A Strategy of Stabilization via Active Energy-Exchange for Bistable Electrochromic Displays

Open AccessCCS ChemistryRESEARCH ARTICLE5 Aug 2022A Strategy of Stabilization via Active Energy-Exchange for Bistable Electrochromic Displays Chang Gu†, Xiaojun Wang†, Ai-Bo Jia, Hongzhi Zheng, Weiran Zhang, Yuyang Wang, Minjie Li, Yu-Mo Zhang and Sean Xiao-An Zhang Chang Gu† State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 †C. Gu and X. Wang contributed equally to this work.Google Scholar More articles by this author , Xiaojun Wang† National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210009 †C. Gu and X. Wang contributed equally to this work.Google Scholar More articles by this author , Ai-Bo Jia State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Hongzhi Zheng National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210009 Google Scholar More articles by this author , Weiran Zhang State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Yuyang Wang State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Minjie Li State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Yu-Mo Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author and Sean Xiao-An Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101180 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail As future energy-saving optoelectronics, bistable electrochromic (EC) materials/devices have high energy efficiency for potential applications as smart windows, displays, and information/energy storage, due to their ability to maintain optical states without consuming energy. However, further development is hindered by the lack of in-depth understanding of related key factors and universally applicable design strategies to achieve bistability. Herein, we report a new strategy based on active energy-exchange with the aid of proton-coupled electron transfer, which can dynamically adjust the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels of materials to obtain good bistability from traditional nonbistable materials. This strategy was thoroughly studied and proven by taking quinone derivatives and bromocresol green derivatives as examples. The device obtained after further polymerization and optimization showed remarkable bistability, coloration efficiency, and application potential for energy-saving flexible displays. The success, challenges, and cognitive gains of this strategy not only accelerate the development of various energy-saving optoelectronic materials/devices, but are also likely to stimulate progress in physics, chemistry, and materials. Download figure Download PowerPoint Introduction The rapid development of modern society is inseparable from the contribution of electronic technology. Countless photoelectric devices not only provide nonnegligible convenience in our daily lives, but inevitably consume enormous amounts of energy. Thus, developing a novel energy-saving material/device, such as a bistable material/device that can maintain its working state (physical properties) without supplying electricity, can be considered a highly energy-efficient method to balance energy consumption and the requirements of modern digital life. Bistable electrochromism, which has two or more stable optical states switched by an electric field and is also known as electrochromism with attractive memory effect, has demonstrated this possibility due to the fact that no additional energy consumption is required to maintain expected optical states. The fabricated electrochromic devices (ECDs) have shown remarkable potential in smart windows,1–4 recordable information displays,5,6 and visual energy storage.7–9 Unfortunately, their practical application is still far away. The main impediments to this leap-forward development and industrialization of this field are the lack of a clear and in-depth understanding of the intrinsic key factor of bistability, as well as universally applicable design strategies for the relevant materials/devices (especially at the molecular or atomic scale). As is well known, the optical color/transmittance switch of EC materials is derived from the electroredox reaction stimulated by an external electric field. And their energy states will inevitably change from stable low-energy states (natural states) to unstable high-energy states after electroredox. Therefore, how to stabilize the related high-energy states is the core as well as the difficulty of designing bistable ECDs. To achieve this goal, most researches are aimed at optimizing the structures of EC materials by physical or chemical methods, such as: stabilizing after-redox states by inter-/intramolecular hydrogen bonds5,6; limiting unwanted thermal diffusion of active EC molecules by polymerization, or being anchored to nanomaterials10–12; reducing leakage current by introducing insulation layers (Ta2O5)13; adjusting the ion matching of electrolytes to improve the reaction rate and stability14; dynamically regulating ion conductivity by controlling the arrangement of liquid crystal15; and so on.16,17 One of the most representative studies, reported by Kim et al.,18,19 achieved improved bistability by controlling interfacial charge transport between EC materials and indium tin oxide (ITO) electrodes via the well-designed functional group for low highest occupied molecular orbital (HOMO) energy levels. However, those bistable EC materials/devices still exist in the relatively unstable high-energy states after electroredox. To solve this problem, we must find a novel way to rapidly convert these high-energy states to stable low-energy states without affecting EC properties. That is, if we can achieve a distinctive energy stabilization in different optical states with active energy-exchange, the inherent disadvantages of high-energy states of the traditional EC mode would be eliminated fundamentally. Proton-coupled electron transfer (PCET) is an efficient energy-transfer and adjustment strategy in photosynthesis and other life processes, which can rapidly change/adjust the reaction potential and energy state of active elements in an orderly fashion according to the needs of life activities.20,21 In our previous work, the PCET has been successfully applied in designing bistable EC materials/devices by well-designed hydrogen bonds.5,6 However, the specific hydrogen bonds still limited the extension of bistable EC materials. Herein, we fabricated bistable ECDs from nonbistable materials by PCET, taking quinone derivatives (electroactive molecules) and bromocresol green derivatives (proton donors) as examples (Scheme 1a), which are classic materials without bistability by known EC methods. The role of PCET in adjusting the frontier molecular orbital levels [HOMO/lowest unoccupied molecular orbital (LUMO)] of these EC materials was studied systemically. Fortunately, we found that such an active energy-exchange mode successfully induced the relevant EC materials to the expected low-energy (stable) states before/after electroredox and effectively endow the traditional nonbistable EC materials with the long-awaited bistability. In addition, the factors of bistability, including thermal diffusion, leakage current, and chemical stability, were comprehensively studied and optimized. Finally, we demonstrated a high-performance flexible ECD (8 × 8 cm2) with remarkable bistability (5.9 h) and high coloration efficiency (597 cm2/C), and discussed its multiple potential applications for next-generation energy-saving flexible displays. Scheme 1 | (a) Chemical structures of quinone derivatives and bromocresol green derivatives. (b) The PCET mechanism of BQ + BG-Na system. Download figure Download PowerPoint Experimental Methods The materials involved for the semisolid ECD To fabricate classic semisolid ECDs, quinone derivatives and bromocresol green derivatives were used as EC materials. Polymethyl methacrylate (PMMA), propylene carbonate (PC), and tetrabutylammonium hexafluorophosphate (TBAPF6) were introduced as the supporting polymer skeleton, plasticizer, and electrolyte, respectively. 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) was selected as an electroactive anode material to balance charges in the ion storage layer due to its excellent redox reversibility ( Supporting Information Figure S4). Furthermore, poly(ionic liquid)-bromocresol green (PIL-BG) and p-TEMPO were synthesized to replace BG-Na and TEMPO, respectively, to inhibit thermal diffusion of small organic molecules. Relative molecular design and synthesis are listed in Supporting Information Method 4, Figures S15–S19, S27–S35, and Tables S1–S4. The fabrication of the semisolid ECD First, the ion storage layer material was deposited by blade coating on the first ITO. Then, the ion conductive layer material was deposited by blade coating on the top of the ion storage layer. Second, the EC layer material was deposited by drip coating on the second ITO. Extra solvent was removed by heating. Finally, the two ITO were assembled together, as shown in Supporting Information Figure S3. It should be noted that polyethylene terephthalate (PET)-coated ITO was used for flexible devices. And detailed material parameters are listed in Supporting Information Method 5. Results and Discussion Active energy-exchange for bistable electrochromism In this work, p-benzoquinone (BQ, electroactive molecule) and bromocresol green derivatives (pH-sensitive molecules, proton donors) containing thymol blue sodium salt (TB-Na), bromothymol blue sodium salt (BB-Na), bromocresol purple sodium salt (BP-Na), and bromocresol green sodium salt (BG-Na) with different donor and acceptor groups were chosen to form required PCET-induced cathode EC systems (Scheme 1a). Taking BQ and TB-Na as an example, spectroelectrochemical tests were performed ( Supporting Information Method 3 and Scheme S2). The mixture solution was yellow (absorption peak: 391 nm), which corresponded to the characteristic absorption of TB-Na (Figure 1a). When −0.45 V (vs Ag wire, the reduction potential of BQ under this condition according to the follow-up experiment) was applied, a new strong absorption at 613 nm appeared, along with the decrease of absorbance at 391 nm and a color change from yellow to green (green curve). The spectral and color changes were almost identical with the stimulation of the chemical base (tBuONa) for transforming into TB-2Na. As a contrast, the UV–vis spectrum of TB-Na barely changed under −0.45 V, if there was no BQ added (red curve). This indicated that TB-Na had no EC phenomenon under this voltage. In addition, although the solution of BQ was reduced by electrical stimulation, the intensity of spectral change was relatively weak (brown curve) and did not occur at 613 nm (belonging to TB-2Na). Those phenomena also appeared in solutions of BQ and BB-Na/BP-Na/BG-Na with typical color changes from yellow to green ( Supporting Information Figure S1), which meant that the proton of bromocresol green derivatives could be captured by reduced BQ as a typical PCET process.22,23 This process was further verified by monitoring the spectral kinetics during the redox process ( Supporting Information Note 1 and Figure S2). Figure 1 | Active energy-exchange and bistable electrochromism. (a) UV–vis spectra of TB-Na (8.0 × 10−4 mol/L), BQ (2.0 × 10−4 mol/L), and their mixture solution before and after electrical stimulation (−0.45 V) in CH3CN, compared with TB-Na (8.0 × 10−4 mol/L) stimulated by 1.0 equiv tBuONa. TBAPF6 (0.1 mol/L) was used as supporting electrolyte. Insert: photos of the BQ + TB-Na solution before and after electrical stimulation. (b) The change of absorbance (TB-Na: 613 nm, BB-Na: 632 nm, BP-Na: 599 nm, and BG-Na: 623 nm) of the semisolid ECDs with different EC materials under the electrical stimulation with −1.2 V for 3.0 s and corresponding maintain ratio of colored states after 200 s. (c) The CVs of BQ (1.0 × 10−3 mol/L), BG-Na (1.0 × 10−3 mol/L), and the mixture of BQ (1.0 × 10−3 mol/L) and 4.0 equiv BG-Na in CH3CN. All the scan rates are 100 mV/s. TBAPF6 (0.1 mol/L) was used as supporting electrolyte. (d) aThe HOMO energy level of the reduced state (colored state) and bthe LUMO energy level of the neutral state (bleached state) of different EC systems. (e) The scheme of self-coloring and -bleaching phenomena in ECDs due to interfacial electron transfer between EC materials and ITO electrodes, and relative energy-level relationships for unstable/bistable cathode EC materials. (f) The scheme of active energy-exchange strategy for bistability via the dynamically adjusting energy levels induced by PCET. Download figure Download PowerPoint To our surprise, different pH-sensitive molecules presented distinguishing maintain times when BQ was used as the electroactive molecule in classic semisolid ECDs (Figure 1b). And from TB-Na to BG-Na, the maintain ratios of colored states showed stepped improvement. This tendency was consistent with the enhancement ability of the proton donors to release protons. To understand these phenomena in depth, the redox properties of the aforementioned EC systems were fully measured by cyclic voltammetry (CV) ( Supporting Information Method 3 and Scheme S1), represented by “BQ + BG-Na” (Figure 1c). BQ had a typical reduction peak at −0.52 V [vs saturated calomel electrode (SCE)], and the reduction peak of BG-Na existed in more negative potential, which meant that BQ was easier to electroreduce than BG-Na. As BG-Na was added, the redox peaks of BQ shifted gradually ( Supporting Information Note 2 and Figure S5). And it required at least 3.7 equiv of BG-Na for BQ to achieve full regulation. These phenomena made us realize that the PCET process must involve a synergistic interaction of multiple molecules, rather than a simple one-to-one interaction. That is probably due to the powerful ability of BG-Na to produce protons and form hydrogen bonds, along with the excellent proton capture of the reduced BQ. In addition, there are a series of functional groups (such as –SO3−, –Br, –OH, –C=O, etc.) that produce strong steric hindrance and various strong intra-/intermolecular supramolecular interactions (e.g., hydrogen bonds, electrostatic interactions of ions, dipole–dipole interactions). Therefore, such an EC molecular system is likely to form a molecular aggregate (maybe a “supramolecular cage”) by self-assembly of multimolecules with a ratio of “1:3.7” (“BQ:BG-Na”). These interactions are also very likely to be the reason why this EC system exhibits enhanced PCET and remarkable electrochemical redox regulation, which has not been found in our previous work. Unfortunately, the above inferences about the dynamic structural state of the “supramolecular cage” were not confirmed experimentally in real time, due to the fragile nature of the molecular aggregate structure and the lack of suitable testing equipment. In fact, this is an indisputable fact that has long hindered the deeper understanding and faster development of the materials and life sciences. We look forward to more and more researchers joining this technological challenge to quickly improve the precision of the testing instruments, achieve more in-depth understanding of the materials and processes, and develop the tremendous potential of bistable EC displays. In the subsequent experiments, an approximate ratio of 1∶4 (BQ:BG-Na) was used. Under this ratio, the reduction potential of BQ shifted from −0.52 to −0.40 V (shift range: 0.12 V), and the relative reoxidation potential of reduced BQ shifted from −0.45 to 0.50 V (shift range: 0.95 V), as shown in Figure 1c. The difference in the shift range between reduction potential and re-oxidation potential was caused by the different hydrogen bonds and proton-capturing capabilities of BQ and reduced BQ. And we propose that this is about a proton coupled two-electron transfer process of BQ and BG-Na, inspired by previous reports.24–27 In this process, BQ would be electroreduced and capture the proton of BG-Na to form hydroquinone, and BG-Na would be transformed into BG-2Na, as shown in Scheme 1b. Similarly, other EC materials also showed similar regulation of redox potentials ( Supporting Information Figure S6). Meanwhile, the stronger the ability of bromocresol green derivatives (proton donors) to release protons from TB-Na to BG-Na, the stronger the strength of the PCET effect and the potential-shifting regulation it produced which also corresponded to better bistable properties (Figure 1b). To further understand bistability, the LUMO energy level of the neutral state (bleached state) and the HOMO energy level of the reduced state (colored state) of these cathode EC systems were calculated based on aforementioned CVs (Figure 1d and Supporting Information Method 6 and Table S5). Results revealed that all related LUMOs and HOMOs presented a step-down trend from BQ + TB-Na to BQ + BG-Na. And the change in HOMOs was more obvious (from −4.25 to −4.71 eV, compared with its LUMOs change from −3.95 to −4.17 eV). Interestingly, all LUMOs were higher than EF of ITO (−4.7 eV),18,28 which were used as the electrodes in the ECDs. Thus, for all these EC systems, the interfacial electron transfer from ITO to EC materials in their neutral states was limited, and the unwanted self-colorings were efficiently avoided. However, for most EC systems, their HOMO energy levels of reduced states were higher than EF of ITO. So, self-bleaching happened due to spontaneous electron transfer from EC materials to ITO, as shown in Figure 1e. But for BQ + BG-Na, its HOMO (−4.71 eV) is lower than EF, which means that the unwanted interfacial electron transfers could be forbidden in both colored and bleached states. And the as-prepared ECD precisely showed remarkable bistable properties (Figure 1b). Moreover, controlling the proton-capturing ability of proton acceptors could also be applied in adjusting related energy levels of EC materials and bistable performance of ECDs. Hereinto, tetrachlorobenzoquinone (BQCl4) and coenzyme Q0 (Q0) as electroactive molecules were introduced to replace BQ. The newly-introduced electroactive molecules, combined with BG-Na, produced similar PCET and EC abilities (as shown in Supporting Information Figure S7), and achieved regulating ability in HOMO/LUMO energy levels (brown and turquoise stars in Figure 1d). Besides, due to the lower HOMO of the reduced state (colored state) and higher LUMO of the natural state (bleached state) than EF of ITO for Qo + BG-Na, this EC system also showed considerable bistability as we expected ( Supporting Information Figure S8), just like BQ + BG-Na. Based on the above results for bistable ECDs, the spontaneous unwanted electron transfer between electrodes and EC materials should be forbidden to stabilize both neutral states and after-redox states of EC materials. Therefore, cathode EC materials should be designed with higher LUMO of the neutral state and lower HOMO of the reduced state than EF of electrodes. Similarly, anode bistable EC materials can be built by designing lower HOMO of the neutral state and higher LUMO of the oxidized state. A complete mechanism model is shown in Figure 1e and Supporting Information Figure S9. However, based on Laviron equations,29 the separation of the aforementioned HOMO/LUMO energy levels for reversible electroactive molecules, which is consistent with potential separation in CVs (ΔE), is about 59/n mV only. Here, n represents the transferred electron number during the redox reaction of each molecule. So, it is hard to distribute the two-frontier molecular orbital (HOMO/LUMO) levels to different sides of EF of electrodes. That is to say, for most organic EC materials based on direct electron transfer mechanisms, either self-bleaching or self-coloring is generally exhibited. In this case, our “active energy-exchange” strategy possesses its own advantage in adjusting the energy levels of after-redox molecules to ideal low/stable states for bistability through rapid and active energy exchange (just like a cascade to release/transfer energy), which is induced by multimolecular synergistic PCET (Figure 1f). It should be noted that this strategy has a wide range of material compatibility (not depending on fixed molecular structures, but only the intermolecular interaction and modulated energy levels). Furthermore, not only this PCET process, but the rest of the potential pathways for dynamic adjustment of energy levels, including complexation/dissociation interaction, bond formation/broken conversion, energy transfer, and so on, may also be developed in future as triggering mechanisms of the bistability of ECDs. At the same time, we believe that more and more optoelectronic systems, such as traditional EC systems, dye-sensitized solar cells, batteries and capacitors, and so on, will also be developed into better functionality and energy-saving characteristics, inspired by this mechanism.30,31 Optimization of bistable EC properties To achieve optimal performances and meet the requirements of future energy-saving displays, other factors influencing bistability were systemically considered and further optimized based on the aforementioned BQ + BG-Na system. First, we found that this system had excellent chemical stability, which could maintain its optical states more than a week in both bleached and colored solutions ( Supporting Information Figures S10 and S11 and Scheme S3). However, its EC performance was still unsatisfactory, including its optical modulation (ΔT), bistability, and color-erasing ability ( Supporting Information Note 3 and Figure S12), due to the unwanted thermal diffusion of small organic molecules inside the device.32,33 Thus, PIL-BGs were designed and synthesized to replace BG-Na (Figure 2a and Supporting Information Method 4, Figure S15–S17, S28–S34, and Tables S1–S3).34 The PIL backbone of PIL-BGs provided good film-forming ability and ion conductivity, along with the electrostatic interaction with BG− for reduced thermal diffusion compared with BG-Na. Meanwhile, p-TEMPO was introduced into ion storage materials by a similar method ( Supporting Information Method 4, Figures S18, S19, and S27).35 Thermogravimetric analysis (TGA) was performed to investigate the thermal stability of the as-prepared polymers ( Supporting Information Figures S13 and S14). Results showed that the decomposition temperatures for PIL-BGs and p-TEMPO were above 240 °C, which was adequate for EC displays. The semisolid ECD based on PIL-BGs and p-TEMPO showed significant color change between yellow and green, along with the transmittance change at 623 nm (Figure 2b), in accordance with a small molecule-based device. More importantly, the device exhibited exciting bistable properties (Figure 2c). Its transmittance in the colored state was almost unchanged after 1200 s (ΔT decay ratio < 1.7%). Figure 2 | The further optimization of bistable ECDs. (a) The structure of the optimized semisolid ECD including PIL-BG in EC layer and p-TEMPO in ion storage layer. (b) The UV–vis spectra of the bistable ECD without electrical stimulation, with electrical stimulation of −1.2 V for 5 s and then +1.1 V for 60 s. Inset: photos of the bistable ECD. (c) The transmittance changes at 623 nm of the device based on PIL-BG (bottom) under relative electrical stimulation (top). The transmittance, coloring-switch times (red solid star), and decay ratios over 1200 s (green hollow star and green word) of the device based on different PIL-BGs (d and e), different ratios of BG−:BQ (f), and different thicknesses of EC layers (g) under the electrical stimulation with −1.2 V for 5 s. (h) The maintain time of the colored state (the decay ratio of ΔT = 10%) after −1.3 V for 10 s, and corresponding leakage current of ECDs with different thicknesses of ion conductive layers under the electrical stimulation with −1.3 V. Download figure Download PowerPoint Then, a series of PIL-BGs with different ratios of acrylonitrile and imidazolium salt containing PF6− and BG− anion (as “x:y:n”) were synthesized to explore the best composition ( Supporting Information Method 4). Optical modulation ability (ΔT), response speed (switch time), and the bistability (decay ratio) of ECDs (defined in Supporting Information Note 4 and Figure S20) were chosen as evaluation indices. As shown in Figure 2d, when fixing the similar ratio of PF6− and BG− (x:y), the ΔT increased initially with the increasing ratio of acrylonitrile due to the improvement of film-forming ability ( Supporting Information Figure S21), and then decreased due to the reducing proportion of BG−. The different ratios of PF6− and BG− showed similar trends (Figure 2e and Supporting Information Figure S22). Thus, the best ratio of x:y:n in PIL-BGs was confirmed as 6.5:14.8:100, and the weight ratio of BG− was about 29.7 wt % (PIL-BG-7 in Supporting Information Table S3). Since the bistable electrochromism was caused by the PCET between BQ and BG−, its ratio undoubtedly affected EC performance of as-prepared ECDs. As shown in Figure 2f, ΔT increased with the increase of BQ content, and the BG−:BQ with 4∶4 had the shortest coloring-switch time, which was selected for subsequent studies. On the other hand, the thickness of the EC layer determined the total amount of EC materials in the per-unit area, which also influenced the device properties, for example, ΔT and the response speed. Thus, the optimal thickness of the EC layer was also confirmed as 1.32 μm, due to its relatively large ΔT, small decay ratio, and short coloring-switch time (Figure 2g). Furthermore, the effect of polymer molecular weight (Mw) on EC performance ( Supporting Information Figure S24) was explored. Here, a series of PIL-BGs with similar aforementioned optimal composition, but different Mw (from 76,545 to 122,948), were successfully synthesized by controlling the amount of initiator in the polymerization process ( Supporting Information Method 4, Figure S35, and Table S4). Results showed that the EC performance of these PIL-BGs, including ΔT, coloring-switch time, and decay ratio over 300 s, exhibited a slight difference. Therefore, it can be concluded that the Mw of related materials does not seriously affect the performance in this range. Besides, we also explored the optimal ratio of PMMA/[Bmim][PF6] inside the device for its dramatic influence in ion conductivity and mobility ( Supporting Information Note 5 and Figure S23). To optimize the maintain time of the colored state, the leakage current, as an important basis for judging the charge exchange inside ECDs, should be avoided. The intensity of leakage current is dependent on the generated inner electric potential during the color-changing process, and the distance between electrodes.13,36,37 Many novel strategies for reducing leakage current may be used to optimize future bistability, for instance, developing the film (electrolyte) with high ion conductivity but low electron conductivity. Herein, bistable ECDs with different thicknesses of ion conductive layers were fabricated, and their corresponding maintain time of the colored stat