Highly uniform electrochromic tungsten oxide thin films deposited by e-beam evaporation for energy saving systems

In the last few decades, there has been a surge of interest in using tungsten oxide thin films as an active layer of electrochromic device. These devices have several practical applications such as smart window of buildings and automobile glazing for energy saving. The main objective of this work was to construct highly homogeneous and uniform e-beam evaporated amorphous WO_3-x based films into electrochromic devices, which were fully characterized for switching speed, coloration efficiencies and cycling voltammetry responses. Fabricated devices contain indium doped transparent oxide coated glass as the transparent conductive electrode, ～200?nm thickness of WO_3-x as the cathodically coloring material and a lithium perchlorate based conducting gel electrolyte. X-ray diffraction patterns indicate that all as-deposited films are amorphous. Experimental results showed that both solid and liquid electrolyte electrochromic devices are initially very transparent that exhibit perfect optical modulation and coloration efficiency (up to 68.7?cm²/C and 52.6?cm²/C at 630?nm, respectively) due to easier intercalation of the Li⁺ within their structure. One of the more significant findings to emerge from this study is that e-beam coated electrochromic devices based on tungsten oxide thin films showed superior performance among to other coating methods. Therefore, excellent reversibility of color change behavior is attractive for pertinent use in electrochromic energy storage devices.     

Electro-optic spectroscopy of electrochromic processes in tungsten trioxide

The nature of the electrochromic processes in solid-state electrochromic devices (ECD) ITO/a-WO₃/a-SiO₂/Au is investigated by analysing their electro-optical behavior during the current pulses and the linear sweeping of the bias voltage. It is shown that the character of the current-coltage curve of the ECD is determined by passivation, by hydrogen/oxygen evolution on the counter electrode (Au) and by redox reactions on the electrochronic electrode (amorphous WO₃ film: a-WO₃). The light transmittance versus voltage and the high-frequency differential impedance of the active component versus voltage curves are dependent mainly on electrochromic reactions.     

Characterization of tungsten-titanium oxide electrode for electrochromic applications

Titanium oxide films are of critical importance for the electrochromic device technology. The substrate, a conductive glass being coated with indium tin oxide (ITO) thin films, was deposited tungsten and titanium oxide by pulsed co-sputtering deposition system. The film thickness increased with the ion beam power. However, the slope of the curve of thickness against power at an ion beam power of less than 300 W was greater than that at a power of 400 or 500 W. A high ion beam power resulted produced a crystalline structure, as revealed by X-ray diffraction (XRD). Moreover, increasing the ion beam power resulted in the high Li-ions transport. The electrochromic behavior was optimal at an ion beam power of 200 W.     

Preparation of UV curing crosslinked polyviologen film and its photochromic and electrochromic performances

Polyether urethane diacrylate matrix (PEUDA) and acrylate-functional viologen (ACV²⁺) were successfully synthesized and characterized in detail by FTIR and ¹H NMR spectra, respectively. Subsequently, they were used to prepare UV curing crosslinked polyviologen film in combination with 2-hydroxyethyl methacrylate (HEMA), trimethylolpropane ethoxylate triacrylate (TMPTA) and diphenyl (2, 4, 6-trimethylbenzoyl) phosphine oxide (TPO). UV curing approach confined the polyviologen film on ITO electrode, which imparted the film excellent adhesion ability to ITO glass, good solvent resistance, excellent chemical stability, excellent optical and electrochemical properties. The crosslinked PACV²⁺ film exhibited excellent photochromic and electrochromic performances. After UV illumination for 60 s, the crosslinked PACV²⁺ film can swiftly change its color from pale yellow to deep blue, while the optical transmission of crosslinked PACV²⁺ film at 610 nm did not change significantly and still retained about 63.6% after 30 cycles. Simultaneously, the cyclic voltammetry experiment showed the PACV²⁺ film can undergo repeatable electrochemical redox reactions with good reversibility beyond the 10th scan. Furthermore, the electrochromic device composed of the PACV²⁺ film and gel electrolyte film can undergo reversible color change in response to the external voltages of −2.0 V and 2.0 V, respectively, while the contrast of EC device at 610 nm did not change significantly and still retained about 39.5% after 10 cycles. This UV curing approach to preparing viologen-functional film offers a method to preparing large-scale photo- and electrochromic device, which is relatively simple, high productivity, energy saving, and environmental protection.     

An electrochromic device (ECD) cell characterization on electron beam evaporated MoO3 films by intercalating/deintercalating the H+ ions

Thin films of molybdenum oxide (MoO₃) is one of the most interesting layered intercalation materials because of its excellent application in solid state batteries, large-area window and display systems. In recent years there has been considerable interest in variable transmittance electrochromic devices (ECD) based on Li⁺, H⁺ and K⁺ intercalation in transition metal oxide (MoO₃) thin films. In the present investigation, thin films of MoO₃ were prepared by electron beam evaporation technique on microscopic glass and fluorine doped tin oxide (FTO) coated glass substrates for the application in electrochromic device cells. The compositional stoichiometry of the films was studied by X-ray photoelectron spectroscopy (XPS). The electrochromic nature of the films has been analyzed by inserting H⁺ ions from the H₂SO₄ electrolyte solution using the cyclic-voltammetry (CV) technique. We studied the electrochromic device cells (ECD) incorporating an evaporated MoO₃ thin films as electrochromic layers. The devices exhibit good optical properties with low transmittance values in the colored state, which make them suitable for large-area window applications. The maximum coloration efficiency of the cell was observed at about 70 cm²/C.     

Towards a thin films electrochromic device using NASICON electrolyte

The optimisation of the morphology of WO₃ thin films allowed a more efficient electrochromic colouring using Na⁺ ions than H⁺ ones. Therefore, sodium superionic conductor (Na₃Zr₂Si₂PO₁₂, NASICON) films may be used as electrolyte in inorganic electrochromic devices. In this paper, the structure, chemical composition, morphology and electrochromic properties of WO₃, ZnO:Al and Na₃Zr₂Si₂PO₁₂ thin films were studied to develop a novel type of electrochromic device. WO₃, ZnO:Al and Na₃Zr₂Si₂PO₁₂ thin films were deposited using reactive magnetron sputtering of tungsten, zinc and aluminium and Zr–Si and Na₃PO₄ targets, respectively. For transparent conductive oxide coatings, a correlation was established between the deposition parametres and the film’s structure, transmittance and electrical resistivity. Classical sputtering methods were not suitable for the deposition of NASICON films on large surface with homogenous composition. On the other hand, the use of high-frequency pulsed direct current generators allowed the deposition of amorphous films that crystallised after thermal annealing upon 700 °C in the Na₃Zr₂Si₂PO₁₂ structure. Amorphous films exhibited ionic conductivity close to 2 × 10⁻³ S cm⁻¹. Finally, preliminary results related to the electrochromic performance of NASICON, WO₃ and indium tin oxide devices were given. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, France, Sept. 9–15, 2007.     

Superior performance characteristics for the poly(2,5-dimethoxyaniline)–poly(styrene sulfonic acid)-based electrochromic device

We have fabricated an electrochromic (EC) device with poly(2,5-dimethoxyaniline), PDMA, entrapped in poly(styrene sulfonic acid) (PSS) as an electrochromic layer. The device showed improved performances like stability, optical contrast, etc., over the device with a PDMA layer doped by H₂SO₄. In the process of fabrication of the EC device with a sandwich configuration, indium tin oxide (ITO)/PDMA–PSS||poly(ethyleneimine) (PEI)/orthophosphoric acid (H₃PO₄)/WO₃/ITO, electrochemical polymerization of 2,5-dimethoxyaniline (DMA) was performed with PSS as electrolyte and ITO coated glass as working electrode. The performance characteristics of EC device, like optical contrast, stability, switching time, etc., were followed by cyclic voltammetry, double potential step chronoamperometry and in-situ spectroelectrochemistry. The device was operated in between − 1 V and + 1 V, and absorption characteristics were followed by in-situ UV–visible spectroscopy. A visible contrast in color upon switching the potential from − 1 V to + 1 V was noticed for the device. The device was pale yellow at − 1 V and dark green at + 1 V. Incorporation of PSS into PDMA resulted enhancement in the performance of the complementary electrochromic device. The optical contrast of the device was improved by incorporating PSS into PDMA matrix. The device retained nearly 50% of their optical contrast after 10,000 double steps informing the superior performance of PDMA–PSS in the EC device.     

Gelatin-based protonic electrolyte for electrochromic windows

Proton-conducting gel polymer electrolytes based on gelatin plasticized with glycerol and containing acetic acid were investigated, characterized, and applied to electrochromic window. For glycerol contents varying from 7% to 48%, the conductivity of the uniform and predominantly amorphous gel electrolyte was found to follow a Vogel–Tamman–Fulcher behavior with the temperature. Typically, for the electrolyte chosen to make 7 × 2 cm² electrochromic smart window with the configuration: glass/fluor-doped tin oxide (FTO)/WO₃/gelatin electrolyte/CeO₂–TiO₂/FTO/glass and containing 28% of glycerol, the conductivities were found to be of the order of 5 × 10⁻⁵ S/cm at room temperature and 3.6 × 10⁻⁴ S/cm at 80 °C. The device was characterized by spectroelectrochemical techniques and was tested up to 10,000 cycles showing a fast coloring/bleaching behavior, where the coloring process was achieved in 10 s and the bleaching in 2 s. The transmission variation at the wavelength of 550 nm was about 15%. The cyclic voltammograms showed a very good reversibility of the cathodic/anodic processes, and the charge density was about 3.5 mC/cm². The memory tests showed that the transmittance in the colored state increased by 8% in 90 min after removing the potential.     

From beads-to-wires-to-fibers of tungsten oxide: electrochromic response

Suitable host lattice and morphology for easy intercalation and deintercalation process are crucial requirements for electrochromic device. In this investigation, the evolution of structural and morphological changes and their effect on electrochromic (EC) properties of spray-deposited WO₃ thin films are studied. Films of different morphologies were deposited from an ammonium tungstate precursor solution using a novel pulsed spray pyrolysis technique (PSPT) on tin-doped indium oxide (ITO) coated glass substrates by varying quantity of spraying solution. Interesting morphological transition from beads-to-wires-to-fibers as a function of quantity of sprayed solution has been demonstrated. The porosity, crystallinity and “open” structures in the films consisting of beads, wires, and fiber-like morphology enabled us to correlate these aspects to their EC performance. WO₃ films comprising wire-like morphology (20 cc spraying quantity) exhibited better EC properties both in terms of coloration efficiency (42.7 cm²/C) and electrochemical stability (10³ colored/bleached cycles) owing to their adequate open structure, porosity, and amorphicity, compared with the films having bead/fiber-like morphology.     

Sol-Gel derived CeO2:TiO2 coatings for electrochromic devices

Thin films of mixed CeO₂-TiO₂ with different Ce/Ti mole ratios were prepared following an alcohol based sol-gel route via the spin coating technique using mixed inorganic-organic [CeCl₃.7H₂O and Ti(OPr)₄] precursors. Ion storage capacity for films obtained from aged sols was observed to be high. Enhanced titanium oxide content improved the insertion capacity of the corresponding films as was evident from inserted charge determined by multiple step chronoamperometric measurements. Electrochemical, optical, structural and thermal performances showed the suitability of the films in an all solid state electrochromic (transmissive) device with tungsten trioxide (WO₃) as electrochromic material and a conductive polymeric electrolyte based on lithium. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

The effect of temperature on the preparation of electrochromic nickel oxide by an electroless method

The advantages of nickel oxide as an electrochromic material are due to its good contrast of transmittance and its suitable use as a secondary electrochrome. Compared to other methods of depositing eletrochromic nickel oxide, coating nickel oxide by electroless is simple and easy to scale-up for industrial application. This study presents the preparation of nickel oxide film on an ITO substrate by an electroless method and oxidizing it with heat treatment. The influence of oxidizing temperature in heat treatment procedures is notable. The morphology of the film was analyzed by a scanning electron microscope (SEM) and X-ray diffraction (XRD). Heat temperature at 380 °C obtained optimal of electrochromic properties. The transmittance difference (ΔT) of the film at 630 nm was maintained at 69%, and the amount of transferred charge during cyclic voltammetry was approximately 0.8 mC/cm² after 1000 cycles of redox, which was operated between −1.5 and +1.5 V potential step. The degradation of the film was decreased by increasing the oxidizing temperature. The evidences showed electrochromic abilities of nickel oxide were affected by heat-treatment procedures. All these analyses provided a novel method for preparing the electrochromic nickel oxide in a low-cost way.     

Structural, optical and electrochromic properties of nickel oxide thin films grown from electrodeposited nickel sulphide

Nickel oxide thin films were grown onto FTO-coated glass substrates by a two-step process: electrodeposition of nickel sulphide and their thermal oxidation at 425, 475 and 525 °C. The influence of thermal oxidation temperature on structural, optical, morphological and electrochromic properties was studied. The structural properties undoubtedly revealed NiO formation. The electrochromic properties were studied by means of cyclic voltammetry. The films exhibited anodic electrochromism, changing from a transparent state to a coloured state at +0.75 V versus SCE, i.e. by simultaneous ion and electron ejection. The transmittance in the coloured and bleached states was recorded to access electrochromic quality of the films. Colouration efficiency and electrochromic reversibility were found to be maximum (21 mC/cm² and 89%, respectively) for the films oxidized at 425 °C. The optical band gap energy of nickel oxide slightly varies with increase in annealing temperature.     

An All-Thin-Film Electrochromic Device Composed of MoO3--LiBSO--NiOx Multilayer Structure

An all-thin-tilm （ATF） electrochromic device for modulating the optical transmittance is manufactured using magnetron sputtering. The devices consists of MoO3 as the main electrochromic layer, LiBO2 ＋Li2SO4 （LiBSO） as the ion conductor layer, and NiOx as the complementary electrochromic layer. Glass covered with indium tin oxide （ITO） is used as the substrate and the ITO film is used as the bottom electrode. The ITO film deposited on the top of the devices is used as the other electrode. The structure and morphology of the films are characterized by x-ray diffraction （XRD） and scanning electron microscopy （SEM）. The devices exhibit good optical properties with low transmittance values in the coloured state, and the optical modulation is measured by spectrophotometer in the wavelength range from 400 to 800nm. The average visible light transmittance reaches 50.2% and 3.7% in bleached and coloured state, respectively. The results indicate that such a monolithic system has great potential to be applied in smart windows.     

Coupling of crystalline colloidal arrays with intrinsically conductive polymers: Reflection-type electrochromic devices

The attenuation of the reflected light from a polystyrene (PS) polymerized crystalline colloidal array (PCCA) is achieved through the electrochemical switching of the electrochromic material polyaniline-poly(2-acrylamido-2-methyl-1-propanesulfonicacid) (PANI-PAMPS). For this purpose, layered electrochromic devices (ECDs) were fabricated that employed a PS PCCA film, acting as both a reflecting substrate and polymeric electrolyte, and the conducting polymer PANI-PAMPS, which served as the electrochromic layer. The resulting ECDs exhibited the attenuation of reflectance peak at the stop band position of the PCCA with a 3% of reflectance contrast ratio.     

The linear electrochromic effect in metanitroaniline

An electric field applied along the polar axis of the molecular crystal meta-nitroaniline causes a change in the optical transmission at the band edge between 505 and 540 nm. The size of the incremental change was found to vary linearly with applied field, and to reverse with field polarity. We have termed this ‘the linear electrochromic effect’. We suggest here, that it arises from a shift in the transition energy of the visible-near u.v. electronic absorption band, which is implied by the observed linear electro-optic behaviour of this material. A theoretical model is proposed for the linear electrochroism of meta-nitroaniline, and from this a maximum band shift of 2·5 × 10^?23 J per (kV mm^?1) is predicted.     

Annealing induced microstructural evolution of electrodeposited electrochromic tungsten oxide films

A significant influence of microstructure on the electrochromic and electrochemical performance characteristics of tungsten oxide (WO₃) films potentiostatically electrodeposited from a peroxopolytungstic acid (PPTA) sol has been evaluated as a function of annealing temperature. Powerful probes like X-ray diffractometry (XRD), transmission electron microscopy (TEM), UV-vis spectrophotometry, multiple step chronoamperometry and cyclic voltammetry have been employed for the thin film characterization. The as-deposited and the film annealed at 60 °C are composed of nanosized grains with a dominant amorphous phase, as well as open structure which ensues from a nanoporous matrix. This ensures a greater number of electroactive sites and a higher reaction area thereby manifesting in electrochromic responses superior to that of the films annealed at higher temperatures. The films annealed at temperatures ≥250 °C are characterized by a prominent triclinic crystalline structure and a hexagonal phase co-exists at temperatures ≥400 °C. The deleterious effect on the electrochromic properties of the film with annealing is ascribed to the loss of porosity, densification and the increasing crystallinity and grain size. Amongst all films under investigation, the film annealed at 60 °C exhibits a high transmission modulation (ΔT ∼ 68%) and coloration efficiency (η ∼ 77.6 cm² C⁻¹) at λ = 632.8 nm, charge storage capacity (Q_ins ∼ 21 mC cm⁻²), diffusion coefficient (6.08 × 10⁻¹⁰ cm² s⁻¹), fast color-bleach kinetics (t_c ∼ 275 s and t_b ∼ 12.5 s) and good electrochemical activity, as well as reversibility for the lithium insertion-extraction process upon cycling. The remarkable potential, which the film annealed at 60 °C has, for practical “smart window” applications has been demonstrated.     

Study of the potassium ion insertion of the electrodeposited electrochromic tungsten trioxide thin films

Various types of dynamic adjustable optical shutters have been proposed; they are called as “smart windows”. The electrochromic smart window utilizes the phenomenon of electrochromism. Electrochromism is the property of a material such that its color is changed by an electrochemical redox reaction. Numerous inorganic and organic electrochromic materials have been examined. During the color-bleach process, the redox reaction of the host material causes injection or ejection of both cation (or anion) and an electron (or hole). So it behaves as a mixed ion conductor and hence it has recently attracted interest in the field of solid state ionics. At present, several prototypes of electrochromic smart windows have been proposed and some of them are commercially available. The cathodic electrochromic oxides consist of n-type semiconductors such as TiO₂, V₂O₅, WO₃ and MoO₃. Among them, WO₃ has been intensively examined and used for most electrochromic devices. These materials are cathodically colored in blue. The electrochromic reaction is expressed by $$xA^ + + MO_y \left( {transparent} \right) + x e^ - \Leftrightarrow A_x MO_y \left( {blue} \right)\left( {A = H, Li, Na, K \ldots , MO_y = metal oxide} \right)$$ where the cation A⁺ and electron e^? are co-injected into the host oxide MO_y which results in the formation of the nonstiochiometric compound A_xMO_y. Here the electrodeposition method is used to deposite WO₃ films under by galvanostatic conditions. The parameters like deposition time, deposition temperature, electrolyte concentration, pH and bath temperature are optimized. The XRD results show the triclinic structure for the as-deposited film. The film shows the same structure after intercalation but the peak intensity is different. The tungsten trioxide thin films show the colour change during the intercalation and retrace the original colour. The diffusion coefficient for K⁺ ion is calculated by using the Randles-Servick equation. In this paper the electrochromic properties, the optical properties of the as deposited, ion intercalated and deintercalated WO₃ films and the diffusion coefficient values are presented.     

Monolithic electrochromic devices using lithium ion conducting perovskite-type oxides

The electrical conductivities of several perovskite-type lithium ion conductors in the Li-Sr-Nb-Ta-Ti-O system have been investigated. The Li⁺-ion conductivities of the Ta-compounds were found to be higher than those of the corresponding Nb-compounds, i.e., Li_0.3Sr_0.6Ta_0.5Ti_0.5O₃ exhibits a bulk ionic conductivity of 1.7×10⁻⁴ S/cm at 30 °C, while Li_0.3Sr_0.6Nb_0.5Ti_0.5O₃ shows a value of 5.4×10⁻⁶ S/cm at the same temperature. Substitution of Fe in Li_0.3Sr_0.6Ta_0.5Ti_0.5O₃ decreases the Li⁺-ion conductivity slightly. The operation of a monolithic (single element) electrochromic devices was demonstrated using perovskite-type Li_0.3Sr_0.6B_0.5Ti_0.5O₃ (B=Nb, Ta). The tantalum compound exhibited the largest coloration at the positive electrode side by the application of a voltage of 1.5 V and was bleached under short-circuit conditions at 350 °C. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Complementary electrochromic windows with conductive nano-composite thin films

All-solid-state electrochromic windows which have complementary color transition were assembled by employing poly(aniline-N-butylsulfonate)s (PANBS), photo polymerizable electrolyte and poly(3,4-ethylenedioxythiophene) (PEDOT) in sequence. Each electrochromic layer thickness was controlled in nano-scale in order to balance charges between the electrodes generated from the electroactive layers. The electrochromic (EC) properties were characterized using cyclic voltammetry, chronocoulometry and UV–Vis spectroscopy. Absorbance change and coloration efficiency were dependant on charge density and an all solid state EC display based on an optimized thickness showed EC response at 2.4 V within 5 s with a stable memory effect.