A novel bis(dihydroxypropyl) viologen-based all-in-one electrochromic device with high cycling stability and coloration efficiency

Polyvinyl Butyral Solid Electrolyte Film and Its Electrochromic Laminated Safety Glass

In recent years, the application of electrochromic laminated safety glass has attracted more and more attention, relying on polyvinyl butyral (PVB) solid electrolyte film. Herein, the ionic conductivity (σ) of the PVB film has been improved by a cross-linked structure and blended with LiClO4, which can reach as high as 1.78 × 10-4 S cm-1 at room temperature. In addition, their excellent comprehensive characteristics have been confirmed, such as mechanical strength, high visible light transmittance (>90%), high bond strength (4.2 MPa), and excellent thermal stability. Based on the PVB film above, WO3-laminated electrochromic devices with 5 × 5 cm2 and 10 × 10 cm2 areas are constructed. They can remain stable after 20 000 cycles monitored by cyclic voltammetry curves, indicating the PVB solid polymer electrolyte (PSPE) with a cross-linked structure has the potential commercial viability of large-area electrochromic devices (ECDs).

Resonant-Cavity-Enhanced Electrochromic Materials and Devices

With rapid advances in optoelectronics, electrochromic materials and devices have received tremendous attentions from both industry and academia for their strong potentials in wearable and portable electronics, displays/billboards, adaptive camouflage, tunable optics, and intelligent devices, etc. However, conventional electrochromic materials and devices typically present some serious limitations such as undesirable dull colors, and long switching time, hindering their deeper development. Optical resonators have been proven to be the most powerful platform for providing strong optical confinement and controllable lightmatter interactions. They generate locally enhanced electromagnetic near-fields that can convert small refractive index changes in electrochromic materials into high-contrast color variations, enabling multicolor or even panchromatic tuning of electrochromic materials. Here, resonant-cavity-enhanced electrochromic materials and devices, an advanced and emerging trend in electrochromics, are reviewed. In this review, w e will focus on the progress in multicolor electrochromic materials and devices based on different types of optical resonators and their advanced and emerging applications, including multichromatic displays, adaptive visible camouflage, visualized energy storage, and applications of multispectral tunability. Among these topics, principles of optical resonators, related materials/devices and multicolor electrochromic properties are comprehensively discussed and summarized. Finally, the challenges and prospects for resonant-cavity-enhanced electrochromic materials and devices are presented.

Synergistic Interaction of Dual-Polymer Networks Containing Viologens-Anchored Poly(ionic liquid)s Enabling Long-Life and Large-Area Electrochromic Organogels

Viologens-based electrochromic (EC) devices with multiple color changes, rapid response time, and simple all-in-one architecture have aroused much attention, yet suffer from poor redox stability caused by the irreversible aggregation of free radical viologens. Herein, the semi-interpenetrating dual-polymer network (DPN) organogels are introduced to improve the cycling stability of viologens-based EC devices. The primary cross-linked poly(ionic liquid)s (PILs) covalently anchored with viologens can suppress irreversible face-to-face contact between radical viologens. The secondary poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) chains with strong polar groups of -F can not only synergistically confine the viologens by the strong electrostatic effect, but also improve the mechanical performance of the organogels. Consequently, the DPN organogels show excellent cycling stability (87.5% retention after 10 000 cycles) and mechanical flexibility (strength of 3.67 MPa and elongation of 280%). Three types of alkenyl viologens are designed to obtain blue, green, and magenta colors, demonstrating the universality of the DPN strategy. Large-area EC devices (20 × 30 cm) and EC fibers based on organogels are assembled to demonstrate promising applications in green and energy-saving buildings and wearable electronics.

Cobweb-Inspired Quintuple Network Structures toward High-Performance Wearable Electrochromic Devices with Excellent Bending Resistance

Flexible electrochromic devices (FECDs) have been regarded as an ideal stratagem for wearable displays. However, it remains a great challenge to achieve long-term stability for high-performance FECDs due to their severe electrolyte deformation/leakage under repeated bending. Herein, inspired by the rough and fluffy microstructure of cobwebs, we prepared a porous polylactic acid (PLA) network through electrospinning and nonsolvent-induced phase separation. This loosely interlaced PLA network can be well infiltrated by electrolytes and exhibits extraordinarily high transparency; in addition, its surface contains numerous tiny holes to effectively load electrolytes to mitigate deformation. Furthermore, we also introduced silver nanowires (AgNWs) as the supporting network to load and connect electrochromic materials. After assembling them with graphene (GR) electrodes, a wearable FECD with a quintuple network structure (two GR networks, two AgNW networks, and one PLA network) was successfully prepared. The resulting FECD can realize high optical modulation (more than 70%), excellent cyclic stability (retain 95% after 1000 cycles), and innovative bending resistance (retain 84.8% after 6000 bending cycles). This work not only solves the long-lasting challenge of developing FECD with high optical modulation and bending resistances but also provides an energetic paradigm for diverse soft electronics used in harsh environments.

Applications of Viologens in Organic and Inorganic Discoloration Materials

Viologen derived from 4,4'-bipyridine has attracted much attention because of its color changing properties with electron transfer, unique redox stability and structural diversity. These characteristics have led to its successful use in various applications, in particular in color-changing materials. In the past few years, researchers have been working on the syntheses of viologen-based color-changing functional materials, and such materials have been widely used in many fields. In photochromic materials, it is used as anti-counterfeiting material; in thermochromic, it is used as memory storage material, and in electrochromic, it is used as a battery material. This Review discusses the progress of viologen in organic and inorganic discoloration materials in recent years. The syntheses of viologen and its derivatives are summarized, and its application in the field of discoloration materials is introduced.

Photochromic and electrochromic properties of a viologen-based multifunctional Cd-MOF

A pair of novel multifunctional MOF materials [Cd₂(L)₂(D/L-Lm)(H₂O)₂]·5H₂O (denoted as D/L-Cd-MOF) has been synthesized by combining the viologen-functionalized ligand H₂L⁺Cl^- and chiral D/L-tartaric acid (H₂Lm) with a simple solvothermal method. Due to the unique photoelectric response properties of the viologen units, reversible photochromic and electrochromic properties have been combined in D/L-Cd-MOF, which points to a new way of designing and constructing multifunctional photoelectric materials.

Chromism-Integrated Sensors and Devices for Visual Indicators

The bifunctionality of chromism-integrated sensors and devices has been highlighted because of their reversibility, fast response, and visual indication. For example, one of the representative chromism electrochromic materials exhibits optical modulation under ion insertion/extraction by applying a potential. This operation mechanism can be integrated with various sensors (pressure, strain, biomolecules, gas, etc.) and devices (energy conversion/storage systems) as visual indicators for user-friendly operation. In this review, recent advances in the field of chromism-integrated systems for visual indicators are categorized for various chromism-integrated sensors and devices. This review can provide insights for researchers working on chromism, sensors, or devices. The integrated chromic devices are evaluated in terms of coloration-bleach operation, cycling stability, and coloration efficiency. In addition, the existing challenges and prospects for chromism-integrated sensors and devices are summarized for further research.

Intercalation-free, fast switching of mesoporous antimony doped tin oxide with cathodically coloring electrochromic dyes

Mesoporous nanoparticle layers of transparent conductive oxides (TCOs) with anchored organic dyes are of great interest for electrochromic applications. Herein, we prepared mesoporous layers of antimony doped tin oxide (ATO) consisting of only 5 nm large particles with a low Sb concentration (2% antimony). The particles were prepared via a modified synthesis procedure based on hexahydroxostannate and pure Sb(v) hexahydroxoantimonate(v). We show that the ATO layers benefit from using a non-intercalating electrolyte such as tetrabutylammonium perchlorate (TBAP) compared to lithium perchlorate. Especially in the negative potential range, negative side effects, such as degradation due to lithium intercalation, are reduced. Furthermore, comparing the behavior of particles with varying antimony doping concentrations showed that the particles doped with 2% Sb are most suitable with respect to their conductivity and transparency. When modified with an electrochromic dye (viologen), the hybrid electrodes allow fully reversible (de)coloration with the non-intercalating electrolyte. Similar viologen/TiO2 electrodes on the other hand show severely restricted performance with the non-intercalating electrolyte as the oxidation of the dye is partially inhibited. Finally, we built a full electrochromic device composed of two ATO electrodes, each bearing a different electrochromic dye with TBAP as the electrolyte. Despite the dense morphology of the layers due to the small particle size as well as the large size of the electrolyte cation, the device displays remarkable switching times below 0.5 s.

Zinc(ii) phthalocyanine–viologen dyads: synthesis, electrochemistry, spectroelectrochemistry, electrodeposition, and electrochromism

Electrochromic properties have been enriched by combining the anodic colorant zinc phthalocyanine and cathodic colorant viologen moieties into a dyad system via a flexible chain.

Polymeric Ion Conductors Based on Sono-Polymerized Zwitterionic Polymers for Electrochromic Supercapacitors with Improved Shelf-Life Stability

Monolithic electrochromic supercapacitors (ECSs) have attracted increasing interest in recent electrochemical electronics due to their simplicity and unique ability to visually indicate stored energy levels. One crucial challenge for practical use is the improvement of shelf-life. Herein, zwitterionic (ZI) ionogels are proposed as effective electrolytes to reduce the self-discharging decay of ECSs. All-in-one ZI electrochromic (EC) gels are produced by one-pot sono-polymerization. The presence of ZI moieties in the gel does not affect the EC characteristics of chromophores. In addition, excellent capacitive properties in areal capacitance and coulombic efficiency are presented owing to the alignment of ZI units under an electric field and the formation of ion migration channels where rapid ion transport is allowed. Furthermore, the shelf-life of the ZI gel-based ECS is significantly improved by adjusting the interaction between polymeric gelators and ion species. The ZI gel-based ECS is expected to be a key platform for future smart energy storage devices.

Post-Synthetic Color Tuning of the Ultra-Effective and Highly Stable Surface-Confined Electrochromic Monolayer: Shades of Green for Camouflage Materials

We report here on the strategy for the preparation of a series of electrochromic (EC) materials in green shades designed for camouflage purposes. This top-down post-synthetic modification provides access to new EC materials by fine modulation of the color of the surface-confined metalorganic monolayer pre-deposited on indium tin oxide screen-printed supports. Selective on-surface N-quaternization of the outer pyridine unit of the EC metal complex covalently embedded onto an enhanced surface area electrode results in a bathochromic shift of the absorbance signal as well as visual color change from blue to different shades of green. When assembled into solid-state EC devices (ECDs), the materials demonstrate high color differences between colored and bleached states and significant differences in optical density. Upon electrochemical switching, the ECDs initially featuring different shades of green become yellowish or clay. The accessible gamut of colors, fulfilling the requirements for chameleon-like camouflage materials, is able to mimic conditions of various natural environments including forests and sands. Notably, ECDs demonstrate high long-term durability (95% retention of the performance after 3300 cycles), fast coloration (0.6-1.1 s), and bleaching (1.2-3.3 s) times and outstanding coloration efficiencies of 1018-1513 cm²/C. Importantly, post-synthetic N-quaternization/color tuning does not deteriorate the performance of the resulting EC materials and devices as judged by cyclic voltammetry, spectroelectrochemistry, and electrochemical impedance spectroscopy. This work adds to the limited number of reports that explore color tuning of EC molecular layers via on-surface modification with the aim to access new non-symmetric materials. Notably, the facile and straightforward technology presented here allows the creation of green-colored EC materials that are difficult to prepare in other ways.

Dual-Cation Electrolytes Crosslinked with MXene for High-Performance Electrochromic Devices

MXene, a 2D material, is used as a filler to manufacture polymer electrolytes with high ionic conductivity because of its unique sheet shape, large specific surface area and high aspect ratio. Because MXene has numerous -OH groups on its surface, it can cause dehydration and condensation reactions with poly(4-styrenesulfonic acid) (PSSA) and consequently create pathways for the conduction of cations. The movement of Grotthuss-type hydrogen ions along the cation-conduction pathway is promoted and a high ionic conductivity can be obtained. In addition, when electrolytes composed of a conventional acid or metal salt alone is applied to an electrochromic device (ECD), it does not bring out fast response time, high coloration efficiency and transmittance contrast simultaneously. Therefore, dual-cation electrolytes are designed for high-performance ECDs. Bis(trifluoromethylsulfonyl)amine lithium salt (LiTFSI) was used as a source of lithium ions and PSSA crosslinked with MXene was used as a source of protons. Dual-Cation electrolytes crosslinked with MXene was applied to an indium tin oxide-free, all-solution-processable ECD. The effect of applying the electrolyte to the device was verified in terms of response time, coloration efficiency and transmittance contrast. The ECD with a size of 5 × 5 cm2 showed a high transmittance contrast of 66.7%, fast response time (8 s/15 s) and high coloration efficiency of 340.6 cm2/C.

Water soluble organic electrochromic materials

Organic materials in electrochromic device applications possess a number of advantages over transition metal oxides like WO3 such as ease of synthesis and tunability, flexibility, and derivability from renewable feedstocks. However, these advantages are offset by the need to use organic solvents in their processing which are often flammable and/or toxic. Therefore, it is of paramount importance to the longterm economic and environmental sustainability of organic electronics research to develop water soluble organic materials. Herein, we describe the advances made in developing water soluble organic electronic materials for electrochromic applications. We here classify electrochromic materials into two broad categories: those that transition between colourless and coloured states (Type I) and those that transition between differently coloured states (Type II). The methods by which organic electrochromes are made water soluble are described in detail along with their potential applications in order to promote research in water soluble organic electronic materials in general.

Mixed-Valence Compounds as a New Route for Electrochromic Devices with High Coloration Efficiency in the Whole Vis-NIR Region

Electrochromic devices (ECDs) that allow the modulation of light transmission are very attractive in the research field of energy saving. Here all-in-one gel switchable ECDs based on mixed-valence electroactive compounds were developed. The use of the thienoviologen/ferrocene couple as cathode and anode, respectively, leads to a significant electrochromic band in the visible range (550–800 nm), with a color change from yellow to green, and to a lower band in the NIR region (1000–1700 nm), due to the presence of one electroactive-chromic species. Replacement of the electroactive ferrocene with a fluorene-diarylamine electroactive-chromic species, allows to extend and intensify the absorption in the NIR region, thus affording modulation of the solar radiation from 500 up to 2200 nm. High optical contrast, fast coloration and bleaching times and outstanding coloration efficiencies were measured for all observed absorption bands upon the application of small potential differences (1.4 V < DV < 2 V).

Design of intrinsically stretchable and highly conductive polymers for fully stretchable electrochromic devices

Stretchable materials are essential for next generation wearable and stretchable electronic devices. Intrinsically stretchable and highly conductive polymers (termed ISHCP) are designed with semi interpenetrating polymer networks (semi-IPN) that enable polymers to be simultaneously applied to transparent electrodes and electrochromic materials. Through a facile method of acid-catalyzed polymer condensation reaction, optimized ISHCP films show the highest electrical conductivity, 1406 S/cm, at a 20% stretched state. Without the blending of any other elastomeric matrix, ISHCP maintains its initial electrical properties under a cyclic stretch-release of over 50% strain. A fully stretchable electrochromic device based on ISHCP is fabricated and shows a performance of 47.7% ∆T and high coloration efficiency of 434.1 cm2/C at 590 nm. The device remains at 45.2% ∆T after 50% strain stretching. A simple patterned electrolyte layer on a stretchable electrochromic device is also realized. The fabricated device, consisting of all-plastic, can be applied by a solution process for large scale production. The ISHCP reveals its potential application in stretchable electrochromic devices and satisfies the requirements for next-generation stretchable electronics.

Photophysical and electrochemical properties of newly synthesized thioxathone–viologen binary derivatives and their photo-/electrochromic displays in ionic liquids and polymer gels

Photo- and electrochromic devices based on thioxathone–viologen derivatives were constructed in ionic liquid and gels, which displayed a good transmittance and reversible colour change behaviour under visible light radiation or a bias of −2.4 V.

Multicolored electrochromic and electrofluorochromic materials containing triphenylamine and benzoates

Multicolored electrochromic and electrofluorochromic materials containing triphenylamine and benzoates were developed.

Viologen-Based Electrochromic Materials: From Small Molecules, Polymers and Composites to Their Applications

Organic materials have gained considerable attention for electrochromic (EC) applications owing to improved EC performance and good processability. As a class of well-recognized organic EC materials, viologens have received persistent attention due to the structural versatility and property tunability, and are major active EC components for most of the marketed EC devices. Over the past two decades, extensive efforts have been made to design and synthesize different types of viologen-based materials with enhanced EC properties. This review summarizes chemical structures, preparation and EC properties of various latest viologen-based electrochromes, including small viologen derivatives, main-chain viologen-based polymers, conjugated polymers with viologen side-chains and viologen-based organic/inorganic composites. The performance enhancement mechanisms are concisely discussed. The current marketed viologens-based electrochromic devices (ECDs) are briefly introduced and an outlook on the challenges and future exploration directions for viologen-based materials and their ECDs are also proposed.

In situ preparation and determination of electrochemical and electrochromic properties of copper phthalocyanine-polyaniline nanocomposite films

Copper phthalocyanine (CuPc) films with different morphologies were electrodeposited on the surfaces of ITO electrodes. Then, in each case, a polyaniline (PANI) film was electrochemically polymerized in situ on the surface of the copper phthalocyanine film to form a CuPc-PANI composite film. The electrochemical properties of the CuPc-PANI composite film were observed to be much better than those of the film without CuPc. With the modification involving the CuPc nanowires, the composite film formed a finer particle surface and an increased interface area between the PANI and the electrolyte. Compared to the single-component PANI film, the CuPc-PANI composite film exhibited better performance with a higher optical contrast (58% at 730 nm), a faster response speed (coloring time of 1.02 s, discoloring time of 1.96 s), and better cycling stability (68.71% of the initial electrochemical activity after 500 cycles, in contrast to only about 48.02% for PANI). Moreover, the CuPc-PANI film shows a new feature that can be used as a supercapacitor (specifically a capacitance value of about 5.4 mF cm-2 at typical currents). Our results demonstrate that the prepared CuPc-PANI composite film is one of the best candidates for multiple potential applications such as high-performance polymer electrochromic materials and supercapacitors.

Dual-colored 4,4′,4′′,4′′′-(cyclobutane-1,2,3,4-tetrayl)-tetrabenzoate electrochromic materials with large optical contrast and coloration efficiency

This paper reports novel ester-containing electrochromic materials, 4,4′,4′′,4′′′-(cyclobutane-1,2,3,4-tetrayl)tetrabenzoate derivatives, with dual-colored electrochromism, high color contrast and coloration efficiency.