Electrochromic Thin Films Research Articles

Study on the role of rGO in enhancing the electrochromic performance of WO3 film

In the present work, an improved electrochromic thin film has been developed with faster color switching time on incorporation of rGO in WO3 film. Here, pristine WO3 and rGO-WO3 (rGO concentration varying from 1–7wt%) based nanocomposite electrochromic films were fabricated on ITO coated glass substrate, by utilizing a facile sol-gel dip-coating technique. Detailed structural and morphological analyses of the films were carried out using XRD, FESEM, TEM and Raman Spectroscopy. Assimilating the electrochromic properties of all the films showed that, 5wt% rGO incorporated WO3 film gave a maximized electrochemical performance with minimum degradation in optical transmittance. The film also exhibited an optical modulation of ∼ 50% and a better switching response having coloration time (tc) ~ 5.3 s and bleaching time (tb) ~ 6.2 s as compared to the pristine film (tc~ 9.6 s, tb ~ 10.4 s). Incorporation of rGO also resulted in an enhancement of coloration efficiency from ∼ 81 to ∼ 386 cm2/C. The Electrochemical Impedance Spectroscopy (EIS) analysis of pristine and rGO loaded film clearly revealed better charge transfer on incorporation of rGO. The cyclic stability study exhibited ∼ 25% deterioration in optical transparency of the bare WO3 film which was < 10% for the rGO impregnated film. An ex-situ XRD analysis demonstrated that a crystal dislocation occurring in pristine WO3 sample was responsible for the same.

Tunable Capacitive Behavior in Metallopolymer-based Electrochromic Thin Film Supercapacitors.

Volumetric capacitance is a more critical performance parameter for rechargeable power supply in lightweight and microelectronic devices as compared to gravimetric capacitance in larger devices. To this end, we report three electrochromic metallopolymer-based electrode materials containing Fe2+ as the coordinating metal ion with high volumetric capacitance and energy densities in a symmetric two-electrode supercapacitor setup. These metallopolymers exhibited volumetric capacitance up to 866.2 F cm-3 at a constant current density of 0.25 A g-1. The volumetric capacitance (poly-Fe-L2: 544.6 F cm-3 > poly-Fe-L1: 313.8 F cm-3 > poly-Fe-L3: 230.8 F cm-3 at 1 A g-1) and energy densities (poly-Fe-L2: 75.5 mWh cm-3 > poly-Fe-L1: 43.6 mWh cm-3 > poly-Fe-L3: 31.2 mWh cm-3) followed the order of the electrical conductivity of the metallopolymers and are among the best values reported for metal-organic systems. The variation in the ligand structure was key toward achieving different electrical conductivities in these metallopolymers with excellent operational stability under continuous cycling. High volumetric capacitances and energy densities combined with tunable electro-optical properties and electrochromic behavior of these metallopolymers are expected to contribute to high performance and compact microenergy storage systems. We envision that the integration of smart functionalities with thin film supercapacitors would warrant the surge of miniaturized on-chip microsupercapacitors integrated in-plane with other microelectronic devices for wearable applications.

Advantageous optical characteristics of tantalum vanadium oxide as counter electrode in electrochromic devices

Smart windows are an important technology in terms of energy saving potential in the building sector due to their ability to control visible light and thermal radiation. The essential component of this type of window glazing is an electrochromic thin film. In addition to the widely established tungsten oxide as the optically active material, in particular the counter electrode offers significant potential for improving the overall device performance. In this study, tantalum vanadium oxide films are prepared by reactive radio-frequency sputtering on fluorine-doped tin oxide substrates and optimized in terms of their spectro-electrochemical properties as ion storage layer. We show that an oxide-based tantalum-vanadium alloy is a promising approach to address the open challenges of pure vanadium pentoxide. The coatings exhibit color neutrality in combination with a high transmittance of up to 80% in the as-prepared state and suitable optical transmittance switching. Additionally, we find both a sufficient stability upon cycling and a suitable charge density of about 35 mC cm−2. Thus, the presented oxide-based alloy offers a beneficial performance as an ion storage layer in electrochromic devices.

Long-term cycling and stability of crystalline WO3 electrochromic thin films prepared by spray pyrolysis

Long-term cycling and stability of crystalline WO3 electrochromic thin films prepared by spray pyrolysis

Effect of pH on the structure and morphology of W18O49 nanowires and their electrochromic properties

In wet-chemistry processes, pH value is an important parameter that can be used to control the microstructures of the as-grown nanostructures. In this work, W18O49 nanowires with crystalline@amorphous core-shell structures were synthesized by controlling the pH of the precursor solutions in a hydrothermal reaction. Due to surface functionalization by ethylene glycol (EG) groups, the hydrothermally grown W18O49 nanowires could be easily dispersed in aqueous environment, thus enabling the fabrication of electrochromic thin films of W18O49 nanowires by a simple spray-drying process. The optimized W18O49 nanowire films showed a large optical modulation (ΔT) of 81.1% at 633 nm, short response time (coloring/bleaching time τc/τb = 7.9 s/3.9 s), good cycle stability (ΔT retention was 84.5% after 1000 cycles), and the excellent memory effect (the color withstood for 36000s after turning off the voltage). Furthermore, the electrochromic device (ECD) (4 × 4 cm2) based on the W18O49 nanowires exhibits a large optical modulation of 56.6%, short response time (τc = 7.6 s, τb = 2.7 s), and superior coloration efficiency (CE) of 64 cm2 C−1. Finally, by simply amplifying the developed processes, a large-area electrochromic device (10 × 10 cm2) was successfully prepared, which implies the possibility of large-area spraying process for practical applications.

Cycling Durability and Potentiostatic Rejuvenation of Electrochromic Tungsten Oxide Thin Films: Effect of Silica Nanoparticles in Liclo4–Propylene Carbonate Electrolytes

Cycling Durability and Potentiostatic Rejuvenation of Electrochromic Tungsten Oxide Thin Films: Effect of Silica Nanoparticles in Liclo4–Propylene Carbonate Electrolytes

Characterization of nanoparticulated WO3 electrochromic thin films prepared via precipitation reaction of peroxotungstic acid solution

Amorphous WO3 thin films with a nanoparticulated morphology were fabricated using a combination of electrodeposition and electrophoretic deposition via the precipitation reaction of a peroxotungstic acid (PTA) solution. Due to the presence of the precipitated WO3 nanoparticles in the aged PTA solution, the electrophoretic deposition and electrodeposition occur simultaneously, which results in a grainy surface morphology in which nano-sized WO3 particles are embedded on a smooth WO3 thin film. The WAT-5 sample prepared with a PTA solution aged for 5 days exhibited faster ion intercalation/de-intercalation kinetics (coloration time of 3 s and bleaching time of 1 s), a greater transmittance modulation (68.36%), and a higher coloration efficiency (41.29 cm2/C) with better cyclic stability (91% retention during 300 cycles) than a WO3 thin film fabricated using the PTA solution without the aging process. The nanoparticulated morphology enhanced the active reaction area by expanding the interface between the electrolyte and the electrochromic material. Such results indicate that the electrochromic characteristics of WO3 thin films can be improved simply by adjusting the aging period of the PTA solution.

In-situ electro-polymerization of fluorescent electrochromic thin films based on charge-transfer complexes

Fluorescent electrochromic (FEC) materials, which reversibly change their both fluorescence and absorption via electrochemical reactions in response to applied potentials, have received extensive attention. Herein we directly obtained poly (2Z,2′Z)-3,3′-(1,4-phenylene)bis(2-(4-((6-(9H-carbazol-9-yl)hexyl)oxy)phenyl)acrylonitrile) P(CHDCS) and poly (2Z,2′Z)-3,3′-(2,5-dimethoxy-1,4-phenylene)bis(2-(4-((6-(9H-carbazol-9-yl)hexyl)oxy)phenyl)acrylonitrile) P(CHDCSM) as thin films on electrodes via electro-polymerization (EP) from linear donor-acceptor (D-A) monomers with isolated π-conjugation. Surprisingly, interchain charge-transfer complexes (CTCs) with mixed D-A stacks were generated between electroactive bis-carbazole (D) and luminescent dicyanodistyrylbenzene (A) units during the EP process. Consequently, significantly redshifted and bright orange fluorescence (∼600 nm) originated from intermolecular CT excitons were observed from both polymers, which could be reversibly switched upon applied potentials. This is the first time that the CTCs have been prepared via EP and applied as FEC materials. Moreover, the stronger intermolecular CT effect gives rise to an enhancement of electrical conductivity and FEC response in P(CHDCS), resulting in high optical contrasts (T = 33.6%, Ion/Ioff = 28.3) and fast FEC switching (coloring 1.25 s, fading 0.53 s, quenching 2.40 s, brightening 6.17 s) as well as good reversibility. In addition, the CTCs improve the spin delocalization of bis-carbazole radical cations, leading to increased color stability of the FEC films by a decay of only 1.2% in 2 h with no power supply after coloration. This work not only affords an innovative CTC design strategy for FEC materials but also paves the way for constructing highly-performed and energy-efficient FECs via one-step electrochemical methods.

Radiation exposure effects on structural, optical, and electrical properties of self-powered prussian blue electrochromic thin films

Prussian blue (PB) electrochromic (EC) thin films were placed on the ITO substrates with thicknesses of 500, 600, and 700 nm, produced by handmade Spray pyrolysis method. Self-powered electrochromic system based on Aloe Vera gel electrolyte. The bleaching appeared voltages of Prussian blue and aluminum electrode with the Aloe Vera gel electrolyte are 0.23, 0.29 and 0.34 V respectively with bleaching time 20 s. All thin films were irradiated by gamma ray (60CO) of dose 18 Gy/ hr with 1.17 MeV of energy for half an hour Prior to and following irradiation.After irradiation, Ultraviolet spectroscopy shows absorbance increases for (525–750) nm wavelength, and optical energy gab is decreased. AVO measurements show increase in the voltages after irradiation which are 0.25, 0.32 and 0.36 V for three thicknesses 500, 600 and 700 nm of PB thin films respectively; furthermore bleaching time has become 30 s. It is noticed that coloring needs long time so the application bias voltages are 2.5 V and 2.8 V before and after irradiation. Self-powered smart systems are useful for many applications, especially with using green electrolyte (Aloe Vera) is useful for smart systems because of its benefits such as low cost, simplicity of availability, and non-toxicity.

A review on the prominence of porosity in tungsten oxide thin films for electrochromism

Transition metal oxides have gained substantial attention in the previous decade by virtue of their distinctive physical and chemical properties. Amidst all, tungsten oxide (WO3) is emerging as distinct metal oxide on account of its outstanding electrochromic performance. Furthermore, optical-switching speed has topmost importance in the actual device. The sole approach to enhance this attribute is to escalate surface area of the electrochromic coatings, which can be done by instigating porosity within bulk material. Porosity can be considered as one of the pivotal facets of functionalized electrochromic thin films. Cations can be made to get transported into the depth of the host with the presence of high porosity in a structure. This featured article is attempting to pivot the path as how porosity is upgrading the electrochromic attribute of WO3 films. Furthermore, various methods to achieve optimum porosity of WO3 films leading to best electrochromic performance are also discussed.

Electron and Ion Transport in Mixed Electrochromic Thin Films of Perfluorinated Phthalocyanines

Thin films of a mixture of the perfluorinated phthalocyanines 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadeca-fluorophthalocyaninato copper(II) (F16PcCu) and 1,4,8,11,15,18,22,25-octakis-fluoro-2,3,9,10,16,17,23,24-octakis-perfluoro(isopropyl) phthalocyaninato copper(II) (F64PcCu) of different thicknesses and mixing ratios were prepared by simultaneous vapor deposition. A moderate extent of the intermolecular coupling between F16PcCu and F64PcCu within the mixed films comparable to that reported earlier for tetrakisperfluoroisopropyl-perfluoro phthalocyanine (F40PcCu) was revealed by UV/Vis spectroscopy. The electrochromic properties as well as the transport of electrons and counter cations in the 1:1 F16PcCu:F64PcCu mixed films were investigated by electrochemical and spectroelectrochemical characterization using an aqueous solution of KCl as electrolyte. The transport coefficients of electrons and ions of these films were comparable to those reported for F40PcCu films, for which a well-balanced, equally fast transport of both electrons and ions had been seen. Fast, reversible, and stable switching characteristics of the mixed films, upon reduction and intercalation of K+ counterions and re-oxidation with extraction of the counterions, were observed by cyclic voltammetry, chronoamperometry, and simultaneously measured optical absorption spectra.

Synthesis of high-performance electrochromic thin films by a low-cost method

Metal-doping is an effective method to adjust the physical and chemical properties of semiconductor metal oxides. This work adopts a simple solvothermal method to synthesize Mo-doped tungsten oxide nanoparticles. The high-performance electrochromic films can be homogenously formed on ITO glass without post-annealing. Compared with pure WO3 films, the optimized Mo-doped WO3 films show improved electrochromic properties with significant optical contrast (68.3% at 633 nm), the short response time (6.3 s and 3.9 s for coloring and bleaching, respectively), and excellent coloration efficiency (107.2 cm2 C−1). The improved electrochromic behavior is mainly due to the increasing diffusion rate of Li+ in Mo-doped WO3 films (increased 20% than that of pure WO3 films). The porous surface of Mo-doped WO3 film shortens the diffusion path of Li+. Besides, Mo doping reduces the resistance and improves conductivity. Furthermore, 2at% Mo-doped WO3 films indicate satisfactory energy-storage properties (the specific capacitance is 73.8 F g−1), resulting from the enhanced electrochemical activity and fast electrical conductivity. This work presents a practical and economical way of developing high-performance active materials for bifunctional electrochromic devices.

Effect of Sputtering Power on Optical Properties of Nickel Oxide Electrochromic Thin Films

The preparation and characterization of nickel oxide (NiO) thin film for electrochromic smart window applications are studied. The NiO thin film was prepared by the DC magnetron sputtering from a pure nickel target. The sputtering power was varies in the interval 50–200 W. The crystallinity and physical morphology of NiO films are characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), respectively. The XRD result revealed that polycrystalline NiO thin films with preferred growth directions along (111) and (200) planes are obtained. Moreover, the electrochromic property of NiO thin films was investigated with a UV-Visible spectrophotometer. The colored state of the electrochromic cell was obtained by the ion insertion at the 1-V external applied bias in 0.1 M KOH. The reversibility between the colored and bleached states is confirmed by the optical transmittance. The result shows the optical contrast as high as 28.68.

Aqueous Processing and Spray Deposition of Polymer-Wrapped Tin-Doped Indium Oxide Nanocrystals as Electrochromic Thin Films

Plasmonic metal oxide nanocrystals are interesting electrochromic materials because they display high modulation of infrared light, fast switching kinetics, and durability. Nanocrystals facilitate solution-based and high-throughput deposition, but typically require handling hazardous nonaqueous solvents and further processing of the as-deposited film with energy-intensive or chemical treatments. We report on a method to produce aqueous dispersions of tin-doped indium oxide (ITO) by refunctionalizing the nanocrystal surface, previously stripped of its native hydrophobic ligands, with a hydrophilic poly(acrylic acid) polymer featuring a low density of methoxy-terminated poly(ethylene oxide) grafts (PAA-mPEO4). To determine conditions favoring the adsorption of PAA-mPEO4 on ITO, we varied the pH and chemical species present in the exchange solution. The extent of polymer wrapping on the nanocrystal surface can be tuned as a function of the pH to prevent aggregation in solution and deposit uniform, smooth, and optical quality spray coated thin films. We demonstrate the utility of polymer-wrapped ITO nanocrystal thin films as an electrochromic material and achieve fast, stable, and reversible near-infrared modulation without the need to remove the polymer after deposition provided that a wrapping density of ∼20% by mass is not exceeded.

Electrochromic properties of silver nanowire-embedded tungsten trioxide thin films fabricated by electrodeposition

One of the main drawbacks of tungsten trioxide (WO3) electrochromic materials is slow response time due to the low electrical conductivity of WO3. In this work, a silver nanowire (AgNW)-embedded WO3 composite structure was designed to improve electrochromic performance. The AgNW-embedded WO3 electrochromic thin films were fabricated by an electrodeposition method. In addition to phase and morphology analyses, electrochromic performance was evaluated via an examination of transmittance modulation, response time, and coloration efficiency. The electrochromic study reveals that embedding AgNWs into the WO3 has a significant impact on electrochromic performance. Due to the presence of a percolated AgNW network with high electrical conductivity and surface roughness, the AgNW-embedded WO3 thin films exhibit faster Li-ion intercalation/de-intercalation kinetics (coloration time of 12 s and bleaching time of 2 s) and a greater coloration efficiency (45.3 cm2/C) with better cyclic stability than a single-layer WO3 thin film. Such results indicate AgNW-embedded WO3 electrochromic thin films as attractive for practical application of electrochromic devices in energy-saving smart windows.

Label-free optical detection of bioelectric potentials using electrochromic thin films

Understanding how a network of interconnected neurons receives, stores, and processes information in the human brain is one of the outstanding scientific challenges of our time. The ability to reliably detect neuroelectric activities is essential to addressing this challenge. Optical recording using voltage-sensitive fluorescent probes has provided unprecedented flexibility for choosing regions of interest in recording neuronal activities. However, when recording at a high frame rate such as 500 to 1,000 Hz, fluorescence-based voltage sensors often suffer from photobleaching and phototoxicity, which limit the recording duration. Here, we report an approach called electrochromic optical recording (ECORE) that achieves label-free optical recording of spontaneous neuroelectrical activities. ECORE utilizes the electrochromism of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) thin films, whose optical absorption can be modulated by an applied voltage. Being based on optical reflection instead of fluorescence, ECORE offers the flexibility of an optical probe without suffering from photobleaching or phototoxicity. Using ECORE, we optically recorded spontaneous action potentials in cardiomyocytes, cultured hippocampal and dorsal root ganglion neurons, and brain slices. With minimal perturbation to cells, ECORE allows long-term optical recording over multiple days.

Electrochromic Thin Films Based on NiAl Layered Double Hydroxide Nanoclusters for Smart Windows and Low-Power Displays

To develop electrochromic thin films for smart windows and low-power displays, homogeneous thin films were produced by spin coating colloidal NiAl layered double hydroxide (LDH) nanocluster solutions on a fluorine-doped tin oxide (FTO) glass substrate. The nano-LDH dispersion was synthesized via a facile one-step epoxide mediated process at room temperature. In situ relative transmittance measurements at 400 nm, recorded during potential scans in cyclic voltammetry or differential pulse chronoamperometry, showed the electrochromic behavior of the homogeneous thin films. The impact of various parameters on the electrochromic properties of the films was detailed, such as the thickness of the film, the nature of the electrolyte, and the presence of electroactive anions. The electrochromic properties, namely, the change in transmittance (ΔT) and the bleaching reversibility, were greatly improved using alkaline metal hydroxide as the electrolyte and in the presence of Fe(CN)64–. The performances that we reached from the deposition of nanoclusters can be advantageously compared to the literature with ΔT = 70% and fast and good reversibility. Upon calcination, the NiAl-LDH film converted into mixed oxides (NiO and NiAl2O4). The higher the calcination temperature, the lower was the ΔT value. The presence of porosity within the thin films, through the introduction of sacrificial polymeric beads and then decomposition at 400 °C, was investigated, highlighting an enhancement of the ΔT value attributed to better accessibility to the nickel redox sites.

Coloration of tungsten oxide electrochromic thin films at high bias potentials and low intercalation levels

The optical absorption of amorphous tungsten oxide thin films was studied at low intercalation levels. The electronic density of states was obtained by an electrochemical method and bias potential regions were assigned to the conduction band and gap states according to the experimentally estimated conduction band edge. Differences between the coloration in the conduction band and gap states were observed when comparing the experimental results to a theoretical site-saturation model that considers electronic transitions between localized tungsten sites. The model could reproduce the optical response due to conduction band states. However, it underestimated the rate of change of the optical absorption coefficient with intercalation level for gap states. This discrepancy is discussed in the context of small-polaron optical absorption.

Flexible electrochromic thin films with ultrafast responsion based on exfoliated V2O5 nanosheets/graphene oxide via layer-by-layer assembly

Abstracts Flexible electrochromic (EC) display devices have received extensive attention due to their application in the new generation of smart and wearable devices. 2D ultrathin nanosheets are the promising structure to allow EC materials (such as V2O5, WO3) to be incorporated into flexible EC electronics owing to their surface-to-surface contact with the flexible substrate. Herein, the graphene-like V2O5 nanosheets (V2O5 NSs) with thicknesses ranging from few-layers (4 nm) to 40 nm are synthesized by liquid-phase exfoliation (LPE) method. To enhance the electronic conductivity and decrease the agglomeration of V2O5 NSs, graphene oxide (GO) is introduced to fabricate V2O5 NSs/GO films by layer-by-layer (LBL) assembly method. The composite films exhibit ultrafast coloring responsion time (1.6 s) and bleaching time (2 s) attributed to the reduced charge transport distances of 2D ultrathin structure. The LBL assembly structure accompanied by larger Li+ ions storage surface areas facilitates the superior transmittance contrast (ΔT) of 57.5% at 425 nm and reversible yellow/green/blue-grey color changes. Besides, the flexible V2O5 NSs/GO films on ITO/PET substrate still display rapid responsion speed (2.7 s/4.3 s) after bending 100 cycles with the bending angle of 90°, which manifests their potentially application for flexible EC devices.

Highly Stable and Rapid Switching Electrochromic Thin Films Based on Metal-Organic Frameworks with Redox-Active Triphenylamine Ligands.

Metal-organic frameworks (MOFs), known for their tailorable porous structures and large specific surface areas, are appealing for electrochromic applications as their abundant pores may greatly benefit the charge transport required for electrochromic switching. Herein, for the first time, a simple, scalable, and cost-effective electrochemical deposition method for fabrication of high-performance and durable MOFs-based electrochromic films with redox-active ligands was developed. The fabricated film can achieve rapid switching speed (both coloration and bleaching time <5 s) because the inherent cavities of the MOFs greatly facilitate ion insertion and extraction. In addition, the film constructed with optimized parameters shows a high optical contrast of 65%@700 nm and can be stably switched for 1000 cycles with <5% contrast attenuation, which is by far the best cycling performance for MOFs-based electrochromic materials ever reported. Furthermore, our method enables the scalable preparation of large-area MOFs-based electrochromic thin films without using large high-pressure reaction vessels, and the as-prepared film in this work could be switched well between colored and bleached states. This new method, therefore, opens up a new avenue to broaden the use of MOFs-based thin films for electrochromic applications.