Incorporation of a stable radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) in an electrochromic device

A stable organic radical, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), was studied. We employed TEMPO as a cathodic radical provider in propylene carbonate (PC) and poly(3,4-propylenedioxythiophene) derivatives (PProDOT-Et₂) as an anodic electrochromic (EC) thin film, which was obtained through electropolymerization. On assembling them together in a device, the electrochemical and optical performances of this hybrid electrochromic device (ECD) showed reversible cycling stability and high absorbance attenuation in the visible range. By selecting proper electrolytes (LiClO₄/PC) and controlling the deposited charge of the PProDOT-Et₂ thin film, it was possible to obtain a transmittance change (ΔT) of up to 59% at 590 nm with no noticeable degradation after operating between 0 and 0.9 V for 1000 cycles. Furthermore, an electrochemical quartz crystal microbalance (EQCM) was used to investigate ion migrations in the PProDOT-Et₂ thin film, which influenced its long-term stability.     

Improved electrochromic devices with an inorganic solid electrolyte protective layer

The durability problem of electrochromic devices (ECD) in Li ion-conducting electrolytes may be due to the degradation of the ECD electrode matrix and irreversibly reacting Li ions during cycling. With this hypothesis, we investigated the performance of WO₃ film with an inorganic solid electrolyte, lithium phosphorous oxynitride (LiPON), protective layer and of the ECD composed of WO₃ and V₂O₅ with a LiPON protective layer prepared by RF magnetron sputtering. WO₃ and ECD (glass/ITO/V₂O₅/LiPON/electrolyte/LiPON/WO₃/ITO/glass) with a protective layer not only showed improved durability with continuous potential cycling, but also demonstrated a good memory effect under voltage-off, good response times and high coloration efficiency (CE). Our results demonstrated that LiPON layers are electrochemically stable, enhance the electrochromic properties in Li ion-conducting electrolytes, and can be used as a protective layer for ECD.     

A complementary electrochromic device based on Prussian blue and poly(ProDOT-Et2) with high contrast and high coloration efficiency

A complementary electrochromic device (ECD) based on Prussian blue (PB) and poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et₂) has been systematically investigated. PB is regarded as an anodic coloring material with high electrochemical stability, while PProDOT-Et₂ is a cathodic coloring polymer with high contrast and high coloration efficiency (η). The electro-optical properties of the two electrochromic (EC) materials are characterized separately in a 0.1 M LiClO₄ in propylene carbonate (PC). A complementary ECD is assembled based on the two EC materials. The maximum transmittance of the ECD at 590 nm can be changed reversibly from 11.3% to 70.6% at the applied voltages of 1.2 and −1.3 V, and achieved a high coloration efficiency of 1214 cm²/C. Moreover, this ECD still remains at 98% of its maximum transmittance window (ΔT_max) even after 1,200 cycles, namely, the ΔT value decreases from 59% to 58%.     

All-solid-state electrochromic device based on poly(butyl viologen), Prussian blue, and succinonitrile

A new complementary electrochromic device (ECD) is described; it is based on poly(butyl viologen) (PBV) and Prussian blue (PB) confined to the electrode surfaces. PBV is a cathodically colored organic polymer, while PB is an anodically colored inorganic material. The two electrochromic materials were individually characterized in a 0.5 M KCl aqueous solution. On the basis of their properties, a PBV–PB ECD containing a solid-state electrolyte prepared by adding lithium tetrafluoroborate (LiBF₄) as a salt to succinonitrile (SN) was investigated. This all-solid-state ECD system showed good optical contrast with a coloration efficiency of ca. 163 cm²/C at 650 nm and good stability during 4000 cycles. The transmittance of the ECD at 650 nm changed from 73% (bleached) to 8% (darkened), with an applied potential of 1.7 V (−1.0 to 0.7 V) across the two electrodes. After 4000 cycles, the transmittance attenuation (ΔT) of the device was still at 86% of its original value, i.e. the ΔT value had decreased from 65% to 56%.     

Electrochromic thin films prepared by sol–gel process

New methods are shown for lower temperature preparation of amorphous tungsten oxide thin film and preparation of crystalline iridium oxide thin film by sol–gel process using metal chloride as the starting materials and ethanol as a solvent. These electrochromic materials were combined with gel solid electrolyte, and preparation of fully solid-state electrochromic display (ECD) was made. The transmittance of the ECD could be made to change by 35% by applying a voltage of 3 V for 0.2 sec.     

A complementary electrochromic device based on polyaniline and poly(3,4-ethylenedioxythiophene)

In this study, two conducting polymers, polyaniline (PANI) and poly(3,4-ethylenedioxythiophene) (PEDOT), were used to construct an electrochromic device (ECD). PANI was employed as the anodic coloring polymer while PEDOT was used as the cathodic coloring polymer. The electrochemical and optical properties of PANI, which has a coloration efficiency of 25 cm²/C at 570 nm, were coupled with the complementary coloring material, PEDOT, which has a coloration efficiency of 206 cm²/C at 570 nm. A suitable operating potential window was switched between −0.6 and 1.0 V to explore the cycle life of the ECD. We tested the PANI–PEDOT ECD, which consisted of PANI, PEDOT, and an organic electrolyte containing 0.1 M LiClO₄ in propylene carbonate and 1 mM HClO₄. The transmittance of the ECD at 570 nm changed from 58% (−0.6 V) to 14% (1.0 V) with a coloration efficiency of 285 cm²/C. Within the selected operating voltage range, the PANI–PEDOT ECD could be cycled for up to 2×10⁴ cycles.     

Cycling and at-rest stabilities of a complementary electrochromic device containing poly(3,4-ethylenedioxythiophene) and Prussian blue

PEDOT-based electrochromic devices (ECDs) have been investigated intensively in recent years. In order to obtain an ECD having long cycle life, the counter electrode and electrolyte used should be compatible in the electrochemical environment. Prussian blue (PB) is proven to be electrochemically stable when cycling in non-aqueous solutions. Thus a new organic-inorganic complementary ECD was assembled in combination with a PMMA-based gel polymer electrolyte. This ECD exhibited deep blue-violet when applying −2.1 V and became light blue when applying 0.6 V. Under these conditions, the transmittance of the ECD at 590 nm changed from 13.8% (−2.1 V) to 60.5% (+0.6 V) with a coloration efficiency of 338 cm²/C. The cell retained 55% of its maximum transmittance window (ΔT_max) after 50,640 repeated cycles. Moreover, the at-rest stability test revealed a transmittance window (ΔT) decay of 9.6% over a period of 107 days. Therefore, the proposed PEDOT-PB ECD may have potential for practical applications.     

Progress toward roll-to-roll processing of inorganic monolithic electrochromic devices on polymeric substrates

As electrochromic device (ECD) emerges into the market, the necessity of cost reduction via high-throughput manufacturing is unavoidable. Inorganic monolithic ECD has compatibility in its manufacturing steps, allows continuous processing and hence low-cost ECD. A roll-to-roll (R2R) production is the industrial standard for high-throughput processing and was chosen to synthesize ECD on polymeric substrates. Preliminary success by implementing methods compatible to the aforementioned R2R processing has been achieved; however, continuous test-runs on an R2R machine are still limited.     

Influence of coloring voltage on the optical performance and cycling stability of a polyaniline–indium hexacyanoferrate electrochromic system

An electrochromic system based on the multielectrochromic polyaniline (PANI) and pseudo-transparent indium hexacyanoferrate (InHCF) thin-film electrodes was studied in this work. In combination with a hybrid H⁺/K⁺-conducting solid polymer electrolyte—KCl-doped poly(2-acrylamido-2-methylpropanesulfonic acid) (K-PAMPS), a precoloring-free PANI–InHCF electrochromic device (ECD) with an active area of 3×3 cm² was fabricated and exhibited yellowish-green-blue multicolor electrochromism. From in situ spectroelectrochemical experiments, we found that the performance of a PANI/K-PAMPS/InHCF ECD was significantly affected by the operating voltages, especially by the coloring voltage. Both the bleached and yellowish state of the ECD could be attained reversibly by applying a voltage ranging from +1.5 to +1.7 V (InHCF vs. PANI). Different coloring voltages resulted in different optical properties and cycling stabilities, however. For instance, the device biased at −1.6 V (InHCF vs. PANI) showed a deep blue color, but the optical activity decayed quickly (less than 50 cycles) when the device was switched between +1.6 and −1.6 V. Nevertheless, the device could be reversibly operated between +1.6 and 0 V for several hundred cycles, although a narrower electrochromic extent (yellowish-to-green) was observed correspondingly. The optimization of the coloring voltage is therefore of paramount importance to the PANI/K-PAMPS/InHCF ECD.     

Bi-based electrochromic cell with mediator for white/black imaging

We studied electrochemical properties of bismuth deposit, which showed reversible color change from colorless clear to black by electrochemical reaction, toward paper-like electronic imaging device. Bismuth salt in an electrolyte solution is colorless clear, but turned to black by the electrodeposition on an electrode. Namely, bismuth ion (Bi³⁺) dissolved in the electrolyte solution (colorless clear) is electrochemically reduced on the electrode to deposit the Bi metal showing black color. The Bi deposit on the electrode is electrochemically oxidized to dissolve the deposit into the solution. These processes enable the color change between colorless clear and black. However, the stability of the switching between colorless and black state is not sufficient due to less electroactivity of bismuth deposit. To improve the switching stability of the present electrochromic cell, various mediators were employed to add into the electrolyte solution. Reversible white/black display cell has been successfully demonstrated.     

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.     

Phthalate-derivative/TiO2-modified electrode for electrochromic application

We studied electrochromic properties (EC) of phthalate derivatives (TP), which showed reversible color change from colorless clear to three primary colors by electrochemical reaction, from a viewpoint of color electronic paper. To improve the ability of keeping coloring state under an open circuit, we successfully prepared terephthalate-derivative/TiO₂-modified electrode and evaluated its EC properties. TP film also showed reversible color change from clear to magenta and enabled not only keeping the coloring state longer under open-circuit condition but also good bleaching response under short-circuit condition.     

Electrochromic emissivity modulator for spacecraft thermal management

A novel electrochromic device (ECD) working in mid- to long-wave infrared (IR) region is presented, comprising of a solid-state monolithic thin film system for adjusting heat rejection/receiving levels on attached surfaces. The system is an electrically controllable active emissivity modulator. EclipseVED™, variable-emissivity ECD, is designed for satellite and spacecraft thermal control, using an active ECD system for long-wave infrared (LWIR) modulation and a passive cold mirror for solar rejection. Emissivity modulation of the system is 0.8 for 7–12 μm region while average solar rejection is 80% in the vis–NIR region. Device properties and initial space test results are also presented.     

Environmental assessment of electrochromic glazing production

The life cycle analysis method was used to determine the environmental impacts associated with the production of an electrochromic (EC) glazing (called ECD). This paper describes the inventory analysis for all the basic materials used during the manufacture of the ECD, i.e. K-Glass, tungsten oxide (WO₃), poly-methyl methacrylate (PMMA), propylene carbonate (PC), lithium perchlorate (LiClO₄) and acetic silicone sealant. K-Glass, PC and PMMA account for the 98% of the total device mass and the CO₂ emissions during their production processes are 810 g. The total embodied energy was estimated to be 49 MJ/ECD, with 32.1 MJ/unit of them derived from the K-Glass. The comparison of the total embodied energies of the ECD and various insulating glass units concluded that mass-produced EC glazings could easily compete with them in terms of environmental performance, anticipating cost attenuation and overall thermal and optical behavior. The above analysis could be implemented for the reduction of the embodied energy of the ECD life cycle, since it is proposed as an energy saving device.     

Electrochromic device of PEDOT–PANI hybrid system for fast response and high optical contrast

An organic–organic hybrid system composed of the polyaniline (PANI) and poly-(3,4-ethylenedioxythiophene) (PEDOT) with controlled thickness was developed successfully in order to realize synergetic effects in electrochromic (EC) properties such as optical contrast and color-switching rate. From the UV transmittance spectra, we found that the optical contrast (Δ%T) was enhanced up to 6–72% at the wavelength of 580 nm compared with the previous PANI–PEDOT ECDs. Furthermore, the optimized ECD showed an extremely fast response time of less than 160 ms. It is therefore concluded that such a complementary full .cell system of PEDOT–PANI ECD is applicable as an optical device.     

Effect of modified ITO substrate on electrochromic properties of polyaniline films

In this work, we report the morphological and electrochromic properties of electrochemically synthesized polyaniline (PANI) thin films on bare and modified indium–tin oxide (ITO) glass substrates. In the last case, the surface of ITO glass was covered by a self-assembled monolayer of N-phenyl-γ-aminopropyl-trimethoxysilane (PAPTS). Atomic force microscopy images and perfilometry show that smoother and thinner PANI films were grown on PAPTS-modified ITO substrates. PANI-based electrochromic devices (ECDs) were assembled by using a viscous polymeric electrolyte (PE) of LiClO₄ and polymethyl methacrylate (PMMA) co-dissolved in a mixture of propylene and ethylene carbonate. The architectural design of the devices was glass/ITO/PANI/PE/ITO/glass. A dual ECD was also prepared by collocating a poly(3-methylthiophene) (P3MT) thin film as a complementary electrochromic element. The effect of the PAPTS-modified ITO substrate is reflected in a higher optical transmittance at bleach state and a little less color change at 550 nm of PANI-based ECDs.     

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.     

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.     

Electrochemically induced electrochromic properties in nickel thin films deposited by DC magnetron sputtering

Nickel oxide thin films and other electrochromic materials receive particular attention due to the great variety of practical applications in energy conservation and in semitransparent optical devices. In this work, nickel thin films were produced by DC magnetron sputtering on ITO substrates. The nickel–ITO thin films were studied by electrochemical techniques, and electrochromic properties were induced in the films after several different cyclic voltammetry runs. The cyclic potential range was set from −400 to 600 mV and the scan rates were varied from 6.6 to 10 mV/s. The electrochromic phenomena was observed just after 80 cycles as derived from voltammograms and color changes in the nickel oxide films were observed close to 100 cycles. The optical properties of as-deposited films and of the ones tested in the electrochemical cell were determined by optical spectrophotometry in the visible range. The structural properties of the films were studied by X-ray diffractometry, scanning and transmission electron microscopy in conventional and high-resolution modes. The electrochemical properties were studied principally by the cyclic voltammetry technique. Noticeable differences in induced electrochromic behavior were observed between the nickel films deposited on two sets of ITO substrates, prepared by DC magnetron sputtering and spray pyrolysis.     

Fabricating red-blue-switching dual polymer electrochromic devices using room temperature ionic liquid

In this work, room temperature ionic liquid (RTIL)—1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIM]PF₆)—was employed to fabricate dual polymer electrochromic devices (DPECDs). [BMIM]PF₆ was used as the electrolyte both in the electrochemical synthesis of conducting polymers (CPs) and in the fabrication of DPECDs. The electrochemically deposited poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-methylthiophene) (PMeT) were employed to serve as two complementary coloring electrochromic thin films. Through combining these two electrochromic layers, the assembled DPECDs were found to switch between deep red and deep blue, which are two primary colors for a display. By employing RTIL as electrolyte, the devices retained 65% of their optical contrast and electroactivity after 5×10³ deep double potential steps, showing enhanced stability and durability. The DPECDs also exhibited stable electrochromic performance, with a maximum optical contrast of 26% at 665 nm, and achieved a high coloring efficiency of 460 cm² C^-1.