Phase transitions of the WO3 layer in photoelectrochromic devices (original) (raw)
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Phase transitions of the WO3 layer in photoelectrochrornic devices
Photoelectrochromic systems are a combination of a dye solar cell and an electrochromic material, usually WO 3 . They change their transmittance on illumination. We prepared a particularly advantageous configuration, which can be coloured and bleached under illumination, and bleached in the dark. During colouring, electrons and Li + cations are intercalated into the electrochromic WO 3 layer. Due to this intercalation, the WO 3 crystals change their structure from monoclinic through tetragonal to cubic. The intercalation and the phase transition are completely reversible. We investigated these phase transitions with the help of electrochemical potential measurement, IR spectroscopy and X-ray diffractometry, applied to the separate layers. We discuss the effect of the phase transition on the colouring and bleaching characteristics of the photoelectrochromic device, i.e. the time dependence of the voltage and the optical density.
Photochromic coloration of WO3 with visible light
Applied Physics Letters, 1996
Thin amorphous films of tungsten oxide (WO3) are well known to change their optical absorption behavior upon light exposure, usually referred to as photochromic effect. Since the formation of color centers is closely related to the optical creation of electron-hole pairs the sensitivity of the photochromic effect in WO3 is limited to energies above its band-gap energy of 3.25 eV, corresponding to the near-ultraviolet range. We will demonstrate that in case of a thin cadmium sulfide (CdS) layer underneath the tungsten oxide film the sensitivity of photochromism is shifted into the visible range. This result is interpreted in terms of charge carrier injection from the CdS into the WO3. Apart from a more detailed understanding of the photochromic effect this may have also relevance for technical applications.
Electrochromic properties of WO3 and WO3:P thin films
Journal of Solid State Electrochemistry, 2003
WO 3 and WO 3 :P (5 mol% H 3 PO 4) thin films were prepared using the sol-gel route and the electrochromic properties of the films were investigated using in situ spectroelectrochemical methods. The measurements were performed in propylene carbonate solution with 0.1 M LiClO 4 as electrolyte. During the cathodic polarization at-0.8 V a blue coloration is observed with a reversible variation between 14% and 84% of the transmittance at k=633 nm. The kinetics for the bleaching process is faster for the WO 3 :P film than for the undoped WO 3 film.
Preparation and Characterisation of Nano-Structured WO3-TiO2 Layers for Photoelectrochromic Devices
Journal of Sol-Gel Science and Technology, 2005
Nano-structured WO 3 -TiO 2 layers were prepared by the sol-gel route. To obtain transparent, porous and crack free layers up to 0.8 µm with a single dipping cycle a templating strategy was used. As a template three-dimensionally network based on organically modified silane was introduced to the WO 3 and TiO 2 sols. The WO 3 layers were dip-coated onto the conductive glass substrate (TCO) and the TiO 2 layers on the top of the WO 3 layer. The morphology and the structure of the layers were determined by Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HR-TEM), Energy Dispersive X-Ray Spectroscopy (EDXS), Auger and Infrared spectroscopy. SEM image of the WO 3 -TiO 2 layer confirmed the nano-porosity of the layers and give the size of the particles of about 10 nm for TiO 2 and 30 nm for WO 3 layer. Further analysis indicated that the titanium sol penetrates the WO 3 layer. Particles in the WO 3 layer consist of a crystalline monoclinic WO 3 core surrounded by a 5-10 nm amorphous phase consisting of WO 3 , TiO 2 and SiO 2 .
Electrochromism of WO3 in acid solutions
Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 1991
The electrochromic reaction of a tungsten trioxide electrode prepared by electrochemical deposition was studied. Chronopotentiometric experiments associated with transmittance measurements were carried out for several film thicknesses. The dependence of electrochromic efficiency on the coloration/bleaching current density, the wavelength of the light source and the film thickness were analysed. The mass and structural changes due to the electrochromic reaction were followed using a quartz crystal microbalance and stress experiments respectively. Film formation and dissolution processes for WOs in acidic media are also discussed.
Materials Chemistry and Physics, 2010
The phenomenon of electrochromism in tungsten trioxide (WO 3 ) thin films has recently attained considerable interest due to their enormous applications in inorganic thin film electrochromic devices. We have investigated the compositional, optical, and electrochromic properties of the WO 3 thin films grown at different substrate temperatures by the thermal evaporation of WO 3 powder. The thin films were characterized using X-ray diffraction (XRD), X-ray photo-emission spectroscopy (XPS), and electrochemical techniques. The XPS analysis suggested that the oxygen to tungsten (O/W) ratio decreases, i.e., the oxygen deficiency increases, on increasing the substrate temperature up to 500 • C. The electrochemical analysis provided a comparative study of the coloration efficiency (CE) of the WO 3 thin films intercalated with three different ions viz. H + , Na + , and K + . The effect of the variation of the substrate temperature on the CE and the switching time have also been investigated for the WO 3 thin films intercalated with H + ions; the thin films deposited at RT and intercalated with H + ions are found to possess adequate electrochromic properties viz. CE and switching time from device point of view.
Infrared and UV-visible ellipsometric study of WO3 electrochromic thin films
Thin Solid Films, 1998
Infrared Fourier Transform phase-modulated and UV-visible ellipsometry techniques have been applied to study the colouringrbleaching process of WO electrochromic thin films. The optical absorption of these films can be changed markedly 3 in a reversible process induced by hydrogen incorporation. The colouringrbleaching process was carried out by using the sample as one electrode of an electrochemical cell filled with an electrolytic solution of H SO . The spectroscopic 2 4 ellipsometric angles ⌿ and ⌬ were obtained ex situ by an infrared Fourier Transform phase-modulated ellipsometer working y1 Ž . in the wavenumber range from 800 to 4000 cm and a rotating analyzer ellipsometer in the UV-visible range 1.7᎐4.8 eV . Significant optical absorption and vibrational changes associated with the colouringrbleaching process which follow the hydrogen incorporation in the film were observed in the UV-visible range from 1.7 to 3.8 eV and in the IR range at 1605 cm y1 and from 3000 to 3800 cm y1 . The strong changes can be seen by changes in the spectroscopic dielectric function. For the coloured WO layer, the main absorption contribution associated with the colour effect was found at 6032 cm y1 , outside the 3 analysed region. ᮊ 1998 Elsevier Science S.A.
Post-annealing effect on the electrochromic properties of WO3 films
Optical Materials, 2020
The idea of Internet of Things (IoT) has brought about the innovation of smart materials and devices which affect our everyday life. One of the technologies associated to the IoT is the electrochromic (EC) smart windows and the key material is tungsten oxide (WO 3). WO 3 is considered as the best candidate due to its superior EC properties for example high optical modulation and high colouration efficiency. In this work, the structural properties of WO 3 films were manipulated by varying the post annealing temperature between 27 • C and 450 • C. The WO 3 films annealed at different temperature were characterised for their structural, morphological, optical and EC properties using X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), optical transmittance study, cyclic voltammetry (CV) and chronoamperometry (CA) measurements. The effect of post annealing temperature will be analysed and investigated which is important for smart window performance optimisation.
Influence of the substrate on the electrochromic characteristics of lithiated α-WO3 layers
Surface Science, 2004
The influence of the substrate on the electrochromic properties of electrochemically lithiated a-WO 3 layers is investigated by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and laser Raman scattering measurements. The oxide films have been evaporated on three different substrates: ITO (In 2 O 3 :Sn) coated glass (the most common electrode used in electrochromic arrays), aluminium-foil (very often used as a flat conductive support for surface sensitive spectroscopies) and Al thin film deposited on a glass sheet. It is demonstrated that the substrate determines the microstructure of the evaporated films which consequently influences the number of ''active'' lithium ions necessary for coloration, as well as the coloration stability during exposure to the atmosphere. A homogeneous distribution of lithium was observed at the oxide layer deposited on ITO, while in the case of the films deposited on Alfoil and on Al/glass sheets a considerable amount of lithium atoms was concentrated on the films surface, where the selfbleaching procedure was accelerated due to the contact with atmospheric oxygen.
Comparative study of the electrochromic properties of WO3 thin films
Displays, 1988
This paper deals with a comparison between the electrochromic properties of tungsten oxide obtained by vacuum evaporation of WO 3 powder, anodic oxidation of W sheets and thermal oxidation of tungsten oxides obtained by chemical vapour deposited (CVD) onto SnO 2 substrate. Good electrochromic performances have been obtained with CVD layers despite their polycryst~llirte morphology and also with anodic oxidized W sheets despite the stoichiometry of the material. The results are compared with those obtained with amorphous tungsten oxide films prepared by vacuum evaporation. Moreover, we have shown that the electrochromic properties of the oxide (colouring efficiency, switching speed) depend on the nature of the substrate used. An AC complex impedance measurement was used to show the influence of this substrate on the behaviour of the electrical interfaces.