Chromogenic Materials and Devices

Resume : Electrochromic films, able to change their color upon application of small electrical potentials, are promising candidates for smart window systems. The latter are used for the dynamic control of the amount of solar radiation entering buildings, thus reducing heating from solar irradiation in the hot season while ensuring adequate levels of illumination. The active electrochromic layer usually consists of an inorganic transition metal oxide such as WO3. Our activity focuses on the fabrication of time-durable, fast-switching electrochromic devices produced by simple and low-cost manufacturing processes. In order to ensure low production costs and industry-standard durability, all materials were deposited by screen printing, and glass frit was used as sealing material. Tungsten hexachloride was used as the starting material in ain-house low-temperature ethanolysis process to obtain a stable, light-yellow and transparent screen-printable paste. 500 nm-thick transparent electrochromic layers were then obtained by annealing the screen printed layers. An in-house GF paste was screen-printed to obtain the sealing pattern of the devices, while platinum counter-electrodeswere screen-printed from a commercial platinum paste. An electrolyte containing lithium ions and the redox couple I-/I3-(ion storage layer) provides the necessary Li+ source for fast coloring/bleaching of the electrochromic layer. Changes in optical density were evaluated through time-drive spectrophotometric analysis, while coloring current and charge densities were recorded through chrono-amperometry and chrono-coulometry measurements. Our future activities will be focused on the integration ofdye-sensitized solar cells (DSSCs) and electrochromic cellsto produce self-powered smart windows.

Resume : Nb2O5thin films were prepared on the 1737F glass substrate and ITO coated glass substrate by RF magnetron sputtering using a Nb2O5circular disk target in an Ar atmosphere. The argon pressure was changed between 3 – 15 mTorr during the filmdeposition. The RF power was maintained at 75 W. The substrate temperature was 300 oC.Kim Oscillator Model was used to determine the wavelength dependence of refractive index n (λ) and extinction coefficient k (λ) andfilm thickness by fitting the optical transmission and reflection measurements of the films over the spectral range from 350 to 1100 nm at 30˚ of angle of incidence simultaneously.The electrochemical and optical absorption measurements of the films were done by an electrochemical analyzer equipped with three electrode cells (using Pt as counter electrode and Ag/AgClKOH as reference electrode in 0.1 M LiClO4/PC electrolyte) and a diode array spectrophotometer during the bleaching and coloration process concomitantly. All as-deposited Nb2O5 thin films were amorphous, transparent and had more than 90% transmission in the visible and near IR region of the spectrum. The refractive index of the films was decreasing from 2.29 to 1.99 with increasing deposition pressure as a result of loose-packing film density. The colouration efficiency of the films were recorded varying between 10.20 to 33.18 cm2/C. The film prepared at 6 mTorrAr pressure has the best electrochromic performance with a colouration efficiency of 33.18 cm2/C and an optical modulation of 59% between +2V (bleached state) and -2V (coloured state). Acknowledgement This work was supported by the Scientific and Technological Research Council of Turkey (Project Number: 111T252).

Resume : Titanium dioxide-based thin films find applications on a large number of devices and components: optical waveguides, piezoelectric transducers, gas sensors, transparent conductive electrodes. TiO2 can be doped by rare-earth elements such as Nd, that leads to intense emissions in IR domain under UV excitation making such films very promising candidates as optically active electrodes for photonic conversion by down-shifting process in silicon based solar cells. Nevertheless the electrical conductivity of Nd-doped TiO2 films has to be improved to be used as a transparent conductive electrode. In this work we attempt to increase the conductivity of Nd-doped TiO2 films by doping with a metal element (i.e. Nb). The Nb/Nd-codoped TiO2 films were grown by nanosecond pulsed-laser deposition. The doping concentration, composition, structure, microstructure and morphology of films were precisely studied. These results are correlated with optical (transparency, photoluminescence under UV excitation) and electrical (resistivity, concentration and mobility of charge carriers) properties.

Resume : Polycrystalline c-axis oriented ZnO:Al (AZO) thin films were deposited onto amorphous SiO2 substrates by pulsed laser deposition (PLD) method. Films were deposited at 300 oC in vacuum and oxygen environment using targets doped with 1-10 at% Al. The properties of thin films were studied from the standpoint of transparent electrode applications. According to XRD analysis, Al doping level and deposition atmosphere does not have considerable influence on crystalline quality of our AZO films. As determined by XRF analysis, Al concentration in the films was substantially lower than nominal Al concentration in ZnO:Al PLD targets. Compared to the films grown in oxygen, films deposited in vacuum environment had substantially lower resistivity (σ) and showed significantly different σ dependence on Al concentration – oxygen-deposited films had minimal σ at 0.6 % Al doping concentration, compared to 4.3 % for vacuum deposited films. Also, the carrier concentration (ne) and mobility (μ) dependence on Al doping level were considerably different – a) at all doping levels, ne of vacuum deposited films was higher than of films deposited in oxygen, b) carrier mobility of oxygen deposited films decreased steadily with the increase of Al doping from 0.6 to 5.2 % (μmax at 1% Al) but for vacuum deposited films, mobility was highest at 2.8 % doping level. The results can be explained by higher effective solubility of Al for the films deposited in vacuum conditions. Oxygen deposited films showed high optical transmittance in the visible and near-IR region while transmittance of the vacuum deposited films decreased in near-IR region due to increased reflectance caused by larger carrier concentrations in these films.

Resume : The ZnO films properties can be enhanced or new properties can be obtained by the use of well adapted dopants. For example Nd can be used to increase the ZnO conductivity (Nd3+ in substitution of Zn2+) and to convert the solar spectrum (down conversion of UV photons in near IR photons), or to induce magnetic properties. Transparent Nd doped ZnO (ZnO:Nd) thin films were grown either by pulsed-laser deposition (PLD) or pulsed-electron beam deposition (PED) on Si and c-cut sapphire single crystal substrates at various oxygen pressures and substrate temperatures. The composition, surface morphology and structure of Nd doped ZnO thin films were investigated by means of Rutherford Backscattering Spectrometry, SEM and X-ray diffraction analyses. The ZnO:Nd films are smooth, dense, with the wurtzite phase. Epitaxial relationships between films and c-cut sapphire substrates were evidenced by asymmetric X-ray diffraction measurements. These results have been correlated with optical and electrical measurements. A dielectric model including band gap transitions and free electron excitations was used to simulate the transmittance spectra of ZnO:Nd films in 300-2500 nm range. The resulting optical parameters were compared with those obtained by Hall effect measurements for different growth conditions and Nd doping concentrations.

Resume : In a recent study the photochromic properties in reactively sputtered yttrium hydride thin films were reported [1]. The incorporation of a substantial amount of oxygen appeared vital for the observation of this effect. Exposure to visible and ultraviolet (UV) light at moderate intensity triggers a uniform decrease in the optical transmission of visible and infrared (IR) light. The reaction is reversible and samples that are left in the dark return to the initial transparent state. Potential applications range from smart windows to sunglasses. The effect observed in YOyHx is unique, since it shows reversible photochromism induced by light from the visible part of the optical spectrum and displays colour neutrality: The transmission decreases for all wavelengths with energies E < Egap. This suggests some opto-electronic phase segregation of metallic entities in a dielectric matrix. Here, we investigate the wavelength dependence of the photochromism on samples with slightly varying bandgaps. We observe in all case a substantial darkening at wavelengths below the gap. References 1. T. Mongstad, C. P. Bjorkman, J. P. Maehlen, L. P. A. Mooij, Y. Pivak, B. Dam, E. S. Marstein, B. C. Houback, S. Zh. Karazhanov, Sol. Energy Mater. Sol. Cells 95 (2011) 3596.

Resume : The binary Cu-O system contains two stable phases (Cu2O and CuO) and one metastable phase (Cu4O3) that may be suitable as p-type TCO. These three copper oxides can be deposited by adjusting the oxygen flow rate introduced into the sputtering chamber. Although some of the properties of the Cu4O3 phase have been described, the relationship between the film microstructure and their properties is scarcely described. This presentation aims to study the effect of the deposition conditions on the films microstructure and on films properties. As a function of the oxygen flow rate and the total pressure, 3 main deposition zones corresponding to the 3 oxides have been evidenced by X-ray diffraction and Raman spectrometry. Within these domains, the films exhibit a columnar microstructure. Pole figure analyses show that the texture of the 3 oxides can be tuned by adjusting the deposition parameters. For Cu2O films, the preferred orientation changes from [100] to [111] by increasing the total pressure. At the border of the 3 zones, a biphased domain has been systematically evidenced by Raman spectrometry (Cu2O + Cu4O3 or Cu4O3 + CuO). The relationship between the microstructure and the electrical and optical properties of the films is discussed. Finally, a homoepitaxial growth of Cu2O has been evidenced by transmission electron microscopy observations in cross section.

Resume : Electrochromic materials, which can reversibly change their optical properties under an applied electric field, have attracted in various applications such as smart windows, optical displays and rear-view mirrors. Electrochromic devices (ECDs) contain an electrochromic film in contact with an electrolyte adjacent to an ion storage layer, and these three layers are sandwiched between the two transparent conducting films. For the simple and large-scale fabrication, ECDs which consist of all solid-state materials are advantageous. In this study, monolithic solid-state form of ECDs was fabricated on single glass substrate by magnetron sputtering. The ECDs exhibited the multilayer structure of ITO/WO3/Ta2O5/NiO/ITO on glass substrate. The WO3, Ta2O5 and NiO were used as electrochromic, solid electrolyte and ion storage layer, respectively. Sputtering conditions, such as DC/AC power, Ar/O2 gas flow rate and deposition time were optimized for the enhanced performance of ECDs. The cyclic voltammetry was performed using a potentiostat with a standard three-electrode configuration. The WO3 and NiO films were deposited on ITO glasses served as the working electrodes, respectively. Pt was used as the counter electrode, Ag/AgCl was the reference electrode and the electrolyte was 1M LiPF6/PC. The transmittance spectrum of as prepared and after colored ECDs was characterized by a UV/VIS/NIR spectrophotometer.

Resume : Oxides of certain transition metals show reversible colour change as a function of temperature. The so-called thermochromism was observed in CoAl2O4-Bi2O3 blends, in TiO2- V2O5 and V2O5-Nb2O5 systems. The three separate systems studied in this work show reversible thermochromism in the temperature range of 150 °C to 500 °C. All three systems exhibited hysteresis behaviour in their transition temperature. CoAl2O4-Bi2O3 blends allowed a certain degree of tuning for the transition temperature. As the ratio of the CoAl2O4:Bi2O3 was increased, the temperature of perceived colour change shifted to higher temperature from 200 °C to 400 °C. The V0.48Ti0.52O2 oxide system show reversible colour change (yellow to deep orange) in the temperature range 150-160 °C with the smallest hysteresis in the transition temperature. The phase analysis of the oxide powders were done by using XRD and electron diffraction studies. The origin of thermochromism was investigated with the help of the UV-VIS Spectroscopy and electron microscopy.

Resume : Nickel oxides thin films and especially one form NiO (bunsenite) are very promising and useful material due it interesting properties which allow to use it in such applications as antiferromagnetic films, chemical and gas sensors, cathodes of batteries, catalyst, varistors and electrochromic devices, called “smart glass”. Our work is focused on synthesis NiO thin films, which could be useful for one of these devices elements. Problems to get quality NiO films using magnetron sputtering are lying in some aspects. The main is that Ni cathode is ferromagnetic. It complicates sputtering process due to shortening magnetic field lines on magnetron surface and causes instabilities in plasma processes. The synthesis of nickel oxide thin films using reactive magnetron sputtering technique and post annealing in atmosphere as deposited films were analyzed in this work. Nickel oxide electrical, optical, crystallographic and surface morphology properties before annealing and after were measured and analyzed. Comparing as deposited properties of films before annealing and after the conclusions were made, that nanocrystal Ni2O3 phase grows during deposition, but after annealing in 550oC temperature (50 min) this phase transforms to quality nanocrystal NiO films.

Resume : Thin films of thermochromic vanadium dioxide were deposited using a novel electric field assisted chemical vapor deposition methodology at atmospheric pressure onto glass substrates. Electric fields were generated during the deposition reaction by applying a potential difference across the inter-digitated electrodes of the gas sensor substrate or buy applying an electric field between two transparent conducting oxide coated glass substrates. The deposited films were analyzed and characterized using scanning electron microscopy, Raman spectroscopy, X-ray diffraction, atomic force microscopy, variable temperature optical spectroscopy and contact angle measurements. It was found that applying an electric field led to large changes in film microstructure, preferential orientation and changes in the film growth rate. This led to significant changes in materials properties such as decreased crystallite size, nano-structuring, increased surface roughness and enhanced wetting behavior. The deposited films were evaluated using Energy Plus for their energy demand reduction characteristics in a variety of architectural settings and compared against existing glazing products. It was found that thermochromic thin films can offer significant energy demand reduction benefits.

Resume : Vanadium dioxide (VO2) is a thermochromic material that presents an abrupt first order phase transition at the critical temperature (Tc) of 68 °C. VO2 thin films have been deposited using reactive sputtering on silicon substrates using three kinds of processes. In the first one, the films have been deposited at high temperature as usually encountered in the literature. In the second process, the VO2 coatings have been synthesised in a two steps method: deposition at room temperature followed by an ex situ annealing in air. Finally, the last process studied can be described as the deposition at room temperature of a VO2 layer covered by a thin film of silica and the air annealing of this bilayer. High temperature deposited films are well crystallized and exhibited a pronounced metal insulator transition at 68°C. On the other hand, the films deposited at room temperature are X-ray amorphous and do not exhibit a thermochromic effect. Furthermore, an oxidation of the films occurs during the air annealing of the vanadium oxide layer. Such oxidation step is avoided by the use of the silica layer that acts as a barrier for oxygen diffusion. The monitoring of the bilayer crystallization by X-ray diffraction shows that the crystallisation occurs at temperature as low as 330 °C and that the crystallisation is governed by a two dimensional growth mechanism. The crystalline bilayer films clearly exhibit switching properties in the infrared domain at a temperature close to 68 °C.

Resume : The interest in electrochromic vanadium oxide coatings has increased during the last few years, because of their use in windows that can control the solar light transmission, contributing therefore significantly to energy saving in buildings. Large area uniformity, low production cost and durability of the coatings can play a very important role in the fabrication of such “smart” windows. In that respect, an aerosol assisted chemical vapor deposition process is more advantageous over the other conventional techniques, which are already used in industry for the fabrication of these coatings. Since, it is easily integrated to float glass plants, no expensive vacuum systems are required and high production rates may be realised. In this paper, the effect of the growth parameters and coating thickness on the structural and morphological characteristics of vanadium pentoxide are considered. Finally, their electrochemical activity, comprising charge storage capability and corresponding specific capacitance, reversibility and repeatability of the charge transfer through the as-deposited and annealed vanadium oxides are investigated.