S. Papaefthimiou

Effect of the tungsten oxidation states in the thermal coloration and bleaching of amorphous WO3 films

Abstract The structural, optical and electronic changes caused by thermal annealing in vacuum and in oxygen atmosphere on evaporated WO 3 films, were investigated by visible and infrared transmission spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The luminous transmittance of the WO 3 films was related to their state of oxidation at each annealing temperature, both during thermal coloration and bleaching. It was found that annealing in vacuum progressively reduces the oxide to the lower oxidation states W 5+ , W 4+ and changes the film color. Both states contribute to the coloration of the film. However, the presence of the W 4+ state was found to play a dominant role in deep blue coloration. Thus, a two step procedure model with polaron transitions from W 5+ to W 4+ and also from W 6+ to W 5+ would better describe the phenomenon.

Structural and electrochemical properties of opaque sol-gel deposited WO3 layers

Abstract The preparation of thick and opaque WO3 layers by a simple sol–gel method is presented. The structure and morphology of these films has been assessed by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The sol–gel WO3 layers were found to have oxygen replaced by a significant amount of hydroxyls and water incorporated in their structure. They consist of particles with dimensions of several microns, have high porosity and increased surface area, which renders them suitable for gas sensing applications. Their color changes reversibly from yellowish green to dark blue upon insertion of lithium ions. They are electrochemically stable and can withstand more than 1000 voltammetric coloration–bleaching cycles.

Development of multilayer transparent conductive coatings

In order to improve the properties of transparent conductive thin films we have developed a range of multilayer coatings consisting of three to five alternating layers of metals and high refractive index dielectrics. We have used physical vapor deposition (electron gun and thermal evaporation) to prepare optimized coatings with excellent optical and electrical properties, improved thermal endurance and long-term stability. They compare favorably with equivalent doped metal oxide films. We have used ZnS/Ag/ZnS films successfully as electrodes in electrochromic devices prepared in our laboratory. These coatings suppress emissive heat losses and prevent overheating by solar IR radiation, thus improving the performance of electrochromic windows. They can also be used as low emittance coatings in double glazings, especially for solar control applications.

Advanced electrochromic devices based on WO3 thin films

We present work on the development of advanced materials suitable for use as electrochromic thin films (EC), ion storage layers and transparent conductors (TC) in electrochromic devices. These thin film layers were prepared in our laboratory by thermal evaporation and electron gun deposition. They were incorporated into electrochromic devices, which were subsequently characterized by optical and electrochemical techniques such as transmission spectroscopy, cyclic voltammetry and galvanostatic intermittent titration. WO3 films 300–600 nm thick have been used as EC layers. They are amorphous and near-stoichiometric with a packing density of about 0.8. We have also developed textured WO3 films. Their structure enhances Li+ intercalation into the oxide matrix, thus doubling the diffusion coefficient. V2O5 ion storage thin films were Li+ doped both electrochemically and in vacuum leading to a 23% increase of the EC device coloration efficiency. We have fabricated electrochromic devices with ZnS/Ag/ZnS coatings as TCs. The use of multilayer ZnS/Ag/ZnS films was found to improve the electrical characteristics and to lower the emittance of the devices. All the devices described above can withstand more than 5000 coloration–bleaching cycles and have an open-circuit memory of several days. They are suitable for advanced glazing and other switching applications.

Effect of the tungsten and molybdenum oxidation states on the thermal coloration of amorphous WO3 and MoO3 films

Transmission and X-ray photoelectron spectroscopy (XPS) was used to investigate the optical and electronic changes caused by thermal annealing in UHV of amorphous WO 3 and MoO 3 thin films suitable for electrochromic applications. The core level XPS measurements showed that annealing in UHV progressively reduces the oxides to the lower oxidation states and causes changes in their color. Their luminous transmittance was related to their state of oxidation at each annealing temperature during thermal coloration. Only the 02p peak characterizes the valence band spectra of the oxides before coloration. After annealing, the 5d and 6s states of tungsten and 4d, 5s of molybdenum are partially filled by electrons released due to the reduction of the oxides. The MoO 3 film exhibits deep blue coloration below 400°C, associated to a continuous distribution between Mo 6+ and Mo 5+ states. For the WO 3 film the W 4 state, which appears above 400°C, plays the dominant role in deep coloration. At lower temperatures distinct W 5+ states are formed with a limited contribution to coloration.

CPC Solar Collectors With Flat Bifacial Absorbers

Abstract The design, construction and test results of non-evacuated stationary CPC solar collectors with flat absorbers are presented and discussed. The proposed collector design is based on a truncated asymmetric CPC reflector, consisting of a parabolic and a circular part. A flat bifacial absorber is installed at the upper part of the collector, parallel to the glazing to form a thermal trap space between the reverse absorber surface and the circular part of the mirror. Two prototypes based on the same collector geometry were constructed and tested. The first model consists of one mirror–absorber unit and the second of three smaller units integrated in one collector device. The truncated CPC mirror and the installation of the absorber parallel to the glazing keep the optical efficiency at a satisfactory level. The reduction of radiative thermal losses by using selective absorbers and the suppression of convection thermal losses from the reverse absorber surface to the collector cover result to a significant decrease of the total collector thermal losses. The experimental results showed that the proposed CPC collector could achieve a maximum efficiency of 0.71 and a stagnation temperature of about 180°C, with the multiunit collector device being more efficient and practical.

Study of WO3 films with textured surfaces for improved electrochromic performance

Abstract We present a study of electrochromic (EC) devices based on WO 3 thin films with textured surfaces. Surface patterns were generated by electron gun deposition of WO 3 stripes (173-μm wide and 150-nm high) on a previously grown ‘flat’ WO 3 film, 150-nm thick. The films were characterized by SEM, XPS and FTIR. They are amorphous and near-stoichiometric and have a packing density of 0.8. The textured films were used for the fabrication of EC devices of the configuration: K-glass/WO 3 /1 M LiClO 4 in PC/K-glass. These devices were characterized by optical and electrochemical techniques such as transmission spectroscopy, cyclic voltammetry and GITT. They exhibit considerable reduction of transmittance in the visible with the application of a potential of 4 V for 30 s, stability and reversibility of their electrochromic properties with an open-circuit memory of more than a week. Compared to equivalent EC devices with ‘flat’ WO 3 films, the textured ones exhibit nearly double diffusion coefficient for Li + and up to 30% more current during cyclic voltammetry tests. Hence, they present a significant improvement over ‘flat’ films, due to better access of Li ions into the WO 3 matrix. The textured WO 3 films are suitable for EC applications as they exhibit sharp contrast between regions with different shades of blue in the colored state. Other more complicated texture patterns are possible.

Study of electrochromic cells incorporating WO3, MoO3, WO3-MoO3 and V2O5 coatings

Abstract We studied electrochromic cells incorporating evaporated WO 3 , MoO 3 and mixed WO 3 -MoO 3 thin films as electrochromic layers. Li + doped V 2 O 5 films served as an ion storage layer. The devices exhibit good optical properties with very low luminous transmittance values in the colored state, which make them suitable for large-area window applications. Addition of V 2 O 5 films as an ion storage layer was found to improve the cell coloration performance up to 74%.

Integrated low-emittance-electrochromic devices incorporating ZnS/Ag/ZnS coatings as transparent conductors

We have prepared and tested integrated low-emittance–electrochromic devices using ZnS/Ag/ZnS coatings as transparent electrodes and WO3 films as electrochromic layers. These devices exhibit adequate coloration and can withstand more than 1000 bleaching-coloration cycles, provided that the metal layer is protected from the liquid electrolyte by a combination of thick dielectric films (ZnS/WO3). We have also predicted the optimum configuration of the WO3/ZnS/Ag/ZnS/Glass stack that maximizes transmission in the visible. Integration of low emittance and electrochromic films into one device could improve the performance and reduce the cost of electrochromic windows.

Cost effective asymmetric CPC solar collectors

Low cost CPC solar collectors were designed, constructed and tested. The collectors consist of two separate absorbers, which are horizontally incorporated in a stationary asymmetric CPC mirror. The efficient operation of the proposed collectors is due to the direct absorption of a large part of the incoming solar radiation and to the thermal losses suppression by the inverted surface of both absorbers. Two collector types with the same basic design are presented. The first type has tubular absorbers which are used for direct water heating and the second has flat fin type absorbers with pipe. Test results showed that the proposed collectors operate efficiently and are suitable for hot water applications.