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.     

Effect of molybdenum doping on the electrochromic properties of tungsten oxide thin films by RF magnetron sputtering

Thin films of pure and molybdenum (Mo)-doped tungsten trioxide (WO₃) were deposited on indium tin oxide (ITO)-coated glass and Corning glass substrates by RF magnetron sputtering technique. The effect of Mo doping on the structural, morphological, optical and electrochromic properties of WO₃ films was studied systematically. The energy dispersive X-ray analysis (EDAX) revealed that the films consist of molybdenum concentrations from 0 to 2 at.%. X-ray diffraction (XRD) studies indicated that with the increase of Mo concentration the structural phase transformation takes place from polycrystalline to amorphous phase. The crystallite size of the films decreased from 24 to 12 nm with increase of doping concentration of Mo in WO₃. Scanning electron microscope (SEM) analysis revealed that Mo dopant led to significant changes in the surface morphology of the films. The electrochemical and electrochromic performance of the pure and Mo-doped WO₃ were studied. The WO₃ films formed with 1.3 at.% Mo dopant concentration exhibited high optical modulation of 44.3 % and coloration efficiency of 42.5 cm²/C.     

Electrochromism in tungsten trioxide films obtained by reactive magnetron sputtering of a tungsten target

The results of structural investigations of electrochromic films of WO₃, obtained by reactive magnetron sputtering of a tungsten target, are presented. It is shown that at low temperatures an amorphous film of WO₃ is formed on the substrate. At a substrate temperature greater than 250°C the film that is formed is polycrystalline and has the structure of hydrotungstic bronze H_0.33WO₃. When annealed in air it transfers into the triclinic modification of WO₃. Spectral investigations showed that the electrochromic properties are most pronounced in amorphous films. Films of WO₃, having a polycrystalline structure after annealing in air, practically lose their ability for electrochromic coloration. Institute of High-Current Electronics, Sib. Otd., Russ. Akad. Nauk. Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 5, pp. 3–7, May, 1996.     

Electrochromic properties of nanocrystalline WO3 thin films grown on flexible substrates by plasma-assisted evaporation technique

Thin films of Tungsten trioxide (WO₃) were deposited on ITO-coated flexible Kapton substrates by plasma-assisted activated reactive evaporation (ARE) technique. The influence of growth and microstructure on optoelectrochromic properties of WO₃ thin films was studied. The nanocrystalline WO₃ films grown at substrate temperature of 250°C were composed of vertically elongated cone-shaped grains of size 65 nm with relative density of 0.71. These WO₃ films demonstrated higher optical transmittance of 85% in the visible region with estimated optical band gap of 3.39 eV and exhibited better optical modulation of 66% and coloration efficiency of 52.8 cm²/C at the wavelength of 550 nm.     

Nanostructured WO3 thin film as a new anode material for lithium-ion batteries

Nanostructured WO₃ thin film has been successfully fabricated by radio-frequency magnetron sputtering method and its electrochemistry with lithium was investigated for the first time. The reversible discharge capacity of WO₃/Li cells cycled between 0.01 V and 4.0 V was found above 626 mAh/g during the first 60 cycles at the current density 0.02 mA/cm². By using X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and selected-area electron diffraction measurements, the reversible conversion of WO₃ into nanosized metal W and Li₂O was revealed. The high reversible capacity and good recyclability of WO₃ electrode made it become a promising cathode material for future rechargeable lithium batteries.     

Aluminum-assisted crystallization and <Emphasis Type="Italic">p</Emphasis>-type doping of polycrystalline Si

Aluminum-doped p-type polycrystalline silicon thin films have been synthesized on glass substrates using an aluminum target in a reactive SiH₄+Ar+H₂ gas mixture at a low substrate temperature of 300 °C through inductively coupled plasma-assisted RF magnetron sputtering. In this process, it is possible to simultaneously co-deposit Si–Al in one layer for crystallization of amorphous silicon, in contrast to the conventional techniques where alternating metal and amorphous Si layers are deposited. The effect of aluminum target power on the structural and electrical properties of polycrystalline Si films is analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and Hall-effect analysis. It is shown that at an aluminum target power of 100 W, the polycrystalline Si film features a high crystalline fraction of 91%, a vertically aligned columnar structure, a sheet resistance of 20.2 kΩ/□ and a hole concentration of 6.3×10¹⁸ cm⁻³. The underlying mechanism for achieving the semiconductor-quality polycrystalline silicon thin films at a low substrate temperature of 300 °C is proposed.     

Fundamental absorption edge of evaporated amorphous WO3 films

The fundamental absorption edge of evaporated WO₃ films is investigated. The optical gap of the virgin film is estimated to be 3.41 eV at room temperature and it decreases with increase of annealing temperature up to 200°C. Annealing at 300°C leads to change in the spectral shape, which is caused by crystallization. For the films annealed at 200°C, temperature coefficient of the optical gap is estimated to be ?2×10^?4 eV/K and the slope of Urbach's tail is found to be independent of measuring temperature up to 200°C. With electrolytic coloration, shift of the optical gap toward higher energy is observed. Magnitude of this shift is estimated to be 0.05 eV at the color center concentration of 7.5×10²¹ cm^?3 when H⁺ electrolyte is used. If Li⁺ electrolyte is used, the magnitude of this shift is about three times larger than in the case of H⁺ electrolyte. This fact is interpreted by a small change in the host matrix structure owing to the injection of proton or Li⁺ during coloration.     

Electrochromic performance of the mixed V2O5–WO3 thin films synthesized by pulsed spray pyrolysis technique

Vanadium pentoxide (V₂O₅) mixed tungsten trioxide (WO₃) thin films have been synthesized by a novel pulsed spray pyrolysis technique (PSPT) on glass and fluorine doped tin oxide (FTO) coated glass substrates at 400 °C. Aqueous solutions of equimolar vanadium chloride and ammonium tungstate were mixed in volume proportions (5%, 10% and 15%) for the deposition of V₂O₅–WO₃ thin films. The structural, morphological, optical and electrochemical properties of V₂O₅–WO₃ thin films were investigated by FT-IR, XRD, SEM, cyclic voltammetry, chronoamperometry and chronocoulometry techniques. The results showed that the electrochemical properties of V₂O₅ were altered by mixing WO₃. All the films exhibited cathodic electrochromism in lithium containing electrolyte (0.5 M LiClO₄ + propylene carbonate (PC)). Maximum coloration efficiency (CE) of about 49 cm² C⁻¹ was observed for the V₂O₅ film mixed with 15% WO₃. The electrochemical stability of the sample was examined and it was found to be stable up to 1000 cycles.     

Fundamental absorption edge of evaporated amorphous WO3 films

The fundamental absorption edge of evaporated WO₃ films is investigated. The optical gap of the virgin film is estimated to be 3.41 eV at room temperature and it decreases with increase of annealing temperature up to 200°C. Annealing at 300°C leads to change in the spectral shape, which is caused by crystallization. For the films annealed at 200°C, temperature coefficient of the optical gap is estimated to be −2×10⁻⁴ eV/K and the slope of Urbach's tail is found to be independent of measuring temperature up to 200°C. With electrolytic coloration, shift of the optical gap toward higher energy is observed. Magnitude of this shift is estimated to be 0.05 eV at the color center concentration of 7.5×10²¹ cm⁻³ when H⁺ electrolyte is used. If Li⁺ electrolyte is used, the magnitude of this shift is about three times larger than in the case of H⁺ electrolyte. This fact is interpreted by a small change in the host matrix structure owing to the injection of proton or Li⁺ during coloration.     

Effective utilization of spray pyrolyzed CeO2 as optically passive counter electrode for enhancing optical modulation of WO3

Nanocrystalline cerium oxide (CeO₂) thin films were deposited onto the fluorine doped tin oxide coated glass substrates using methanolic solution of cerium nitrate hexahydrate precursor by a simple spray pyrolysis technique. Thermal analysis of the precursor salt showed the onset of crystallization of CeO₂ at 300 °C. Therefore, cerium dioxide thin films were prepared at different deposition temperatures from 300 to 450 °C. Films were transparent (T ~ 80%), polycrystalline with cubic fluorite crystal structure and having band gap energy (Eg) in the range of 3.04–3.6 eV. The different morphological features of the film obtained at various deposition temperatures had pronounced effect on the ion storage capacity (ISC) and electrochemical stability. The larger film thickness coupled with adequate degree of porosity of CeO₂ films prepared at 400 °C showed higher ion storage capacity of 20.6 mC cm^? 2 in 0.5 M LiClO₄ + PC electrolyte. Such films were also electrochemically more stable than the other studied samples. The Ce⁴⁺/Ce³⁺ intervalancy charge transfer mechanism during the bleaching–lithiation of CeO₂ film was directly evidenced from X-ray photoelectron spectroscopy. The optically passive behavior of the CeO₂ film (prepared at 400 °C) is affirmed by its negligible transmission modulation upon Li⁺ ion insertion/extraction, irrespective of the extent of Li⁺ ion intercalation. The coloration efficiency of spray deposited tungsten oxide (WO₃) thin film is found to enhance from 47 to 53 cm² C^? 1 when CeO₂ is coupled with WO₃ as a counter electrode in electrochromic device. Hence, CeO₂ can be a good candidate for optically passive counter electrode as an ion storage layer.     

Effect of the excimer laser irradiation on sol–gel derived tungsten–titanium dioxide thin films

A novel technique based on the excimer laser induced crystallization and modification of TiO₂ thin films is being reported. W⁺⁶ ions loaded TiO₂ (WTO) precursor films were prepared by a modified sol–gel method and spin-coated onto microscopic glass slides. Pulsed KrF (248 nm, 13 ns) excimer laser was used to irradiate the WTO amorphous films at various laser parameters. Mesoporous and nanostructured films consisting of anatase and rutile were obtained after laser irradiation at room temperature. The effect of varying W⁺⁶ ions concentrations on structural and optical properties the WTO films was analyzed by X-ray diffraction, field-emission scanning electron microscope, UV-Vis spectrophotometer and transmission electron microscope before and after laser treatment. Films irradiated for 10 pulses at 65–75 mJ/cm² laser fluence, exhibited anatase whereas higher parameters promoted the formation of rutile. XPS results revealed WO₃ along with minor proportion of WO₂ compounds after laser irradiation. Photo-absorbance of the WTO films was increased with increase in W⁺⁶ ions concentration in the film. TEM results exhibited a crystallite size of 15 nm which was confirmed from SEM results as well.     

Lithium conducting solid polymer electrolytes based on polyacrylonitrile copolymers: Ion solvation and transport properties

The systems poly(butadiene-co-acrylonitrile) (PBAN) - lithium salts have been studied by means of X-ray and IR spectroscopy, optical microscopy and ac- and dc-conductivity measurements. X-ray and microscopy studies have confirmed that PBAN dissolves LiClO₄ up to [CN]/[Li] ≈ 2: 1. IR spectra of the samples with LiAsF₆, LiCF₃SO₃ and LiClO₄ have indicated the coordination between Li⁺ and the polar CN groups of PBAN. So, PBAN was found to be a suitable polymer matrix for SPE. The polymer films exhibited predominant ionic conductivity. Measurements of conductivity and Li transport numbers versus temperature over a wide range of salt concentrations revealed the existence of two concentration regions (within the limits of salt solubility) corresponding to liquid-like and glass-like ion transport mechanisms. New solid polymer electrolyte with lithium single-ion conductivity of 10⁻³ S cm⁻¹ at 25 – 95 °C was obtained. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

A comparison of electrochromic properties of sol–gel derived amorphous and nanocrystalline tungsten oxide films

A pristine acetylated peroxotungstate sol with and without 4 wt% of oxalic acid dihydrate (OAD) yielded nanocrystalline and amorphous tungsten oxide (WO₃) films respectively by dip coating technique. Contrary to the expected trend, whereby, the nanostructured 4% OAD film with a triclinic modification should have shown superior electrochromic efficiency, its amorphous 0% OAD counterpart exhibits higher optical modulation and coloration efficiency at photopic wavelengths. This anomalous behavior of the amorphous 0% OAD film was correlated to a higher W⁵⁺ content in its colored state. The colored nanocrystalline 4% OAD film contained a lower proportion of the W⁵⁺ color centers. Under the same level of lithiation, while the 4% OAD film was also constituted by W⁴⁺ states, its 0% OAD counterpart did not contain any 4+ states of tungsten. X-ray photoelectron spectroscopic (XPS) investigations also confirmed that the single peak at ∼1.4 eV in the absorption coefficient–wavelength spectrum of the colored 0% OAD film arises from the small polaronic transitions between the W⁶⁺ and W⁵⁺ states of tungsten whereas the W⁶⁺–W⁵⁺ and the W⁵⁺–W⁴⁺ charge transitions produce two distinct peaks at 1.2 and 1.6 eV in the α–λ spectrum of the colored 4% OAD film. The microstructure of the 4% OAD film, characterized by an interconnected network of nanocrystallites and pores promotes rapid ion insertion and extraction. Therefore, the larger magnitudes of NIR reflectance modulation, diffusion coefficient for lithium, electrochemical activity and faster color–bleach kinetics observed for the 4% OAD film, are a direct consequence of its structure. Band gap widening upon lithium insertion observed for both films, is a repercussion of Burstein–Moss effect and structural changes that occur upon coloration.     

ArF excimer laser deposition of wide-band gap solid electrolytes for thin film batteries

High quality solid electrolyte thin films was grown by pulsed laser deposition (PLD) using a high photon energy ArF excimer laser. Various amorphous thin films were successfully deposited on glass substrates from oxide targets; such as Li₃PO₄, LiBO₂, Li₂SiO₃, Li₂CO₃, Li₂SO₄, Li₂ZrO₃, LiAlO₂, Li₂WO₄ and Ohara glass ceramics. The morphology, optical property and ionic conductivity of these films were examined by optical microscope, UV–VIS spectroscopy and impedance analysis. Dramatic improvement of the film morphology was observed by using a high photon energy laser, while the broken film with many droplets was obtained by using lower ones. Ionic conductivity of the films was examined by impedance spectroscopy and dc polarization method. For example, an ionic conductivity of a Li₃PO₄ film was 4.6 × 10^? 6 S cm^? 1 at 25 °C with activation energy of 0.57 eV. Electronic conductivity measurements revealed that most of the films were pure lithium ion conductors, while a Li₂WO₄ film was a mixed conductor.     

Rapid anisotropic diffusion of lithium in electrochromic thin films based on the hexagonal tungsten bronze structure

The chemical diffusion coefficient of lithium in oxidized potassium hexatungstate and reduced hexagonal potassium tungsten bronze films was measured by the galvanostatic transient method. Two-dimensional anisotropic diffusion was found in the hexagonal hexatungstates, while no appreciable anisotropy was observed in samples with the potassium hexagonal tungsten bronze structure. The chemical diffusion coefficients along the c-axis in thin films of the tungstates K_0.33WO_3.165 and K_0.3WO_3.15 (about 6000 Å thick) are about 10^?10 cm²/s, while those along the a-axis are about 10^?7 cm²/s. This latter value is about the same to those measured in a potassium hexagonal tungsten bronze single crystal of composition K_0.28WO₃ which was grown electrochemically. It is most likely that the presence of the additional oxygen atoms in the tunnels within the hexatungstate structure is responsible for the large decrease in the rate of motion of lithium along the c-axis that leads to the anisotropy in the macroscopic diffusion coefficient in this crystal structure.     

Efficient electrochromic device based on sol–gel prepared WO<Subscript>3</Subscript> films

Tungsten oxide (WO₃) films were prepared on indium–tin oxide (ITO) glass by sol–gel method. The influence of annealing temperature on the structural, morphological, optical, electrochemical, and electrochromic properties has been investigated. The film annealed at 250 °C with an amorphous structure exhibits a noticeable electrochromic performance, such as the highest optical modulation of 58.5 % at 550 nm, high electrochemical stability, and excellent reversibility (Q _b/Q _c?=?96.3 %). An electrochromic (EC) device based on WO₃/NiO complementary structure shows improved performance. It exhibits high optical transmittance modulation of 62 % at 550 nm, excellent cycling stability, and relatively fast electrochromic response time (10 s for coloration and 19 s for bleaching).     

The order of addition of corn starch/lithium perchlorate/glycerol affects the optical,mechanical, and electrical properties of a solid polymer electrolyte

Optical, mechanical, and electric properties of solid polymer electrolyte (SPE) were affected by the order of addition of corn starch (S), lithium perchlorate (Li), and glycerol (G) during the preparation process. Four formulations were made based on whether Li was added prior to S gelatinization (simultaneous formulations SGLi and SLi+G) or whether it was added after S was gelatinized (sequential formulations SG+Li and S+LiG). Simultaneous formulations produced films with smaller elongation-at-break response (60–75%) relative to their sequential counterparts (75–82%). The simultaneous formulations exhibited higher electrical conductivity (～0.7 mS cm^?1) and capacitance (～0.017 F cm^?2) and electrochemical stability than the sequential formulations (～0.9 mS cm^?1 and ～0.012 F cm^?2) at room temperature. Results from FTIR and DSC analyses indicated that starch re-crystallization in casting phase could lead to variations on electrical properties for the different SPE formulations. It was postulated that Li cations replace hydrogen ions inside starch molecules, retarding the re-crystallization of starch molecules.     

Structural-optical study of high-dielectric-constant oxide films

High-k polycrystalline Pr₂O₃ and amorphous LaAlO₃ oxide thin films deposited on Si(0 0 1) are studied. The microstructure is investigated using X-ray diffraction and scanning electron microscopy. Optical properties are determined in the 0.75-6.5 eV photon energy range using spectroscopic ellipsometry. The polycrystalline Pr₂O₃ films have an optical gap of 3.86 eV and a dielectric constant of 16-26, which increases with film thickness. Similarly, very thin amorphous LaAlO₃ films have the optical gap of 5.8 eV, and a dielectric constant below 14 which also increases with film thickness. The lower dielectric constant compared to crystalline material is an intrinsic characteristic of amorphous films.     

Precursor derived LiMn2O4 thin films as ionic conductor

Thin films of spinel LiMn₂O₄ have been fabricated using a metallorganic precursor. Crystalline films have been deposited on Au substrates to exhibit as the cathode in rechargeable thin film lithium batteries. The nucleation and growth of spinel LiMn₂O₄ crystallites were investigated with heat treatment of the deposited thin films. Film capacity density as high as 22 μAh/cm² was measured for LiMn₂O₄. The film heat treated at 700 °C were cycled electrochemically up to 30 cycles against Li metal without any degradation of the capacity. There were neither open area nor amorphous layers which prevent the Li⁺ions transfer at the boundaries in the LiMn₂O₄ thin film. The microscopic study revealed that (111) planes in the two grains directly bonded at the grain boundary which could proceed the lithium ion intercalation or deintercalation smoothly.     

Electrochromism in WO3 thin films. I. LiClO4-propylene carbonate-water electrolytes

The performances of electrochromic cells containing evaporated amorphous WO₃ thin films as electrochromic material in 1M LiClO₄-propylene carbonate-water electrolytes are presented. Much attention has been paid to some parameters such as the thickness of the layer, the overpotential applied to WO₃ electrode during the electrochemical coloration and the amount of water contained in the electrolyte (from 50 ppm to 10% in weight). Simultaneous electrical and optical in situ measurements have been carried out to study electrochromism. The optical data were stored into a microprocessor and restituted after treatment. The method used here gave us the possibility to rapidly test electrochromic materials.