Thin Films For Electrochromic Applications Research Articles

Tungsten Oxide Thin Films for Electrochromic Applications: Pulse Width‐Controlled Deposition by High‐Power Impulse Magnetron Sputtering

Tungsten oxide (WO3) thin films have been of prime interest among electrochromic materials because of their chemical stability, strong adherence to various substrates, and high coloration efficiency. High‐power impulse magnetron sputtering (HiPIMS) holds great potential in fabricating durable WO3‐based electrochromic layers. However, the tungsten target–plasma interactions in reactive‐HiPIMS deposition of WO3 and their role in modulating the electrochromic function of the resulting WO3 coatings are yet to be understood. Herein, by controlling the HiPIMS pulse length, the stoichiometry of tungsten oxide structures can be tuned to optimize the transparency and electrochromic function of the coatings. X‐ray photoelectron spectroscopy data shows that at pulse lengths shorter than 85 μs, the concentration of suboxide compounds is less than that of tungsten trioxide, while for pulse lengths longer than 100 μs, this balance is reversed. The average optical transparency of the coatings in the range of visible light is higher than 80%. The optical transmittance modulation (ΔT) of 38.1, 36.2, and 34.3% and coloration efficiency of 41.3, 38.4, and 35.9 cm2 C−1 are measured for the WOx samples deposited at pulse lengths of 70, 85, and 100 μs, respectively. Tuning the HiPIMS pulse characteristics is a simple strategy to deposit tungsten oxide films with tuned electrochromic properties for an array of applications, from smart windows to wearable displays.

Structural, optical, and electrochromic properties of RT and annealed sputtered tungsten trioxide (WO3) thin films for electrochromic applications by using GLAD technique

Structural, optical, and electrochromic properties of RT and annealed sputtered tungsten trioxide (WO3) thin films for electrochromic applications by using GLAD technique

Cobalt-doped MoO3 thin films and dual-band electrochromic devices with excellent cyclic stability

Electrochromic (EC) technology has played a pivotal role in elevating the prominence of smart windows by enabling them to alter their color and effectively manage the level of transparency. Nanostructured molybdenum trioxide (MoO3) thin films are highly promising for the development of EC technology. However, the production of uniform, crack-free and durable MoO3 thin films through conventional deposition methods is still a challenge though these properties are critical for achieving prolonged cyclic stability. In this work, we demonstrate the use of ultrasonic spray deposited bare MoO3 and cobalt-doped MoO3 (MoO3:Co) thin films for EC applications. Cobalt (Co) doping is shown to significantly improve the cyclic stability when compared to the bare MoO3 films. Furthermore, MoO3:Co films have exhibited significant performance in novel dual-band electrochromic devices (ECD). After 5000 cycles, the MoO3:Co electrodes showed significantly higher capacitance retention (92%) compared to the bare MoO3 thin film electrodes (54%). Moreover, MoO3:Co electrodes exhibited a higher maximum transmittance change than the bare MoO3 electrodes. Dual-band ECDs showed excellent cycling performance up to 5000 cycles with a capacitance retention of 99%. Obtained results demonstrated the potential of the ultrasonic spray deposition method for MoO3 thin film based EC windows and dual-band ECDs with excellent cyclic stability.

Effect of partial pressure of oxygen, target current, and annealing on DC sputtered tungsten oxide (WO3) thin films for electrochromic applications

Tungsten oxide (WO3) thin films were prepared on Corning (CG) and Fluorine doped tin oxide (FTO) glass substrates at partial pressure of oxygen (pO2) 4 × 10−2 Pa and 8 × 10−2 Pa using DC magnetron sputtering (DCMs). In this work, we have varied the deposition parameters like pO2, target currents, and temperature. At pO2 4 × 10−2 Pa and 8 × 10−2 Pa samples were deposited at target currents of 50 mA and 100 mA, the maintained growth conditions are RT (substrate temperature 28 °C), Pre annealed (substrate temperature 400 °C), and Post annealed (annealing temperature 400 °C). The samples were systematically characterized for vibrational, structural, optical, and Electrochromic (EC) properties by using Raman, XRD, Uv-Vis spectrometer, and Electrochemical analyzer respectively. XRD analysis reveals that RT-deposited samples show amorphous nature and pre & post-annealed samples show a crystalline nature for both pO2. Optical transmittance was higher at RT-deposited samples at 50 mA (94% & 92%) and lower at 100 mA (87% & 85%) at the wavelength of 600 nm for both pO2. From CV analysis higher cathodic peak current density was observed in RT-deposited samples at 50 mA (−6.49 mAcm−2 & -13.80 mAcm−2) and lower at 100 mA (−5.25 mAcm−2 & -12.88 mAcm−2) for both pO2. The diffusion coefficient was observed at a higher target current at 100 mA (1.06 × 10−7 cm2/s & 9.20 × 10−8 cm2/s). For annealed samples optical and EC properties were decreased for both pO2.

Structural and electrochemical studies of WO3 coated TiO2 nanorod hybrid thin films for electrochromic applications

This work presents the fabrication of Titanium dioxide (TiO2) nanorods/Tungsten oxide (WO3) hybrid thin films and the influence of nanostructures on the electrochromic behavior of the films. WO3 thin films were deposited at a constant substrate temperature of 673 K under various partial pressures of oxygen (ppO2). Also, Tungsten nanorods were grown on Fluorine doped tin oxide (FTO) coated glass substrate by hydrothermal process. Optimized WO3 films were coated on the TiO2 nanorod film by a sputter deposition method. Material, optical, electrical, and electrochemical analysis have been performed on the films using XRD, Raman, XPS, UV-Visible spectrometer and Cyclic voltammetry. The enhanced coloration efficiency (η) of the optimized WO3 films is found to be attributed to its large active surface area which favored H+ ions intercalation in the films. Also, the TiO2 nanorods/WO3 hybrid films exhibited a good electrochemical property in terms of the diffusion coefficient of 1.8 × 10−7 cm2/s better than those of pure WO3 and TiO2 nanorods.

Comparison of Al3+ and Li+ liquid electrolytes for as deposited and heat treated WO3 thin films for electrochromic applications

Al3+ is considered as one of the alternative charge carrier ions to Li+ for WO3 based electrochromic (EC) devices. In this study, WO3 films have been grown by Magnetron Sputtering method and tested in Li+ and Al3+ liquid electrolytes for the comparison of EC performances. In order to see the effects of structure and heat treatment on the EC performance of WO3 films in both liquid electrolytes, films were annealed at 250°C, 350°C and 450°C. As deposited film and films heat treated up to 350°C were amorphous while the film annealed at 450°C crystallized. Surface modifications were observed with annealing, especially for the film annealed at 450 °C. X-ray photoelectron spectroscopic analysis indicated that the binding energies of W 4f did not vary much with annealing. However, noticeable changes were observed after the EC test performed on the WO3 films in trivalent Al3+ compared to the films tested in monovalent Li+ liquid electrolyte. The optic transmittance of the pristine and annealed films was basically the same except the film annealed at 450 °C for which the transmittance decreased by 25%. The Li+ ions have higher insertion and extraction in and out of the annealed films except the film annealed at 450 °C. The optical modulation parameter was highest for the film annealed at 350 °C tested in Al3+ while that of the pristine film was the highest for the films tested in Li+ electrolyte. The diffusion coefficients of Al3+ ions is calculated to be smaller compared to Li+ ions. The WO3 film annealed at 350 °C was found to be the most durable when tested in Al3+ while both pristine and heat-treated films had relatively better long-time durability for the cycle times used.

Glancing angle sputter deposited tungsten trioxide (WO3) thin films for electrochromic applications

Glancing angle sputter deposited tungsten trioxide (WO3) thin films for electrochromic applications

Simulation and fabrication of tungsten oxide thin films for electrochromic applications

Electrochromics is the emerging technology that is used in sunlight control window glazing for buildings, automobiles and it can also control indoor climate through smart windows. Electrochromism is the mutable change in optical properties of an electrochromic material caused by redox reactions due to the application of voltage. Easy intercalating the H + ions on a dense electrochromic material (WO3) is the most important parameter as far as the reaction kinetics is concerned. The goal of our work is to improve the electrochemical response of electrochromic material by constructing nano-pillars rather than using dense electrochromic materials. Electrochemical performance of both the dense (planar) and porus (nano pillars) structures were simulated and experimentally proved with a systematic discussion in the present work. It is proven and shown here the increase in the electrochemical kinetics through easy diffusion of ions into the nanostructured electrochromic material.

Effect of annealing and oxygen partial pressure on the RF sputtered WO3 thin films for electrochromic applications

the electrochromic thin layer of Tungsten trioxide (WO3) was RF sputtered on FTO (fluorine-doped tin oxide) slide. In a reactive Ar + O2 gas environment with varying oxygen partial pressures, the deposition continued. The samples were air annealed at 400 °C for 2 h after being deposited at room temperature. SEM, XRD, UV–Visible spectrometer, and electrochemical analyzer characterization methods were employed to analyze the surface, structural morphology, optical, and electrochromic behaviour of the deposited material after annealing. The Optical Bandgap and Transmittance were found to be of a higher value for air annealed samples than RT deposited samples because RT deposited samples are amorphous whereas air annealed samples exhibit crystalline nature with Oxidation, reduction peak currents variation with respect to the temperature.

Influence of applied voltage on optimal performance and durability of tungsten and vanadium oxide co-sputtered thin films for electrochromic applications

In this research, tungsten and vanadium oxide co-sputtered thin films are evaluated for electrochromic properties based on the effects of applied cyclic voltages of −0.7 V, −1.0 V and −1.3 V for 9600 s. XRD patterns reveal the appearance of new peaks after completion of −1.3 V cycles, these peaks are not seen after lower voltage scans. SEM images also verify severely damaged morphologies. Chronoamperometry (CA) scan of −1.3 V shows a drastic reduction in current density value to as low as 32% for bleaching and 26% for coloring respectively. This is because of the lesser intercalation and de-intercalation as a result of damaged and disturbed orientation. Whereas the films with −1.0 V testing cycles shows the current density drop to 93% and 88% for bleaching and coloring respectively. A remarkable discovery in switching times of WO3-V2O5 co-sputtered thin films is also achieved. Coloring and bleaching times are drastically reduced to 5.5 s and ‹1.3 s as compared to only WO3 films having these values as 13.4 and 4.1 s respectively. We further investigates electrochromic films for different states and concentrations by XPS, electrochemical impedance spectroscopy (EIS) for electronic/ionic conductivity comparison and visible spectroscopy to verify optical modulation and calculation of some optical constants.

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.

On the preparation of Tri-vanadium hepta-oxide thin films for electrochromic applications

Nanostructured Tri-vanadium hepta-oxide (V3O7) thin films for electrochromic devices having high optical modulation, enhanced optical density, multi-coloration, fast switching time, and better reversibility, were prepared by nebulizer spray pyrolysis technique with 400 °C substrate temperature. A monoclinic structured, V3O7 films having nano rod-like morphology have been observed from structural and surface morphological analyzes, respectively. Surface chemical analyzes revealed the mixed vanadium oxidation states that favored the formation of relatively stable V3O7 thin films. Electrochromic performance of the films was examined by electrochemical cyclic voltammetry (CV) studies with multiple CV scan rate. The multiple peaks present in the anodic and cathodic curves prove the multi-coloration behavior of the films with appreciable optical modulation in the visible range of light.

High coloration efficiency, high reversibility and fast switching response of nebulized spray deposited anatase TiO2 thin films for electrochromic applications

Nebulized spray deposited electrochromic oxide thin films have shown better performance in electrochromic application due to the formation of finer particles on the substrate surface and improved contact between the substrate surface and film. In the present work, anatase phase TiO2 thin films with excellent electrochromic performance were prepared using nebulized spray deposition technique. The influence of substrate temperature on the electrochromic performance of TiO2 thin films was studied. In addition, various physical properties of the films were studied. The film prepared at lower substrate temperature of 200°C exhibit a high coloration efficiency of 260.7mCcm−2 at 600nm, fast switching kinetics (bleaching time=1.7s, coloration time=2.37s), high reversibility of 99% and consistent stability of about 1000 cycles. The present report suggests that the nebulized spray deposited TiO2 thin films can perform efficiently in electrochromic applications.

Characterization of MAPLE deposited WO3 thin films for electrochromic applications

Tungsten trioxide (WO3) is a widely studied material for electrochromic applications. The structure, morphology and optical properties of WO3 thin films, grown by matrix assisted pulsed laser evaporation (MAPLE) from monoclinic WO3 nano-sized particles, were investigated for their possible application as electrochromic layers. A KrF* excimer (λ=248 nm, ζFWHM=25 ns) laser source was used in all experiments. The MAPLE deposited WO3 thin films were studied by atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXRD) and Fourier transform infrared spectroscopy (FTIR). Cyclic voltammetry measurements were also performed, and the coloring and bleaching were observed. The morpho-structural investigations disclosed the synthesis of single-phase monoclinic WO3 films consisting of crystalline nano-grains embedded in an amorphous matrix. All thin films showed good electrochromic properties, thus validating application of the MAPLE deposition technique for the further development of electrochromic devices.

Electrochemical synthesis of polyaniline/inorganic salt binary nanofiber thin films for electrochromic applications

Polyaniline conductive polymer thin films without or with LiClO4 inorganic salt were coated by electrochemical deposition method on ITO surface. Electrochromic stability test, electrochemical behavior, coloring performance, optical properties and surface formations of the produced films were studied in detail. The deposited films were found to have a good stability. Electrochemical analysis showed that the conductivity of LiClO4 doped PANI film was higher than that of the PANI. Oxidation and reduction peaks, and potentials differed significantly depending on the content of the electrolytes. SEM studies demonstrated that the surface of each film consisted of completely nanofibers. Addition of LiClO4 to the deposition electrolyte decreased substantially the diameter of these nanofibers. PANI film with LiClO4 exhibited four colors, while pure PANI film had three different colors. The EDX results obtained from the surface of LiClO4 doped PANI material indicated that PANI and LiClO4 could be successfully linked to form PANI/LiClO4 binary thin film.

Propylene glycol-assisted seed layer-free hydrothermal synthesis of nanostructured WO3 thin films for electrochromic applications

This research article presents an overview of the hydrothermal synthesis of nanostructured tungsten oxide (WO3) and its electrochromic (EC) performance. A remarkable evolution in the past few years of producing pure and fine WO3 nanostructures using mild hydrothermal synthesis has received great attention. The hydrothermal process is highly suited for producing monodispersed nanoparticles with control over size and morphology, low processing temperature, and easy synthesis. In this article, we developed a facile seed layer-free hydrothermal approach for preparing WO3 thin films with improved EC performance. Structural and morphological properties were studied using X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. The electrochemical stability of the propylene glycol-assisted nanostructured WO3 film was examined in lithium per chlorate-propylene carbonate (LiClO4-PC) electrolyte for prolonged color/bleach cycles. The results showed an improvement in electrochemical stability with fast response time.

AA-CVD and ALD of Vanadium Pentoxide Thin Films for Electrochromic Applications

Despite the high potential impact of vanadium based electrochromic devices, there are still many issues to be resolved before widespread market implementation can be realised. The current limitations arise from cost, the slow electrochemical kinetics and the poor cycling stability of vanadium pentoxide. In this work we initially employed aerosol assisted chemical vapor deposition (AACVD) at 400°C in combination with a 600°C anneal in air to grow films on fluorine doped tin dioxide (FTO) coated glass. The aerosol was generated from a methanolic solution of vanadyl (IV) acetlyacetonate using an ultrasonic humidifier and a nitrogen carrier gas. All annealed films were found to be monoclinic vanadium pentoxide but only the thicker films demonstrated a viable electrochemical response. The lack of response from the thinner films was proposed to be due to either the formation of an unstable interfacial oxide or non-uniform growth due to the aerosol method. To determine the reasons for this limitation two experiments were performed, firstly the addition of a sub-nm alumina barrier layer by atomic layer deposition between the FTO and the V2O5 to inhibit the formation of any interfacial mixed oxide; Secondly, the low temperature growth of V2O5via atomic layer deposition using vanadyl triisopropoxide and oxygen as reagents. Initial indications are that the inclusion of an alumina barrier layer does not improve the uniformity of the thinner AACVD films. The ALD grown V2O5 films however demonstrated significant improvements in uniformity even at very high optical transparencies. These observations are discussed with reference to electron microscopy, UV-visible spectroscopy, XRD and the samples’ electrochemical responses.

Influence of spray pyrolysis deposition parameters on the optoelectronic properties of WO3 thin films

The electrochromic (EC) properties of tungsten oxide (WO3), such as coloration efficiency, cyclic durability and reversibility strongly depend on the structural and morphological properties, which are influenced by the deposition method and parameters.This paper presents the steps for optimizing the deposition parameters (substrate temperature, air flow pressure and precursor solution molarity) for improving the optical and electrical properties of WO3 thin films for EC applications. WO3 thin films were deposited by spray pyrolysis using tungsten hexachloride (WCl6) dissolved in ethanol as precursor solution. The EC properties of optimized films were tested in two different electrolytes (H2SO4 1M and acetic acid/sodium acetate buffer with pH=4) and changes in structure, composition and morphology of the films after coloration/bleaching cycles were discussed.The deposition temperature, carrier gas pressure and solution molarity were optimized at 250°C, 120kPa and 0.14M respectively. Under these condition a dense, uniform film, with homogenous distribution of particles, good adhesion to the substrate, low roughness (9.02nm), high transparency (>70% in the 500–1100nm range) and conductivity was obtained. Transmission modulation is higher for the sample cycled in H2SO4 1M (64% at 630nm) compared to that cycled in the buffer (21% at 630nm), whereas opposite results were obtained for coloration efficiencies 28cm2C−1 (at 630nm) and 35cm2C−1 (at 630nm), respectively. Changes in surface chemistry and morphology of the optimized sample were observed after cycling in H2SO4.

Micro-Raman spectroscopy on polyethylene-glycol assisted sol–gel meso and macroporous WO 3 thin films for electrochromic applications

Tungsten oxide (WO 3) is one of the most promising materials for electrochromic devices and is currently investigated for applications in catalysis, ferroelectricity and in sensor technology. Here a molecular assembly approach to synthesize by sol–gel meso and macroporous thin films of tungsten oxide using polyethylene-glycol as a templating agent is reported. Micro-Raman spectroscopy has been employed to investigate the structure of WO 3 thin films, deposited by dip-coating on glass, as a function of thermal annealing. This modified sol–gel route promotes the coexistence of a sub-stoichiometric monoclinic phase (most probably the WO 2.72 phase) together with some hydrous phases (WO 3·nH 2O) even in samples heat treated up to 500 °C.

Synthesis and structural characterization of mesoporous V2O5 thin films for electrochromic applications

Mesoporous vanadium pentoxide (V2O5) films have been synthesized by hydrolysis of vanadium tri-isopropoxide (VO(OC3H7)3) in the presence of polyethylene glycol (PEG) as a structure-directing agent. The structure, the stoichiometry and the morphology of the films have been studied as a function of the thermal annealing by X-ray diffraction (XRD), micro-Raman spectroscopy, optical microscopy, scanning electron microscopy and atomic force microscopy. XRD patterns and Raman spectra show the presence of two previously unreported crystalline phases. The PEG:V2O5 molar ratio affects the temperature of phase formation, the amount and even the order in which the phases appear. The morphological characterization underlines the role of the surfactant to promote porous networks, formed by micrometric clusters of controlled shapes and patterns embedded in a homogeneous host matrix.