Reflective Electrochromic Device Research Articles

Recent progress in transmissive and reflective electrochromic devices for multi-color modulation

Multi-color electrochromic devices (M-ECDs), which dynamically and reversibly regulate the visible optical properties of the objects, have received significant interest in various fields such as displays, flexible intelligent electronics, smart windows and military camouflage. This review provides comprehensive summaries of the latest advances in inorganic electrochromic materials and M-ECDs. We discuss the basic principles for achieving M-ECDs, from the intrinsic multicolor characteristics of materials to a combination with optical structures. The progress on the construction of multicolor electrochromic thin-films and associated devices are unfolded in detail based on their transmissive or reflective working scenarios. Moreover, the limitations and challenges are also outlined for consideration for the future development of M-ECDs.

Fully spray-coated electrochromic devices containing octa-viologen substituted polyhedral oligomeric silsesquioxane

Here we present the fully spray-coated electrochromic devices (ECDs) containing octa-viologen substituted polyhedral oligomeric silsesquioxane (OHV-POSS). By adopting various compositions and layer configurations, transmissive and reflective ECDs were fabricated by spray-coating on polyester and polycarbonate, respectively, substrates. As an electrode, the mixture of silver nanowires and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) was used in the presence of divinyl sulfone. The electrical conductivity and figure-of-merit values of the electrodes were estimated depending on the compositions. The ion gel containing OHV-POSS with ionic liquid was prepared for an electrochromic layer. An electrolyte layer containing lithium salt and a curable polymer matrix was inserted between the electrochromic and electrode layers to minimize solvent infiltration. The spray-coated plastic ECDs exhibited a low redox potential of -1.5 V and reversible color changes. The devices maintained the switching times are less than 10 s and the cyclic stability for up to 90,000 s. Using all spray-coating methods, we demonstrate a proof-of-concept for electrochromic paint. With the current achievements, it would be possible to avoid the restriction imposed on the substrate and the resulting device structure. Various device structures can be constructed as the current approach does not require an upper substrate, which is further extended to diverse solid surfaces that exhibit on-demand color-changing capabilities.

Electrochromic Displays Having Two‐Dimensional CIE Color Space Tunability

AbstractElectrochromic devices with a wide color gamut distribution have long been sought after for non‐emissive display technologies. The current state‐of‐the‐art multicolor electrochromic displays utilize a single electrochromic layer, which restricts their color tunability within a linear or curved segment scope in International Commission on Illumination (CIE) color space and thus leads to limited color hues. Herein, it is demonstrated vivid electrochromic displays with broadened color hues via fabricating Zn‐based multicolor electrochromic displays having 2D CIE color space tunability. In addition, it is revealed that a Fabry–Perot nanocavity structure can further tune the color hues via altering the coordinate of the 2D CIE color space. It is known that this is the first demonstration of 2D CIE color space tunability realization from a single transparent or reflective electrochromic device. These findings represent a novel strategy for fabricating multicolor electrochromic displays and are expected to advance the development of electrochromic displays.

A feasible strategy of Prussian blue reflective electrochromic devices capable of reversible switching between sand-yellow and leaf-green

A flexible reflective-type electrochromic device based on PB films has been constructed. Based on the subtractive color-mixing theory, the demonstrated electrochromic device exhibits a unique reversible color changes from leaf-green to sand-yellow by electrochemical operation. Fast response time of 0.9 s for bleaching and 1.3 s for coloring is observed, which is faster than most reports. In addition, the device exhibits high electrochromic stability of more than 100 cycles without compromising its performance. The flexible device shows promise for a broad range of potential application in the wearable electrochromic displays and camouflage.

A Self-Bleaching Electrochromic Mirror Based on Metal Organic Frameworks.

Metal-organic frameworks (MOFs) are considered to be the most promising positive anode materials to store charge for electrochromic devices. Nevertheless, a detailed mechanism of the electrochemical and ions storage process has not yet been revealed. Herein, the electrochemical mechanism of the highly porous ZIF-67 films and the electrochromic performance of electrochromic mirrors constructed from ZIF-67 and WO3 electrodes were investigated. The mechanism of the charge storage was revealed in the kinetic analysis of the Li-ion behavior based on the cyclic voltammetry curves and electrochemical impedance spectra. Impressively, the electrochromic mirrors with the self-bleaching effect and self-discharge behavior showed a unique electrochromic performance, such as a high coloration efficiency of 16.47 cm2 C−1 and a maximum reflectance modulation of 30.10% at 650 nm. This work provides a fundamental understanding of MOFs for applications in electrochromic devices and can also promote the exploration of novel electrode materials for high-performance reflective electrochromic devices.

Air-stable, high contrast solution-phase electrochromic device based on an A-D-A viologen derivative

Electrochromic (EC) devices with reversible transmittance switching have promising prospects in the applications of photoelectric displays. Herein, a novel air-stable and high contrast solution-phase electrochromic device based on new viologen derivative (DPB) with A-D-A structure (A: acceptor; D: donor) was demonstrated. Devices possessed simple configuration of ITO/EC solution/ITO. Ferrocene and polycarbonate (PC) were selected as the complementary material and solvent, respectively. All of the fabrications and performance testing work were conducted in normal air condition. The DPB-based device showed deep magenta color in colored state and exhibited high optical contrast of ΔT of 72.2% (517 nm) and good coloration efficiency of 102.2 cm2/C at 1.8 V. The device appeared only slight degradation of ΔT (ΔT = 69.6% or retained 96.7% of its initial ΔT after 5000 s constant switching) under the normal air atmosphere, which was even better than some reported well-packaged viologen-based solution ECDs. Additionally, the UV–vis reflective spectra were also conducted to assess the reflectance property of the devices. The reflectance peak (ΔR = 2.6%) and absorption peak matched accurately, which indicated that the new viologen derivative possesses the potential application in reflective electrochromic devices.

High‐Performance Reflective Electrochromic Device by Integrating White Reflector and High Optical Density Electrochromic System

AbstractThis paper reports an efficient reflective electrochromic device (ECD) by integrating a highly reflective micro‐porous titanium dioxide–based white reflector film and an optimized transparent to black convertible electrochromic (EC) system. This reflective ECD can reflect almost 58.5% of light in the white state and 4.7% in the dark black state at 550 nm. Similarly, it also exhibits a very high ambient contrast ratio of 38:1 under office ambient light condition and good driving stability of about 83% in the white state (0 V) and 98% in the black state (1.7 V) after 10 000 driving cycles. In addition, the EC system is optimized by selecting good proton generator and suitable solvent for better coloration property as well as a low driving voltage via lactone ring opening reaction of leuco dye and supporting polymer for high optical density through suppressing the movement of oxidized leuco dye to the electrode. An ECD with the optimized EC system shows significant improvement in average optical density from 0.62 to 1.9 (at 400 to 700 nm) and faster switching speeds of about 1.6 s.

An optically efficient full-color reflective display with an electrochromic device and color production units

ABSTRACTIn this paper, we report a full-color reflective display device by combining reflective electrochromic device (ECD) and different color production units (conventional color filter (CF) and transmittance controllable electrochromic color filter (TCECF)). A full-color reflective device with TCECF showed an excellent diffuse reflectance of 47.2% in the white state owing to the high transmittance of TCECF in the bleached state than that of CF (19.5%). This device structure can easily provide various colors with high brightness and saturation with a broad grayscale. Particularly, in the colored state, TCECF and CF-based full-color reflective device displayed color coordinates of (0.59, 0.34), (0.31, 0.66), (0.24, 0.30) and (0.65, 0.33), (0.27, 0.61), (0.15, 0.06), for red, green and blue, respectively, and alongside also exhibited color gamut of 36.8% (for TCECF) and 73.1% (for CF) for full color reflective devices.

Study of ionically conducting nanocomposites for reflective electrochromic devices

New, nanocomposite, and ionically conducting membranes have been developed by using plasticized and crosslinked gelatin and clay (montmorillonite SCa-3) with concentration of 1–20 wt%. The samples were studied by FTIR, X-ray diffraction, UV-Vis (transmittance and reflectance), CW-EPR, HYSCORE, and ionic conductivity properties. The obtained results have revealed that the incorporation of clay has promoted changes in the physical properties of the samples. The transparency of theses membranes decreased, while the reflectance augmented with the rise of the clay content. The best ionic conductivity result of 3.49 × 10−4 S/cm2 at 25 °C was obtained for the sample with 15 wt% of clay. CW-EPR spectra of Gel-SCa-3-Na samples doped with Cu2+ ions showed that SCa-3-Na had no significant impact on the Cu2+ local coordination surroundings and HYSCORE spectra showed one and two proton signals depending on the sample. Finally, the samples were inserted in the electrochromic devices (ECDs) composed of glass/ITO/Prussian blue/gelatin-SCa-3/CeO2-TiO2/ITO/glass. The best results were registered with the ECD containing 15 wt% of clay, which changed its reflectance from 15 to 8% at 450 nm. Moreover, this ECD demonstrated to be completely reversible. The obtained results confirmed the impact of the clay on the ionic conductivity, transparency, and performance of the ECDs. In summary, the nanocomposite electrolytes seem to be very interesting materials, mainly because of their possible practical use in electrochromic devices.

A Reflective Inorganic All-Thin-Film Flexible Electrochromic Device with a Seven-Layer Structure**Supported by the Science and Technology on Vacuum Technology and Physics Laboratory for the Research Project under Grant No 6142207040104.

A reflective electrochromic device is fabricated on a 10 cm×10 cm flexible PI/Al substrate using magnetron sputtering. The device has a complementary all-thin-film structure and consists of seven layers. Indium tin oxide (ITO) acts as a transparent electrode deposited on the top, meanwhile, an aluminum (Al) film is adopted as an inter-counter bottom electrode and provides high reflectance. Tungsten oxide (WO3) is used as the main electrochromic layer and nickel oxide (NiO) acts as the complementary electrochromic layer. Lithium niobate (LiNbO3) is applied as a Li+ ion conductor layer. Especially, in the seven-layer structure, two tantalum oxides (Ta2O5) are added as transition layers to prevent Li+ escaping from LiNbO3 when the potential is not applied on the device. When the device is in an electrochromic process, both Ta2O5 provide excellent conductivity for Li+ ions and act as the dielectric of electrons. The complementary device with structure Al/NiO/Ta2O5/LiNbO3/Ta2O5/WO3/ITO exhibits good optical properties, and the reflectance modulation reaches up to 55% measured by a spectrophotometer in the range of 400–1600 nm. The cyclic stability of the electrochromic device is investigated. The results indicate that the charge density involved in the electrochromic process decreases and the electrochromic response time increases with the cycle number because of the Li+ insertion in WO3.

Reflective Electrochromic Device with Gelatin-Nanocomposite Electrolyte

Reflective electrochromic device with glass/Cr/PB/electrolyte/CeO2-TiO2/ITO/glass configuration was assembled and characterized. Prussian blue (PB) thin film was electrodeposited galvanostaticaly at -40 µA/cm2 for 180 s from a solution of K3[Fe(CN)6] (0.05 mol/L), FeCl3 .6H2O (0.05 mol/L), and HCl (0.1 mol/L) on chrome coated glass as working electrode. CeO2-TiO2 thin film was obtained from sol-gel method and deposited by dip-coating on ITO conductive glass at 20 cm/min. This coating was heat treated at 450°C for 15 min. As electrolyte, there was used gelatin-nanocomposite membrane prepared by casting from mixture of gelatin, LiClO4, and clay (SCa-3) at concentrations of 10, 15, and 20 wt% solution. The electrolyte was characterized by ionic conductivity properties, and the one with 15 wt% of clay was applied in ECD. This electrolyte displayed the ionic conductivity of 6.46x10-5 and 6.02x10-4 S/cm2 at 25 and 70°C, respectively. The ECD was subjected to reflectance measurements that revealed a change from 8 to 15% at 450 nm for bleached and colored states, respectively. The picture of colored and bleached states is shown in Fig. 1. Moreover, this ECD demonstrated to be completely reversible. Finally, it was shown that reflective ECD with nanocomposite electrolyte based on natural macromolecule and clay have promising performance for display application. Figure 1

Optical constants of electrochromic films and contrast ratio of reflective electrochromic devices

This study investigates the optical constants of WO3 electrochromic films and NiO ion-storage films in bleached and colored states and that of a Ta2O5 film used as an ion conductor. These thin films were all prepared by electron-beam evaporation and characterized using a spectroscopic ellipsometer. The spectra obtained using a spectrophotometer and those calculated from the optical constants agreed closely. An all-solid thin-film reflective electrochromic device was fabricated and discussed. Its mean contrast ratio of reflectance in the range of 400-700 nm was 37.91.

A green-yellow reflective electrochromic device

All solid state green-yellow electrochromic devices (ECD) performing in reflexion and built with Prussian Blue (PB) as electrochromic (EC) layer, CeO2–TiO2 as counter-electrode and PVB-based membrane as electrolyte have been assembled and characterized. The PB films were deposited by galvanostatic method and stabilized by cyclic voltammetry evidencing two cathodic and two anodic peaks and confirming their blue, green and transparent states. The analysis of PVB-based polymer electrolyte revealed an Arrhenius type ionic conductivity with values increasing from 7.7×10−4Scm−1 at 27°C to 3.6×10−3Scm−1 at 70°C. The Raman spectra of the membrane evidenced two peaks, centered at 113 and 145nm, attributed to I3− and I5−, respectively, and related to the yellow color of the sample. The ECD changed its reflectance from 46% (at −2.0V) to 23% (at +2.0V) at 550nm. The digital system for color management of pictures confirmed the change of color and the CIE LAB 1976 color scale evaluation revealed the total color difference between green (+2.0V) and yellow (−2.0V) of ΔE=40.34. After 400 green-yellow color chronoamperometric cycles between −1.5V and +1.5V one observed a decrease of the charge density from −5.5mCcm−2 to −4.9mCcm−2; moreover a memory test realized with the device in open circuit conditions revealed an increase of the reflectance from 27.5% to 30% for the deep green state and a decrease from 36.5% to 33% for the yellow state of, during 120min. All the obtained results suggest that the investigated green-yellow reflective ECD performed well and can be a promising solution for display applications.

Preparation of a reflective‐type electrochromic device based on monodisperse, micrometer‐size‐range polymeric microspheres and viologen pendants

AbstractA reflective electrochromic device (ECD) based on polymeric microspheres was proposed, and its feasibility for a display was investigated. The viologen moiety was introduced onto the surface of chloro‐functionalized polymeric microspheres via refluxing in toluene. The existence of the viologen pendants was confirmed with X‐ray photoelectron spectroscopy, and their redox reactions were examined with cyclic voltammetry. The ECD had an inherent white background without an applied potential because the micrometer size range of the substrate particles scattered the light, whereas a deep violet color appeared when 3.0 V was applied to the cell. This color change could have been caused by the reduction of viologen pendants on the surface of the polymeric microspheres. The reflectance values of the ECD were observed with a spectrophotometer, and the response times and cyclic stability of the ECD, depending on the potential, were also examined. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6562–6572, 2005

Microporous Patterned Electrodes for Color-Matched Electrochromic Polymer Displays

The emergence of electroactive and conducting polymers offers new opportunities for the design of materials for electrochromic devices (ECDs). Of these, poly(3,4-alkylenedioxythiophene)s (PXDOTs) and their derivatives exhibit the most promising electrochromic (EC) properties. Here, we report the use of highly porous metallized membranes which allow the production of patterned, rapid-switching, reflective ECDs. Using poly(3,4-ethylenedioxithiophene) (PEDOT), poly(3,4-propylenedioxythiophene) (PProDOT), and the dimethyl-substituted derivative PProDOT-Me2 as the active EC materials, we have obtained switching times of 0.1−0.2 s (5−10 Hz) to achieve full EC contrast. These polymers yield reflective contrast values of up to 90% in the NIR and ∼60% in the visible regions. In addition, the ECDs were switched repetitively 180 000 times with less than 10% contrast loss. We have also demonstrated a 2 × 2 pixelated display device built using shadow mask patterning. Two cathodically coloring polymers which exhibit two distinct colors in their neutral states (blue and red) are patterned on a porous metallized electrode to yield a highly contrasted surface. Upon simultaneous oxidation of these polymers, the bi-color surface is rapidly bleached presenting a uniform shiny gold surface.