Voltage-Tunable Multicolor, Sub-1.5 V, Flexible Electrochromic Devices Based on Ion Gels

3D-Printed Hydrogel-Based Flexible Electrochromic Device for Wearable Displays

Flexible electrochromic devices (FECDs) are widely explored for diverse applications including wearable electronics, camouflage, and smart windows. However, the manufacturing process of patterned FECDs remains complex, costly, and non-customizable. To address this challenge, a strategy is proposed to prepare integrated FECDs via multi-material direct writing 3D printing. By designing novel viologen/polyvinyl alcohol (PVA) hydrogel inks and systematically evaluating the printability of various inks, seamless interface integration can be achieved, enabling streamlined manufacturing of patterned FECDs with continuous production capabilities. The resultant 3D-printed FECDs exhibit excellent electrochromic and mechanical properties, including high optical contrast (up to 54% at 360 nm), nice cycling stability (less than 5% electroactivity reduction after 10 000 s), and mechanical stability (less than 19% optimal contrast decrease after 5000 cycles of bending). The potential applications of these 3D-printed hydrogel-based FECDs are further demonstrated in wearable electronics, camouflage, and smart windows.

Unveiling the Impact of Tailoring Ionic Conductor Characteristics on the Performance of Wearable Triboelectric Nanogenerators

This work reveals the correlation between the performance of triboelectric nanogenerators (TENGs) and the characteristics of deformable solid-state ionic conductors (referred to as ionogels). For this purpose, we modify ionogel characteristics by incorporating additional plasticizers (propylene carbonate) and solid salts (lithium bis(trifluoromethylsulfonyl)imide) into the ionogels. We conclude that the high capacitance of the ionogel is crucial for achieving a high-performance TENG platform. The optimized ionogel-based TENG (i-TENG) exhibits a power density of ∼372.4 mW·m-2 (based on 95 V and 36 mA·m-2 outputs) with outstanding long-term stability over 2 weeks. Additionally, successful demonstrations of wearable nanogenerators are performed by leveraging the high stretchability (up to ∼1000%) and optical transparency (∼90%) of the ionogels. Overall, the results provide insight into the design of deformable ionic conductors for high-performance, reliable, and wearable TENGs.

Full-Color Generation via Phototunable Mono Ink for Fast and Elaborate Printings

Unlike pigment-based colors, which are determined by their molecular structure, diverse colors can be expressed by a regular arrangement of nanomaterials. However, existing techniques for constructing such nanostructures have struggled to combine high precision and speed, resulting in a narrow gamut, and prolonged color fabrication time. Here, this work reports a phototunable mono ink that can generate a wide range of colors by controlling regularly arranged nanostructure. Core-shell growth controlled by polymerization time precisely regulates the distance between arranged particles at a nanometer-scale, enabling the generation of various colors. Moreover, the wide and thin arrangement induces constrained out-of-plane growth, thus facilitating the intricate color generation at the desired location via photopolymerization. Upon terminating polymerization by oxygen gas, the generated colors are readily fixed and kept stable. Utilizing programmed ultraviolet illumination, large-scale and high-resolution (≈1 µm) full-color printings are demonstrated at high speed (100 mm2 s-1 ).

High-Voltage Pulse-Assisted Operation of Single-Layer Electrochromic Systems for High Performance and Reliability

A single-layer electrochromic device (SL-ECD) based on ionic conductors containing EC chromophores provides a very simple platform that can be readily fabricated by sandwiching the EC layer between two electrodes. The operation of SL-ECDs is governed by the diffusion of redox species due to their SL structure, which causes a relatively slow dynamic response. In this study, we propose an effective high-voltage pulse injection strategy to improve the performance of SL-ECDs. Applying a programmed voltage wave composed of DC and high-voltage pulses promotes coloration/bleaching switching without degrading device stability, which is more advantageous than applying high DC voltages. We modified the input voltage profile by considering fundamental parameters, such as the amplitude and duty ratio of additional voltage pulses. The coloration and bleaching dynamic responses with the optimized voltage wave are ∼62 and ∼20% faster, respectively, compared with those with the simple DC input. Furthermore, the additionally injected pulse aids in increasing the coloration efficiency from ∼95.3 to ∼168.6 cm2 C-1. Another notable feature of this system is that the device operates stably when a programmed voltage wave is used. These results indicate that the concept of high-voltage pulse-assisted operation of SL-ECDs is a straightforward but effective method for improving device performance without changing the EC chromophore or device structure.

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.

Survey of Recent Advances in Molecular Fluorophores, Unconjugated Polymers, and Emerging Functional Materials Designed for Electrofluorochromic Use

In this review, recent advances that exploit the intrinsic emission of organic materials for reversibly modulating their intensity with applied potential are surveyed. Key design strategies that have been adopted during the past five years for developing such electrofluorochromic materials are presented, focusing on molecular fluorophores that are coupled with redox-active moieties, intrinsically electroactive molecular fluorophores, and unconjugated emissive organic polymers. The structural effects, main challenges, and strides toward addressing the limitations of emerging fluorescent materials that are electrochemically responsive are surveyed, along with how these can be adapted for their use in electrofluorochromic devices.

Advanced liquid crystal-based switchable optical devices for light protection applications: principles and strategies

With the development of optical technologies, transparent materials that provide protection from light have received considerable attention from scholars. As important channels for external light, windows play a vital role in the regulation of light in buildings, vehicles, and aircrafts. There is a need for windows with switchable optical properties to prevent or attenuate damage or interference to the human eye and light-sensitive instruments by inappropriate optical radiation. In this context, liquid crystals (LCs), owing to their rich responsiveness and unique optical properties, have been considered among the best candidates for advanced light protection materials. In this review, we provide an overview of advances in research on LC-based methods for protection against light. First, we introduce the characteristics of different light sources and their protection requirements. Second, we introduce several classes of light modulation principles based on liquid crystal materials and demonstrate the feasibility of using them for light protection. In addition, we discuss current light protection strategies based on liquid crystal materials for different applications. Finally, we discuss the problems and shortcomings of current strategies. We propose several suggestions for the development of liquid crystal materials in the field of light protection.

Recent progress in electrochromic energy storage materials and devices: a minireview

Integration of several functionalities into one isolated electrochemical body is necessary to realize compact and tiny smart electronics. Recently, two different technologies, electrochromic (EC) materials and energy storage, were combined to create a single system that supports and drives both functions simultaneously. In EC energy storage devices, the characteristic feature of EC materials, their optical modulation depending on the applied voltage, is used to visually identify the stored energy level in real time. Moreover, combining energy-harvesting and EC storage systems by sharing one electrode facilitates the realization of further compact multifunction systems. In this minireview, we highlight recent groundbreaking achievements in EC multifunction systems where the stored energy levels can be visualized using the color of the device.

Emerging Electrochromic Materials and Devices for Future Displays

With the rapid development of optoelectronic fields, electrochromic (EC) materials and devices have received remarkable attention and have shown attractive potential for use in emerging wearable and portable electronics, electronic papers/billboards, see-through displays, and other new-generation displays, due to the advantages of low power consumption, easy viewing, flexibility, stretchability, etc. Despite continuous progress in related fields, determining how to make electrochromics truly meet the requirements of mature displays (e.g., ideal overall performance) has been a long-term problem. Therefore, the commercialization of relevant high-quality products is still in its infancy. In this review, we will focus on the progress in emerging EC materials and devices for potential displays, including two mainstream EC display prototypes (segmented displays and pixel displays) and their commercial applications. Among these topics, the related materials/devices, EC performance, construction approaches, and processing techniques are comprehensively disscussed and reviewed. We also outline the current barriers with possible solutions and discuss the future of this field.

Switchable Adhesion: On-Demand Bonding and Debonding

Adhesives have a long and illustrious history throughout human history. The development of synthetic polymers has highly improved adhesions in terms of their strength and environmental tolerance. As soft robotics, flexible electronics, and intelligent gadgets become more prevalent, adhesives with changeable adhesion capabilities will become more necessary. These adhesives should be programmable and switchable, with the ability to respond to light, electromagnetic fields, thermal, and other stimuli. These requirements necessitate novel concepts in adhesion engineering and material science. Considerable studies have been carried out to develop a wide range of adhesives. This review focuses on stimuli-responsive material-based adhesives, outlining current research on switchable and controlled adhesives, including design and manufacturing techniques. Finally, the potential for smart adhesives in applications, and the development of future adhesive forms are critically suggested.

DNA Optoelectronics: Versatile Systems for On-Demand Functional Electrochemical Applications

Herein, we propose innovative deoxyribonucleic acid (DNA)-based gels and their applications in diverse optoelectronics. We prepared the optoelectronic DNA-based gels (^OpDNA Gel) through molecular complexation, that is, groove binding and ionic interactions of DNA and 1,1'-diheptyl-4,4'-bipyridinium (DHV). This process is feasible even with sequence-nonspecific DNA extracted from nature (e.g., salmon testes), resulting in the expansion of the application scope of DNA-based gels. ^OpDNA Gel possessed good mechanical characteristics (e.g., high compressibility, thermoplasticity, and outstanding viscoelastic properties) that have not been observed in typical DNA hydrogels. Moreover, the electrochromic (EC) characteristics of DHV were not lost when combined with ^OpDNA Gel. By taking advantage of the facile moldability, voltage-tunable EC behavior, and biocompatibility/biodegradability of ^OpDNA Gel, we successfully demonstrated its applicability in a variety of functional electrochemical systems, including on-demand information coding systems, user-customized EC displays, and microorganism monitoring systems. The ^OpDNA Gel is a promising platform for the application of DNA-based biomaterials in electrochemical optoelectronics.

Donor-acceptor-type poly[chalcogenoviologen-alt-triphenylamine] for synaptic biomimicking and neuromorphic computing

Polymer memristors are preeminent candidates for low-power edge computing paradigms. Poly[chalcogenoviologen-alt-triphenylamine] (PCVTPA) has been synthesized by direct coupling of chalcogeno-viologen as electron acceptor and 4-(bromomethyl)-N-(4-(bromo-methyl)phenyl)-N-phenylaniline as electron donor. The introduction of chalcogen atoms (S, Se, Te) into viologen scaffolds can greatly improve electrical conductive, electrochemical, and electrochromic properties of the materials when compared with the conventional viologens. Taking PTeVTPA as an example, the as-fabricated electronic device with a configuration of Al/PTeVTPA/ITO exhibits excellent multilevel storage and history-dependent memristive switching performance. Associated with the unique memristive behavior, the PTeVTPA-based device can not only be used to emulate the synaptic potentiation/depression, the human's learning and memorizing functions, and the transition from short-term synaptic plasticity to long-term plasticity but also carry out decimal arithmetic operations as well. This work will be expected to offer a train of new thought for constructing high-performance synaptic biomimicking and neuromorphic computing system in the near future.

Dynamically Cross-Linked Hydrogel Electrolyte with Remarkable Stretchability and Self-Healing Capability for Flexible Electrochromic Devices

It is desired to develop self-healing gel electrolytes for flexible electrochromic devices (ECDs) due to the demand of healing damages caused during operations. We here report a hydrogel electrolyte with remarkable self-healing capability, excellent stretchability, and ionic conductivity. The hydrogel electrolyte was synthesized via one-step copolymerization of glycerol monomethacrylate (GMA) and acrylamide (AAm) in the presence of borate. Within the hydrogel electrolyte, dynamic cross-linking is expected to be formed due to the borate-didiol complexation and hydrogen-bonding interactions. As a result, the hydrogel electrolyte demonstrates an excellent self-healing efficiency of up to 97%, a fracture strain of 1155%, a fracture toughness of 136.6 kJ m^-3, and a fracture stress of 13.0 kPa. Additionally, a flexible ECD based on the hydrogel electrolyte and an electrochromic layer of poly(3,4-(2,2-dimethyl-propylenedioxy)thiophene) (PProDOT-Me₂) was assembled and evaluated. The device is found to be stable in both mechanical and optical properties over 1000 operation cycles. This study may provide a promising way for self-healing electrolyte gels to be utilized in a variety of flexible electrochemical devices, including ECDs, supercapacitors, and batteries.

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.

Engineered Ionene/PNIPAM Hybrid Dual-Response Material Generating Tunable and Unique Optical Modes for Adaptive Solar Transmittance Modulation

A hybrid smart window exhibiting dual chromic response properties based on an ionene/polymer material is successfully engineered. Thermochromic poly(N-isopropylacrylamide) is integrated with an electrochromic viologen-tethered ionene, also acting as an electrolyte, to produce a smart window that can adaptively control solar visible light transmittance in response to multiple stimuli. This new blend allows the formation of unique reversible optical states, namely, "clear", "amber", "cloudy", and "grainy" states, which are passively triggered by environmental temperature and actively induced by external potential or simultaneously by both. This hybrid material shows tunability in terms of its electrochemical and optical properties, switching kinetics, and coloration efficiency and can also achieve a nearly absolute zero-transmissive state. With the material's excellent solubility and film-forming ability, the smart device can be fabricated with much flexibility and ease. Finally, this device has an all-in-one layer configuration, creating a more compact and simplified design. With all these properties combined, the development of a next-generation multifunctional smart window device, which can efficiently control incoming solar light for energy-saving in buildings and also provide visual comfort, is possible.

Redox of Viologen for Powering and Coloring

Viologen is among the most attractive and easiest-to-use organic redox active group in many functional molecular assemblies. It plays crucial roles as an electron transfer mediator in the artificial photo-energy conversion systems and electron-transfer protein assemblies and as a building block of supramolecules. Its features include electrochemically reversible redox activity and stability. Strong blue color and tendency to dimerization of the one-electron reduced form, viologen mono-radical mono-cation, are remarkable. In this Account, we describe the use of viologen to give a powered movement of small molecules and motion of millimetre-sized macroscopic soft-matters and the use of viologen ionic liquid as electrochromic materials. Attractivities of the use of viologen units for powering and coloring are demonstrated and discussed. In particular, we highlight driving of mechanical movements by π-π stacking dimerization, incorporation in a hydrogel to attain highly deformable material, induction of 2D phase transformation, and sharp color change of very thin ionic liquid layer in a compartment-less electrochromic display.

Substituent-Adjusted Electrochromic Behavior of Symmetric Viologens

As a promising electrochromic material, viologens have attracted increasing attention due to their high redox activity and adjustable electrochromic capability. In order to investigate the effect of alkyl substituents on electrochromic behavior, four alkyl-substituted viologens and a benzyl-substituted viologen were synthesized, namely 1,1′-dioctyl-4,4′-bipyridinium dibromide (OV), 1,1′-didekyl-4,4′-bipyridinium dibromide (DeV), 1,1′-didodecyl-4,4′-bipyridinium dibromide (DoV), 1,1′-dihexadecyl-4,4′-bipyridinium dibromide (HV), and 1,1′-dibenzyl-4,4′-bipyridinium dibromide (BV). The different photophysical and electrochemical properties of these viologens were attributed to their deviation in spatial structure caused by different substituents. Compared with benzyl-substituted BV, a slight blueshift occurred for the absorption peaks of alkyl-substituted viologens from 262 to 257 nm with the increase in alkyl chain length. Moreover, the first redox couple increased positively, and the dimerization of the compound decreased gradually, accompanied by the decrease in optical contrast and distinct chromatic difference. A comparison of chromatic and optical contrasts indicated that OV had the longest coloring response time (R_Tc), while it was shortest for HV. The bleaching response time (R_Tb) of viologen films gradually decreased with the alkyl chain length, and the OV film had the shortest R_Tb. Furthermore, when increasing the length of the alkyl chain, the cycling stabilities of alkyl viologens increased gradually. In addition, the OV film exhibited the best contrast after 200 continuous cycles.

Automatic light-adjusting electrochromic device powered by perovskite solar cell

Electrochromic devices can modulate their light absorption under a small driving voltage, but the requirement for external electrical supplies causes response-lag. To address this problem, self-powered electrochromic devices have been studied recently. However, insensitivity to the surrounding light and unsatisfactory stability of electrochromic devices have hindered their critical applications. Herein, novel perovskite solar cell-powered all-in-one gel electrochromic devices have been assembled and studied in order to achieve automatic light adjustment. Two alkynyl-containing viologen derivatives are synthesized as electrochromic materials, the devices with very high stability (up to 70000 cycles) serves as the energy storage and smart window, while the perovskite solar cell with power-conversion-efficiency up to 18.3% serves as the light detector and power harvester. The combined devices can automatically switch between bleached and colored state to adjust light absorption with variable surrounding light intensity in real-time swiftly, which establish significant potentials for applications as modern all-day intelligent windows.

Reliable, High-Performance Electrochromic Supercapacitors Based on Metal-Doped Nickel Oxide

Herein, high-performance, reliable electrochromic supercapacitors (ECSs) are proposed based on tungsten trioxide (WO₃) and nickel oxide (NiO) films. To maximize device performance and stability, the stoichiometric balance between anode and cathode materials is controlled by carefully adjusting the thickness of the anodic NiO film while fixing the thickness of WO₃ to ∼660 nm. Then, a small amount (≤10 mol %) of metal (e.g., copper) is doped into the NiO film, improving the electrical conductivity and electrochemical activity. At a Cu doping level of 7 mol %, the resulting ECS exhibited the highest performance, including a high areal capacitance (∼14.9 mF/cm²), excellent coulombic efficiency (∼99%), wide operating temperature range (0-80 °C), reliable operation with high charging/discharging cyclic stability (>10,000 cycles), and good self-discharging durability. Simultaneously, the change in transmittance of the device is well synchronized with the galvanostatic charging/discharging curve by which the real-time energy storage status is visually indicated. Furthermore, the practical feasibility of the device is successfully demonstrated. These results imply that the ECS fabricated in this work is a promising potential energy storage platform and an attractive component for future electronics.

White-Light-Emitting AIE/Eu3+-Doped Ion Gel with Multistimuli-Responsive Properties

A white-light-emitting ion gel composed of a poly[(2-(4-vinylphenyl)ethene-1,1,2-triyl)tribenzene-b-ethylene glycol-b-(2-(4-vinylphenyl)ethene-1,1,2-triyl)tribenzene] aggregation-induced emission (AIE) network and a poly([2,2':6',2″-terpyridin]-4'-yl methacrylate-co-methyl methacrylate) Eu³⁺-doped network was fabricated via a solution mixing process. This ion gel exhibits special multistimuli-responsive properties, and it can change its luminescent color by changing pH, temperature, or the solvent. The unique color-changing property is attributed to the different luminescent mechanisms of the AIE/Eu³⁺-doped polymer networks. The former is affected by changes in its aggregation state, while the latter is controlled by the dynamic metal-ligand cross-linking bonds. Furthermore, owing to the interpenetrating networks formed by the two polymers, the hybrid gel has both good mechanical strength and flexibility. It may be used in the fields of sensors, probes, and light-emitting materials.

Multichromic Monolayer Terpyridine-Based Electrochromic Materials

The article describes novel electrochromic materials (ECMs) that are based on a monolayer consisting of two or three isostructural metal complexes of 4'-(pyridin-4-yl)-2,2':6',2''-terpyridine simultaneously deposited on surface-enhanced support. The support was made by screen printing of indium tin oxide (ITO) nanoparticles on ITO-glass and has a surface area sufficient for a monolayer to give color visible to the naked eye. The ability to separately electrochemically address the oxidation state of the metal centers on the surface (i.e., Co²⁺/Co³⁺, Os²⁺/Os³⁺, and Fe²⁺/Fe³⁺) provides an opportunity to achieve several distinct color-to-color transitions, thus opening the door for constructing monolayer-based multicolor ECMs.

Nofal M, Karim WO, Brevik I, Brza MA, Abdulwahid RT, Al-Zangana S, Kadir MFZ. Structural, Impedance and Electrochemical Characteristics of Electrical Double Layer Capacitor Devices Based on Chitosan: Dextran Biopolymer Blend Electrolytes

This report presents the preparation and characterizations of solid biopolymer blend electrolyte films of chitosan as cationic polysaccharide and anionic dextran (CS: Dextran) doped with ammonium iodide (NH4I) to be utilized as electrolyte and electrode separator in electrical double-layer capacitor (EDLC) devices. FTIR and XRD techniques were used to study the structural behavior of the films. From the FTIR band analysis, shifting and broadening of the bands were observed with increasing salt concentration. The XRD analysis indicates amorphousness of the blended electrolyte samples whereby the peaks underwent broadening. The analysis of the impedance spectra emphasized that incorporation of 40 wt.% of NH4I salt into polymer electrolyte exhibited a relatively high conductivity (5.16 × 10-3 S/cm). The transference number measurement (TNM) confirmed that ion (tion = 0.928) is the main charge carriers in the conduction process. The linear sweep voltammetry (LSV) revealed the extent of durability of the relatively high conducting film which was 1.8 V. The mechanism of charge storage within the fabricated EDLC has been explained to be fully capacitive behavior with no redox peaks appearance in the cyclic voltammogram (CV). From this findings, four important parameters of the EDLC; specific capacitance, equivalent series resistance, energy density and power density were calculated as 67.5 F/g, 160 ohm, 7.59 Wh/kg and 520.8 W/kg, respectively.

Multicolored Cathodically Coloring Electrochromism and Electrofluorochromism in Regioisomeric Star-Shaped Carbazole Dibenzofurans

In this work, a series of fluorescent cathodically coloring electrochromic (EC) small molecules o-, m-, and p-DBFDCz with 3,5-di(9H-carbazol-9-yl)benzene (DCz) linked to dibenzofuran (DBF) at different substitutional positions were synthesized and fully characterized. These compounds are electroactive and undergo quasi-reversible two-step single-electron reduction generating radical anions and dianions. The absorptions of o-, m-, and p-DBFDCz in the neutral states lie in the UV region (λ_onset ≈ 350 nm), showing high transparency, while the absorption of their reduced states can be largely tuned across the visible region through driving voltage and substitutional positions. Initially generated spectroelectrochemically radical anions show absorption in the short-wavelength region of ∼380-500 nm with weak broad absorptions at longer wavelengths. On further reduction, these bands disappear on the cost of growing intense bands from dianions at longer wavelengths of 500-700 nm with some tail absorptions in the shorter-wavelength region. This renders the colors of the EC devices based on these materials, which are changed from green to red, yellow to magenta, and light to deep blue for o-, m-, and p-DBFDCz, respectively, covering four legs of the L*a*b* color space. Besides excellent optical contrast (>90%) and high coloration efficiency (up to 504 cm² C^-1), the fluorescence observed in solution of neutral o-, m-, and p-DBFDCz can be modulated between the fluorescence and quenched states by direct electrochemical redox reactions. Both EC and electrofluorochromic (EFC) processes are reversible on cycling. This research demonstrates the feasibility of developing multifunctional EC/EFC materials with multicolored electrochromism through exploiting electrochemical properties of traditional fluorescent small molecules.

Voltage-Tunable Dual Image of Electrostatic Force-Assisted Dispensing Printed, Tungsten Trioxide-Based Electrochromic Devices with a Symmetric Configuration

Electrostatic force-assisted dispensing (EFAD)-printed tungsten trioxide (WO₃)-based electrochromic devices (ECDs) displaying a dual image depending on the applied voltage are demonstrated. We first print WO₃ via EFAD printing, in which the width of the printed lines can be tuned by adjusting the printing speed. The performance of the ECDs is characterized while varying the thickness of the printed WO₃ film. It is determined that ∼550 nm thick WO₃ is the optimal film considering maximum transmittance contrast (ΔT_max), device dynamic responses, efficiency, and long-term coloration/bleaching cyclic stability. More significantly, the coloration of the devices in this work can alternatively appear due to the use of electrolyte-soluble anodic species (here, dimethyl ferrocene, dmFc), for which WO₃ films should be deposited on both electrodes and a part of the electrodes should be exposed to the electrolyte for the oxidation of dmFc. To take advantage of such features of the devices, we successfully demonstrate EFAD-printed, flexible WO₃ ECDs alternately displaying a dual image, which is expected to have high potential as a functional component of printed 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.

A transparent-to-gray electrochromic device based on an asymmetric viologen

A transparent-to-gray electrochromic device (ECD) based on an asymmetric viologen was fabricated and characterized.

Asymmetric molecular modification of viologens for highly stable electrochromic devices

Asymmetric viologens are proposed for highly stable electrochromic devices.

User-Customized, Multicolor, Transparent Electrochemical Displays Based on Oxidatively Tuned Electrochromic Ion Gels

Transparent displays have emerged as a class of cutting-edge electronics. Here, we propose user-customized, design-it-yourself (DIY) transparent displays based on electrochromic (EC) ion gels including viologens. To achieve multiple colors and enhance the functionality of EC displays (ECDs), the incorporation of several EC chromophores is inevitable. However, the issue related to the discrepancy of coloration voltages is inherent due to the different electrochemical characteristics of each material, causing unbalance of the color contrast. To overcome this problem without significantly affecting the performance of ECDs, we suggest a simple but effective strategy by adjusting the oxidation activity of electrolyte-soluble anodic species (i.e., ferrocene (Fc) derivatives) by modifying pendant groups. We systematically investigated the effects of the employed Fc derivatives on the EC behaviors of ECDs in terms of the coloration voltage, maximum transmittance contrast, device dynamics, coloration efficiency, and operational stability. We determine the conditions for implementing red-green-blue (RGB) colors with comparable intensities at similar voltages. Last, we draw images using RGB EC inks for conceptual demonstration of the DIY transparent displays. The fabricated ECDs exhibit transparent bleached states and user-customized images in the colored states. Overall, this result implies that the extremely simple DIY ECDs, which do not require conventional lithography or printing, have great potential as future transparent displays that can be easily customized.

Non-volatile, phase-transition smart gels visually indicating in situ thermal status for sensing applications

Non-volatile smart gel platforms that change optical properties according to temperature are successfully prepared based on a random copolymer, poly(styrene-ran-benzyl methacrylate-ran-methyl methacrylate) [(P(S-r-BzMA-r-MMA)]. The P(S-r-BzMA-r-MMA) copolymers are judiciously designed to serve as both polymer hosts for a gel, and thermoresponsive materials. In contrast to typical lower critical solution temperature (LCST) or upper critical solution temperature (UCST) systems including sol-gel (or gel-sol) transition, the thermoresponsive gels consisting of the P(S-r-BzMA-r-MMA) and ionic liquids (ILs) maintain an elastic gel-state due to their network structure irrespective of phase transitions. We investigate the effects of the copolymer composition and copolymer/IL ratio on the gel properties. Temperature dependent self-assembly of the gel is revealed using small-angle X-ray scattering (SAXS). Thermal response of the gel is examined optically and dynamically. Moreover, we propose a simple method to additionally control the response temperature and the mechanical robustness of the gels by incorporating additional salts. Lastly, we successfully demonstrate practical feasibility of the thermoresponsive gels in high-temperature safety indicators and temperature range indicators, in which the gels visually display in situ thermal status by a change in optical transmittance.

Mechano-thermo-chromic device with supersaturated salt hydrate crystal phase change

Active control of transparency/color is the key to many functional optoelectric devices. Applying an electric field to an electrochromic or liquid crystal material is the typical approach for optical property control. In contrast to the conventional electrochromic method, we developed a new concept of smart glass using new driving mechanisms (based on mechanical stimulus and thermal energy) to control optical properties. This mechano-thermo-chromic smart glass device with an integrated transparent microheater uses a sodium acetate solution, which shows a unique marked optical property change under mechanical impact (mechanochromic) and heat (thermochromic). Such mechano-thermo-chromic devices may provide a useful approach in future smart window applications that could be operated by external environment conditions.

Stretchable and reflective displays: materials, technologies and strategies

Displays play a significant role in delivering information and providing visual data across all media platforms. Among displays, the prominence of reflective displays is increasing, in the form of E-paper, which has features distinct from emissive displays. These unique features include high visibility under daylight conditions, reduced eye strain and low power consumption, which make them highly effective for outdoor use. Furthermore, such characteristics enable reflective displays to achieve high synergy in combination with wearable devices, which are frequently used for outdoor activities. However, as wearable devices must stretch to conform to the dynamic surfaces of the human body, the issue of how to fabricate stretchable reflective displays should be tackled prior to merging them with wearable devices. In this paper, we discuss stretchable and reflective displays. In particular, we focus on reflective displays that can be divided into two types, passive and active, according to their responses to stretching. Passive displays, which consist of dyes or pigments, exhibit consistent colors under stretching, while active displays, which are based on mechanochromic materials, change their color under the same conditions. We will provide a comprehensive overview of the materials and technologies for each display type, and present strategies for stretchable and reflective displays.

Tetrathiafulvalene: effective organic anodic materials for WO3-based electrochromic devices

Finding a new, effective anodic species is a challenge for achieving simpler low-voltage tungsten trioxide (WO₃)-based electrochromic devices (ECDs). In this work, we utilize tetrathiafulvalene (TTF) and demonstrate its reversible redox behaviors as an electrolyte-soluble anodic species. The concentration of TTF in the electrolyte is varied to optimize device performance. When the TTF concentration is low (0.01 M), a smaller maximum transmittance difference (ΔT_max ∼ 34.2%) and coloration efficiency (η ∼ 59.6 cm² C⁻¹) are measured. Although a better performance of ΔT_max ∼ 93.7% and η ∼ 74.5 cm² C⁻¹ is achieved at 0.05 M TTF, the colored state could no longer return to its original form. We conclude that 0.03 M of TTF is the appropriate concentration for high-performance WO₃ ECDs with high optical contrast and reversible EC behaviors. The irreversible EC transition at high concentrations of TTF is attributed to the agglomeration of TTF molecules.

Tetrathiafulvalene (TTF) is employed as an effective electrolyte-soluble anodic species for achieving low-voltage tungsten trioxide (WO₃)-based electrochromic devices (ECDs).

Voltage Stimulated Anion Binding of Metalloporphyrin-induced Crystalline 2D Nanoflakes

Voltage-stimulated redox-active materials have received significant attention in the field of organic electronics and sensor technology. Such stimuli-responsive materials trigger the formation of crystalline nanostructures and facilitate the design of efficient smart devices hitherto unknown. Herein, we report that free-base and metallo-tetratolylporphyrin-linked ferrocene derivatives (H₂ TTP-Fc and ZnTTP-Fc) undergo distinct proton/anion binding mechanism in CHCl₃ during bulk electrolysis at applied voltage of 1.4 V to give [H₄ TTP-Fc]⁺ Cl^- and H⁺ [(Cl)ZnTTP-Fc]^- followed by nanospheres and crystalline 2D nanoflakes formation, confirmed by SEM and TEM images, by methanol vapor diffusion (MVD) approach. Moreover, X-ray diffraction analysis suggest that protonated H₂ TTP-Fc aggregates exhibit amorphous nature, whereas H⁺ [(Cl)ZnTTP-Fc]^- depict crystalline nature from layer-by-layer arrangement of nanoflakes assisted by π-π stacking and ion-dipole interactions.

Electrochromism of Ferrocene- and Viologen-Based Redox-Active Ionic Liquids Composite

Redox-active ionic liquids (RAILs) require no other additional reagents such as solvent and supporting electrolyte for electrochemical reactions under undiluted condition. Viologen-based RAILs are one of the electrochromic (EC) ionic liquids with sharp color contrast and high chemical stability. An operation of an EC cell requires two electroactive elements, an EC material and a charge compensating material. In this study, an equimolar composite of a viologen-based RAIL as the EC material and a ferrocene-based RAIL as the charge compensation material, was synthesized and applied to an EC cell. The EC cell with the composite RAIL of as high concentration as 0.92 M each redox species showed good coloration efficiency (91.4 cm² C^-1 at 540 nm on 1.0 V). The coloration process of the EC cell was diffusion-limited process. The current and absorbance of the EC cell reached constant values at large enough bias voltage because of the charge recombination between reduced viologens and oxidized ferrocenes. The recombination affected rapid color erasing process. Almost no deterioration of the composite RAIL was found by ¹H NMR after 13 000 potential cycle durability experiment.

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.

Microporous organic polymer-induced gel electrolytes for enhanced operation stability of electrochromic devices

Gel electrolytes were synthesized by the formation of microporous organic polymers in the presence of LiClO₄ and NMP, showing enhanced operation stability of electrochromic cells, compared with liquid electrolytes.

Dual-Function Electrochromic Supercapacitors Displaying Real-Time Capacity in Color

Dual-function electrochromic supercapacitors (ECSs) that indicate their real-time charge capacity in color are fabricated using tungsten trioxide (WO₃) and Li-doped ion gels containing hydroquinone (HQ). The ECSs can simultaneously serve as either electrochromic devices or supercapacitors. The coloration/bleaching and charging/discharging characteristics are investigated between 0 and -1.5 V. At the optimal HQ concentration, large transmittance contrast (∼91%), high coloration efficiency (∼61.9 cm²/C), high areal capacitance (∼13.6 mF/cm²), and good charging/discharging cyclic stability are achieved. Flexible ECSs are fabricated on plastic substrates by exploiting the elastic characteristics of the gel electrolytes, and they exhibit good bending durability. Moreover, practical feasibility is evaluated by demonstrating the use of the ECSs as an energy storage device and a power source.

The Effect of Different Mixed Organic Solvents on the Properties of p(OPal-MMA) Gel Electrolyte Membrane for Lithium Ion Batteries

A solvent is a key factor during polymer membrane preparation, and it is directly related to application performance as a separator for lithium ion battery (LIB). In this study, different mixed solvents were employed to prepare polymer (p(OPal-MMA)) membranes by the phase inversion technique. The polymer membrane then absorbed liquid electrolytes to obtain gel electrolytes (GPEs). The surface morphologies and porosities of these membranes were investigated, and lithium ion transferences and electrochemical performances of these GPEs were also measured. The membrane displayed an interconnected three-dimensional framework structure with uniformly distributed pores when using DMF as a porogen. When combined with acetone as the component solvent, the prepared GPE displayed the largest lithium ion transference number (0.706), the highest porosity (42.6%) and ion conductivity (3.99 × 10−3 S/cm). Even when assembled as Li/GPE/LiFePO4 cell, it exhibited the highest initial specific capacity of 167 mAh/g and retained most capacity (162 mAh/g) after 50 cycles. The results presented here probably provide reference for choosing an appropriate mixed solvent in fabricating polymer membranes.