintain charge balance, thus constraining the PBI core at the stable anion radical state. Therefore, for its high reaction degree and fast response, the TEABF4 electrolyte with small cation size was chosen. Its stability contrast was also investigated and is discussed in the following section. Figure 2 | Spectroelectrochemistry of poly(Th-Cl-PBI) (110 nm) in different monomer-free electrolytes (a) TEABF4, (b) TBABF4, and (c) TOABF4. Download figure Download PowerPoint As is mentioned above, poly(Th-Cl-PBI) can be prepared with various thicknesses for electrochromism. But it should be noted that optical contrast does not exactly increase monotonically with the increase of thickness, especially for materials having absorption simultaneously in colored and bleached states in the same wavelength range. The relationship between optical properties and thickness can be determined by the Lambert–Beer law, from which the optimized value of optical contrast can be estimated,44,45 as eq 1 shows. Δ T max = 10 − A b − 10 − A c = 10 − k b D − 10 − k c D (1)where ΔTmax is the maximum optical contrast. Ab and Ac represent absorption of bleached and colored states, respectively. kb and kc represent equivalent linear absorption coefficients of bleached and colored states respectively. D is the thickness of the film. As optical contrast represents the difference between the transmittance of two states, “colored state” and “bleached state” are used to represent the maximum and minimum absorption states at a specific wavelength, respectively, for the convenience of calculation. To be specific, they represent the neutral state and dianion state at 520 nm (the maximum optical contrast originating from the contrast of spectra at 0 and −1 V.), radical anion state and neutral state at 760 nm (the maximum optical contrast originating from the contrast of spectra at 0 and −0.7 V.), and dianion state and neutral state at 680 nm (the maximum optical contrast originating from the contrast of spectra at −1 and 0 V). Thus, equivalent linear absorption coefficient k can be calculated according to k = A/D, which is derived from Lambert–Beer’s law, using the absorbance of the thinnest film (70 nm), as shown in Table 1. At the thinnest state, poly(Th-Cl-PBI) is assumed to be completely doped or dedoped. In addition, the thickness measurement of several samples before and after doping proves that doping does not cause obvious thickness change in this system, as shown in Supporting Information Figure S4. Table 1 | Equivalent Linear Absorption Coefficients of PBI, PBI−, and PBI2− by Calculation Wavelength (nm) kb (cm−1)a kc (cm−1)b 520 4831 42,875 680 450 64,083 760 530 60,833 aEquivalent linear absorption coefficients of Th-Cl-PBI in bleached state. bEquivalent linear absorption coefficients of Th-Cl-PBI in colored state. Figure 3 shows theoretical curves of optical contrast with thickness established by eq 1. According to the theoretical curve, the optical contrasts at three wavelengths, 520, 680, and 760 nm, all reach the maximum at the thickness of about 240 nm. To verify this, we selectively tested the optical contrasts of several EP films with different thicknesses (70, 140, 160, 240, and 350 nm). Experimental data fit well with theoretical predictions. Only at the thickness of 350 nm was the experimental value slightly lower than the theoretical value, which was probably caused by incomplete doping and dedoping when film thickness was very high. For the optimized EP film of 240 nm, detailed square-wave potential step tests were performed to investigate the transmittance change with corresponding color changes in Figures 4a–4d. The optical contrast between colored and bleached states was found to be 69.1% at 520 nm, 94.1% at 760 nm, and 95.7% at 680 nm. Commission Internationale de L'Eclairage 1931 chromaticity is shown in Figure 4e. The corresponding chromatic coordinates are shown in Supporting Information Table S1. Figure 3 | Optical contrast of poly(Th-Cl-PBI) in TEABF4 varying with thickness by calculations. Download figure Download PowerPoint In addition to high optical contrast, poly(Th-Cl-PBI) also exhibited excellent performances in other aspects, as shown in Table 2. It had a fast response of 1.0 s at 520 nm, 2.6 s at 760 nm, 3.1 s at 680 nm for coloring; and 2.3 s at 520 nm, 1.4 s at 760 nm, 2.1 s at 680 nm for bleaching. Besides, the CEs at these three wavelengths were 316.02, 503.81, 564.69 cm2 C−1, respectively. Compared with other high performance EC materials in recent years, poly(Th-Cl-PBI) not only had high optical contrast, but also had high CE that was superior to inorganic EC materials and most organic EC materials, as shown in Table 3. Table 2 | EC Properties of Poly(Th-Cl-PBI) (240 nm) in TEABF4 λ (nm)a ΔT (%) tc (s)b tb (s)b ΔODc Qd (mC cm−2)d CE (cm2 C−1)e 520 69.1 1.0 2.3 0.914 2.892 316.02 760 94.1 2.6 1.4 1.457 2.892 503.81 680 95.7 3.1 2.1 1.689 2.991 564.69 aWavelength of the absorption maximum. btc and tb denotes the coloring time and bleaching time, respectively. cOptical density (ΔOD) = log(Tbleached/Tcolored), where Tbleached and Tcolored denote the maximum transmittance in the neutral and reduced states, respectively. dQd is the injected charge for color change. eCE = ΔOD/Qd. Table 3 | Comparison between Poly(Th-Cl-PBI) and Other Reported High Performance EC Materials Materials Electrolyte Wavelength (nm) ΔT (%) tc (s) tb (s) Retention (%)/cycles Thickness (nm) CE (cm2 C−1) Film Preparation Method “Nano to nano” WO346 LiClO4 633 92.0 9 15 76/1000 – 51 EDa DPB47 Fc 517 72.2 8.1 13.3 96.7/50 – 102 CPb PPHEN48 TBAPF6 1460 99.0 1 1 – – 174 CP ProDOTBz249 TBABF4 632 89.0 0.4 ∼ 0.6 0.4 ∼ 0.6 – – 600 EP ECP black50 LiClO4 580 85.5 1.5 1.8 77/500 1800 44 CP PI-2a51 TBAP 760 96.2 1.3 1.1 – 360 78 CP Poly(Th-Cl-PBI) TEABF4 680 95.7 3.1 2.1 90.0/2000 240 565 EP aElectrodeposition. bChemical polymerization. Figure 4 | (a) Spectroelectrochemistry of poly(Th-Cl-PBI) (240 nm) in TEABF4 electrolyte; square-wave potential step absorptiometry of poly(Th-Cl-PBI) films monitored at (b) 520 nm, from 0 to −1.0 V vs Ag+/Ag, (c) 760 nm, from 0 to −0.7 V vs Ag+/Ag, and (d) 680 nm, from 0 to −1.0 V vs Ag+/Ag. (e) CIE 1931 chromaticity diagram of poly(Th-Cl-PBI) varying with potential. Download figure Download PowerPoint The cyclic stability of the EC film was investigated by CV. Cyclic stability between dianion and neutral states is shown in Figure 5a. As the LUMOs of the anion and dianion states of PBI got close to each other, overlaps in two redox process appeared in the CV curve. In addition, with the increase of the thickness of the film, the conductivity decreased, and the polarization increased, which made the effect more obvious. Finally, the initial two peaks in thin film (105 nm) merged into one. Nevertheless, according to the characteristic spectra and color change, the film reached the complete dianion state under the applied potential of −1.0 V. The reduction peak current decreased to 90.0% after 2000 cycles. As discussed above, the cation size of the electrolyte had quite an influence on the doping degree, which strongly affected the cyclic stability. Therefore, the TOABF4 electrolyte was also selected to test the cyclic stability. During the first 1000 cycles, the current gradually increased and then decreased in the following cycles, as shown in Figure 5a. This initial increase was attributed to the unbound structure with the electronic percolation inside the electrode reaching its maximum after 1000 cycles.52 As a result, after 4000 cycles, the electrochemical activity still maintained 90.2%. However, due to the relatively slower kinetic behavior and lower doping degree caused by TOABF4, poly(Th-Cl-PBI) showed inferior EC performance to that in TEABF4, as indicated in Supporting Information Figure S5 and Table S2. Therefore, there was a tradeoff between EC performace metrics and longterm cyclic stability. The spectra cyclic stability under different conditions is shown in Figures 5b–5d. The retention ratio of optical contrast after the stability test is shown in Supporting Information Table S3. At the same potential, −1.0 to 0 V (switching time 20 s), the stability in TOABF4 (Figure 5c ) was much better than that in TEABF4 (Figure 5b). Poly(Th-Cl-PBI) in TEABF4 maintained 87.7% of the initial optical contrast at 520 nm and 94.4% at 680 nm after 500 cycles (Figure 5b). While in TOABF4, after 2000 cycles, the optical contrast did not decrease, but increased (138.2% at 520 nm and 102.2% at 760 nm) (Figure 5c). This was also consistent with the effect of current increase in CV curves (Figure 5a). The doping degree of the films in the two electrolytes was almost the same, that is, the initial optical contrast was the same, and there was no attenuation in TEABF4 after 2000 cycles as well (113.0% at 520 nm and 103.7% at 760 nm) (Figure 5d). The corresponding transmittance and current changed with time as shown in Supporting Information Figures S6 and S7. In addition, by comparing the spectra before and after the cyclic stability test, it was found that a high doping degree (dianion state) will lead to side reaction, resulting in irreversible formation of non-EC species, as shown in Supporting Information Figure S8. Because it is not easy to precisely control the potential in practical applications compared with the approach of reducing working potential, using large-size TOABF4 electrolyte can effectively inhibit the overdoping and thus improve stability. Figure 5 | (a) CV curves of poly(Th-Cl-PBI) (240 nm) in 0.1 M TEABF4 and TOABF4 electrolyte, scan rate 100 mV s−1. Transmittance of poly(Th-Cl-PBI) (240 nm) varying with number of cycles under square-wave potential step absorptiometry with switching time 20 s in (b) TEABF4 at 520 and 680 nm, −1.0 to 0 V, (c) TOABF4 at 520 and 760 nm, −1.0 to 0 V, (d) TEABF4 at 520 and 760 nm, −0.63 to 0 V. Propylene carbonate with high boiling point was used as solvent to prevent the interference of solvent volatilization during the stability test. Download figure Download PowerPoint Conclusion High performance cathodic electrochromism has been realized by poly(Th-Cl-PBI), which changes from orange red to nearly transparent to turquoise. The thiophene group with high polymerization efficiency was utilized as the oxidation coupling unit to prepare film as thick as possible to achieve high optical contrast. The maximum optical contrast was obtained at the thickness of 240 nm, reaching as high as 69.1% at 520 nm, 94.1% at 760 nm, and 95.7% at 680 nm, respectively. Therorectical simulation based on the Lambert–Beer law also verified the optical contrast dependence on film thickness. Poly(Th-Cl-PBI) EP film also showed high CE of 564.69 cm2 C−1. Meanwhile, excellent cyclic stability of 90.0% retention after 2000 cycles was observed. Better electrochemical stability of 90.2% retention after 4000 cycles and stable transmitance spectra after 2000 cycles can be achieved when electrolytes of larger cation size are used to constrain poly(Th-Cl-PBI) in a moderate doping degree. Therefore, we report a cathodic EC polymer based on thick PBI film with high performance that regulates electrolyte selectivity on doping degree and stability. Supporting Information Supporting Information is available and includes synthesis of the monomer, comparison of the spectra of the monomer and the polymer and spectroelectrochemical data. Conflict of Interest There is no conflict of interest to report. Acknowledgments This research supported by the National Natural Science Foundation of China (grant nos. 51521002 and 21905098), the China Postdoctoral Science Foundation (grant no. 2018M643067), and the 21Hong Kong-Macao Joint Laboratory of Optoelectronic and Magnetic Functional Materials (grant no. 2019B121205002).