Optical properties of electrochromic iridium oxide and iridium-tantalum oxide thin films in different colouration states

Electrochromic iridium oxide (IrOx) and iridium-tantalum oxide (IrTaOx) thin films were prepared by sputtering. Complex refractive indices were determined for samples deposited on indium-tin oxide covered glass in different colouration states, and for as-deposited samples on sapphire and Corning glass. The refractive index was found to be practically constant for both IrOx (∼1.3) and IrTaOx (∼2). The extinction coefficient was found to vary between the coloured and bleached states with ∼35% for IrOx and ∼55% for IrTaOx at 660 nm. This is believed to be a result of the removal of intraband transitions within the Ir t_2g band during bleaching.     

Electrochemical investigations on spray deposited tin oxide thin films

Tin oxide (SnO₂) thin films were prepared by a simple and inexpensive spray pyrolysis technique from an aqueous solution at various substrate temperatures viz. 300, 400 and 500 °C, and their electrochemical studies have been carried out. The thin films have been optically and electrochemically characterized by means of transmittance, cyclic voltammetry and chronoamperometry. The mechanism of reduction and oxidation reactions that took place during the potential cycling is presented. The samples deposited at 500 °C exhibit better performance in terms of coloration efficiency, reversibility, contrast ratio and response time.     

Promotion of electrochromism in spray-deposited molybdenum oxide-doped iridium oxide thin films

Electrochromic molybdenum oxide-doped iridium oxide thin films were prepared by using a pneumatic spray pyrolysis technique onto fluorine-doped tin oxide (FTO) coated conducting glass substrates. An aqueous solution of 0.01 M ammonium molybdate was mixed with 0.01 M iridium trichloride in different volume proportions and the resultant solution was used as a precursor for spraying. An aqueous electrolyte (0.5 N H₂SO₄) was used to study electrochromic properties of thin films using cyclic voltammetry (CV), chronoamperometry (CA) and spectrophotometry. During the potential scan the iridium oxide electrode switches between coloured and bleached state due to Ir⁺⁴–Ir⁺³ intervalency charge transfers. The optical density difference (ΔOD)_λ=630 nm and colouration efficiency was maximum for 2% molybdenum oxide-doped sample. Moreover, loss in charge density during extended cycling is less than undoped and other doped (>2%) samples.     

Electrochromic thin films prepared by sol–gel process

New methods are shown for lower temperature preparation of amorphous tungsten oxide thin film and preparation of crystalline iridium oxide thin film by sol–gel process using metal chloride as the starting materials and ethanol as a solvent. These electrochromic materials were combined with gel solid electrolyte, and preparation of fully solid-state electrochromic display (ECD) was made. The transmittance of the ECD could be made to change by 35% by applying a voltage of 3 V for 0.2 sec.     

Sol–gel deposited nickel oxide films for electrochromic applications

The electrochromic (EC) behavior, the microstructure, and the morphology of sol–gel deposited nickel oxide (NiO_x) coatings were investigated. The films were produced by spin and dip-coating techniques on indium tin oxide (ITO)/glass and Corning glass (2947) substrates.The coating solutions were prepared by reacting nickel(II) 2-ethylhexanoate as the precursor, and isopropanol as the solvent. NiO_x was heat treated at 350 °C for 1 h. The surface morphology, crystal structure, and EC characteristics of the coatings were investigated by scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), atomic force spectroscopy (AFM), X-ray diffractometry (XRD), and cyclic voltammetry (CV).SEM and AFM images revealed that the surface morphology and surface characteristics of the spin- and dip-coated films on both types of substrate were different. XRD spectra revealed that both films were amorphous, either on ITO or Corning glass substrates. CV showed a reversible electrochemical insertion or extraction of the K⁺ ions, cycled in 1 M KOH electrolyte, in both type of film. The crystal structure of the cycled films was found to be XRD amorphous. Spectroelectrochemistry demonstrated that dip-coated films were more stable up to 1000 coloration–bleaching cycles, whereas spin-coated films gradually degraded after 500 cycles.     

Electrochromic W–M–O (M=V, Nb) sol-gel thin films: a way to neutral colour

Mixed vanadium and niobium tungsten oxide W–M–O (M=V, Nb) thin films were prepared by sol-gel from solution of tungsten oxychloride (WOCl₄) and vanadium oxypropoxide (VO(OPrⁱ)₃) or niobium oxybutoxide (Nb(OBuⁿ)₅), respectively. The annealing temperature and time were optimized to 200°C and 10 min. W–M–O (M=V, Nb) thin films were characterized by means of optical (transmittance) and electrochemical (cyclic voltammetry) methods. Evidence of different electrochemical behaviour is given where the presence of either vanadium or niobium leads to more neutral coloured films.     

Electrochromism of sol–gel derived niobium oxide films

Niobium oxide films are promising cathodic electrochromics that in many aspects can compete with the more frequently studied WO₃ films. The films reported herein were prepared using the sol–gel route from a NbCl₅ precursor. The electrochromic properties were pronounced for crystalline films heat-treated at 500°C and exhibited transmittance changes between coloured and bleached states of 60% in the ultraviolet (UV) and 80% in the visible (VIS) and near-infrared (NIR) regions. Improved bleaching and more reversible electrochromism of thick niobium oxide films (thickness (d)>250 nm) were obtained by lithiation.Electrochromic (EC) devices were also prepared by assembling niobium oxide and lithiated niobium oxide films of different thicknesses with a hybrid inorganic/organic Li⁺ ionic conductor (organically modified electrolyte-ormolyte) and a molybdenum and antimony doped tin oxide (SnO₂ : Sb(7%) : Mo(10%) counter electrode films. The EC devices exhibited adequate colouring/bleaching kinetics (<2 min) and colouring/bleaching changes up to 40–50%.     

Thickness-dependent electrochromic properties of solution thermolyzed tungsten oxide thin films

Tungsten oxide (WO₃) thin films are prepared by using a simple and inexpensive solution thermolysis technique. Thin film samples of different thickness are obtained by varying quantity of ammonium tungstate solution sprayed onto the preheated conducting glass substrate. A simple three-electrode cell has been formed to study the electrochemical and electrochromic properties. The electrochemical parameters of the cell such as anodic peak current, anodic peak potential, threshold voltage, amount of H⁺ ions intercalated into and deintercalated out of the WO₃ samples are calculated. The effect of film thickness on these parameters are studied. The extent of electrochromism and reversibility of the colouration/bleaching processes of various WO₃ samples are described. The colouration efficiencies at 633 nm are calculated. The maximum colouration efficiency obtained for thicker film, is 56 cm²/C. The samples were found to be stable in 0.05N H₂SO₄ electrolyte up to 1×10³ colour/bleach cycles.     

Optical properties of chemical vapor deposited thin films of molybdenum and tungsten based metal oxides

Thin films of tungsten oxide, molybdenum oxide and mixed MoO₃–WO₃ oxides were obtained by atmospheric pressure chemical vapor deposition (CVD). All the films were prepared using identical technological parameters and through investigation of the optical properties of as deposited and annealed at 400°C a comparative study is reported. Raman, IR and VIS spectrophotometry and spectral ellipsometry methods were used. The mixed MoO₃–WO₃ films have higher optical absorption with maxima at a closer position with respect to the human eye sensitivity peak at 2.5 eV. The observed electrochromic effect is better expressed in the mixed films; the electrical charge inserted is higher.     

Optical properties of tungsten and titanium oxide thin films prepared by plasma sputter deposition

Tungsten oxide and titanium oxide thin films were prepared by RF reactive magnetron sputter deposition. The stationary and rotating substrate holders were applied to analyze the rotating effect. The optical properties and thicknesses of oxide films were determined by a proposed optical model and the measured transmittance spectra. The dispersed refractive indices of thin films have a wide range distribution in different sputtering conditions. In the situation of rotating substrate holder, the refractive index was lower than that of the stationary substrate holder. Also, amorphous TiO₂ structure can be prepared by using rotating substrate holder. The transmittance spectrum of crystalline TiO₂ reveals that the textured structure on the film surface affects the transmittance characteristic.     

Ellipsometric spectroscopy study of cobalt oxide thin films deposited by sol–gel

Due to their unique optical properties, solar selective coatings enhance the thermal efficiency of solar photothermal converters. Hence it seems to be interesting to study the optical properties of promising materials as solar selective coatings. In an earlier work, it was demonstrated that sol–gel deposited cobalt oxide thin films possess suitable optical properties as selective coatings. In this work, cobalt oxide thin films were prepared by same technique and their optical properties were analyzed as a function of the dipping time of the substrate in the sol, using the spectroscopy ellipsometry, atomic force microscopy and X-ray photoelectron spectroscopy techniques. The optical constants (n and k) for these films, in the 200–800 nm range, are reported as a function of the dipping time. The fitting of ellipsometric data, I_s and I_c, for the glass substrate and the cobalt oxide thin film, as modeled with the Lorentz and Tauc–Lorentz dispersion relations, indicated that the film microstructure resembles a multilayer stack with voids. From these results, the Co₃O₄ and void percentages in the film were estimated. Both, thin film thickness and void/Co₃O₄ percentage ratio, were determined to be strongly dependent on the immersion time. Furthermore, the total thickness of a multilayered film was found to be the sum of thickness of each individual layer.     

Studies on electrochromism of spray pyrolyzed cobalt oxide thin films

Cobalt oxide thin films were prepared by spray pyrolysis technique on to fluorine doped tin oxide (F.T.O.) coated glass substrates. The electrochromic cell was formed by using these films as working electrodes and the electrochromic characteristics were determined by using cyclic voltammetry and chronoamperometry. The films exhibited anodic electrochromism; changing colour from grey to pale yellow.     

Characterization of ion diffusion and transient electrochromic performance in PECVD grown tungsten oxide thin films

Electrochromic tungsten oxide thin films were synthesized by plasma-enhanced chemical vapor deposition (PECVD). Film density and electrochromic performance were controlled by the degree of ion bombardment. A moderate degree of ion bombardment was optimal, and the refractive index was shown to be a sensitive indicator of electrochromic performance. Chronoamperometry in concert with optical transmission was used to determine diffusion and absorption coefficients using both H⁺ and Li⁺ containing electrolytes. The absorption coefficients were similar for both ions, scaling with the degree of intercalation to 50,000 cm⁻¹ in the opaque state. The diffusion coefficients for optimized films were found to be relatively insensitive to the degree of ion intercalation, with values of 10⁻⁹ and 10⁻¹⁰ cm²/s for H⁺ and Li⁺, respectively. These values are about an order of magnitude greater than values reported for vacuum-deposited films, which was attributed to low relative density in the PECVD films. The diffusion and absorption coefficients were incorporated into a model that successfully reproduced transient optical performance.     

Studies on electrochromic properties of nickel oxide thin films prepared by spray pyrolysis technique

Electrochromic nickel oxide thin films were prepared by using a simple and inexpensive spray pyrolysis technique (SPT) onto fluorine-doped tin oxide (FTO) coated glass substrates from nickel chloride solution. Transparent NiO-thin films were obtained at a substrate temperature 350°C. The films were cubic NiO with preferred orientation in the (1 1 1) direction. Infrared spectroscopy results show presence of free hydroxyl ion and water in nickel oxide thin films. The electrochromic properties of the thin films were studied in an aqueous alkaline electrolyte (0.1 M KOH) using cyclic voltammetry (CV), chronoamperometry (CA) and spectrophotometry. The films exhibit anodic electrochromism, changing colour from transparent to black. The colouration efficiency at 630 nm was calculated to be 37 cm²/C.     

Electrochromism and photochromism of reactively DC magnetron sputtered Mo–Ti oxide films

Mo–Ti oxide films with compositions ranging from Mo oxide to Ti oxide were prepared by reactive DC magnetron sputtering. Electrochromic properties were studied in situ during Li ion intercalation. Colored films have a neutral gray appearance. Their stability improved when the Ti content was increased. Photochromic properties of Mo–Ti oxide films were investigated in order to clearly display polaron absorption bands.     

High-rate dual-target DC magnetron sputter deposition of “blue” electrochromic Mo oxide films

Mo oxide films were prepared by dual-target reactive DC magnetron sputtering under conditions leading to films that were blue in as-deposited state. The deposition rate for this “blue” Mo oxide was as high as 1.5 nm/s, to be compared to 0.85 and 0.1 nm/s for highly transparent Mo oxide films made with dual-target and single-target sputtering, respectively. Good electrochromic properties of the “blue” films evolved after 5 colour/bleach cycles in a Li⁺ conducting electrolyte.     

Electrochromism in nickel oxide and tungsten oxide thin films: Ion intercalation from different electrolytes

Electrochromic (EC) NiO_z and WO_y thin films were prepared by sputtering and were used in a feasibility study aimed at investigating mixtures of these two oxides. The object was to identify a suitable electrolyte, compatible with both NiO_z and WO_y. To that end we carried out cyclic voltammetry (CV) in potassium hydroxide (KOH), propionic acid, and lithium perchlorate in propylene carbonate (Li-PC). WO_y could be coloured in propionic acid and Li-PC, while NiO_z could be coloured only in KOH. Both films showed best stability in Li-PC, which hence is well suited for further studies of mixed NiO_z and WO_y.     

Optically passive counter electrodes for electrochromic devices: transition metal–cerium oxide thin films

Films of W–Ce oxide and Ti–Ce oxide were made by reactive DC magnetron sputtering. Li⁺ intercalation/deintercalation showed that the electrochromic coloration efficiency depended on the Ce/W and Ce/Ti ratio, and at proper values of these ratios the films remained transparent irrespective of their ion content. Ti–Ce oxide combines this property with good electrochemical cycling durability, and this material can serve as a counter electrode in transparent electrochromic devices.     

Electrochromic tungsten oxide films: Review of progress 1993–1998

W oxide films are of critical importance for electrochromic device technology, such as for smart windows capable of varying the throughput of visible light and solar energy. This paper reviews the progress that has taken place since 1993 with regard to film deposition, characterization by physical and chemical techniques, optical properties, as well as electrochromic device assembly and performance. The main goal is to provide an easy entrance to the relevant scientific literature.     

Electrical and optical properties of Al doped cadmium oxide thin films deposited by radio frequency magnetron sputtering

Cadmium oxide thin films with different percentages of aluminum doping have been synthesized via radio frequency magnetron sputtering technique. Thin films were deposited on glass and silicon substrates with different percentages of aluminum at a substrate temperature of 573 K and pressure of 0.1 mbar in Ar+O₂ atmosphere. The deposited films were characterized by studying their structural, electrical and optical properties. The X-ray diffraction pattern revealed good crystallinity with preferred (1 1 1) orientation in the films. Aluminum doping in CdO thin films were confirmed by X-ray photoelectron spectroscopic studies and actual doping percentages were also measured from it. The optical band gap was found to decrease first and then increase with increasing percentages of aluminum concentrations. The electrical conductivity was found to increase with increase of aluminum doping concentration up to 5% but for higher doping concentration (>5%) the conductivity was found to decrease.