Viologen-based flexible electrochromic devices

Electrochromic technology has gained significant attention in various fields such as displays,smart windows,biomedical monitoring,military camouflage,human-machine interaction,and electronic skin due to its ability to provide reversible and fast color changes under applied voltage.With the rapid development and increasing demand for flexible electronics,flexible electrochromic devices(FECDs)that offer smarter and more controllable light modulation hold great promise for practical applications.The electrochromic material(ECM)undergoing color changes during the electrochemical reactions is one of the key components in electrochromic devices.Among the ECMs,viologens,a family of organic small molecules with 1,1'-disubstituted-4,4'-dipyridinium salts,have garnered extensive research interest,due to their well-reversible redox reactions,excellent electron acceptance ability,and the ability to produce multiple colors.Notably,viologen-based FECDs demonstrate color changes in the liquid or semisolid electrolyte layer,eliminating the need for two solid electrodes and thus simplifying the device structure.Consequently,viologens offer significant potential for the development of FECDs with high optical contrast,fast response speed,and excellent stability.This review aims to provide a comprehensive overview of the progress and perspectives of viologen-based FECDs.It begins by summarizing the typical structure and recent exciting developments in viologen-based FECDs,along with their advantages and disadvantages.Furthermore,the review discusses recent advancements in FECDs with additional functionalities such as sensing,photochromism,and energy storage.Finally,the remaining challenges and potential research directions for the future of viologen-based FECDs are addressed.

An overview of recent progress in the development of flexible electrochromic devices

Electrochromic materials are capable of reversibly switching their colors or optical properties through redox reactions under applied voltages,which have shown great potential applications including smart windows,nonemissive displays,optical filters,among others.Although the current rigid electrochromic devices have shown emerging interest and developed rapidly,many applications(e.g.,wearable/deformable optoelectronics)are blocked due to their inflexible features.Herein,the adaption of rigid electrochromic devices to flexible ones is of particular interest for the new era of smart optoelectronics.In this review,the current state-of-the-art achievements of flexible electrochromic devices(FECDs)are highlighted,along with their design strategies and the choice of electrochromic materials.The recent research progress of FECDs is reviewed in detail,and the challenges and corresponding solutions for real-world applications of FECDs are discussed.Furthermore,we summarize the basic fabrication strategies of FECDs and their potential applications.In addition,the development trend,the perspectives,and the outlook of FECDs are discussed at the end of this Review,which may provide recommendations and potential directions to advance the practical applications of FECDs.

PREPARATION OF STAR NETWORK PEG-BASED GEL POLYMER ELECTROLYTES FOR ELECTROCHROMIC DEVICES

An amorphous,colorless,and highly transparent star network polymer with a pentaerythritol core linking four PEG-block polymeric arms was synthesized from the poly(ethylene glycol)(PEG),pentaerythritol,and dichloromethane by Williamson reaction.FTIR and ~1H-NMR measurement demonstrated that the polymer repeating units were C[CH_2-OCH_2O-(CH_2CH_2O)_m-CH_2O-(CH_2CH_2O)_n-CH_2O]_4.The polymer host held well mechanical properties for pentaerythritol cross-linking.The gel polymer electrolytes based on Lithium pe...

Deep eutectic solvent-based gel electrolytes for flexible electrochromic devices with excellent high/low temperature durability

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℃),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.

Bifunctional flexible electrochromic energy storage devices based on silver nanowire flexible transparent electrodes

Flexible electrochromic energy storage devices(FECESDs)for powering flexible electronics have attracted considerable attention.Silver nanowires(AgNWs)are one kind of the most promising flexible transparent electrodes(FTEs)materials for the emerging flexible devices.Currently,fabricating FECESD based on AgNWs FTEs is still hindered by their intrinsic poor electrochemical stability.To address this issue,a hybrid AgNWs/Co(OH)_(2)/PEDOT:PSS electrode is proposed.The PEDOT:PSS could not only improve the resistance against electrochemical corrosion of AgNWs,but also work as functional layer to realize the color-changing and energy storage properties.Moreover,the Co(OH)_(2)interlayer further improved the color-changing and energy storage performance.Based on the improvement,we assembled the symmetrical FECESDs.Under the same condition,the areal capacitance(0.8 mF cm^(−2))and coloration efficiency(269.80 cm^(2)C−1)of AgNWs/Co(OH)_(2)/PEDOT:PSS FECESDs were obviously higher than AgNWs/PEDOT:PSS FECESDs.Furthermore,the obtained FECESDs exhibited excellent stability against the mechanical deformation.The areal capacitance remained stable during 1000 times cyclic bending with a 25 mm curvature radius.These results demonstrated the broad application potential of the AgNWs/Co(OH)_(2)/PEDOT:PSS FECESD for the emerging portable and multifunctional electronics.

All-solid-state electrochromic devices based on WO3‖NiO films:material developments and future applications

Electrochromism refers to the persistent and reversible change of optical properties by an applied voltage pulse.Electrochromic(EC)devices have been extensively studied because of their commercial applications in smart windows of green buildings,display devices and thermal control of equipments.In this review,a basic EC device design is presented based on useful oxides and solid-state electrolytes.We focus on the state-of-the-art research activities related to the structures of tungsten oxide(WO_3)and nickel oxide(NiO),summarizing the strategies to improve their EC performances and further applications of devices.

CaF_(2):A novel electrolyte for all solid-state electrochromic devices

The energy consumption in building ventilation,air,and heating conditioning systems,accounts for about 25%of the overall energy consumption in modern society.Therefore,cutting carbon emissions and reducing energy consumption is a growing priority in building construction.Electrochromic devices(ECDs)are considered to be a highly promising energy-saving technology,due to their simple structure,active control,and low energy input characteristics.At present,Hþ,OH-and Liþare the main electrolyte ions used for ECDs.However,Hþand OH-based electrolytes have a high erosive effect on the material surface and have a relatively short lifetime.Liþ-based electrolytes are limited due to their high cost and safety concerns.In this study,inspired by prior research on Ca^(2+)þbatteries and supercapacitors,CaF_(2)films were prepared by electron beam evaporation as a Ca^(2+)þ-based electrolyte layer to construct ECDs.The structure,morphology,and optical properties of CaF_(2)films were characterized.ECDs with the structure of ITO(indium tin oxide)glass/WO3/CaF_(2)/NiO/ITO show short switching times(22.8 s for the coloring process,2.8 s for the bleaching process).Additionally,optical modulation of the ECDs is about 38.8%at 750 nm.These findings indicate that Ca^(2+)þbased ECDs have the potential to become a competitive and attractive choice for large-scale commercial smart windows.

Self-powered electrochromic devices with tunable infrared intensity

Triboelectric nanogenerator （TENG） is an efficient way to convert ambient mechanical energy into electricity to power up portable electronics. In this work, a flexible inflared electrochromical device （IR-ECD） with stable performances was assembled with a TENG for building self-powered infrared detector with tunable intensity. As driven by TENG, the electrochromic device could be operated in the mid-lR region due to the reversible electrochromic reactions. An average infrared reflectance contrast of 46% was achieved in 8-14 μm regions and as well a clear thermal image change can be observed. This work indicates that the TENG-driven infrared electrochromical device has potential for use in self-powered camouflage and tbermal control.

Preparation of a Star Network PEG-based Gel Polymer Electrolyte and Its Application to Electrochromic Devices

A star network polymer with a pentaerythritol core linking four PEG-block polymeric arms was synthesized, and its corresponding gel polymer electrolyte based on lithium perchlorate and plasticizers EC/PC with the character being colorless and highly transparent has been also prepared. The polymer host was characterized and confirmed to be of a star network and an amorphous structure by FTIR, ^1H NMR and XRD studies. The polymer host hold good mechanical properties for pentaerythritol cross-linking. Maximum ionic conductivity of the prepared polymer electrolyte has reached 8.83 × 10 ^-4 S·cm^-1 at room temperature. Thermogravimetry （TG） of the polymer electrolyte showed that the thermal stability was up to at least 150 ℃. The gel polymer electrolyte was further evaluated in electrochromic devices fabricated by transparent PET-ITO and electrochromically active viologen derivative films, and its excellent performance promised the usage of the gel polymer electrolyte as ionic conductor material in electrochrornic devices.

The Synthesis of 1-Ethyl-1'-(4-vinylbenzyl)-4,4'-bipyridinium Chloride and Iodide and its Electrochromic Property

Abstract： A new compound 1-ethyl-1＇-（4-vinylbenzyl）-4, 4＇-bipyridinium chloride and iodide has been synthesized. The cyclic voltammogram and impedance spectra indicated that a layer of viologen＇s electrochromic （EC） film could be deposited on conductive ITO glass working electrode With polyelectrolyte as ionic conduction layer, solid EC devices based on this compound have been assembled and their thickness was about 2.35 mm. When different voltages were added, they showed blue or violet red color. After optimization, its response time was less than 50 ms, the number of redox circulation was over 107 and the color of coloration states could be kept for 3 days. This kind of EC device can meet the demand of electronic ink.

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

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.

A high-performance electrochromic device assembled with WO_(3)/Ag and TiO_(2)/NiO composite electrodes towards smart window

The choice of cathode and anode materials for electrochromic devices plays a key role in the performance of electrochromic smart windows.In this research,WO_(3)/Ag and TiO_(2)/NiO composite thin films were separately prepared by the hydrothermal method combined with electrodeposition.The electrochromic properties of the single WO_(3) thin film were optimized,and TiO_(2)/NiO composite films showed better electrochromic performance than that of the single NiO film.WO_(3)/Ag and TiO_(2)/NiO composite films with excellent electrochromic properties were respectively chosen as the cathode and the anode to construct a WO_(3)/Ag‒TiO_(2)/NiO electrochromic device.The response time(tc=4.08 s,tb=1.08 s),optical modulation range(35.91%),and coloration efficiency(30.37 cm^(2)·C^(-1))of this electrochromic device are better than those of WO_(3)-NiO and WO_(3)/Ag-NiO electrochromic devices.This work provides a novel research idea for the performance enhancement of electrochromic smart windows.

Polymer ionogels and their application in flexible ionic devices

Polymer ionogel(PIG)is a new type of flexible,stretchable,and ion-conductive material,which generally consists of two components(polymer matrix materials and ionic liquids/deep eutectic solvents).More and more attention has been received owing to its excellent properties,such as nonvolatility,good ionic conductivity,excellent thermal stability,high electrochemical stability,and transparency.In this review,the latest research and developments of PIGs are comprehensively reviewed according to different polymer matrices.Particularly,the development of novel structural designs,preparation methods,basic properties,and their advantages are respectively summarized.Furthermore,the typical applications of PIGs in flexible ionic skin,flexible electrochromic devices,flexible actuators,and flexible power supplies are reviewed.The novel working mechanism,device structure design strategies,and the unique functions of the PIG-based flexible ionic devices are briefly introduced.Finally,the perspectives on the current challenges and future directions of PIGs and their application are discussed.

Multicolor V_(2)O_(5)/TiO_(2)electrochromic films with fast switching and long lifespan for camouflage and information display

V_(2)O_(5),which has multicolor and energy storage properties,is a promising electrochromic material for multifunctional electrochromic devices,but its practical application is limited by its poor lifespan and long switching time.In this work,high-performance V_(2)O_(5)/TiO_(2)films were fabricated by spraying a V_(2)O_(5)solution on in situ-grown TiO_(2)nanorods.Due to the porous structure formed between the TiO_(2)nanorods and the remarkable electron transfer performance of TiO_(2),the switching time of the V_(2)O_(5)/TiO_(2)films decreased.Moreover,the strong adhesion between the TiO_(2)nanorods and F-doped tin oxide(FTO)glass and the increased surface roughness of the substrates significantly improved the cycling stability of the V_(2)O_(5)/TiO_(2)films.With a large transmittance modulation(47.8%at 668 nm),fast response speed(τ_(c)=5.1 s,τ_(b)=4.2 s),and long lifespan,V_(2)O_(5)/TiO_(2)films were used as electrodes for the electrochromic energy storage device(EESD),which switched in six colors through color overlay:dark orange,sandy yellow,green-yellow,yellow-green,dark green,and dark brown.Inspired by pixel displays,EESDs were designed by segmenting V_(2)O_(5)films to stagger the display of the electrochromic and ion storage layers,which presented 11 types of information based on different combinations of colors.This work provides inspiration for developing multifunctional electrochromic devices,especially for camouflage and information displays.

In-situ characterization of electrochromism based on ITO/PEDOT:PSS towards preparation of high performance device

Poly（3,4-ethylenedioxythiophene）：poly（styrenesulfonate）（PEDOT：PSS） is usually sandwiched between indium tin oxide（ITO） and a functional polymer in order to improve the performance of the device. However, because of the strong acidic nature of PEDOT：PSS, the instability of the ITO/PEDOT：PSS interface is also observed. The mechanism of degradation of the device remains is unclear and needs to be further studied. In this article, we investigate the in-situ electrochromism of PEDOT：PSS to disclose the cause of the degradation. X-ray photoelectron spectroscopy（XPS） was used to characterize the PEDOT：PSS films, as well as the PEDOT：PSS plus polyethylene glycol（PEG） films with and without indium ions. The electrochromic devices（ECD） based on PEDOT：PSS and PEG with and without indium ions are carried out by in-situ micro-Raman and laser reflective measurement（LRM）. For comparison, ECD based on PEDOT：PSS and PEG films with LiCl, KCl, NaCl or InCl_3 are also investigated by LRM. The results show that PEDOT：PSS is further reduced when negatively biased, and oxidized when positively biased. This could identify that PEDOT：PSS with indium ions from PEDOT：PSS etching ITO will lose dopants when negatively biased. The LRM shows that the device with indium ions has a stronger effect on the reduction property of PEDOT：PSS-PEG film than the device without indium ions. The contrast of the former device is 44%, that of the latter device is about 3%. The LRM also shows that the contrasts of the device based on PEDOT：PSS＋PEG with LiCl, KCl, NaCl, InCl_3 are 30%, 27%, 15%, and 18%, respectively.

Polyoxometalate-MnO_(2)film structure with bifunctional electrochromic and energy storage properties

The polyoxometalates-based electrochromic energy storage devices(POMs-EESDs)were constructed using P2W17O6111
coated TiO_(2)as the working electrode and MnO_(2)film as the counter electrode.The MnO_(2)films with different thicknesses acted as the charge balancing layer.The device showed bifunctional of enhanced EC and energy storage properties along with high durability and cycle stabilities after the introduction of MnO_(2)film.The optimistic thickness of MnO_(2)was found to be 345 nm through balancing the optical modulation and the energy storage properties.For the EC aspect,the optical contrast was enhanced by four times under the potential step of
2.0/1.0 V.The MnO_(2)film also introduces high capacitance leading to the device can be used as a transparent capacitor.It is the first time to construct high-performance POMs-based EESD only by introducing a simple charge balancing(MnO_(2))layer in the device.Moreover,the mechanism of the MnO_(2)layer towards the performance improvement of the POMs-based EESDs was also explained.

Flexible Electrochromic Poly(thiophene-furan) Film via Electrodeposition with High Stability

Flexible electrochromic(EC) materials have an urgent demand in the current electronic equipment market due to their technological interest and applications. However, at present, few flexible EC devices developed by industry exist due to some problems and challenges still to be solved such as flexibility. In this work, we have successfully synthesized a novel thiophene-furan(TFu) monomer via Stille coupling reaction,and facilely electrochemically polymerized in a neutral Bu_4 NPF_6-CH_2Cl2 electrolyte system to afford the corresponding poly(thiophene-furan)(PTFu) polymer film with good flexibility. The electrochemical and photoelectrochemical analyses of the as-prepared PTFu demonstrate that it has achieved the improved EC performance compared with pure polyfuran and polythiophene polymers, and as a result it possesses favorable EC parameters manifested as a reasonable ΔT(32.1%), faster response(1.38 s), excellent coloration efficiency(CE, 300.9 cm^(2)·C^(-1)), and after a continuous redox process up to 2000 s, its optical stability can be maintained at 96%, and even after 3000 s, it can still be maintained at 80%. In addition, the successful assembly of the electrochromic device of PTFu film can easily realize the reversible conversion of the color from orange to gray. All these systematic studies suggest that the as-prepared flexible PTFu film is a promising candidate for EC materials and has great potential interest for versatile EC applications.

Copolymerization of Azobenzene-bearing Monomer and 3,4-Ethylenedioxythiophene(EDOT): Improved Electrochemical Performance for Electrochromic Device Applications

In this study,novel electrochromic copolymers of 3,4-ethylenedioxythiophene(EDOT)and(E)-1,2-bis(2-fluoro-4-(4-hexylthiophen-2-yl)phenyl)diazene(M1)with different monomer feed ratios were designed and synthesized electrochemically.Electrochemical and spectroelectrochemical characterizations were performed using voltammetry and UV-Vis-NIR spectrophotometry techniques to test the applicability of copolymers for electrochromic applications.In terms of electrochemical behaviors,addition of an electron-rich EDOT unit into the azobenzenecontaining copolymer increased the electron density on the polymer chain and afforded copolymers with very low oxidation potentials at around0.30 V.While the homopolymers(P1 and PEDOT)exhibited neutral state absorptions centered at 510 and 583 nm,EDOT-bearing copolymers showed red shifted absorptions compared to those of P1 with narrower optical band gaps.In addition,the poor optical contrast and switching times of azobenzene-bearing homopolymer were significantly improved with EDOT addition into the copolymer chain.As a result of the promising electrochromic and kinetic preperties,Co P1.5-bearing single layer electrochromic device that works between purple and light greenish blue colors was constructed and characterized.

Near-infrared and Multicolor Electrochromic Device Based on Polyaniline Derivative

Electroactive conducting copolymers of aniline （ANI） and diphenylamine （DPA） are prepared on indium tin oxide （ITO） surface from 1 mol/L H2SO4 aqueous solution with different feed ratios of ANI to DPA by using a potentiostatic method. FTIR spectra and SEM measurements are used to confirm the formation of copolymers. Due to the combination of the N,N＇-diphenyl benzidine and aniline units in the molecular chain, the copolymer films exhibit improved electrochemical and electrochromic properties, compared to PANI and PDPA. The copolymer [marked as P（ANI9-co-DPA1）] film prepared at a ratio of 9：1 （ANI/DPA） exhibits novel transmittance modulation both in visible and near-infrared （NIR） region between -0.8 V and 0.8 V （52% and 67% respectively） and fast response time （3.6 s for coloration and 2.3 s for bleaching at 600 nm）. An electrochromic device （ECD） based on P（ANI9-co-DPA1） and PEDOT：PSS is also fabricated and shows a multicolor electrochromic performance, with a good optical contrast （29% in visible region and 40% in NIR region）, acceptable response time （8.3 s for coloration and 7.5 s for bleaching at 600 nm） and long-term stability. Clear color changes from transparent （-0.8 V）, bright green （0 V）, seagreen （0.4 V） to dark slate gray （0.8 V） are demonstrated.

Li_(x)Na_(2-x)W_(4)O_(13) nanosheet for scalable electrochromic device

The printed electronics technology can be used to efficiently construct smart devices and is dependent on functional inks containing well-dispersed active materials.Two-dimensional(2D)materials are promising functional ink candidates due to their superior properties.However,the majority 2D materials can disperse well only in organic solvents or in surfactant-assisted water solutions,which limits their applications.Herein,we report a lithium(Li)-ion exchange method to improve the dispersity of the Na_(2)W_(4)O_(13) nanosheets in pure water.The Li-ion-exchanged Na_(2)W_(4)0_(13)(Li_(x)Na_(2-x)W_(4)O_(13))nanosheets show highly stable dispersity in water with a zeta potential of-55 mV.Moreover,this aqueous ink can be sprayed on various substrates to obtain a uniform LixNa2_xW4O13 nanosheet film,exhibiting an excellent electrochromic performance.A complementary electrochromic device containing a Li_(x)Na_(2-x)W_(4)O_(13) nanosheet film as an electrochromic layer and Prussian white(PW)as an ion storage layer exhibits a large optical modulation of 75% at 700 nm,a fast switching response of less than 2 s,and outstanding cyclic stability.This Na2W4Oi3-based aqueous ink exhibits considerable potential for fabricating large-scale and flexible electrochromic devices,which would meet the practical application requirements.