A comparison of absorptance and reflectance modulation variable transmission electrochromic devices

A comparison is made of the optical properties and electrochromic performance of two types of electrochromic device for use as variable transmission glazing. The devices employ respectively amorphous tungsten oxide and crystalline tungsten oxide as the active electrochromic layer. Both devices exhibit pronounced transmission modulation. Some measure of reflectance modulation is observable for the crystalline tungsten oxide device.     

Highly transparent electrochromic plastic device that changes to purple and to blue by increasing the potential

We report the assembly and characterization of a highly transparent electrochromic device showing dual-coloration (purple and blue color change) using an innovative electrochromic formulation recently patented. This electrochromic formulation is composed of the three electroactive materials needed in a traditional electrochromic device (electrolyte, electrochromic and ion storage material) embedded in a polymeric matrix to reduce the chance of device malfunction by leaking and evaporation of the solvent. Two materials based on viologen, which show different electrochromic behaviors at 1.5 and 2.5 V, have been incorporated into the electrochromic formulation. Therefore, the electrochromic device was transparent in the bleach state but depending on the potential applied the final coloration was pink at 1.5 V and blue at 2.5 V. The maximum transparency of the device was 80% in the bleach state, 25.21% at 1.5 V when showing purple coloration and 3% at 2.5 V when showing blue coloration. The device presented a coloration efficiency (CE) of 136.6 cm²/C and a coloring time of 6.5 s to achieve 90% of the maximum change in absorbance. The device exhibited a memory effect of 8 min estimated as the time needed to obtain the initial transparency.     

An all-solid-state electrochromic device based on NiO/WO3 complementary structure and solid hybrid polyelectrolyte

An all-solid-state electrochromic (EC) device based on NiO/WO₃ complementary structure and solid polyelectrolyte was manufactured for modulating the optical transmittance. The device consists of WO₃ film as the main electrochromic layer, single-phase hybrid polyelectrolyte as the Li⁺ ion conductor layer, and NiO film as the counter electrochromic layer. Indium tin oxide- (ITO) coated glass was used as substrate and ITO films act as the transparent conductive electrodes. The effective area of the device is 5×5 cm². The device showed an optical modulation of 55% at 550 nm and achieved a coloration efficiency of 87 cm² C⁻¹. The response time of the device is found to be about 10 s for coloring step and 20 s for bleaching step. The electrochromic mechanism in the NiO/WO₃ complementary structure with Li⁺ ion insertion and extraction was investigated by means of cyclic voltammograms (CV) and X-ray photoelectron spectroscopy (XPS).     

A complementary electrochromic device based on polyaniline-tethered polyhedral oligomeric silsesquioxane and tungsten oxide

In this paper we report a high-contrast complementary electrochromic device based on polyaniline-tethered polyhedral oligomeric silsesquioxane (POSS-PANI) and tungsten oxide (WO₃). The electrochromic properties, cyclic voltammetry behavior and coloration efficiency of the device are studied. Due to the loosely packed structure of POSS-PANI, it possesses more accessible doping sites and hence gives rise to a significantly higher electrochromic contrast than polyaniline (PANI). Furthermore, the replacement of PANI with POSS-PANI as the complementary layer for WO₃ leads to an enhanced complementary effect, for which the underneath mechanism is also discussed.     

Design and fabrication of electrochromic light modulators

The battery-like structure of electrochromic devices warrants the application of a stoichiometric oxidation-reduction model to interpret and to optimize their operation. Of particular significance is the variation of the potential of electrochromic materials with composition, the bleached state absorption coefficients, their electrochemical stability limits and their coloration functionality versus Faradaic charge. Once these input parameters are known, the optical transmission spectrum of electrochromic light modulating devices can be predicted as a function of terminal voltage given certain assumptions about the thickness and redox state of the electrochromic and counter electrode materials on device assembly. The latter is not readily determined directly, but may be derived from properties of the two terminal device. Furthermore, the electro-optical characteristics of the two terminal device can be used to determine the source of any deviation from its optimal dynamic range.     

Development of a new self-powered electrochromic device for light modulation without external power supply

Despite considerable improvements within the last decades, electrochromic (EC) window coatings are still too expensive to be applied in buildings on a large scale. Beside the manufacturing costs, wiring costs have to be added which may exceed the fabrication expenses of the electrochromic window. Therefore, self-powered electrochromic windows have been considered, where a semi-transparent photovoltaic (PV) cell provides the power to activate an electrochromic system deposited on top of the solar cell. The whole PVEC device consists of up to eight layers which must be deposited on large scales without short circuits or other failures. Recently, we came up with a much simpler idea where power generation and electrochromic properties are combined rather than just added as in the case of the PVEC cell. The whole device now is obtained by the deposition of only three layers and is highly transparent in the bleached state. Exposing it to sunlight and completing an external circuit the device can be colored within a few minutes, reducing the transmission by about 40%. Bleaching occurs either spontaneously by blocking the sunlight or is induced by a small rechargeable battery which can be incorporated in the external circuit and is charged from the device when exposed to sunlight.     

Electrochromic windows

Electrochromic windows can be described as devices based on electrochromic materials that work as reversible switching optical shutters, different device concepts and their electrochemical principles will be described.The relevance of electrochromic windows to low energy architecture will be discussed. A detailed analysis about the general requirements and the specific performances of electrochromic glazing for architectural applications will be introduced. The challenging issues of the present electrochromic technology approaching architectural glazing will be identified from the technological point of view and from the applications perspective.Emerging electrochromic glazing designs will be critically examined. A final discussion will be introduced about the different technical and marketing problems that slow down the time to market of electrochromic windows.     

Step potential analysis of cobalt oxide-based electrochromic devices

The characterization of electrochemical behavior of electrochromic intercalation device based on cobalt oxide thin film was carried out using the step potential excitation method. A method based on generating plots of current density as a function of passed charge has been applied for characterization of electrochromic cobalt oxide thin films using an aqueous KOH electrolyte. The device resistance and the intercalation capacity of the material are calculated. Dynamic built-in potential estimated from step potential experiment and plots of the built-in potential as function of the passed charge, V_bi(ΔQ), are generated for intercalation process. The intercalation efficiency curve is obtained to confirm the nature of energy distribution of intercalation sites in electrochromic cobalt oxide.     

Multichromic conducting copolymer of 1-benzyl-2,5-di(thiophen-2-yl)-1H-pyrrole with EDOT

Despite the significant progress made in the field of electrochromic polymers, the multichromic facility of current knowledge is restricted. Therefore, as previously proven, electrochemical copolymerization of 1-benzyl-2,5-di(thiophen-2-yl)-1H-pyrrole (SNBS) and 3,4-ethylenedioxythiophene (EDOT) was used as a strategy to achieve desired multichromic properties, where the resultant copolymer displayed distinct color changes between claret red, yellow, green, and blue colors with short switching times and high optical contrast. As an application, absorption/transmission type electrochromic device with indium tin oxide (ITO)/copolymer/gel electrolyte PEDOT/ITO configuration was constructed, where copolymer and PEDOT functioned as the anodically and the cathodically coloring layers, respectively. Results implied the successive use of this copolymer in electrochromic device applications, since the device exhibited short switching times with a wide color variation upon applied potential.     

Electrochromic interference filters fabricated from dense and porous tungsten oxide films

Smart windows offer an opportunity to reduce energy consumption. However, the use of multiple optical elements, such as low emittance coatings and electrochromic devices, is detrimental to the luminous transmittance of these high performance windows. Although the addition of antireflective coatings has helped to reduce this problem, some elements, such as high index of refraction materials still give rise to loss of light. We show that replacing the single WO₃ active coating, the main component of an electrochromic device, by an appropriately designed electrochromic interference filter can significantly increase the transmittance. This active filter is based on a stack of dense and porous WO₃ layers. We first study the effect of porosity on the physical and electrochromic properties of WO₃ prepared by radio frequency magnetron sputtering. We demonstrate that the overlying dense coating does not inhibit the coloration of the underlying porous coating. The best performing films are combined into a 27 layer quarter-wave interference filter which is shown to cycle between bleached and colored states, while providing attractive transmission. Finally, we discuss various filter designs which can increase the transmission of an electrochromic device in its bleached state, as well as the potential use of active filters for optical security devices possessing two levels of authentication.     

A complementary electrochromic device based on polyaniline tethered polyhedral oligomeric silsesquioxane and poly(3,4-ethylenedioxythiophene)/poly(4-styrene sulfonic acid)

A high-contrast complementary electrochromic device based on polyaniline (PANI) tethered polyhedral oligomeric silsesquioxane (POSS) (POSS-PANI) and poly(3,4-ethylenedioxythiophene)/poly(4-styrene sulfonic acid) (PEDOT:PSS) is assembled. The electrochromic properties, cyclic voltammetry behavior and coloration efficiency of the device are studied. Due to the loosely packed structure, POSS-PANI gives rise to a significantly higher electrochromic contrast, coloration efficiency and faster switching speed than PANI. Despite its high contrast, the combination of POSS-PANI with PEDOT:PSS still shows synergy in terms of contrast enhancement, which can be attributed to the additional driving force for the diffusion of dopants into PEDOT:PSS provided by the dedoping of POSS-PANI.     

光电致变色薄膜及其器件

光电致变色器件(Photoelectrochromic device)由染料电池和WO3电致变色薄膜电极组成。本研究采用溶胶一凝胶法分别制备WO3和TiO2纳米薄膜，并组装成光电致变色器件，对不同热处理和薄膜厚度下的器件的光电致变色性能进行测试分析。试验表明用提拉法制成的WO3薄膜和用旋涂法及丝网印刷制成的TiO2薄膜，都具有较好的成膜性，并且由其组装成的器件具有良好的光电致变色效果。     

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.     

Electrochromic properties of nanocomposite WO3 films

A new nanocomposite WO₃ (NWO) film-based electrochromic layer was fabricated by a spray and electroplating technique in sequence. An indium–tin oxide (ITO) nanoparticle layer was employed as a permanent template to generate the particular nanostructure. The structure and morphology of the NWO film were characterized. The optical and electrochromic properties of the NWO films under lithium intercalation are described and compared to the regular WO₃ film. The NWO films showed an improved cycling life and an improved contrast with compatible bleach-coloration transition time, owing to the larger reactive surface area. The nanocomposite WO₃ film-based electrochromic device (NWO-ECD) was also successfully fabricated. Most importantly, the NWO film can be prepared on a large scale directly onto a transparent conductive substrate, which demonstrates its potential for many electrochromic applications, especially, smart windows, sunroof and displays.     

All-plastic electrochromic devices based on PEDOT as switchable optical attenuator in the near IR

Advanced materials for IR applications such as thermal control in spacecraft applications or variable optical attenuators which could replace the present systems have been sought. The use of electrochromic devices based on conducting polymers will add lightness and flexibility to the final device in order to overcome the limitations of the present materials used in IR applications. In this work, we present a new all-plastic electrochromic device with optical contrast (%ΔT) of 44% at 1971 nm in the IR region based on PEDOT formulations and ionic liquid blends as electrolytes. The switching time of the device is in the order of a few seconds, with a t_c 2.7 s and t_b 3.8 s.     

Organic electrochromism for a new color electronic paper

We studied electrochemical properties of terephthalate derivatives, which showed reversible color change from clear to three primary colors by electrochemical stimuli, towards paper-like electronic imaging device. To improve the repetitive stability of the electrochromic cell, ferrocene was added to the electrolyte solution as electron donor (counter redox material). Passive-matrix operation (8×8) of electrochromic display (ECD) has been demonstrated. Moreover, a prototype three-layered ECD, which consisted of cyan, magenta and yellow primary-color layers, was also demonstrated to clarify the potential for full-color electronic paper.     

Progress in chromogenics: New results for electrochromic and thermochromic materials and devices

Chromogenic device technology can be used to vary the throughput of visible light and solar energy for windows in buildings as well as for other see-through applications. The technologies can make use of a range of “chromic” materials - such as electrochromic, thermochromic, photochromic, etc - either by themselves or in combinations. The first part of this paper points at the great energy savings that can be achieved by use of chromogenic technologies applied in the built environment, and that these savings can be accomplished jointly with improved indoor comfort for the users of the building. Some recent data are presented on a foil-type electrochromic device incorporating tungsten oxide and nickel oxide. In particular, we consider the possibilities of controlling the near-infrared transmittance and optimize this property for specific climates. To that end we discuss Au-based transparent conductors for electrochromics as well as high-transmittance thermochromic multilayer films incorporating VO₂ and TiO₂.     

Application issues for large-area electrochromic windows in commercial buildings

Projections of performance from small-area devices to large-area windows and enterprise marketing have created high expectations for electrochromic glazings. As a result, this paper seeks to precipitate an objective dialog between material scientists and building-application scientists to determine whether actual large-area electrochromic devices will result in significant performance benefits and what material improvements are needed, if any, to make electrochromics more practical for commercial building applications.Few in situ tests have been conducted with large-area electrochromic windows applied in buildings. This study presents monitored results from a full-scale field test of large-area electrochromic windows to illustrate how this technology will perform in commercial buildings. The visible transmittance (T_v) of the installed electrochromic ranged from 0.11 to 0.38. The data are limited to the winter period for a south-east-facing window. The effect of actual device performance on lighting energy use, direct sun control, discomfort glare, and interior illumination is discussed. No mechanical system loads were monitored. These data demonstrate the use of electrochromics in a moderate climate and focus on the most restrictive visual task: computer use in offices.Through this small demonstration, we were able to determine that electrochromic windows can indeed provide unmitigated transparent views and a level of dynamic illumination control never before seen in architectural glazing materials. Daily lighting energy use was 6–24% less compared to the 11%-glazing, with improved interior brightness levels. Daily lighting energy use was 3% less to 13% more compared to the 38%-glazing, with improved window brightness control. The electrochromic window may not be able to fulfill both energy-efficiency and visual comfort objectives when low winter direct sun is present, particularly for computer tasks using cathode-ray tube (CRT) displays. However, window and architectural design as well as electrochromic control options are suggested as methods to broaden the applicability of electrochromics for commercial buildings. Without further modification, its applicability is expected to be limited during cold winter periods due to its slow switching speed.     

An all-thiophene electrochromic device fabricated with poly(3-methylthiophene) and poly(3,4-ethylenedioxythiophene)

A novel all-organic electrochromic device (ECD) is presented. By electrodepositing poly(3-methylthiophene) (PMeT) in boron fluoride ethyl ether (BFEE), a strong Lewis acid, a good-quality PMeT film was obtained. On the basis of studies of PMeT, it can be regarded as a pseudo-anodic coloring material for ECDs. On the other hand, poly(3,4-ethylenedioxythiophene) (PEDOT) is an ideal cathodic coloring electrochromic material known for its high optical contrast, long-term stability, and high coloration efficiency. By combining these two thiophene derivatives, the application potential of this device was determined. The color of the ECD switches between deep blue at −1.4 V (PEDOT vs. PMeT) and light red at 0.6 V. The device exhibits stable electrochromic performance, with a maximum optical attenuation (ΔT_max) at 655 nm reaching 46% (from 9% to 55%), and achieves a high coloration efficiency of 336 cm²/C. After 100 repeated cycles, the cell still retained at 91.3% of its ΔT_max at 655 nm.     

Study on the WO3 dry lithiation for all-solid-state electrochromic devices

In this report, a simple WO₃ dry lithiation is proposed for fabrication of all-solid-state electrochromic devices and characterized completely by X-ray photoelectron spectroscopy and electrochemical method. Lithiation is carried out by electron-beam evaporation of metal lithium, and the lithiated films have different components and electrochromic properties with different lithiation degrees. It is found that if Li/W ratio is less than 0.25, tungsten bronze Li_xW0₃ is formed and the lithiated by wet method. Finally, a lithium-based all-solid-state electrochromic device with proper lithiation degree is fabricated using this dry method.