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With the increasing interest in the application of electrochromism to flexible and wearable electronics in recent years, flexible electrochromic devices (ECDs) that can function at extreme temperatures are required. However, the functionalities of flexible ECDs are severely hampered by the inadequate choice of electrolytes, which might ultimately result in performance fading during low‐ and high‐temperature operations. Here, we develop a deep eutectic solvent (DES)‐based gel electrolyte that can maintain its optical, electrical, and mechanical properties over a wide range of temperatures (−40 to 150°C), exhibiting an extremely high visible‐range transmittance over 90%, ion conductivity of 0.63 mS cm−1, and fracture strain exceeding 2000%. Owing to the excellent processability of the DES‐based electrolytes, provided by dynamic interactions such as the lithium and hydrogen bonding between the DES and polymer matrix, a directly written patterning in ECDs is realized for the first time. The fabricated ECDs exhibit an excellent electrochromic behavior superior to the behavior of the ECDs fabricated with traditional gel electrolytes. The introduction of such DES‐based electrolytes is expected to pave the way for a widespread application of electrochromic products. The gel electrolyte constructed with deep eutectic solvent (DES) and acrylate‐based copolymer enables a direct writing electrolyte layer for patterning in flexible electrochromic devices (ECDs), which can maintain its excellent optical transparency, electrical, and mechanical properties over a wide span of temperature ranging from −40 to 150°C.

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. The manufacturing process of patterned flexible electrochromic devices (FECDs) complex and non‐customizable. To address this challenge, a strategy is proposed to prepare integrated FECDs via multi‐material direct writing 3D printing. The FECDs exhibit impressive electrochromic and mechanical properties, including high optical contrast and excellent cycling stability, and is shown to have great potential in wearable displays, camouflage, and smart windows.

Flexible electrochromic devices (FECDs) are promising candidates for the next generation of wearable electronics due to their low operating voltage and energy consumption. For the flexible electrochromic devices, the electrolyte is an important component. Typically, the electrolyte needs to be formulated according to the device structure and usage scenario. A high-performance electrolyte involves consideration of many factors, including choosing the right polymer, solvent, curing agent, and ion type to satisfy particular device specifications. In this work, a ultraviolet-curable solid–liquid host–guest (UV-SLHG) electrolyte is developed. Several aspects of performance are improved by introducing the solid–liquid coexisting microstructure without changing the electrolyte formulation, including excellent adhesion, a 30% increase in tensile characteristics, and a seven-fold increase in ionic conductivity when compared to a fully cured solid-state electrolyte. More importantly, the unique advantage of SLHG electrolytes lies that the thickness will not change significantly during bending. The FECD made by using the UV-SLHG-based electrolyte sustained 10,000 bending cycles at the bending radius of 2.5 mm while maintaining outstanding optical modulation. A wearable ring-type ECD and a battery-free FECD wine label were made as demonstrators. The UV-SLHG strategy is not only suitable for the FECDs but also universally applicable to other electrolyte-based of flexible electronics such as flexible capacitors and batteries.

Three homologous electrochromic conjugated polymers, each containing an asymmetric building block but decorated with distinct alkyl chains, were designed and synthesized using electrochemical polymerization in this study. The corresponding monomers, namely T610FBTT810, DT6FBT, and DT48FBT, comprise the same backbone structure, i.e., an asymmetric 5-fluorobenzo[c][1,2,5]thiadiazole unit substituted by two thiophene terminals, but were decorated with different types of alkyl chain (hexyl, 2-butyloctyl, 2-hexyldecyl, or 2-octyldecyl). The effects of the side-chain structure and asymmetric repeating unit on the optical absorption, electrochemistry, morphology, and electrochromic properties were investigated comparatively. It was found that the electrochromism conjugated polymer, originating from DT6FBT with the shortest and linear alkyl chain, exhibits the best electrochromic performance with a 25% optical contrast ratio and a 0.3 s response time. The flexible electrochromic device of PDT6FBT achieved reversible colors of navy and cyan between the neutral and oxidized states, consistent with the non-device phenomenon. These results demonstrate that subtle modification of the side chain is able to change the electrochromic properties of conjugated polymers.

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%.

Smart electronics and optoelectronics such as smart windows, touch panels and solar cells have profoundly changed our lives over the last decade, as a result of the development of transparent conductive electrodes. In particular, indium tin oxide-based electrodes dominate the market of the electronic and optoelectronic devices, but these electrodes have limitations. For instance, indium ores are only found in a few countries, such as China, Peru and Canada, thus limiting the global competitiveness of some countries. Moreover, indium tin oxide is quite brittle and easy to crack, thus unsuitable in flexible electronics. These issues may be solved by developing metal nanowire-based transparent conductive electrodes, such as Cu, Ag and Au nanowire electrodes. Here, we review the latest advances for metal nanowire-based transparent conductive electrodes, including the strategies for the synthesis of metal nanowires, the preparation of metal nanowire films by various solution-based deposition processes and their practical applications as electrodes in diverse flexible chromic devices.

A recently described flexible polyurethane electrolyte was artificially weathered at 25/50 °C and 50% r.h. in air and at 25 °C in a dry nitrogen atmosphere, each with and without UV irradiation. Different formulations and the polymer matrix, used as a reference, were weathered in order to investigate the influence of the amount of conductive lithium salt and the solvent propylene carbonate. The complete loss of the solvent at a standard climate was already observed after a few days, strongly influencing the conductivity and mechanical properties. The essential degradation mechanism appears to be the photo-oxidative degradation of the polyol’s ether bonds, which leads to chain scission, oxidation products and negative changes in the mechanical and optical properties. A higher salt content has no effect on the degradation; however, the presence of propylene carbonate intensifies the degradation.

A 1,4-bis((9H-carbazol-9-yl)methyl)benzene (DCB)-containing homopolymer (P(DCB)) and four DCB- and ED-derivative (3,4-ethylenedioxythiophene (EDOT) and 3,4-ethylenedioxythiophene-methanol (EDm))-containing copolymers (P(DCB-co-ED), P(2DCB-co-ED), P(DCB-co-EDm), and P(2DCB-co-EDm)) were electropolymerized on ITO-polyethylene terephthalate (PET) substrates and their electrochromic performances were studied. DCB displays a lower Eonset than that of EDOT and EDm, conjecturing that the biscarbazole-containing DCB group shows a stronger electron-donating property than that of the ED derivatives. The P(2DCB-co-ED) film presents slate grey, dark khaki, and dark olive green at 0.0, 1.0, and 1.2 V. Bleaching-to-coloring switching studies of polymers show that P(2DCB-co-EDm) shows a high ΔT (31.0% at 725 nm) in solutions. Five dual-layer flexible electrochromic devices (ECDs) based on P(DCB), P(DCB-co-ED), P(2DCB-co-ED), P(DCB-co-EDm), and P(2DCB-co-EDm) as the anodic materials and PEDOT-PSS as the cathodic material are constructed. The P(2DCB-co-ED)/PEDOT-PSS flexible ECD shows a high ΔT (40.3% at 690 nm) and long-term electrochemical cycling stability, while the P(DCB-co-EDm)/PEDOT-PSS ECD shows a high ΔT (39.1% at 640 nm) and short response time (≤1.5 s). These findings offer us a new structural insight for the valuable design of conjugated polymers in high-contrast, flexible ECDs.

Highlights A flexible dual-band electrochromic device with a high optical modulation and a long cycle life was reported. The device assembled can modulate the visible light and near-infrared independently and effectively, showing higher energy-saving performance than commercial low-emissivity glass in most climatic zones around the world. The flexible device also shows good energy storage and energy recycling performances, recycling 51.4% of the energy consumed in the coloration process for local reusing. Dual-band electrochromic devices capable of the spectral-selective modulation of visible (VIS) light and near-infrared (NIR) can notably reduce the energy consumption of buildings and improve the occupants' visual and thermal comfort. However, the low optical modulation and poor durability of these devices severely limit its practical applications. Herein, we demonstrate an efficient and flexible bifunctional dual-band electrochromic device which not only shows excellent spectral-selective electrochromic performance with a high optical modulation and a long cycle life, but also displays a high capacitance and a high energy recycling efficiency of 51.4%, integrating energy-saving with energy-storage. The nanowires structure and abundant oxygen-vacancies of oxygen-deficient tungsten oxide nanowires endows it high flexibility and a high optical modulation of 73.1% and 85.3% at 633 and 1200 nm respectively. The prototype device assembled can modulate the VIS light and NIR independently and effectively through three distinct modes with a long cycle life (3.3% capacity loss after 10,000 cycles) and a high energy-saving performance (8.8 °C lower than the common glass). Furthermore, simulations also demonstrate that our device outperforms the commercial low-emissivity glass in terms of energy-saving in most climatic zones around the world. Such windows represent an intriguing potential technology to improve the building energy efficiency.

Low‐cost, flexible and thin display technology is becoming an interesting field of research as it can accompany the wide range of sensors being developed. Here, the synthesis of poly(dimethylpropylene‐dioxythiophene) (PProDOT‐Me2) by combining vapor phase polymerization and screen printing is presented. A multilayer architecture using poly(3,4‐ethylenedioxythiophene) (PEDOT) and PProDOT‐Me2 to allow for electrochromic switching of PProDOT‐Me2, thereby eliminating the need for a supporting transparent conductive (metal oxide) layer is introduced. Furthermore, the technology is adapted to a blended architecture, which removes the additional processing steps and results in improved color contrast (∆E* > 25). This blend architecture is extended to other conductive polymers, such as PEDOT and polypyrrole (PPy), to highlight the ability of the technique to adjust the color of all‐printed electrochromic displays. As a result, a green color is obtained when combining the blue and yellow states of PEDOT and PPy, respectively. This technology has the potential to pave the way for all‐printed multicolored electrochromic displays for further utilization in printed electronic systems in various Internet of Things applications. Blends and multilayers of conducting polymers are developed and demonstrated in all‐printed electrochromic displays. The electrochromic layers are synthesized through a combination of vapor phase polymerization and screen printing. Displays with multiple colors and high color contrast are obtained by combining poly(dimethylpropylene‐dioxythiophene) (PProDOT‐Me2) or polypyrrole (PPy) with poly(3,4‐ethylenedioxythiophene) (PEDOT), where the latter obviates the need of transparent conductive oxides.