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.     

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.     

d-Electrons and two active thin film devices for achieving a solar energy economy

In this paper, the marriage of condensed matter science and solar energy technologies will be illustrated by two closely related active thin film devices, a variable reflectivity electrochromic window and a high energy density rechargeable (or secondary) thin film battery. Each device can significantly assist in achieving a solar energy economy. The primary focus of this paper is the important role of d-electrons in two candidate thin film materials, tungsten oxide and lithium cobalt oxide, that could be utilized in one or in both the devices, respectively.     

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.     

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.     

Development and characterisation of electrochromic devices on polymeric substrates

The development of full solid-state electrochromic (EC) devices on polymeric substrate is underway within a CEC BRITE-EURAM project (Project “FREDOPS”, BE-4137) carried out by four industries, two universities and two research centers from Belgium, Denmark, France, and Italy. The specific goal of this project is to develop a Fast Response Electrochromic Device On Polymeric Substrate (FREDOPS); in order to satisfy the required range of specifications in terms of fast response, long term performance and high contrast ratio, several systems based on different materials have been tested. The full cells consist of an electrochromic material layer and a counter electrode, inserted between two PET/ITO layers and separated by a polymeric electrolyte. Different types of polymeric electrolytes, counter electrodes and electrochromic layers have been developed, studied and checked. Full devices have been assembled using different combinations. Voltammetric and spectrophotometric measurements have been executed to check the electrochromic behaviour of the developed layers in half and full cells. Comparison of the electrochromic performances of different materials based cells has led to the rejection of several solutions due to poor performance and incompatibilities between layers. Considering that the electrochromic devices are finalised for different uses (window, sunroof,…), some performance specifications for each application are defined. A testing bench for cycling and ageing was developed. The present paper discusses these results in order to indicate the best performance.     

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.     

Proton diffusion in polyethylene oxide: Relevance to electrochromic device design

Polyethylene oxide is a frequently used component in polymer electrolytes developed for applications in electrochromic devices. The transmittance variation may occur as a result of either proton or lithium ion intercalation into the electrochromic films. Impedance spectroscopy data in the low-frequency space-charge relaxation regime can be used to obtain estimates of ion concentrations and ion diffusion coefficients in ion-conducting materials. We apply this method to literature data for pure polyethylene oxide where the residual conductivity is believed to be due to protons. The obtained diffusion coefficient is found to be in the order of, or higher than, reported lithium ion diffusion coefficients in low molecular weight polyethylene oxide. Hence it is likely that proton intercalation will be of importance for electrochromic devices, provided there is a significant amount of protons present.     

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.     

Mg-Ni thin-film composition dependence of durability of electrochromic switchable mirror glass in simulated environment

Mg-Ni thin films as a switchable mirror material are promising for application in energy-saving windows because they can change their state between reflective and transparent by hydrogenation and dehydrogenation. In our previous work, we found that electrochromic switchable mirror glass based on a Mg₄Ni thin film is affected by environmental factors such as temperature and humidity. In this work, we investigated the effects of the environment on the optical switching properties of electrochromic switchable mirrors with Mg-Ni thin films of various compositions suitable for a broad range of applications and operating environments. When the mirror devices were kept in a simulated environment with a constant temperature 40 °C and a constant relative humidity of 80%, controlled by a thermostat/humidistat bath, their optical switching properties degraded. The degradation was found to be related to the change in the Mg-Ni thin films into nonmetallic states of oxides and hydroxides. The device with a Mg₆Ni thin film kept in the bath for 7 days showed no optical switching property. The mechanism of degradation in the bath was strongly affected by the composition of the Mg-Ni thin-film optical switching layer. The device constructed with an optical switching layer having high magnesium content degraded more rapidly in the test.     

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.     

The influence of the intercalate species on the quasi-static electrochromic behavior of tungsten-oxide-based devices

The influence of the intercalate species on the quasi-static electrochromic behavior of tungsten-oxide-based devices is investigated. Two different electrolytes are used in the devices: an aqueous sulfuric acid solution, from which it is assumed that intercalation of hydrogen occurs; and a solution of lithium perchlorate in propylene carbonate, from which it is assumed that intercalation of lithium occurs. Experiments are performed in which a step-current of small magnitude is imposed through the device, and the corresponding time-dependence of the electrical potential and optical transmission are measured simultaneously. The behavior of the optical efficiency is relatively insensitive to the nature of the intercalate species, but the device potential is appreciably more sensitive to lithium intercalation than to hydrogen intercalation. The disparity in electrical behavior is likely due to increased strain effects and/or a diminished availability of sites associated with the larger lithium intercalate. It is shown that the electrical and optical behavior of the two types of devices may be related by a single linear scaling relation, indicating that the fundamental processes involved in the operation of the devices are similar.     

Intercalation studies on electron beam evaporated MoO3 films for electrochemical devices

Now-a-days a large number of extensive research has been focused on electrochromic oxide thin films, owing to their potential applications in smart windows, low cost materials in filters, low cost electrochemical devices and also in solar cell windows. Among the varieties of electrochromic transition metal oxides, the molybdenum oxide (MoO₃) and tungsten oxide (WO₃), form a group of predominant ionic solids that exhibit electrochromic effect. The electrochromic response of these materials are aesthetically superior to many other electrochromic materials, because WO₃ and MoO₃ absorb light more intensely and uniformly. In the present case, we have discussed about the electrochromic behaviour of electron beam evaporated MoO₃ films. Moreover, the MoO₃ film can also be used as a potential electro-active material for high energy density secondary lithium ion batteries; because it exhibits two-dimensional van der Waals bonded layered structure in orthorhombic phase. The films were prepared by evaporating the palletized MoO₃ powder under the vacuum of the order of 1 × 10⁻⁵ mbar. The electrochemical behaviour of the films was studied by intercalating/deintercalating the K⁺ ions from KCl electrolyte solutions using three electrode electrochemical cell by the cyclic-voltammetry technique. The studies were carried out for different scanning rates. The films have changed their colour as dark blue in the colouration process and returns to the original colour while the bleaching process. The diffusion coefficient values (D) of the intercalated/deintercalated films were calculated by Randle's Servcik equation. The optical transparency of the coloured and bleached films was studied by the UV–Vis–NIR spectrophotometer. The change in bonding assignment of the intercalated MoO₃ films was studied by FTIR spectroscopic analysis. A feasible study on the effect of substrate temperatures and annealing temperatures on optical density (OD) and colouration efficiency of the films were discussed and explored their performance for the low cost electrochemical devices.     

A new approach for design of organic electrochromic devices with inter-digitated electrode structure

We present a new approach for design of organic electrochromic devices (ECD) with inter-digitated electrode (IDE) structure and three-electrode dynamic operation. The advantages of the IDE design include the ability to produce fast and homogenous color change over large areas. In addition, it enables fabrication of multi-color devices. Our method involves photolithographic etching of ITO followed by electrophoretic deposition (EPD) and mechanical compression of porous titania to produce finely patterned electrodes with high surface area. The titania layer is chemically modified by new stable and reversible electrochromic viologen derivatives involving phenylphosphonic acid anchoring moiety. The new device demonstrates reversible and strong color change from colorless to deep blue and yellow.     

Characteristics of laminated electrochromic devices using polyorganodisulfide electrodes

The application of polyorganodisulfides as optically passive counter-electrodes in a variety of electrochromic smart glazing devices are discussed. Characteristic data is presented for electrochromic devices using proton, and lithium coloration ions with modified polyethylene oxide electrolyte and polydimercaptothiadiazole positive electrodes. Solid state devices consisting of molybdenum doped WO₃, amorphous polyethylene oxide electrolyte (a-PEO), and a polyorganodisulfide counter-electrode colored rapidly from a pale yellow to a deep blue-green, upon application of −1.2 V DC. The photopic transmittance (T_p) changes from 61 to 9%, and the solar transmittance (T_s) changes from 45 to 5% during the coloration process. Also, our experiments with polyimidazole are detailed. This family of compounds due to its unique electrical and ion conduction properties allow a single composite ion storage and ion conductor electrode to be made, simplifying the device construction. Devices made from this family of compounds color to deep blue-gray upon application of −1.2–1.5 V DC. Bleaching occurs at −0.4 to −0.5 V DC. The photopic transmittance changed from 55 to 9%, and the solar transmittance from 34 to 4% during coloration. Both coloration and bleaching are quite rapid.     

Self-adjusting smart windows based on polymer-dispersed liquid crystals

The control of sunlight can be achieved either by electrochromic or polymer-dispersed liquid crystal (PDLC) smart windows. We have recently shown that it is possible to homeotropically align fluid mixtures of low molecular mass liquid crystal with a negative dielectric anisotropy, and a liquid crystalline monomer, in order to obtain electrically switchable chromogenic devices. They are new materials useful for external glazing. In fact, they are not affected by the classical drawbacks of PDLCs. In this paper we present a new self-switchable glazing technology based on the light-controlled transmittance in a PDLC device. The self-adjusting chromogenic material, which we obtain, is able to self-increase its scattering as a function of the impinging light intensity. The relationship between the electro-optical response and the physical-chemical properties of the components has been also investigated.     

Electrochromic devices: A comparison of several systems

The great variety of colour contrast achieved with electronically conducting polymers is the most significant characteristic of this class of ion-insertion organic materials. The performance of selected electrochromic devices based on electronically conducting polymers, with organic or inorganic materials as optically passive or electrochromic in complementary mode counter-electrodes, is reported and the advantages and drawbacks of each system discussed.     

Sb–Cu–Li electrochromic mirrors

Switchable mirrors offer significant advantages over traditional electrochromic devices for control of incident light in architectural and aerospace applications due to their large dynamic ranges in both transmission and reflection in the visible and near infrared regimes. Here we describe construction and spectroscopic characterization of a complete electrochromic mirror device consisting of an antimony–copper alloy (40 at% Cu) active electrode coupled with an optically passive vanadium oxide counter electrode and a crosslinked polymer gel electrolyte. Transmittance and reflectance spectra in the visible–near IR (300–2500 nm) in both mirror and transparent states are reported. The photopic transmittance of the complete device varied from less than 3% to more than 20% during cycling, requiring about 40 min for complete switching in each direction. At the same time, the photopic reflectance varied from 40% to 25%. The crosslinked polymer improves the stability of the mirror electrode relative to that in a liquid electrolyte.     

V0.50Ti0.50Ox thin films as counterelectrodes for electrochromic devices

Thin films of V_0.50Ti_0.50O_x have been deposited by RF sputtering from metallic targets. Their use as potential counterelectrodes in electrochromic devices has been investigated. It is found that although they are slightly yellow looking in transmission, the films can reversibly store relatively large amounts of charge, whilst showing a reasonably low electrochromic colouration efficiency. The electrochemistry of V_0.50Ti_0.50O_x is found to be simple, in fact rather similar to that of WO₃, making it an almost ideal material to use in such a variable transmission device.