Electrochromic WO3 Thin Films Research Articles

Transmittance correlated real-time resistivity modulation and insulator-metal transition of electrochromic WO3 thin films

A quantitative relationship between optical and electrical properties of crystalline WO3 films during Li + insertion has been established in this work. Detailly, the exponential relationship between transmittance and resistivity of crystalline WO3 thin films in visible and near-infrared wavebands is determined, and the real-time characterization of the resistivity of WO3 thin films during electrochromic process is realized. The experimental results show the resistivity of WO3 film can be regulated in the range of 0.078–0.98 Ω m with rapid response (∼6 s) when applied ±1 V voltage for 30 s. By analyzing the fitted resistivity-transmittance curve of WO3 thin film, a high-efficiency optical modulation of WO3 film in near-infrared 1000 nm can be achieved. Combining the IMT phenomenon during Li+ insertion, when x of LixWO3 changes from 0.08 to 0.13, the WO3 film is in a transition state from the insulating state to the metallic state with the tetragonal crystalline phase. The symmetry structural evolution of crystalline WO3 thin films during the Li + insertion is further demonstrated by transmission electron microscopy (TEM). These results contribute to the development of WO3 based optoelectronic bifunctional electrochromic devices.

Enhanced electrochromic properties of Ag-incorporated WO3 nanocomposite thin films

WO3 nanocomposite thin films, in which metal nanoparticles are incorporated into a WO3 matrix, offer an enhanced electrochromic response speed with color neutrality. This article reports the structural, optical, and electrochemical properties of Ag nanoparticle-incorporated electrochromic WO3 thin films with different concentrations. Room-temperature-sputtered thin films using composite targets with a low Ag concentration exhibited a random distribution of small nanoparticles. In contrast, in the film with a higher Ag concentration, the particles were merged, resulting in a single or polycrystalline phase. Surface plasmon resonance peaks were observed in the absorption spectra of the Ag–WO3 composite films, indicating the formation of metallic nanoparticles, which was also confirmed via high-resolution transmission electron microscopy. The optical bandgaps decreased gradually from 3.15 eV for WO3-δ to 3.02 and 2.99 eV for W0.97Ag0.03O3-δ and W0.91Ag0.09O3-δ thin films, respectively. The W0.91Ag0.09O3-δ thin film showed a faster switching time of 2.6 s for coloring and 4.8 s for bleaching with a higher coloration efficiency of 66.52 cm2/C than those of the WO3-δ thin film, which were 3.8 s, 6.8 s, and 58.68 cm2/C, respectively. However, transmittance modulation in the W0.91Ag0.09O3-δ thin film was inferior to that in the other films. Furthermore, the W0.91Ag0.09O3-δ thin films were black in the colored state, whereas the bare WO3-δ and W0.97Ag0.03O3-δ thin films were Prussian blue, suggesting the realization of color neutrality through the incorporation of Ag nanoparticles.

Enhancement of electrochromic response and cyclic durability of WO3 thin films by stacking Nb2O5 layers

This article reports on the control of interface morphology degradation between acid-based electrolytes and electrochromic WO3 thin films by stacking Nb2O5 layers to develop highly sustainable and durable electrochromic devices. Thin films of Nb2O5 sputtered at room temperature did not exhibit electrochromic properties, whereas WO3 thin films showed a typical electrochemical response by manifesting reversible color changes from dark blue (under negative potential) to transparent (under reverse potential). As the number of electrochromic cycles in the H2SO4 electrolyte increased, the columnar-structured WO3 thin film considerably changed to a nanoflake structure; however, the microstructure did not significantly change when Nb2O5 layers were stacked on the WO3 layers. The electrochromic performance of bilayer thin films was critically dependent on the stacking order and thickness of each layer. The thin film stack of 100-nm thick Nb2O5/200-nm thick WO3 exhibited enhanced transmittance modulation, response speed, and coloration efficiency compared with that of the 300-nm thick WO3 thin film. Furthermore, up to 1300 cycles, the bilayer thin film exhibited transmittance modulation exceeding 40%; it lost electrochromic switching at the 3500th cycle. In contrast, the single-layer WO3 lost its electrochromic properties at the 700th cycle. This study provides an excellent opportunity for fabricating devices with enhanced electrochromic properties and long-term cyclic durability that can be achieved by stacking Nb2O5 layers.

Adhesive electrochromic WO3 thin films fabricated using a WO3 nanoparticle-based ink

Nanotechnology has advanced rapidly in recent years, mainly due to the unique properties of basic elements, which can be tuned by altering their chemical composition and size. Owing to their unique properties, nanoparticles (NPs) are applied in many fields, such as catalysis, electronics, and coatings. In this study, we synthesized a coatable WO3 NP dispersed ink and aimed to highlight the importance of developing an appropriate design strategy for achieving optimal physicochemical properties of the inks. Polyvinyl alcohol (PVA) was added to the WO3 NP ink to improve its adhesion to indium tin oxide-coated glass substrates. The contact angle of the inks on the substrate decreased with the addition of PVA, indicating their good wettability. Furthermore, the WO3 NP films exhibited an excellent transmittance change of 77 % at 633 nm and a coloration efficiency of 35 cm2/C. Their optical transmittance in the visible and near-infrared regions varied with the applied voltage, indicating their potential for applications in electrochromic devices and smart windows.

Potentiostatic rejuvenation of electrochromic WO3 thin films: Exploring the effect of polyethylene oxide in LiClO4-Propylene carbonate electrolytes

Thin films of electrochromic (EC) W oxide were prepared by reactive DC magnetron sputtering and were immersed in electrolytes of LiClO4 in propylene carbonate with 0.5–3.0 wt% of added polyethylene oxide (PEO). Charge density and optical modulation range were found to diminish monotonically upon prolonged voltammetric cycling with concurrent optical transmittance measurements for up to 500 and 200 cycles in the voltage ranges 2.0–4.0 and 1.5–4.0 V vs. Li/Li+, respectively. Rejuvenation of the degraded EC films was then accomplished by potentiostatic treatment at 6.0 V vs. Li/Li+ for 20 h and has been ascribed to expulsion of trapped Li ions. Importantly, degradation subsequent to rejuvenation was found to progress at a significantly lowered pace than for as-deposited films—in proportion with the amount of PEO—especially for harsh voltammetric cycling in the voltage range 1.5–4.0 V vs. Li/Li+. The results of this explorative study are of considerable value for the development of highly durable EC devices such as polymer-laminated smart glazing for energy efficient buildings with excellent indoor comfort.

Electrochemical pretreatment of electrochromic WO3 films gives greatly improved cycling durability

Electrochromic WO3 thin films have important applications in devices such as smart windows for energy-efficient buildings. Long-term electrochemical cycling durability of these films is essential and challenging. Here we investigate reactively sputter-deposited WO3 films, backed by indium–tin oxide layers and immersed in electrolytes of LiClO4 in propylene carbonate, and demonstrate unprecedented electrochemical cycling durability after straight-forward electrochemical pretreatments by the application of a voltage of 6 V vs. Li/Li+ for several hours.

Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO3 thin films.

Amorphous WO3 thin films are of keen interest as cathodic electrodes in transmittance-modulating electrochromic devices. However, these films suffer from ion-trapping-induced degradation of optical modulation and reversibility upon extended Li+-ion exchange. Here, we demonstrate that ion-trapping-induced degradation, which is commonly believed to be irreversible, can be successfully eliminated by constant-current-driven de-trapping, i.e., WO3 films can be rejuvenated and regain their initial highly reversible electrochromic performance. Pronounced ion-trapping occurs when x exceeds ~0.65 in LixWO3 during ion insertion. We find two main kinds of Li+-ion trapping sites (intermediate and deep) in WO3, where the intermediate ones are most prevalent. Li+-ions can be completely removed from intermediate traps but are irreversibly bound in deep traps. Our results provide a general framework for developing and designing superior electrochromic materials and devices.

Electrochromic WO3 thin films prepared by combining ion-beam sputtering deposition with post-annealing

A method combining ion-beam sputtering deposition with post-annealing was proposed to prepare electrochromic WO3 thin films. It was observed that the intensity of oxygen deficiency phases can be controlled by adjusting the post-annealing temperature and time. It was also observed that as-prepared WO3 films exhibit excellent optical modulations up to 78–87%. With the increasing of oxygen deficiency intensity, the optical modulation increases accordingly. Further, the electrochromic mechanism of WO3 films was also discussed.

Coloration and Depth Distribution of Cations Electrochemically-inserted into Electrochromic WO3 Thin Films

Li+ and H+ ions were electrochemically inserted into amorphous WO3 films prepared on an ITO-coated glass substrate by an R.F. magnetron sputtering method under Ar/O2 flow ratios of 4/1 (SP1)and 1/1 (SP2). The cation distribution was estimated indirectly by depth profiles of refractive-index obtained from prism coupler measurements and was evaluated directly by glow discharge spectrometry (GDS). H+ ions inserted were segregated only at deeper region around ITO electrode, which was independent to the preparing condition. In the case of Li+ insertion into SP1 film, Li+ ions were initially segregated at around ITO electrode, and after further insertions, they were also distributed at around the surface of WO3 film. In SP1 film, Li+ ions at around ITO electrode seemed to contribute to coloration. In SP2 film, however, Li+ ions subsequently inserted, which were uniformly distributed in the film, were only involved in coloration. The difference in depth distribution and coloration was due to the difference in atomic structure of WO3 films.

Ageing of electrochromic WO3 coatings characterized by electrochemical impedance spectroscopy

We have developed a method for characterization of ageing of electrochromic materials by electrochemical impedance spectroscopy (EIS). Electrochromic WO3 thin films have been electrochemically cycled in propionic acid electrolyte and probed by EIS and optical measurements. A very small amount of optical degradation was observed in both the bleached and coloured states. The samples exhibited a few hundred times higher impedance in the bleached state than in the coloured state. It was observed that, in the bleached state, impedance values at low frequencies increased significantly with increasing number of cycles.

Lithium depth profiling in thin electrochromic WO3 films

Li profiles in sol-gel prepared electrochromic WO3 thin films have been characterized using three different techniques: neutron depth profiling (NDP), secondary ion mass spectrometry (SIMS), and elastic recoil detection (ERD). The results have been evaluated with respect to their concentration sensitivity and depth resolution. NDP and SIMS led to comparable results in terms of the shape of the profiles. The NDP results were quantified by using a calibration sample. NDP has the advantage that it is a quantitative nondestructive technique while SIMS has a very high sensitivity and depth resolution. Another advantage of NDP is that there is very little risk of neutron-induced Li migration during the measurement because of the very low energy of the incoming thermal neutrons. ERD was found to have a limited depth resolution, attributable to deterioration of the sample as a result of ion bombardment. However, the absolute amount of Li can be very accurately quantified with this technique.

Optical constants of electrochromic polycrystalline WO3 thin films prepared by chemical vapour deposition

Abstract Reflectance and transmittance spectra were recorded, at normal incidence, of coloured or bleached electrochromic polycrystalline WO 3 thin films prepared by chemical vapour deposition on SnO 2 -coated Pyrex substrates. The optical constants n and k were determined using Fresnel's laws and dispersion relations via an iterative process.

Heat treatment of amorphous electrochromic WO3 thin films deposited onto indium-tin oxide substrates

Electrochromic tungsten oxide thin films, obtained by vacuum evaporation, were studied before and after heat treatment between 25 and 250°C for 2 h in air. Electrochromic properties were investigated in acid electrolyte by simultaneous measurements of the electrical and optical parameters. A.c. complex impedance techniques and voltammetry were used to characterize the films from an electrical point of view. We observed an enhancement of the electrochromic response times during both coloration and bleaching after heat treatment carried out between 150 and 220°C. This phenomenon was associated with a decrease of the ohmic drop in the electrode and a continuous variation of the impedance diagrams of these electrochromic electrodes. Moreover, we observed that the diffusion coefficient of H+ ions into WO3, obtained on colored thin films, increased as the electrochromic kinetics increased.