Flexible Electrochromic Devices Research Articles

SCC/PVA Hydrogel with Self‐Healing Capability for Electrochromic Device

AbstractCracking caused by aging or repeated bending can severely affect the lifetime of electrochromic devices. In this work, a self‐healing electrochromic hydrogel was developed with poly (vinyl alcohol) (PVA)‐sodium carboxymethyl chitosan (SCC) semi‐inter transfer network as the matrix, viologen bromide (HPV2+2Br−) as the electrochromic substance, and borax as the cross‐linking agent. The self‐healing properties of hydrogel stem from the dynamic and reversible borate bonds between borax and PVA molecular chains. As the voltage changes, the color of the device transitions from colorless to light purple and then to dark purple. The optical contrast at 600 nm reaches approximately 51 %. At room temperature, the disconnected hydrogel can fully restore its original state after 30 minutes of contact, effectively addressing issues related to cracks and damage in flexible electrochromic devices. This research holds significant implications for enhancing the reliability and stability of flexible electrochromic devices.

Regulation of Br on molecular aggregation, film morphology and electrochromic performance of polythiophene derivatives

In this work, five polythiophene derivatives were synthesized through electrochemical copolymerization of 3,3-bis(bromomethyl)-3,4-dihydro-2H-thieno[3,4-b] (Deng et al., 2023; Li et al., 2020) [1,4]dioxepine (ProDOT-Br) and 3,3-dimethyl-3,4-dihydro-2H-thieno[3,4-b] (Deng et al., 2023; Li et al., 2020) [1,4]dioxepine (ProDOT-Me) at different mass ratios. DFT results prove that the layer spacing of conjugated blocks was increased from 5.94 Å for PProDOT-Me to 8.89 Å for PProDOT-Br due to the introduction of Br atom into side chain. And, the diverse film morphologies, such as fibre shape, cellular shape, flat shape, etc, were obtained by changing Br content in polymer. Additionally, through optimizing Br content, PBM(1,2) achieved enhanced electrochromic properties relative to two reference polymers [PProDOT-Br and PProDOT-Me], such as high optical contrast (ΔT = 76 %), fast response time (0.5 s), high transparency, and good stability. Finally, the flexible electrochromic device (8 cm × 4 cm) based on PBM(1,2) was constructed, which achieves rapid color change from purple to transparent blue. The results indicate that introducing various contents of Br atom into side chain is an effective strategy for regulating molecular microstructure and optimizing electrochromic properties of conjugated polymers.

Self‐Healing Waterborne Polyurethanes as a Sustainable Gel Electrolyte for Flexible Electrochromic Devices

Self‐Healing Waterborne Polyurethanes as a Sustainable Gel Electrolyte for Flexible Electrochromic Devices

All-Climate Solid-State Flexible Electrochromic Devices Enabled by Layered Hydrated Vanadate/MXene Composite and Hydrogel Electrolyte

All-Climate Solid-State Flexible Electrochromic Devices Enabled by Layered Hydrated Vanadate/MXene Composite and Hydrogel Electrolyte

Recent Advances in Electrochromic Devices: From Multicolor to Flexible Applications

With increasing demand for energy saving and environmental sustainability, electrochromic devices (ECDs) are considered as emerging display devices with low energy consumption. While various reflective‐type displays produce images with low energy, achieving full color displays often involves much device complexity and nonflexibility. Multicolor ECDs aim to realize full color reflective‐type displays, surpassing the current monochromic type or limited coloration capabilities in a 1D color space. Enhancing device flexibility is also highly desirable for use of ECDs in wearable and flexible electronics for health monitoring and advanced textiles with easy visualization. In this review, recent advances in multicolor and flexible ECDs are examined. Several primary strategies to achieve multicolor ECD are described, including material modifications, color overlay, and dye‐mediated colorations. In addition, recent developments in flexible ECDs are explored, emphasizing novel materials and fabrication processes that improve mechanical durability and reliability under deformation. It is expected that this review will provide a comprehensive understanding of multicolor and flexible ECDs for applications in smart windows, displays, and wearable electronics.

In-situ-selective-UV crosslinking fabrication of solid liquid host guest electrolyte: A facile one-step method realizing highly flexible electrochromic device

In-situ-selective-UV crosslinking fabrication of solid liquid host guest electrolyte: A facile one-step method realizing highly flexible electrochromic device

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 360nm), nice cycling stability (less than 5% electroactivity reduction after 10000 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.

A Flexible Electrochromic Device for All-Season Thermal Regulation on Curved Transparent Building Envelopes.

As one of the least energy-efficient components in buildings, transparent building envelopes are responsible for approximately 60% of the total energy losses. Although controlling solar transmittance through electrochromic modulation is an effective method for temperature management in these structures, a dynamic control strategy for solar light on curved transparent building envelopes is still lacking. In this study, we introduce a dual-mode flexible electrochromic device based on reversible silver deposition for curved transparent building envelopes. The device operates by reversibly depositing and dissolving silver on a flexible polyethylene terephthalate-indium tin oxide (PET-ITO) substrate, controlled through the application and removal of pulsed voltage. This mechanism enables rapid switching between radiative cooling and solar heating modes, leading to modulation of solar reflectance from 89.1% to 15.7% and solar transmittance from 0.02% to 72.9%. Under approximately 700 W/m2 of solar irradiance, the device achieves an average temperature reduction of 1.6 °C (with a maximum reduction of 4.3 °C) compared to ambient temperature in radiative cooling mode. In solar heating mode, the device achieves an average temperature increase of 17.1 °C (with a maximum increment of 23.7 °C) compared to ambient temperature. Simulation results show that the dual-mode flexible electrochromic device could offer all-season thermal regulation for curved transparent building envelopes and achieve a maximum of over 50% annual HVAC energy savings.

Self‐Healing Waterborne Polyurethanes as a Sustainable Gel Electrolyte for Flexible Electrochromic Devices

Self‐healing polymers are promising for diverse applications in wearable technology and electronic skin. Polyurethanes are versatile polymers that can incorporate various monomer structures. Waterborne polyurethanes (WPUs) emerge as an environmentally conscious choice due to water usage instead of organic solvents, thereby minimizing the generation of volatile organic compounds. This study introduces a novel approach to enhance the self‐healing properties of WPUs by integrating disulfide bonds. These dynamic disulfide bonds undergo exchange reactions upon heating, facilitating the renewal of cross‐links on damaged film surfaces. Self‐healing WPUs with a low glass transition temperature achieve excellent self‐healing efficiency under mild conditions. Self‐healing adhesives applied to various flexible substrates retain stable peel strength, which confirms their potential as self‐healing solutions. Furthermore, the WPU hydrogel electrolyte is prepared with dihydroxyhexyl viologen (DHHV), and the prepared electrochromic gel exhibits good ionic conductivity while maintaining high transparency. The flexible electrochromic device exhibited excellent performances, including low coloration voltage, high coloration efficiency, and long‐term stability. The transmittance difference is exceptional, with over 99%, and no decay after repeated bending cycles is observed. The current results demonstrate the feasibility of self‐healing WPUs in improving the operation and durability of high‐performance flexible electrochromic devices.

Capillary Force-Induced Graphene Spontaneous Transfer and Encapsulation of Silver Nanowires for Highly-Stable Transparent Electrodes.

Silver nanowires (NWs) (AgNWs) have emerged as the most promising conductive materials in flexible optoelectronic devices owing to their excellent photoelectric properties and mechanical flexibility. It is widely acknowledged that the practical application of AgNW networks faces challenges, such as high surface roughness, poor substrate adhesion, and limited stability. Encapsulating AgNW networks with graphene has been recognized as a viable strategy to tackle these issues. However, conventional methods like self-assembly reduction-oxidation or chemical vapor deposition often yield graphene protective layers with inherent defects. Here, we propose a novel one-step hot-pressing method containing ethanol solution that combines the spontaneous transfer and encapsulation process of rGO films onto the surface of the AgNWs network, enabling the preparation of flexible rGO/AgNWs/PET (reduced graphene oxide/silver NWs/polyethylene terephthalate) electrodes. The composite electrode exhibits outstanding photoelectric properties (T ≈ 88%, R ≈ 6 Ω sq-1) and possesses a smooth surface, primarily attributed to the capillary force generated by ethanol evaporation, ensuring the integrity of the rGO delamination process on the original substrate. The capillary force simultaneously promotes the tight encapsulation of rGO and AgNWs, as well as the welding of the AgNWs junction, thereby enhancing the mechanical stability (20,000 bending cycles and 100 cycles of taping tests), thermal stability (∼30 °C and ∼25% humidity for 150 days), and environmental adaptability (100 days of chemical attack) of the electrode. The electrode's practical feasibility has been validated by its exceptional flexibility and cycle stability (95 and 98% retention after 5000 bending cycles and 12,000 s long-term cycles) in flexible electrochromic devices.

Carbon nanotube-grid infrared transparent electrodes for flexible electrochromic devices with visible to mid-infrared dual-band modulation

Flexible mid-infrared electrochromic devices (MIR-ECDs) have attracted wide attention due to the growing demands for thermal camouflage and body comfort. However, the normal conductive films (e.g., indium tin oxide/polyethylene terephthalate (ITO/PET) films) are unsuitable for MIR-ECDs due to low infrared transmittance. Herein, the infrared transparent carbon nanotube (CNT) grid films were prepared by designing the grid structure. The infrared transmittance of CNT grid film could reach 72 %; meanwhile, the visible transmittance could reach 75 %. The CNTs exhibited infrared modulation under different voltages due to the doping/de-doping of ions in electrolytes into the CNT network. The flexible electrochromic devices with visible to mid-infrared dual-band modulation were prepared by using CNT grid film as the electrode and poly (3,4 ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) as the electrochromic layer, which could realize the infrared synergistic modulation of PEDOT:PSS and CNT grid in the mid-infrared range and reversible color changes between light blue and dark blue in the visible range, showing promising applications in adaptive camouflage and thermal management.

MXene-based all-solid flexible electrochromic microsupercapacitor

With the increasing demand for multifunctional optoelectronic devices, flexible electrochromic energy storage devices are being widely recognized as promising platforms for diverse applications. However, simultaneously achieving high capacitance, fast color switching and large optical modulation range is very challenging. In this study, the MXene-based flexible in-plane microsupercapacitor was fabricated via a mask-assisted spray coating approach. By adding electrochromic ethyl viologen dibromide (EVB) into the electrolyte, the device showed a reversible color change during the charge/discharge process. Due to the high electronic conductivity of the MXene flakes and the fast response kinetics of EVB, the device exhibited a fast coloration/bleaching time of 2.6 s/2.5 s, a large optical contrast of 60%, and exceptional coloration efficiency. In addition, EVB acted as a redox additive to reinforce the energy storage performance; as a result, the working voltage window of the Ti3C2-based symmetric aqueous microsupercapacitor was extended to 1 V. Moreover, the device had a high areal capacitance of 12.5 mF cm−2 with superior flexibility and mechanical stability and showed almost 100% capacitance retention after 100 bending cycles. The as-prepared device has significant potential for a wide range of applications in flexible and wearable electronics, particularly in the fields of camouflage, anticounterfeiting, and displays.

Stretchable interconnected modular electrochromic devices enabled by self-healing, self-adhesive, and ion-conducting polymer electrolyte

Here, we propose a versatile terpolymer electrolyte material, UPy-PEGMA-VBMI, comprised of three monomers: ureidopyrimidinone methacrylate (UPyMA), poly(ethylene glycol) methacrylate (PEGMA), and vinyl benzyl-methylimidazolium (VBMI). The UPy-PEGMA-VBMI, supported by a combination of soft ethylene glycol chains, an ion-conducting imidazole unit, and self-complementary quadruple hydrogen bonds of UPy, exhibits exceptional properties such as high stretchability, rapid self-repair, solution processability, self-adhesion, and ion conduction. The UPy-PEGMA-VBMI was then applied to make ion gel composite films, demonstrating a notable ionic conductivity of 3.84 × 10−3 S cm−1 while maintaining robust stretching and adhesive characteristics. The electrochromic devices (ECDs) fabricated by this ion gel demonstrate outstanding performance, with fast switching times, long-term kinetic stability, and extraordinarily high coloration efficiency of 443.83 cm2 C−1. The stretchable ECDs are fabricated by utilizing the stretchable nature of UPy-PEGMA-VBMI, exhibiting a transmittance contrast of 42.7 % at an extension of 53 % and operational stability during stretching cycles. Moreover, self-healing and self-adhesive features enable the fabrication of interconnected modular ECDs, implementing area-scalable devices. These modular ECDs, each powered independently, provide arbitrary positioning and shifting of individual device patches, overcoming the performance limitations of conventional large-area devices. The unprecedented material and resulting devices open new possibilities for electrochromic applications, particularly in stretchable ECDs, pattern-switching devices, and scalable flexible ECDs.

Factors Impacting the Photostability of a Roll‐to‐Roll Processed PEDOT Derivative for Flexible Electrochromic Devices

AbstractFor the use of polymeric electrochromic devices (ECDs) in architectural or automotive applications, the photostability of the electrochromic (EC) polymer is crucial for long‐term durability. In this study, the photostability of the side‐chain modified poly(3,4‐ethylenedioxythiophene) (PEDOT‐EthC6)thin films deposited by roll‐to‐roll slot‐die coating on indium tin oxide coated polyethylene terephtalate (PET‐ITO) is investigated and characterized by UV–vis and IR spectroscopy. Residual iron salt on the EC polymer layer, necessary for the in situ polymerization, and the variation of storage times between processing steps have no influence on the degradation rate during light exposure. The photostability of the thin films is 30% better in an inert atmosphere than in ambient conditions. Different long‐pass filters are applied to enhance the photostability in the colored and the bleached state. The position of the absorption edge strongly affects the photostability and visual appearance/color of the PEDOT‐EthC6 electrodes. The analysis of the CIE L*a*b* color coordinates reveals that a trade‐off between stability and a desirable color is necessary. To prevent UV degradation of the EC polymer in flexible ECDs, sputtered nickel oxide is chosen as the counter electrode material. The hybrid ECD shows no sign of degradation and loss of electrochromic performance after 350 h of light exposure.

Potential Gradient-Driven Dual-Functional Electrochromic and Electrochemical Device Based on a Shared Electrode Design.

The integration of electrochromic devices and energy storage systems in wearable electronics is highly desirable yet challenging, because self-powered electrochromic devices often require an open system design for continuous replenishment of the strong oxidants to enable the coloring/bleaching processes. A self-powered electrochromic device has been developed with a close configuration by integrating a Zn/MnO2 ionic battery into the Prussian blue (PB)-based electrochromic system. Zn and MnO2 electrodes, as dual shared electrodes, the former one can reduce the PB electrode to the Prussian white (PW) electrode and serves as the anode in the battery; the latter electrode can oxidize the PW electrode to its initial state and acts as the cathode in the battery. The bleaching/coloring processes are driven by the gradient potential between Zn/PB and PW/MnO2 electrodes. The as-prepared Zn||PB||MnO2 system demonstrates superior electrochromic performance, including excellent optical contrast (80.6%), fast self-bleaching/coloring speed (2.0/3.2 s for bleaching/coloring), and long-term self-powered electrochromic cycles. An air-working Zn||PB||MnO2 device is also developed with a 70.3% optical contrast, fast switching speed (2.2/4.8 s for bleaching/coloring), and over 80 self-bleaching/coloring cycles. Furthermore, the closed nature enables the fabrication of various flexible electrochromic devices, exhibiting great potentials for the next-generation wearable electrochromic devices.

Sustainable kiwi fruit derived carbon dots architected WO3 (KF-CDs@WO3) thin films based flexible electrochromic devices with improved stability for next generation smart windows applications

The design and development of new advanced materials or technologies are essential for the construction of high performance smart windows for the next generation of energy efficient buildings. Tungsten trioxide (WO3) based flexible electro-chromic devices f-ECDs have received extensive attention for the development of next generation f-ECDs. Here, we demonstrated the design and fabrication of kiwi fruit peel derived carbon dots (KF-CDs) decorated WO3 films under benign approaches. Furthermore f-ECDs were developed and their performance have been examined in terms of transmittance and coloration efficiency. The amount of KF-CDs in to the WO3 was varied and optimized f-ECDs exhibited the excellent optical contrast (ΔT) of 83.74 %, coloration efficiency of 86.2 cm2/C. The optimized f-ECDs exhibited excellent cyclic stability of 25000th cycles and retained more than 80 % optical contrast compared to the initial performance. This improved stability may be attributed to the presence of strong hydrogen bonding between the functional groups of KF-CDs and WO3.

Structure and material designs of stretchable electrochromic devices

AbstractElectrochromic materials are a significant class of optoelectronic functional materials that can change colour by adjusting the voltage periodically. In recent years, there has been rapid development in electrochromic technology. However, current research predominantly focuses on traditional rigid electrochromic devices (ECDs), typically using conductive glass substrates such as indium tin oxide. These rigid colour‐changing devices face significant challenges, including large thickness, low mechanical strength, and high cost, which hinder the advancement and commercialisation of electrochromic technology. With the rise of wearable devices and electronic skins, among other future technologies, flexible ECDs have garnered much attention due to their foldability, wearability, and even embeddability. They have emerged as a research hotspot in the field of electrochromism. As a further development direction of flexible ECDs, achieving stretchability poses higher difficulties as it requires maintaining high performance under large strains and even distortions. This article provides an overview of the latest advances in stretchable electrochromic devices (SECDs) from the perspectives of structural and material design. Regarding structural design, the ‘island‐bridge’ structure, the ‘longitudinal wave’ structure, and core‐shell structures are discussed. In terms of material design, the design schemes of substrate, conductive layer, electrochromic layer and electrolyte layer are mainly introduced, with particular emphasis on the introduction of gel electrolyte. Finally, the challenges and difficulties faced by the development of SECDs are briefly analysed.

One-step achieving high performance all-solid-state and all-in-one flexible electrochromic supercapacitor by polymer dispersed electrochromic device strategy

Electrochromic devices (ECD) are widely used to regulate the transmittance of sunlight by applying a small voltage, but the drawbacks like complex layer-by-layer preparation procedures and inconvenient assembling process still exist. To address these problems, gel or solution-type all-in-one ECDs were recently developed for the simple structure, however, the leakage risk and absence of flexible large-area production have limited real applications. Herein, a novel all-solid-state and all-in-one flexible ECD was reported by originally developed polymer dispersed electrochromic device (PDECD) strategy. This all-solid-state flexible ECD could be efficiently prepared only by one step of phase separation without any extra treatment, and demonstrated outstanding stability (92.1 % of original ΔT remained after 10,000 cycles), high coloration efficiency (197 cm2/C), low power consumption (86.4 μW/cm2) and satisfied response time (≤12 s). Meanwhile, the stored power in ECD during coloring process could drive a LED with excellent cyclic stability (93 % of original capacity remained after 3000 cycles), implying that ECD could also serve as an idea electrochromic supercapacitor. What’more, a reported largest viologen-based all-solid-state flexible ECD (17.8 × 13.2 cm2) with robust bending resistance (up to 1000 bending cycles) was successfully fabricated with industrial roller coating technique, which indicated the huge potential in real world.

Enhanced Transport Kinetics of Electrochromic Devices by W18O49 NW/Ti3C2Tx Composite Films

AbstractElectrochromic devices can facilitate the realization of a wide set of future applications, ranging from energy‐saving windows to smart wearables and stealth. Historically, tungsten oxides have been the most studied materials for electrochromism, albeit with bottlenecks like limited conductivity, high charge transport barrier, and low ion diffusivity. Here, inspired by the recent MXene materials, the study has engineered an electrochromic composite of MXene nanosheets (Ti3C2Tx) and W18O49 nanowires (NWs). A transparent conductive electrode is fabricated by co‐assembly of Ag and W18O49NWs, followed by depositing W18O49 NW/Ti3C2Tx layers for the fabrication of the electrochromic device. The incorporation of Ti3C2Tx nanosheets enhances the transport of electrons and ions within the electrochromic layer, leading to a significant improvement in the electrochromic performance. Noteworthily, the film structure comprising 15 layers of W18O49 NW/Ti3C2Tx composite reveals enhanced transmittance modulation (61%), rapid response time (4.5 s coloration, 6.5 s bleaching), and high coloration efficiency (139.1 cm2 C−1). Moreover, the electrode also presents a high diffusion coefficient of Li+ and good cycling stability (96.66% after 250 switching cycles). Finally, a large‐scale (15 × 20 cm2) flexible electrochromic device with a solid electrolyte is successfully fabricated and utilized as a smart window and a flexible stealth patch.

A Flexible Electrochromic Device based on PEDOT: PSS Working in Visible and Infrared Band

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), a novel conducting polymer, has gained significant attention in the realm of electrochromism. In this study, we present the fabrication of a simple double-layer device utilizing PEDOT:PSS, which successfully achieves precise control of emissivity in the infrared band. Through comprehensive experimentation, we observe an impressive regulation amplitude of 0.25 and a temperature difference of 10°C, relative to a background temperature of 100°C. Additionally, the device exhibits remarkable flexibility compared to other existing devices. These findings demonstrate the potential of PEDOT:PSS-based double-layer devices as versatile tools for infrared emissivity modulation, holding promise for diverse applications in fields such as thermal management and energy-efficient technologies.