Flexible Electrochromic Research Articles

Dynamically transflective multicolor modulation via single metal-dielectric inorganic electrochromic electrode

Transflective electrochromic (EC) materials with the ability to dynamically adjust color in both reflection and transmission mode are extremely attractive for tunable filter, smart windows, and ultralow-power displays due to their wide applicability in dark and bright environment and unique tunability of optical properties. However, simultaneously brilliant multicolor modulation of both reflection and transmission in one EC film is challenging. Here, integrated and metal/dielectric structured EC electrode composed of three-layered Cr/Ag/WO3 (CAW) films are demonstrated. Benefiting from the synergistic effect of the good conductivity, strong optical interference, and excellent substrate-film adhesion, these well-designed CAW films successfully demonstrate dynamic variegated modulation of both reflective and transmissive colors. This modulation encompasses a broad spectrum of hues, boasts high saturation, rapid response times, and exhibits impressive cycling stability. The flexible CAW film with superior bending stability is also achieved for the successful construction of the patterned flexible EC device.

Flexible Electrochromic Device on Polycarbonate Substrate with PEDOT:PSS and Color-Neutral TiO2 as Ion Storage Layer.

Electrochromic (EC) windows on glass for thermal and glare protection in buildings, often referred to as smart (dimmable) windows, are commercially available, along with rearview mirrors or windows in aircraft cabins. Plastic-based applications, such as ski goggles, visors and car windows, that require lightweight, three-dimensional (3D) geometry and high-throughput manufacturing are still under development. To produce such EC devices (ECDs), a flexible EC film could be integrated into a back injection molding process, where the films are processed into compact 3D geometries in a single automized step at a low processing time. Polycarbonate (PC) as a substrate is a lightweight and robust alternative to glass due to its outstanding optical and mechanical properties. In this study, an EC film on a PC substrate was fabricated and characterized for the first time. To achieve a highly transmissive and colorless bright state, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was used as the working electrode, while titanium dioxide (TiO2) was used as the counter electrode material. They were deposited onto ITO-coated PC films using dip- and slot-die coating, respectively. The electrodes were optically and electrochemically characterized. An ECD with a polyurethane containing gel electrolyte was investigated with regard to optical properties, switching speed and cycling behavior. The ECD exhibits a color-neutral and highly transmissive bright state with a visible light transmittance of 74% and a bluish-colored state of 64%, a fast switching speed (7 s/4 s for bleaching/coloring) and a moderately stable cycling behavior over 500 cycles with a decrease in transmittance change from 10%to 7%.

Self-cross-linkable ferrocenyl-containing polysiloxanes as flexible electrochromic materials

Flexible ferrocenyl-containing silicone rubbers (FSRs) with various ferrocenyl-substituted unit contents of 25, 50, and 75 mol% were obtained by applying a simple method included catalytic hydrosilylation (between polymethylhydrosiloxane and vinylferrocene) and self-cross-linking reactions (dehydrocoupling between Si–H groups). Self-cross-linking is one of the most suitable and simple methods to coat and obtain FSRs on various contact conductive surfaces, such as onto indium tin oxide (ITO) glass or ITO-poly(ethylene terephthalate). The synthesized FSRs exhibit the electrochromic (EC) properties, resulting from the reduction-oxidation of ferrocenyl groups (Fc/Fc+ transformations at E0 c.a. 0.36 V) and leading to color change: oxidation causes new band formation at 634 nm in ultraviolet-visible spectra. FSRs with ferrocenyl-substituted unit content of 50 mol% (FSR50) is the most optimal in terms of EC (coloration efficiency of 10.98 cm2/C, coloring and bleaching times of c.a. 49 and 66 s, which are close to undoped oligoaniline-functionalized polysiloxanes and carbazolyl-modified polysiloxanes) and mechanical properties (the tensile strength, Young's modulus, and elongation at break are 2.80 MPa, 25.33 MPa, and c.a. 50%, respectively). The previously reported EC polysiloxanes have lack of mechanical characteristics. Both electrochromism and flexibility predominantly make FSR50 promising materials for the creation of flexible and stretchable EC devices.

Self-Powered Flexible Multicolor Electrochromic Devices for Information Displays.

The development of self-powered flexible multicolor electrochromic (EC) systems that could switch different color without an external power supply has remained extremely challenging. Here, a new trilayer film structure for achieving self-powered flexible multicolor EC displays based on self-charging/discharging mechanism is proposed, which is simply assembled by sandwiching an ionic gel film between 2 cathodic nickel hexacyanoferrate (NiHCF) and Prussian blue (PB) nanoparticle films on indium tin oxide substrates. The display exhibits independent self-powered color switching of NiHCF and PB films with fast responsive time and high reversibility by selectively connecting the Al wire as anodes with the 2 EC films. Multicolor switching is thus achieved through a color overlay effect by superimposing the 2 EC films, including green, blue, yellow, and colorless. The bleaching/coloration process of the displays is driven by the discharging/self-charging mechanism for NiHCF and PB films, respectively, ensuring the self-powered color switching of the displays reversibly without an external power supply. It is further demonstrated that patterns can be easily created in the self-powered EC displays by the spray-coating method, allowing multicolor changing to convey specific information. Moreover, a self-powered ionic writing board is demonstrated based on the self-powered EC displays that can be repeatedly written freehand without the need of an external power source. We believe that the design concept may provide new insights into the development of self-powered flexible multicolor EC displays with self-recovered energy for widespread applications.

Electrochromic Displays via the Room-Temperature Electrochemical Oxidation of Nickel.

Electrochromism refers to the persistent and reversible change in color by applying an electric field. The phenomenon involves the insertion and extraction of electrons and ions within the active material. There is a keen interest in electrochromic (EC) materials, since they exhibit a wide range of potential applications. In recent years, transition-metal oxides have been widely investigated as EC materials due to their low power requirement, high coloration efficiency, and memory effect under an open-circuit condition. Nickel oxide (NiO), a p-type wide band gap semiconductor, exhibits attractive features such as a high color contrast ratio, good chemical stability, cost-effectiveness, and good compatibility with the cathodically coloring tungsten oxide. NiO thin films have been fabricated by various methods, but these are not cost-effective, scalable, or suitable for flexible applications. With the increasing demand for flexible and soft EC devices, it is essential to find routes to fabricate NiO thin films at lower temperatures. In this work, a NiO/Ni(OH)2-based thin EC layer on fluorine-doped tin oxide-coated glass is developed via an electroless nickel (EN) deposition route, followed by room-temperature electrochemical oxidation. The deposition time is optimized to control the film thickness. The EC performance is investigated in an aqueous alkaline electrolyte (1 M KOH) by means of cyclic voltammetry, chronoamperometry, and transmittance measurements. Both the as-deposited and annealed films, after electrochemical oxidation, exhibit excellent EC properties with an optical modulation of approximately 64% (at 550 nm) and good response times of approximately 3 s (coloration) and 14 s (bleaching). A 2 × 2 display obtained by patterning the EN deposition is also demonstrated as part of this work.

Photo-induced self-reduction enabling ultralow threshold voltage energy-conservation electrochromism

Electrochromic (EC) devices are highly desired for fields like smart windows, sensing, and display because of their great light modulation ability. However, the high driving voltage of the EC device usually leads to the decomposition of internal electrolyte, degradation of cyclability and higher energy consumption. In this work, based on the finding of the photo-induced self-reduction and coloration of light illuminated viologen/TiO2 gels (VTGs), we develop VTG-based devices which show remarkable EC performance under an ultralow voltage (∼0.1 V). The working energy consumption of a VTG device is at least one order of magnitude lower than those of reported EC devices and other display technologies, giving rise to the larger area/scale optical modulation. Furthermore, through the construction of RGB array displays, large-area EC devices, and flexible EC devices, the universal applicability and multifield potential applications of VTGs were demonstrated.

P-doped carbon quantum dot graft-functionalized amorphous WO3 for stable and flexible electrochromic energy-storage devices

The intrinsic structural instability of amorphous WO3 (a-WO3) following repeated Li ion insertion/extraction and bending hinders its application in stable and flexible electrochromic (EC) energy-storage devices. To resolve this problem, developing an elaborate heterostructure of a-WO3 is essential for high-performance flexible EC energy-storage devices. Herein, a novel structure is proposed to introduce P-doped carbon quantum dots (P-CQDs) into a-WO3 through a low-temperature process to induce robust chemical linkages with a-WO3. The functional groups of P-CQDs form strong hydrogen bonds with a-WO3 in the form of quantum dot grafts, which effectively tolerate structural deformation following long, repetitive cycling electrochemical reactions and bending. Moreover, the functional groups and graphitic carbon of P-CQDs improve the Li ion wettability and electrical conductivity, respectively, thereby enhancing the electrochemical activity and kinetics. As a result of the impacts, P-CQD graft-functionalized a-WO3 (P-CQD/a-WO3) electrodes exhibit excellent EC energy-storage performances. Furthermore, a flexible EC energy-storage device fabricated with the P-CQD/a-WO3 electrode exhibits a remarkably enhanced long-term cycle stability (transmittance retention of 87.3% after 4,000 cycles) and flexibility (transmittance retention of 84.1% and specific capacitance retention of 85.3% after 500 bending cycles). Therefore, we believe that the proposed heterostructure-inducing quantum dot graft functionalization with a-WO3 is a breakthrough in the realization of high-performance flexible EC energy-storage devices.

All-in-one wearable electronics design: Smart electrochromic liquid-crystal-clad fibers without external electrodes

Smart textiles and wearable sensors based on liquid crystal materials with highly tunable and controllable discoloration are attracting attention due to their promising functionality. However, products previously reported almost relies on the extra transparent conductive electrode to provide a stable electric field and resulting in complexity and inconvenience. Besides, practical applications remain limited by the lack of continuously processed long electrochromic fibers suitable for industrial weaving. This work presents smart electrochromic liquid-crystal-clad (ECLCC) fibers with multi-environmental stability and long-range controllability that are prepared continuously using simple tailormade instruments. By introducing a variety of electrochromic materials (CLCs) and a distinctive device design (coaxial dual-counter-electrode sandwich structure), multiple rapid and uniform color changes can be achieved over a long duration. The monofilament conductive core provides ECLCC fiber with enhanced color effects without viewing angle dependence, as well as good flexibility and stretchability. The fiberized coaxial electrode structure realizes the flexible electrochromic of textiles compared to the previously parallel sandwich structure of the traditional liquid crystal display. Thus, complex textiles can be easily knitted from fibers that respond to different external electro stimuli, showed the potential applications in intelligent clothing that sense and report the healthy condition of the wearer or the situation of the surrounding environment.

Preparation and electrochromic properties of flexible transparent WO3/AgNW-decorated nanofiber composite film

It is well-known that tungsten oxide (WO3) is extensively used for electrochromic (EC) devices due to its special properties, such as high coloration/bleaching response time, good coloration efficiency, etc. The preparation of flexible EC devices is highly desirable for various applications. However, the fabrication of flexible EC devices with improved performance is still a major task due to their limited experimental protocols. In this study, we have prepared highly flexible transparent films consisting of WO3 and silver nanowires coated on nanofiber-reinforced composite films (WO3/AgNW-NCF). A novel flexible transparent WO3/AgNW-NCF is used for EC applications. The as-fabricated EC device based on WO3/AgNW-NCF demonstrated good coloration efficiency (161.3 cm2 C−1), excellent stability (81.5%) and rapid response time for coloration (2.9 s) and bleaching (3.2 s). The fabricated transparent flexible EC device may potentially be useful for designing an outstanding EC device for smart window applications.

Low-Temperature Deposition of Transparent Conducting Films Applied to Flexible Electrochromic Devices.

Here, we compare two different transparent conducting oxides (TCOs), namely indium tin oxide (ITO) and indium zinc tin oxide (IZTO), fabricated as transparent conducting films using processes that require different temperatures. ITO and IZTO films were prepared at 230 °C and at room temperature, respectively, on glass and polyethylene terephthalate (PET) substrates using reactive magnetron sputtering. Electrochromic WO3 films deposited on ITO-based and IZTO-based ECDs using vacuum cathodic arc plasma (CAP) were investigated. IZTO-based ECDs have higher optical transmittance modulation, ΔT = 63% [from Tbleaching (90.01%) to Tcoloration (28.51%)], than ITO-based ECDs, ΔT = 59%. ECDs consisted of a working electrochromic electrode (WO3/IZTO/PET) and a counter-electrode (Pt mesh) in a 0.2 M LiClO4/perchlorate (LiClO4/PC) liquid electrolyte solution with an active area of 3 cm × 4 cm a calculated bleaching time tc of 21.01 s and a coloration time tb of 4.7 s with varying potential from −1.3 V (coloration potential, Vc) to 0.3 V (bleaching potential, Vb).

Multifunctional polyimides containing triarylamine for electrochromic flexible device, photodetector and resistance memory device

Integrating different functions in one material will decrease the volume, responding time, and price of device. Polyimides (PIs) are the optimal candidates because of excellent flexible mechanics, thermal stability, and transparent properties. In this work, a series of multifunctional flexible PIs were designed and synthesized from a novel functional triarylamine for electrochromic (EC) device. The PIs show promising application in EC with the efficiency of 205 cm2 C−1, memory devices with the ON/OFF ratio of 3.7 × 104, and photodetectors with a limit being of 0.07 V. Furthermore, the flexible EC device of PI-3 (short for PI 3 made of 4,4′-(hexafluoroisopropylidene) diphthalic anhydride) keeps excellent stability, and the optical contrast does not decrease even after 2000 cycles.

Cover Feature: Ion Conductive Phytic Acid‐G Quadruplex Hydrogel as Electrolyte for Flexible Electrochromic Device (ChemNanoMat 6/2021)

Biomolecules assemble to conduct: An ion conducting transparent hydrogel with thermo-reversibility, self-healing and injectable properties can be developed spontaneously through a supramolecular self-assembly of phytic acid with guanosine monophosphate at room temperature. The efficient ionic mobility in the self-assembled phytic acid crosslinked G-quadruplex nanofibres resulted in its applicability as a gel electrolyte for flexible electronic devices. More information can be found in the Full Paper by Tridib K. Sarma et al.

Amorphous-quantized WO3·H2O films as novel flexible electrode for advanced electrochromic energy storage devices

The existing synthetic approaches for high-performance WO3-based electrodes require energy-intensive instrumentation and complex processing, which hinder the development of flexible electrochromic (EC) energy storage devices. Herein, new low-temperature-synthesized amorphous (a)-quantized WO3·H2O films for application in EC energy storage devices are proposed. The WO3·H2O films are fabricated by the spontaneous hydrolysis of a spin-coated WCl6 solution by the water molecules in the surrounding atmosphere followed by annealing at 80 °C. This is an original and unique concept in that the induced quantization of a-WO3 increases the number of electroactive sites, provides abundant oxygen vacancies, and widens the band gap, while the intercalated water molecules stabilize the structure, resulting in efficient charge transfer and stress alleviation during electrochemical reactions. Consequently, the transmittance (53.8% at 633 nm) and specific capacitance (78.5 F g−1 at 1 A g−1) of the flexible electrodes improves. Additionally, the carrier concentration and mobility increase due to a-WO3 quantization, thereby increasing the electrical conductivity, resulting in the rapid switchability (3.7 s for coloration and 2.9 s for bleaching) and high rate capability of the flexible electrodes. The flexible solid-state devices light a 1.5 V white-light-emitting diode and maintain their good EC energy storage performance (76.1% transmittance modulation and 73.1% specific capacitance) even after 300 bending cycles at a bending radius of 1.3 cm. Therefore, the proposed a-quantized WO3·H2O films are promising active electrodes for flexible EC energy storage devices.

Defective impacts on amorphous WO3·H2O films using accelerated hydrolysis effects for flexible electrochromic energy-storage devices

We newly developed amorphous WO3·H2O (a-WO3·H2O) films with porosity and oxygen vacancy (VO) defects through a humidity adjustment causing the accelerated hydrolysis of WOCl4 with H2O during spin-coating and during low-temperature annealing for flexible electrochromic (EC) energy-storage devices. Optimizing the hydrolysis effect in all a-WO3·H2O films, we adjusted the humidity to 25, 35, and 45% in a humid chamber. Specifically, the a-WO3·H2O film fabricated at 35% exhibited a developed porous morphology and an increased number of VO defects, providing increased electrochemically active sites and enhanced electrical conductivity, respectively, due to the accelerated hydrolysis of WOCl4 and the increased intercalation of water molecules. Such behaviors of the a-WO3·H2O film bring about superior flexible EC energy-storage performances of widened transmittance modulation (60.0% at 633 nm), fast switching speeds (3.4 s for coloration speed and 4.2 s for bleaching speed), a high CE (62.7 cm2/C), good specific capacitance (94.2F/g at 2A/g), and rate capability (74.3%). Specifically, the increased transmittance modulation and specific capacitance stem from the increased electrochemical activity caused by the enriched electrochemically active sites. Moreover, the fast switching speeds and good rate capability are generated by electrochemical kinetics improved with the porous morphology and the increased VO.

Ion Conductive Phytic Acid‐G Quadruplex Hydrogel as Electrolyte for Flexible Electrochromic Device

AbstractSelf‐healing and thermo‐reversible hydrogels derived from pristine biomolecules with potential ionic conductivity represent a promising class of functional and flexible materials for a range of applications such as bioelectronics, energy storage, and wearable and implantable devices. Herein, spontaneous formation of an ion conductive hydrogel through the reversible non‐covalent interaction of pristine biomolecules, phytic acid and guanosine‐5'‐monophosphate in aqueous medium is reported that shows excellent self‐healing and thermo‐reversible properties. The phytic acid biomolecule induces effective cross‐linking of the highly ordered G‐tetrad leading to gelation. The homogeneous and mechanically strong hydrogel is used as a gel electrolyte for flexible electrochromic devices taking advantage of the high ionic conductivity. The biocompatibility, robustness, good conductivity and multifunctional applicability of phytic acid induced G‐quadruplex hydrogel, makes it a prospective material for environmental and bioelectronics applications.

Flexible and high-performance electrochromic devices enabled by self-assembled 2D TiO2/MXene heterostructures

Transition metal oxides (TMOs) are promising electrochromic (EC) materials for applications such as smart windows and displays, yet the challenge still exists to achieve good flexibility, high coloration efficiency and fast response simultaneously. MXenes (e.g. Ti3C2Tx) and their derived TMOs (e.g. 2D TiO2) are good candidates for high-performance and flexible EC devices because of their 2D nature and the possibility of assembling them into loosely networked structures. Here we demonstrate flexible, fast, and high-coloration-efficiency EC devices based on self-assembled 2D TiO2/Ti3C2Tx heterostructures, with the Ti3C2Tx layer as the transparent electrode, and the 2D TiO2 layer as the EC layer. Benefiting from the well-balanced porosity and connectivity of these assembled nanometer-thick heterostructures, they present fast and efficient ion and electron transport, as well as superior mechanical and electrochemical stability. We further demonstrate large-area flexible devices which could potentially be integrated onto curved and flexible surfaces for future ubiquitous electronics.

Flexible electrochromic and thermochromic hybrid smart window based on a highly durable ITO/graphene transparent electrode

Abstract A flexible smart window with electrochromic (EC) and thermochromic (TC) properties is fabricated on a highly durable transparent indium-tin oxide (ITO)/graphene/polyethylene terephthalate (PET) electrode. An amorphous WO3 film and crystalline VO2 film are used as the EC and TC material, respectively, in the device. The ITO film deposited via atomic layer deposition acts as a protective layer and suppresses the plasma-induced damage of graphene, which typically occurs during the sputtering process. In this device, the amorphous WO3 film undergoes cathodic electrochromic reactions with high coloration efficiency, while the crystalline VO2 film shows characteristic thermochromic behavior. Thus, the flexible EC–TC hybrid smart window shows the controlled transmission of optical and solar energies according to the surrounding temperature and the applied voltage, allowing selective modulation of the visible and near-infrared light. The flexible EC–TC hybrid device is also very stable under mechanical bending, owing to the robust adhesion of the ITO film on the graphene support and the reduced plasma-induced damage of graphene. The fabricated high-performance, durable ITO/graphene/PET transparent electrode can pave the way for next-generation flexible optoelectronic devices as well as multifunctional flexible smart windows.

Highly Stable Ag–Au Core–Shell Nanowire Network for ITO‐Free Flexible Organic Electrochromic Device

AbstractSolid and flexible electrochromic (EC) devices require a delicate design of every component to meet the stringent requirements for transparency, flexibility, and deformation stability. However, the electrode technology in flexible EC devices stagnates, wherein brittle indium tin oxide (ITO) is the primary material. Meanwhile, the inflexibility of metal oxide usually used in an active layer and the leakage issue of liquid electrolyte further negatively affect EC device performance and lifetime. Herein, a novel and fully ITO‐free flexible organic EC device is developed by using Ag–Au core–shell nanowire (Ag–Au NW) networks, EC polymer and LiBF4/propylene carbonate/poly(methyl methacrylate) as electrodes, active layer, and solid electrolyte, respectively. The Ag–Au NW electrode integrated with a conjugated EC polymer together display excellent stability in harsh environments due to the tight encapsulation by the Au shell, and high area capacitance of 3.0 mF cm−2 and specific capacitance of 23.2 F g−1 at current density of 0.5 mA cm−2. The device shows high EC performance with reversible transmittance modulation in the visible region (40.2% at 550 nm) and near‐infrared region (−68.2% at 1600 nm). Moreover, the device presents excellent flexibility (>1000 bending cycles at the bending radius of 5 mm) and fast switching time (5.9 s).

A flexible and high-performance electrochromic smart window produced by WO3/Ti3C2Tx MXene hybrids

WO3 nanoparticle/Ti3C2Tx (MXene) nanosheet hybrids were prepared to fabricate electrochromic (EC) devices with significantly enhanced EC performance. And a large-sized flexible EC smart window was successfully fabricated and demonstrated.

Embedding Silver Nanowires into a Hydroxypropyl Methyl Cellulose Film for Flexible Electrochromic Devices with High Electromechanical Stability.

Transparent conductive films (TCFs) based on silver nanowires (AgNWs) are becoming one of the best candidates in realizing flexible optoelectronic devices. The AgNW-based TCF is usually prepared by coating AgNWs on a transparent polymer film; however, the coated AgNWs easily detach from the polymer underneath because of the weak adhesion between them. Herein, a network of AgNWs is embedded in the transparent hydroxypropyl methyl cellulose film, which has a strong adhesion with the AgNWs. The obtained TCF shows high optical transmittance (>85%), low roughness (rms = 4.8 ± 0.5 nm), and low haze (<0.2%). More importantly, owing to the embedding structure and strong adhesion, this TCF also shows excellent electromechanical stability, which is superior to the reported ones. Employing this TCF in a flexible electrochromic device, the obtained device exhibits excellent cyclic electromechanical stability and high coloring efficiency. Our work demonstrates a promising TCF with superior electromechanical stability for future applications in flexible optoelectronics.