•Polymer achieving synchronous electrochromic and electrofluorochromic switching.•The polymer can present dual-state emission while remaining electrochromic property.•Fabricated device can realize electrochromic and electrofluorochromic switching.A novel designed polymer with a goal to achieve synchronous electrochromic and electrofluorochromic switching was synthesized. Biphenyl was introduced into thiophene backbone to form twisting conformation, alleviating common aggregation-caused quenching of polythiophenes. The polymer can emit intense fluorescence in both solution and solid state while remaining excellent electrochromic properties. With applied external potential, the sprayed polymer film can turn to a transparent state (non-fluorescent state) from yellow color (fluorescent state) with comparable large transmittance change (35%) and fast response time. Furthermore, the fabricated device containing this polymer can also realize reversible electrochromic and electrofluorochromic switching simultaneously, making the designed polymer promising candidate to be applied in optoelectronic fields.Download high-res image (241KB)Download full-size image

A novel durability-reinforced electrochromic device (ECD) is described, in which the electrochromic materials remain in different physical form. Solution-phase ethyl viologen is used as the working electrochromic material, and modified Ti-doped V2O5 is utilized as counter electrochromic material. Electrochromic and spectral performances of the device are recorded here. The ECD shows good electrochromic performances in the visible and utraviolet region. Particularly, it’s worth mentioning that the ECD could work for 250,000 cycles at 60 °C, with transmittance of 70% and 8% between bleached and colored states at the wavelength of 600 nm, which shows remarkable long term stability. The excellent durability are mainly attributed to the reasonable choice of matching states and durability reinforced Ti-V2O5 film: the color change, charge capacity and switching speed of the viologen solution could be controlled through adjusting the concentration, and the addition of Ti provides excellent stability due to the distorted V2O5 layer structure.

A series of novel conducting conjugated yellow-to-transmissive electrochromic (EC) polymers were designed and synthesized to research their structure–property relationships, achieving electrofluorescent (EF) switching with applied external potential. These bifunctional materials are based on repeat units of bi-phenyl-1,3,4-oxadiazole and thiophene derivatives with different side chains. The polymers present a similar yellow color and emit various fluorescences with applied reduction potential, and they can turn to nearly transparent and non-fluorescent with oxidization potential. The optical contrast, electrochemical, electrochromic and electrofluorescent switching properties were also characterized in detail. A device containing these polymers was also fabricated that can achieve electrochromic and electrofluorescent switching simultaneously, making these polymers promising candidates for electro-bifunctional materials.

We here present a new generation of high-performance photoelectrochromic device, which contains a photoanode of CdS quantum dot sensitized TiO2, an electrochromic electrode of WO3, and aqueous electrolyte containing [Fe(CN)6]4−/3− and Li+ ions. It is found that the CdS quantum dot sensitized TiO2 photoanode exhibits a tunable optical absorption edge through modulating the deposition cycles, which results in a controllable photovoltaic conversion. A maximum transparency of 81% and light modulation of about 70% in the visible light range is readily reached under optimized conditions, which is much improved from that of the conventional dye-based photoelectrochromic device. This study demonstrates the promising potential of the photoelectrochromic device in applications for energy-saving smart windows.

We first doped europium (Eu) ions into WO3 using a hydrothermal method. The effects of Eu ions on the structure, morphology, and electrochromic and photoluminescence performances of WO3 films have been systematically investigated. It is found that a WO3 film doped with Eu ions exhibits the bi-functionality of electrochromic switching and red emission. Compared with an undoped WO3 film, a WO3 film doped with 10%Eu ions shows a remarkably enhanced optical contrast of 80.7% at 680 nm, a faster coloration response of 7.2 s, and more than 3 times higher coloration efficiency of about 57.85 cm2 C−1. Moreover, a WO3 film doped with Eu ions could be excited by 260 nm ultraviolet light and exhibit a tunable red emission as a response to the applied electric field. This novel and bifunctional WO3 film shows potential in preparation of illuminable “smart windows”.

By converting incident light into electric power, self-powered electrochromic window (SP-ECW) can achieve color change in electrochromic layer with no need for external voltage. In this work, a newly-designed SP-ECW is proposed for altering its color between deep blue and colorless state according to on/off state of incident light. The device consists of a working electrode with planar integration of photovoltaic (PV) and electrochromic (EC) elements on one electrode, a platinum counter electrode and a redox electrolyte comprising Br–/Br3– couple. A high transmittance modulation of 41% at 582 nm is obtained. Electrical energy converted from light is not only sufficient to drive the device, but also can be outputted to the external circuit.

•A facile casting technique used to prepare the flexible nanocomposite films by loading the desired amount of ATO into the PVDF solution.•The films were characterized using FTIR, FESEM, DSC and UV–Vis spectra measurement.•The results revealed that incorporation of ATO facilitates the production and adopt the all Trans conformations that lead to the formation of piezoelectric polar crystals and increases the effectiveness of PVDF in shielding UV radiation contributed due to the UV absorption capacity of ATO nanoparticles.In this work, we report on the preparation and characterization of a poly(vinylidene fluoride) (PVDF) film filled with antimony tin oxide nanoparticles (ATO-NPs), films were prepared via a facile solution casting method to investigate the electroactive β-polymorph formation mechanism and the optical properties of the (ATO/PVDF) nanocomposite films. The impact of the ATO loading on the structural and morphological properties of PVDF were investigated by Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy techniques. Designing nanocomposites with a combination of the advantages of both PVDF and ATO nanofiller leads to a feasible route to a remarkable increase of the electroactive phase enriched PVDF films. The (0.05) ATO/PVDF composite film exhibits a maximum β-phase fraction of 86% compared with other loadings this may be assigned to the presence of interfacial interactions at the interface between NPs surface and CH2/CF2 dipoles of PVDF inducing the electroactive phases. Furthermore, the thermal properties of the composites were investigated by differential scanning calorimetry technique revealed the effect of the ATO loading on the structural and morphological properties of PVDF. Studying the optical properties of the nanocomposite films exhibit a strong absorbance in the entire UV region compared with neat PVDF, which is very appealing for UV blocking properties.Download high-res image (144KB)Download full-size image

A novel complementary electrochromic device (ECD) with full spectrum absorption, based on surface-confined tungsten oxide (WO3), and solution-phase N-methyl-phenothiazine (NMP) was described and constructed. Cathodically colored inorganic material WO3 is near-infrared active, and anodicallly colored organic material NMP is visible active. These two electrochromic materials possessed the matching redox potential value and complementary spectra absorption in the full spectrum region. The fabricated ECD combined the electrochromic properties of blue colored WO3 and red colored NMP, and exhibited vivid color change from transparent to dark publish-red with superior optical modulation (ΔT %) both in the visible region (81 % at 535 nm) and near-infrared region (70 % at 1000 nm). It took 10 s to color the device under the application of −1.1 V and 18 s to bleach the device completely with the potential of 0 V. Furthermore, the assembled ECD realized a high coloration efficiency of 110 cm2 C−1 at 535 nm and also achieved excellent long-term cycling stability and maintains 95 % of the initial ΔT % value after 60,000 continuous switchings.

A lightweight wearable piezoelectric membrane device with excellent flexibility and air breathability is assembled for the first time using a one step continuous electrospinning method. The piezoelectric membrane device displays a three-layer structure in which the PVDF nanofiber membrane is sandwiched between two PVDF–rGO electrode membranes. Since the whole piezoelectric device is prepared by electrospinning, both the active layer and electrode layer have high levels of electroactive β-phase crystallinity. The as-prepared device demonstrates high electric output. Open-circuit voltage, short-circuit current and power density can reach up to 46 V, 18 μA and 18.1 μW cm−2, respectively. The electric output is remarkably affected by the frequency and magnitude of the exerted excitation force, specifically, the output demonstrates a linear relationship with the excitation force. Moreover, the piezoelectric membrane device demonstrates a long working stability of over 50000 cycles and is capable of lighting LEDs directly.

A long lifetime of electrochromic (EC) materials is required in real-life applications, and the understanding and alleviation of deterioration are imperative to their commercialization. Here we investigate the stability of poly(3,4-(2,2-dimethylpropylenedioxy)thiophene) (PProDot-Me2) films via cycling 10000 times in 0.1 M LiClO4/propylene carbonate (PC). Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy results reveal that parts of [ClO4]− and Li+ ions were trapped and then adsorbed on polymer fibres, which makes the morphology change from irregular networks to compact microstructures, and the inclusion complex formed reduces the number of electroactive sites and blocks ions channels, preventing the migration of free [ClO4]− from the electrolyte. Based on our hypothesis, the stability was finally improved by optimizing the cycling conditions. This study provides guidance to improve the stability of EC materials.

A pair of solution-processable electrochromic polymers poly(2,3-bis(3,4-bis(dodecyloxy)phenyl)-5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)quinoxaline) (P1) and poly(3,4-di(2-ethylhexyloxy)thiophene-co-3,4-dibutoxythiophene) (P2) were designed and synthesised through a chemical oxidative polymerization and copolymerization approach. According to colour mixing theory, the two polymers with complementary colours (green for P1 and purple-red for P2) were utilised to create black-to-transmissive switching films via co-processing from a solution containing them in an appropriate mass ratio. The films exhibit a deep black colour, moderate optical contrast and robust stability. This method and the films with favourable electrochromic performances could be employed for the realization of potential large area electrochromic device applications.

•A novel black-to-transmissive electrochromic device bearing viologen and triphenylamine derivatives was achieved.•Absorption-complementary viologen and triplenylamine derivatives were designed and synthesized.•Device combining the two materials realized panchromatic absorption over entire visible region.•Device exhibited high optical contrast and excellent stability.A novel black-to-transmissive electrochromic device based on TiO2-supported viologen and triphenylamine derivatives was designed and constructed via the absorption-complementary approach. In the device, cathodically coloring electrochromic material 1,4-bis[((N-phosphono-2-ethyl)-4,4′-bipyridinium)-methyl]-benzene tetrachloride acted as working electrode and novel anodically coloring electrochromic material (4-((4-(dimethylamino)-phenyl)(4-methoxyphenyl)-amino)-benzyl) phosphonic acid acted as counter electrode. The assembled electrochromic device achieved panchromatic absorption over entire visible spectrum with almost zero transmittance in colored state. The optical contrast (ΔT) of the device realized in this work was comparable to the highest value (60%) among all reported black-to-transmissive ECDs. Furthermore, excellent cycling stability was achieved, which maintained almost 80% of the initial ΔT value at 570 nm after continuous 100,000 switchings. These outstanding comprehensive electrochromic performances potentially make this device a promising candidate for electrochromic device applications.Download high-res image (391KB)Download full-size image

•Thiophene-based electrochromic polymers bearing trifluoromethyl have been designed and synthesized.•The polymers without long side chains show better thermal stability compared with non-fluorine EC soluble polymers.•The novel series polymers have proven to provide a new strategy to design electrochromic conjugated polymers.A series of novel thiophene-based electrochromic (EC) polymers bearing trifluoromethyl group were designed and synthesized. Without long-chain alkyl or alkoxy group attached, the polymers still exhibit good solubility in conventional organic solvents, and much better thermal stability comparing with conventional non-fluorine EC soluble polymers. Optical, electrochemical, spectroelectrochemical and electrochromic switching properties of the polymers are characterized in detail as well as the solubility and thermal stability. Combining comprehensive properties of fluorine-containing molecules and electrochromic conjugated polymers, the polymers reported here are proven to provide a new strategy to design novel electrochromic conjugated polymers.

A facile method is introduced for preparation of porous self-polarized piezoelectric thin films (100–300 nm) by depositing polyvinylidene fluoride solution onto the surface of an aqueous solution. A polar β crystalline phase enhancement is realized with the addition of sodium chloride into the aqueous solution. Fully saturated piezoresponse hysteresis loops and out-of-plane piezoresponse phase images imply the high piezoelectricity and self-polarized property of the as-prepared films. Also, we fabricate a device and evaluate its performance. A high sensitivity of 52.2 pC N−1 and an electric output of over 24 V and 16 μA are revealed. Therefore, we provide a convenient and low cost approach to obtain self-polarized piezoelectric films with an enhanced polar β crystalline phase and create a dual functional device for pressure sensing and electricity generation.

A highly regiosymmetric homopolymer based on a diethyl malonate derivatized 3,4-propylenedioxythiophene (ProDOT) monomer was synthesized through FeCl3 oxidative polymerization and postpolymerization functionalization to realize a water-processable blue-to-transmissive switching electrochromic polymer (WPECP-blue). As an electrochromic material, the polymer has a high electrochromic contrast ΔTmax = 56% at 580 nm and a relatively fast switching speed t95 = 1.8 s, and shows only contrast loss of 11% (from 56% to 45%) at square wave potential step of 5 s over 11 000 switching cycles, making it a desirable candidate for electrochromic applications such as windows and displays.Keywords: blue; conjugated polymer; dioxythiophene; electrochromism; transmissive;

A novel alternating polymer, ProDOT-TPE, with aggregation-enhanced fluorescent emission and electrochromic properties based on thiophene and tetraphenylethene derivatives was designed, synthesized, and characterized. The polymer displays weak photoluminescence in tetrahydrofuran, but its corresponding film prepared by spray-coating exhibits yellow-green fluorescent light at 540 nm. The color of the polymer film could be switched from bright yellow to navy blue by applying a relatively low voltage. An electrochromic device (ECD) of the polymer was fabricated that differs from common ECDs because both its color and fluorescent state could be synchronously switched by an applied voltage, making the polymer a unique candidate for electrochemical fluorescence and electrochromic applications.Keywords: aggregation-induced emission; electrochromic; fluorescence; polymer; tetraphenylethene; thiophene

•Ti-doped V2O5 films were created by spin coating a solution on glass.•Adding Ti distorted V2O5 layers structure, proving more free space for Li+ movement.•Doped V2O5 demonstrated better stability and higher transmittance than pure V2O5.•Electrochromic device with optimized doped film exhibited excellent cyclic stability.•Electrochromic device with doped film demonstrated a good memory effect.Vanadium pentoxide (V2O5) is a promising material for constructing electrochromic devices, due to its outstanding properties of lithium ion intercalation/deintercalation. However, its poor cyclic stability limits practical applications. We report here that V2O5 films gain significantly improved cyclic stability by doping with titanium (Ti) and perform well in WO3-based electrochromic devices. V2O5 films were fabricated by spin coating with a vanadium triisopropoxy oxide solution followed by heat treatment. X-ray diffraction studies reveal that introducing the Ti element reduces the crystallinity of a V2O5 film and distorts its layer structure. The cyclic stability of the V2O5 films increases as the doped Ti content increases. The electrochromic device assembled with an optimized V2O5 electrode (V:Ti = 2:1) has lasted 200,000 cyclic switching times between the lowest (2%) and highest (62%) transmittance with no significant degradation of performance.

•Novel copolymers based on thiophene derivatives are designed and synthesized.•The polymers show high optical contrast ranging from 51% to 57%.•All polymers exhibit good long-term stability.•The polymers’ neutral state colors can be tuned by varying monomer ratios.•All polymers have decent solubility in conventional organic solvents.A series of solution-processable red-to-transmissive electrochromic polymers P1, P2 and P3 were designed and synthesized through chemical oxidative copolymerization approach, wherein the mole ratio of 3,4-di(2-ethylhexyloxy)thiophene to 3,4-ethylenedioxythiophene) were 6:1, 2:1 and 1:1, respectively. Their maximum absorption wavelength in the neutral state vary with incorporation ratio of 3,4-ethylenedioxythiophene, giving rise to tunable colors. All polymers exhibit decent solubility in conventional organic solvents, high optical contrast ranging from 51% to 57% and good stability. These excellent comprehensive electrochromic performances make them as promising candidates for large area electrochromic device applications.

•Novel D–π–A polymers bearing thiophene and TPA derivatives are synthesized.•Different acceptor pendent groups exert different impact on polymers’ property.•All polymers display excellent solubility in low-boiling point organic solvents.•Polymers have both fluorescence effect and electrochromic effect.•Polymers show large transmittance changes in near-infrared region.A series of novel donor–π-bridge–acceptor type conjugated polymers bearing 3,3-bis((2-ethylhexyloxy)methyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine and triphenylamine (TPA) derivatives are synthesized via Stille coupling. The optical, fluorescent, electrochemical, spectroelectrochemical and electrochromic switching properties are characterized. The polymers display excellent solubility in low-boiling point organic solvents. Different electron-withdrawing pendent groups attached on TPA exert great influence on polymer’s fluorescent and electrochromic properties. All polymers show large transmittance changes in near-infrared region and reveal fast switching from one second to several seconds.

Novel spray-processable red-to-transmissive electrochromic polymers poly[3,4-di(2-ethylhexyloxy)thiophene-co-thiophene-co-3,4-di(methoxy)thiophene] with different monomer ratios have been synthesized and the electrochromic properties such as optical contrast, switching speed and long term switching stability were characterized. The incorporation of the simple thiophene unit into the polymer with the monomer ratio DEHOT:Th:DMOT = 1:0.2:0.8 successfully improves the switching speed (t95 = 1.6 s) with a high optical contrast ΔTmax = 56% at 518 nm, which constitutes a forward step toward the dynamic electrochromic displays.

•This paper takes the compatibility of electrodes into account.•Well-matched viologen and triphenylamine materials and electrodes were prepared.•Performances of the as-prepared electrodes showed that they matched each other very well.•An electrochromic device using the two electrodes demonstrated good performances.An electrochromic device (ECD) can change color absorption when subjected to an appropriate voltage. Such a device includes three components: a working electrode, a counter electrode and an electrolyte. Compatibility of these three components is important for ECD’s stability. In this study, two novel compatible electrochromic materials, cathodic 1-(9-hexyl-9H-carbazole)-1′-(propylphosphonic acid)-4,4′-bipyridilium dichloride and anodic (4-(diphenylamino)phenyl)methylphosphonic acid were designed, synthesized and fabricated into electrochromic electrodes using a chemisorption method. We characterized the electrochromic performance of these two electrodes, including the degree of color change, color changing voltage and charge capacity; the results indicated that they matched each other very well. An electrochromic device fabricated using these two electrodes, as expected, exhibited rapid, vivid color changes and proved highly stable for up to 100,000 cycles.

•Unconfined ice nanofibers were produced for the first time.•Ice nanofibers were made by electrospray and hyperquenching method.•The formation mechanism of ice nanofibers was proposed.We report here that massive unconfined ice nanofibers are made by electrospraying liquid water into a hyperquenching chamber. Cotton-appeared ice are found on the bottom of the chamber where the temperature is below 140 K and they are confirmed to be ice nanofibers with diameters ranging from several tens to hundreds of nanometers. In contrast to the mechanism for electrospinning polymer nanofibers, we suggest that the formation of the ice nanofibers involves a series of physical processes: droplet evaporation, vapor supersaturation, droplet coalescence and hyperquenching. This study demonstrates for the first time the possibility to make template-free ice nanofibers and may have implication for fundamental studies and industrial applications.

Vanadium pentoxide (V2O5) holds promise as an electrode material for lithium batteries and electrochromic devices, due to its high lithium ion intercalation. However, it remains challenging to make uniform, large-scale thin films with the conventional electrodeposition method. We prepared V2O5 films by ultrasonic spraying (ULS). Our study investigated the structural, surface morphological, electrochemical, and optical properties of films obtained using ULS on ITO glass. Compared with electrodeposited V2O5 films, sprayed films exhibited similar transmittance values but with relatively poor charge storage capacity. We found that the annealing temperature affected the ion storage capacity. After treatment at high temperatures, the films' charge capacities became more stable. The advantages in both cost and control may make ULS a useful method for preparing large-area films for electrochromic devices.Highlights► We prepared vanadium pentoxide films by an ultrasonic spraying method. ► We studied their electrical and optical properties for electrochromic applications. ► The films exhibited similar transmittance to electrodeposited films. ► The annealing temperature influenced electrochemical properties.

We electrochemically deposited one layer of the electrochromic (EC) polymer poly(3,4-(2,2-dimethylpropylenedioxy)thiophene) on ITO-coated PET substrate. Scanning electron microscopy showed that the polymer film was nanostructured, which becomes desirable for improving ion diffusion and color changing. We built an equivalent circuit model to simulate the EC kinetics using the measured alternating current impedance data. Furthermore, we assembled a set of smart goggles with good flexibility; these exhibited the ability to change colors quickly at a low driving voltage and demonstrated their stability through tens of thousands of test cycles.Graphical abstractHighlights► Nanostructured PProDOT-Me2 film improving ion diffusion and color changing. ► An equivalent circuit model for the electrochromic kinetics. ► High-performance smart goggles.

We first doped europium (Eu) ions into WO3 using a hydrothermal method. The effects of Eu ions on the structure, morphology, and electrochromic and photoluminescence performances of WO3 films have been systematically investigated. It is found that a WO3 film doped with Eu ions exhibits the bi-functionality of electrochromic switching and red emission. Compared with an undoped WO3 film, a WO3 film doped with 10%Eu ions shows a remarkably enhanced optical contrast of 80.7% at 680 nm, a faster coloration response of 7.2 s, and more than 3 times higher coloration efficiency of about 57.85 cm2 C−1. Moreover, a WO3 film doped with Eu ions could be excited by 260 nm ultraviolet light and exhibit a tunable red emission as a response to the applied electric field. This novel and bifunctional WO3 film shows potential in preparation of illuminable “smart windows”.

A facile method is introduced for preparation of porous self-polarized piezoelectric thin films (100–300 nm) by depositing polyvinylidene fluoride solution onto the surface of an aqueous solution. A polar β crystalline phase enhancement is realized with the addition of sodium chloride into the aqueous solution. Fully saturated piezoresponse hysteresis loops and out-of-plane piezoresponse phase images imply the high piezoelectricity and self-polarized property of the as-prepared films. Also, we fabricate a device and evaluate its performance. A high sensitivity of 52.2 pC N−1 and an electric output of over 24 V and 16 μA are revealed. Therefore, we provide a convenient and low cost approach to obtain self-polarized piezoelectric films with an enhanced polar β crystalline phase and create a dual functional device for pressure sensing and electricity generation.

A lightweight wearable piezoelectric membrane device with excellent flexibility and air breathability is assembled for the first time using a one step continuous electrospinning method. The piezoelectric membrane device displays a three-layer structure in which the PVDF nanofiber membrane is sandwiched between two PVDF–rGO electrode membranes. Since the whole piezoelectric device is prepared by electrospinning, both the active layer and electrode layer have high levels of electroactive β-phase crystallinity. The as-prepared device demonstrates high electric output. Open-circuit voltage, short-circuit current and power density can reach up to 46 V, 18 μA and 18.1 μW cm−2, respectively. The electric output is remarkably affected by the frequency and magnitude of the exerted excitation force, specifically, the output demonstrates a linear relationship with the excitation force. Moreover, the piezoelectric membrane device demonstrates a long working stability of over 50000 cycles and is capable of lighting LEDs directly.

A long lifetime of electrochromic (EC) materials is required in real-life applications, and the understanding and alleviation of deterioration are imperative to their commercialization. Here we investigate the stability of poly(3,4-(2,2-dimethylpropylenedioxy)thiophene) (PProDot-Me2) films via cycling 10000 times in 0.1 M LiClO4/propylene carbonate (PC). Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy results reveal that parts of [ClO4]− and Li+ ions were trapped and then adsorbed on polymer fibres, which makes the morphology change from irregular networks to compact microstructures, and the inclusion complex formed reduces the number of electroactive sites and blocks ions channels, preventing the migration of free [ClO4]− from the electrolyte. Based on our hypothesis, the stability was finally improved by optimizing the cycling conditions. This study provides guidance to improve the stability of EC materials.

In this work, we first studied the effects of CdS quantum dot modification on the electrochromic properties of TiO2 films in a Li ion containing organic electrolyte. Driven by an external potential, the CdS modified TiO2 (TiO2/CdS) film achieves reversible coloration and discoloration. In particular, the TiO2/CdS film exhibits multistage color switching from yellow (original state), to green (−1.5 V), to black (−2 V) in the coloration process, then to brown (−0.6 V) in the discoloration process. Besides, an improved optical contrast window of about 79.7% at 624 nm, an enhanced coloration efficiency of 70.8% and a half shortened coloration time of 12.7 s are achieved for the TiO2/CdS film, when compared with those of a pure TiO2 film. This work not only presents a novel electrochromic material with enhanced electrochromic properties, but also brings us a new insight into exploring the electrochromic functions of metal-chalcogenides.

Tungsten trioxide (WO3) is a promising electrochromic material for potential applications in architectural and automotive smart windows. Nanostructured WO3 has attracted particular attention over the past few years due to its potential advantages in electrochromic properties. Herein, Ni-doped WO3 nanostructured films were obtained by a seed-free hydrothermal method directly on transparent conducting substrates. The influences of Ni doping on the nanostructure and electrochromic properties of WO3 films were investigated. Compared to pure WO3 films, Ni doping can increase the crystal defects and induce significant changes in the nanostructure, thus affecting the electrochromic performance of WO3 films. Low concentration Ni-WO3 films exhibited a vertically aligned nanorods morphology, ultrahigh optical modulation (over 85% from 600 to 900 nm), high coloration efficiency (60.5 mc2 C−1 at 600 nm) and excellent cycling stability for about 5500 cycles. Finally, the complementary electrochromic device assembled with 1.5% Ni-WO3 and NiO-based films exhibited vivid color change from transparent to black (x = 0.326, y = 0.369, YL% = 3.29%) with maximum transmittance of about 3.5% in the dark state.