Satyen K. Deb

Electrochromic mechanism in a-WO3−y thin films

The electrochromic mechanism in amorphous tungsten oxide films is studied using Raman scattering measurements. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm−1 due to vibrations of the W6+–O and W6+=O bonds, respectively, and a weaker peak at 220 cm−1 that we attribute to the W4+–O bonds. When lithium or hydrogen ions and electrons are inserted, extra Raman peaks due to W5+–O and W5+=O bonds appear at 330 and 450 cm−1, respectively. Comparison of the Raman spectra of sputtered isotopic a-W16O3−y and a-W18O3−y films confirms these assignments. We conclude that the as-deposited films contain mainly the W4+ and W6+ states, and the W5+ states are generated as a result of reduction of the W6+ states when lithium or hydrogen ions and electrons are inserted. We propose that the optical absorption in the colored films is caused by transitions between the W6+ and W5+, and W5+ and W4+ states.

Chromic Mechanism in Amorphous WO 3 Films

The authors propose a new model for the chromic mechanism in amorphous tungsten oxide films (WO{sub 3{minus}y}{center_dot}nH{sub 2}O). This model not only explains a variety of seemingly conflicting experimental results reported in the literature that cannot be explained by existing models, it also has practical implications with respect to improving the coloring efficiency and durability of electrochromic devices. According to this model, a typical as-deposited tungsten oxide film has tungsten mainly in W{sup 6{minus}} and W{sup 4{minus}} states and can be represented as W{sub 1{minus}y}{sup 6+} W{sub y}{sup 4+}O{sub 3{minus}y}{center_dot}nH{sub 2}O. The proposed chromic mechanism is based on the small polars transition between the charge-induced W{sup 5+} state and the original W{sup 4+} state instead of the W{sup 5+} and W{sup 6+} states as suggested in previous models. The correlation between the electrochromic and photochromic behavior in amorphous tungsten oxide films is also discussed.

Electrochromic coloration efficiency of a-WO3−y thin films as a function of oxygen deficiency

We report on how electrochromic coloration is affected by oxygen deficient stoichiometries in sputtered amorphous tungsten oxide (a-WO3−y) films. The electrochromic coloration efficiency increases with increasing oxygen deficiency in (a-WO3−y) films. No coloration is observed in nearly stoichiometric WO3 films. Raman spectroscopic studies reveal that the number of W5+ states generated with lithium insertion increases with the oxygen deficiency. Furthermore, there are no Raman peaks resulting from W5+ states in lithiated a-WO3−y films with near perfect stoichiometry, which is consistent with the absence of electrochromic coloration in those films. We conclude that the coloration efficiency of a-WO3−y films depends on the number of the W5+ states generated by lithium insertion and that the oxygen deficiency plays an important role in generating the W5+ states with lithium insertion.

Electrochromic and optical properties of mesoporous tungsten oxide films

Standard and mesoporous sol–gel tungsten oxide thin films were prepared by a spin-coating technique from an ethanolic solution of tungsten hexachloride. A block copolymer (BASFk Pluronic p123) was employed as a template to generate the mesoporous structure. An ultraviolet (UV) illumination method was employed to remove the polymer templates at room temperature. The electrochromic and optical properties of the mesoporous films are described and compared to standard sol–gel tungsten oxide films. Results are also presented on the samples prepared by thermal treatment. We demonstrate that the UV illumination/ozone treatment is a superior method to remove templates which enables us to more effectively investigate the effect of mesoporosity on the electrokinetics of ion insertion into tungsten oxide films. D 2002 Elsevier Science B.V. All rights reserved.

Raman spectroscopic studies of electrochromic a-WO3

We report on the eAect of electrochromic coloration on the Raman spectra of evaporated and sputtered a-WO3ˇy films. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm ˇ1 due to vibrations of the W 6+ ‐O and W 6+ .O bonds, respectively, and a weaker peak at 220 cm ˇ1 that we attribute to the vibrations of the W 4+ ‐O bonds. When lithium or hydrogen ions are inserted, extra Raman peaks due to W 5+ ‐O and W 5+ .O bonds appear at 330 and 450 cm ˇ1 , respectively. Comparison of the Raman spectra of sputtered a-W 16 O3ˇy and aW 18 O3ˇy isotopic films confirms these assignments. We conclude that the as-deposited films contain mainly the W 4+ and W 6+ states, and the W 5+ states are generated as a result of reduction of the W 6+ states when ions are inserted. We propose that the electrochromism in a-WO3ˇy films originates from reduction of the W 6+ states to W 5+ due to the double insertion/extraction of ions and electrons and the optical absorption in the colored films is caused by transitions between the W 6+ and W 5+ , and W 5+ and W 4+ states. # 1999 Elsevier Science Ltd. All rights reserved.

Alternating current impedance and Raman spectroscopic study on electrochromic a-WO3 films

The chemical diffusion of lithium ions in a-LixWO3 films is investigated using alternating current impedance spectroscopy and Raman scattering measurements. The diffusion coefficients increase with increasing x in a-LixWO3 up to x=0.072 and then decrease. Raman measurements show that the W6+=O/O–W6+–O ratio also increases at the early stage of lithium insertion and then decreases with further lithium insertion. We conclude that the diffusion kinetics of lithium ions in a-LixWO3 films is very closely related to the W6+=O/O–W6+–O ratio.

Stand-alone photovoltaic-powered electrochromic smart window

Three different, innovative approaches have been taken to develop photovoltaic (PV) integrated electrochromic (EC) devices for smart-window applications. These are: (i) a stand-alone, side-by-side PV-powered EC window; (ii) a monolithically integrated PV-EC device; and (iii) a novel photoelectrochromic device based on a dye-sensitized TiO2 solar cell. The compatibility of PV-EC devices has been analyzed, and the potential for large energy savings for building applications has been suggested. The first monolithic, amorphous-silicon based, PV-powered electrochromic window is described in detail. The device employs a wide bandgap a-Si1−xCx/H n–i–p PV cell as a semitransparent power source, and a LiyWO3/LiAlF4/V2O5 EC device as an optical-transmittance modulator. The EC device is deposited directly on top of a PV cell that coats a glass substrate. The a-Si1−xCx/H PV cell has a gap of 2.5 eV and a transmittance of 60–80% over a large portion of the visible light spectrum. Our prototype 16-cm2 PV-EC device modulates the transmittance by more than 60% over a large portion of the visible spectrum. The coloring and bleaching times of the EC device are approximately 1 min under normal operating conditions (±1 V). A brief description of photoelectrochromic windows based on a combination of dye-sensitized TiO2 and WO3 EC-layer is also given.

Comparison of electrochromic amorphous and crystalline tungsten oxide films

Abstract A detailed systematic study of the tungsten oxide thin films has been carried out using WO 3 films after they were annealed at progressively increasing temperatures ranging from 350°C to 450°C in oxygen environments. The structural properties of the films were characterized using X-ray diffraction and Raman spectroscopy. The amorphous WO 3 films remain as an amorphous phase up to 385°C and begin to crystallize at 390°C and then are completely crystallized at 450°C. Absorption peaks of the films are found to shift to a higher energy side with increasing annealing temperature up to 385°C and then shift abruptly to a lower energy as the films begin to crystallize at 390°C. Deconvolution of the absorption spectra shows that there are two different polaron transitions in the amorphous WO 3 films.

Raman spectroscopic studies of electrochromic a-MoO3 thin films

We report the effects of electrochromic coloration on the Raman spectra of sputtered a-MoO3 films. The Raman spectra of as-deposited films show two strong peaks at 828 and 951 cm−1 due to vibrations of the OMo6+O and Mo6+O bonds, respectively. A shoulder on the right side of the Raman peak at 951 cm−1 is observed, and we attribute it to the Mo6+O bonds which originate from the singly coordinated oxygen atoms in α phase layered MoO3. When lithium (or hydrogen) ions and electrons are intercalated into a-MoO3, the overall Raman intensity decreases due to electrochromic coloration and an extra Raman peak due to the bonds between Mo5+ and oxygen atoms appears at ∼400 cm−1. In addition, the force constant for the OMo6+O bonds increases due to the compressive stress induced by lithium (or hydrogen) ion insertion. We conclude the Mo5+ states are generated as a result of the reduction of the Mo6+ states with ion/electron insertion and the optical absorption in the colored state is caused by transitions between the Mo6+ and Mo5+.

Raman spectroscopic studies of Ni-W oxide thin films

We report on a Raman spectroscopic study of sputtered nickel–tungsten oxide films. Their crystallographic and chemical identification with electrochemical lithium insertion and extraction are obtained by Raman spectroscopy. The Raman spectra of as-deposited Ni–W oxide films show a strong peak at 525 cm−1 due to vibrations of the NiO bonds and two weaker peaks at 875 and 950 cm−1, which we attribute to the vibrations of the WO bonds. The Raman spectrum of the as-deposited Ni–W oxide film shows a blueshift of the NiO stretching mode compared to that of a NiOx film. When lithium ions and electrons are inserted, the overall Raman intensity increases due to electrochromic bleaching and the relative intensity of the peak at 875 cm−1 also increases. By comparing these results with results from the same measurements using sodium ions, we conclude that this reversible peak at 875 cm−1 is not directly related to the Li or Na ions.