Electrochromic Displays

Abstract

Electrochromic displays (ECDs) have been the subject of extensive research due to their excellent potential for application in novel paperlike display devices, the so-called e-paper. Although ECDs have been studied since the 1980s, a commercially available full-color ECD has not been realized to date. EC materials exhibiting clear color changes from colorless to the three primary colors (red, green, and blue or cyan, magenta, and yellow) are required for full-color e-paper imaging devices. On the basis of this concept, studies of electrochromism (EC) to realize full-color reflective display are gradually increasing. This chapter is a brief review of electrochromic materials and displays toward full-color reflective display. This chapter also includes mechanism, characteristics, and recent progress of various EC materials and future applications in addition to display to clarify the advantages of phthalate derivative-based ECDs.

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Introduction

Phenomena in which reversible color tone changes are induced by a stimulus are referred to as “chromism” and have been long known. In recent years, “chromogenics,” defined as various reversible optical changes including color, scattering, and reflection, has been recognized as a distinct technical field. Chromism includes a variety of phenomena that are named on the basis of the relevant stimulus: “photochromism” induced by photon stimulus, “electrochromism” by electric energy, “thermochromism” by heat, “solvatochromism” by solvent, and so on. Widely known examples of photochromism include the photoisomerization of azobenzene, spiropyran, and diarylethene derivatives and the photochemical reaction of rhodopsin in retinal tissue and sunglasses that change optical density in response to light intensity; photochromism is also expected to find application in ultrahigh-density memory, photoresponsive devices, and so on. Thermochromism is utilized in the well-known card thermometer with black background which employs a cholesteric liquid crystal and the erasable gel pen (Pilot Frixion Ball^TM) which employs thermosensitive reaction of leuco dyes and developer. Solvatochromism is widely utilized as a pH indicator. These examples demonstrate that many chromic phenomena are utilized in our everyday activities.

Although electrochromism (EC) refers strictly to a reversible optical change induced by electric energy as described above, it is more generally defined as a reversible color change induced by an electrochemical redox reaction. This color change is based on a change in the electronic state of a material caused by electron transfer between the material and an electrode. The EC phenomenon is similar to the discharge and charge of a secondary battery, and thus, EC is classified in a unique group different from electric field systems such as liquid crystal devices and electrophoresis. Tungsten oxide has been long recognized to exhibit EC, and an electrochemical color change in tungsten oxide powder was reported in the 1930s. Recently, many EC materials have emerged, such as inorganic, organic, and conducting polymer materials. These materials offer many advantages including multicolor, low operation voltage, memory effects, and so on. With the development of these materials, EC was expected to be applied to an imaging device called an electrochromic display (ECD) from as early as the 1980s.

The ECD is a light-receiving and reflective display. Unlike light-emitting displays such as the transmissive liquid crystal display (LCD), organic light-emitting diode (OLED), and plasma display panel (PDP), the ECD offers high visibility under daylight conditions and a wide view angle and can reduce eye strain in the case of continuous use. Consequently, the ECD was expected to be applied to novel display systems. In fact, ECDs such as a 7-segment clock and an information board for displaying stock prices were test-marketed in the 1980s. However, production of these items was discontinued because of the slow response time of the EC reaction, which occurs with diffusion of substances (ions). Subsequently, liquid crystal displays studied in the same period emerged for quick response displays such as TV and PC monitors.

Because of these factors, EC was applied to devices that do not necessitate a quick response time. In recent years, the use of natural energy has attracted attention from the perspective of environmental concerns, and the application of EC to a passive solar system for controlling the transmission of and for utilizing solar energy became an active area of focus. The passive solar EC system was expected to be applied to “curtainless” smart windows (Rosseinsky and Mortimer 2001; Granqvist et al. 2007), which control the amount of transmitted sunlight and increase air-conditioning efficiency. Smart windows have in fact already been put into practical use in Europe and the United States (Fig. 1a). Particularly, light-modulating window which is called dimmable window was installed in Boeing 787 Dreamliner. Another universal application of EC for light control systems is an antiglare mirror for automobiles (Fig. 1b); these are also already available on the market for use in motor vehicles.

Fig. 1

Electrochromic applications: (a) dimmable and smart windows, (b) antiglare devices

Nevertheless, it is still expected that EC will be applied to information displays. The slow response of EC previously precluded the development of EC-based information displays. However, in recent times, EC has reemerged as a strong candidate for paperlike reflective displays, such as an e-paper, which require the memory property without the need for quick response. The demand for novel paperlike information display media has reinitiated the study of EC for such display uses.

Structure of Electrochromic Display

Generally, an EC cell is constructed by sandwiching an electrolyte solution between a pair of electrodes, similar to a battery (Fig. 2). At least one of the electrodes is transparent, allowing observation of the color change of the cell. EC materials are dissolved in the electrolyte solution or attached as modifications to the electrode surface as a coloration layer. Although solutions, inorganic solids, polymers, gels, and ionic liquids have been used as the electrolyte layer, high-ion conductivity materials are desired, as is the case with a battery.

Fig. 2

The typical structure and coloration mechanism of EC cell (cathodic coloration cell)

The EC cell is a two-electrode cell composed of an anode and a cathode. The electrochemical reduction of a reducible material on one electrode should be accompanied by electrochemical oxidation of an oxidizable material on the other electrode with an equivalent amount of charge. Therefore, the EC cell requires a counter material that compensates the charge consumed at the coloration electrode on the counter electrode. Since EC is current-driven system like OLED, energy-saving ability is often debated in comparison with voltage-driven electrophoretic and liquid crystal system. Even in ECD, consumed energy increases with increasing a number of writing and erasing processes. However, the EC with sufficient memory properties does not require energy to keep image (just like to keep charged state in a secondary battery), leading a possibility to show very low energy consumption rather than voltage-driven system if the number of writing and erasing is not so frequent. Therefore, consumed energy depends on the number of writing and erasing processes. EC (coloration) efficiency is commonly used as a measure of energy consumption when its value is compared with other EC values. EC efficiency is defined as the change in optical absorption per injected charge amount (the so-called change in absorbance in coloration per injected charge density). Since charges required for writing in EC (mC/cm²) are about 1,000 times larger than that of LCD (μC/cm²), design of EC materials, counter materials, and device structure with and for high EC efficiency is considerably important to decrease energy consumption. The EC efficiency of EC cells is improved by employing suitable counter electrode systems that undergo electrochemical reactions that compensate the charge consumed at the coloration electrode. In the event that the counter material exhibits a color change associated with the electrochemical reaction, it may interfere with the color of the coloration electrode, and a white scattering/reflective layer must be introduced, especially in the case of a reflective display such as e-paper. On the other hand, the counter electrode does not need to exhibit an electrochemical reaction for the charge compensation. For example, an electrochemical capacitor can accumulate the charge as space charge by using an electric double layer. In this way, many kinds of counter electrode systems become available, and an effective combination of the coloration electrode system and the counter electrode system is important for improving the characteristics of the EC cell.

EC Materials

Inorganic EC Materials

One typical inorganic EC material is tungsten oxide (WO₃) (Granqvist 1995). An EC cell employing WO₃ thin film was reported by S. K. Deb (1973) in 1969. The redox process of WO₃ can be expressed in the following electrochemical reaction:

$$ {\mathrm{WO}}_3\left(\mathrm{clear}\ \mathrm{and}\ \mathrm{colorless}\right) + {\mathrm{x}\mathrm{M}}^{+} + {\mathrm{x}\mathrm{e}}^{-}\iff {\mathrm{M}}_{\mathrm{x}}{\mathrm{WO}}_3\left(\mathrm{blue}\right) $$

When the WO₃ layer modified onto an electrode receives an electron from the electrode, a cation in the electrolyte is introduced into the WO₃ layer for charge compensation (Fig. 3). Specifically, the WO₃ crystal has a perovskite structure comprised of W⁶⁺ and O²⁻. A certain amount of W⁶⁺ is converted to W⁵⁺ by electrochemical reduction. WO₃ comprising the mixed valence states of W⁶⁺ and W⁵⁺ is strongly colored by an intervalence transfer band. Cations having a small ionic radius (H⁺, Li⁺, etc.) can be introduced into the crystal lattice of WO₃ for charge compensation. Based on the film-forming method employed, quick response times of less than 100 ms can be achieved in the WO₃ film, along with long-term switching stability exceeding 10 (Hagfeldt et al. 1994) cycles.

Fig. 3

Mechanism of coloration of WO₃-based EC material

Many other inorganic substances (Monk 2007) such as Prussian blue (PB), NiO, Ir(OH)_x, V₂O₅, and CeO₂ are also known to exhibit EC. These inorganic substances have already been applied to optical shutters, antiglare mirrors, and smart windows based on their long-term switching stability and absorption in the infrared region. Recently, studies on PB are gradually increased from a viewpoint of color display because PB is known to expend its color variation by changing central metal ions. However, in the case of e-paper requiring low energy consumption and flexibility, inorganic substances may be disadvantageous. Depending on the film-forming method and cell structure, the coloration efficiencies of inorganic substances are within the range of 20–100 cm²/C. EC efficiency is defined as the change in absorbance in coloration per injected charge density. These values are lower than those of organic EC materials and conducting polymers. Low energy consumption driving can be achieved by the development of high EC efficiency inorganic EC materials and improvement of the cell structure.

Organic Electrochromic Materials

As previously described, inorganic EC materials including metal oxide are suitable for binary displays such as blue and transparent or white presentations. However, in the case of color displays, organic EC materials offer a number of advantages. Recently, increasing research efforts have been dedicated to the EC of organic compounds such as dyes, metal complexes, and conductive polymers from the viewpoint of multicolor and coloration efficiency. Examples of the application of organic EC materials to increase color palette leading to full-color e-paper are emerging. Compared with inorganic oxide systems, organic EC materials offer the advantages of color variety and EC efficiency. Organic EC materials present various colors including the three primary colors (such as red, green, and blue (RGB) or cyan, magenta, and yellow (CMY)) and colors intermediate between these. Organic EC systems, in which various colors may be derived from single materials and multicolor may be achieved by stacking the cells, are anticipated to be strong candidates for satisfying the rising demand for e-paper such as in the case of black and white, multicolor, and full-color representations.

Viologen derivatives, N,N′-dialkylated di-cations of 4,4′-bipyridine, exhibit various colorations upon variation of the quaternization agent. The viologen derivatives and analogous molecules are known as EC materials (Monk 1998). These organic materials show high EC efficiency compared to inorganic EC materials. The di-cationic derivatives of viologen can be dissolved in water and organic solvents depending on the molecular structure. The color change of viologen derivatives is induced by electrochemical reduction, and the reduced derivatives are deposited on the cathode as a result of the change in the solubility on turning from the di-cationic state to the mono-cationic state. However, like many other organic EC materials simply dissolved in electrolyte solution, viologen derivatives do not exhibit quick response in common EC systems because the diffusion of the molecules and charge transfer between molecules are kinetically limited. In addition, viologen derivatives suffer from the disadvantage of bleaching because the colored states of the viologen derivatives sometimes crystallize and detach from the electrode as the colored precipitates.

To obviate the disadvantages of the viologen system, viologen derivatives were adsorbed onto the surface of titanium dioxide (TiO₂) nanocrystals modified on the electrode (Hagfeldt et al. 1994). TiO₂ was prepared on the transparent electrode as a thin film of a few micrometers in thickness by using the same method employed in the dye-sensitized solar cell. This TiO₂ thin film is porous, and its effective surface area is more than a thousand times the area of the geometrical electrode surface. Therefore, the viologen derivative adsorbed onto the TiO₂ surface (Fig. 4) has a high surface concentration. An experimental EC device constructed with a viologen derivative adsorbed onto TiO₂ nanocrystal electrode exhibited long-term switching stability, more than several thousand times in −1.3 V driving. The charge transfer between TiO₂ and viologen is more efficient than that of the unmodified transparent electrode system; thus, the viologen-modified TiO₂ nanocrystal system demonstrated quick response and high EC efficiency. In fact, the EC efficiency of the TiO₂ nanocrystal system was about 20 times higher than that of the systems without the TiO₂ nanocrystal. The viologen-modified TiO₂ nanocrystal system achieved a quick response time of several milliseconds due to the high mobility of charge compensation ions within the porous electrode, in addition to the high electron transfer rate between viologen and TiO₂.

Fig. 4

Schematic representation of viologen derivative-modified TiO₂ nanocrystal electrode

Recently, this system has been widely studied and has been applied to the almost successful development of electronic paper, such as an electronic book using the viologen system. NTERA has already developed an ECD termed NanoChromics^TM using the viologen-modified TiO₂ nanocrystal system (Fig. 5). For full-fledged application to electronic paper, prototype EC devices of the TiO₂ nanocrystal system fabricated by the screen printing method were also reported. Thus, the TiO₂ nanocrystal system has potential for the further development of ECDs.

Fig. 5

NanoChromics^TM displays developed by NTERA

Examples aiming for the application of full-color e-paper have also been reported by the combination of three-primary-color EC. Since e-paper should be a reflective display, the three primary colors cyan, magenta, and yellow (CMY) are suitable for full-color representation of e-paper as in the case of photography and printing. On this basis, the electrochemical properties of phthalate derivatives were evaluated. The three primary colors (cyan, magenta, and yellow) were electrochemically obtained in the ITO sandwich cell using diacetyl benzene, dimethyl terephthalate, and biphenyl dicarboxylic acid diethyl ester, respectively (Fig. 6). Each of the colors obtained with the various derivatives is regarded as one of the three primary colors based on the CIE 1931 colorimetric measurement (Urano et al. 2004). It was revealed that the anion radical of each derivative generated at the cathode exhibited each of the three primary colors (in the case of dimethyl terephthalate, monomeric structure of polyethylene terephthalate, PET, shows magenta color) and that the coloration was affected by the supporting electrolyte and solvent. Red, green, blue, and black colors have also been achieved by stacking two- or three-primary-color EC cells (Kobayashi et al. 2008). Figure 7 displays a photograph of the three-layered ECD. The color of this three-layered ECD was based on the subtractive color mixing process. Blue, red, and green pixels were obtained by Blue, red, and green pixels were obtained by M + C, Y + M, and Y + C combination, respectively. The black color of this three-layered ECD was not adequate because the CMY subtractive color mixture process cannot fully represent a chromatic black. Therefore, a full-color reflective display based on the present system requires four layers (cyan, magenta, yellow, and black) as is the case with color printing. It was anticipated on the basis of the abovementioned facts that the phthalate derivative-based EC cell should be a candidate for a multi- or full-color e-paper.

Fig. 6

Digital camera images of colored state of terephthalate derivative-based ECD. Colorimetric analysis of its colored states with CIE 1931 Yxy color diagram

Fig. 7

Digital camera image of the three-layered ECD with an 8 × 8 pixel resolution and passive matrix drive

The feasibility of fabricating flexible EC cells has been demonstrated using an ITO-coated flexible plastic electrode and a gel electrolyte. The flexible EC cell exhibited a clear color change, as is the case with the glass electrode EC cell.

For the practical application, cycle stability of the phthalate derivative-based sandwich EC cell is very important. In the case of a two-electrode sandwich cell, a counter material is necessary to improve the cycle stability of the EC cell. Several counter materials such as ferrocene, carbon, nickel oxide, and so on in the phthalate-based EC cell were examined to improve cycle stability. Carbon is not electroactive but is effective to form reversible electric double layer by ion adsorption and desorption on its surface. Therefore, it works as a capacitive layer to hold charges consumed at working electrode. Particularly, nickel oxide was reported to work as an excellent counter material in phthalate-based EC cell. Cycle stability after over 5,000 coloring and bleaching cycles was reported (Watanabe et al. 2011). This clearly indicates that even organic phthalate derivatives are not so weak in an EC cell by adopting suitable counter material and charge compensation system.

The memory property is also an important factor for e-paper application to reduce energy consumption and to realize continuous tone (gray scale) representation. If EC materials show excellent memory properties, it is easy to realize continuous tone because the EC material keeps a certain absorbance (coloration) without power supply, depending on the injected charges, as is the case with a variation of charging level in a secondary battery. However, the memory properties of phthalate derivatives are inefficient even in open-circuit state of the EC cell. Application of continuous DC voltage with different magnitude to the EC cell can realize continuous tone, but this results in the consumption of large amount of energy and a possibility of decomposition of the EC materials and cell. Therefore, continuous tone representation was realized in phthalate-based EC cell by applying a suitable frequency rectangular wave voltage with various duty ratios, instead of a DC voltage. According to a subtractive color mixture process, multiple colors including intermediate colors between cyan, magenta, and yellow were successfully demonstrated by stacking EC cells with different tones (Fig. 8; Watanabe et al. 2012).

Fig. 8

Color representation of stacked phthalate-based EC cells, two of which with different tones are stacked together. The coloration area of the EC cells was 1.0 × 1.0 cm²

For full-color e-paper application of EC, innovative progress was made in 2011 (Yashiro et al. 2011). Multilayered EC consisting of one display unit was developed, and TiO₂-modified electrode system and three-primary-color organic dye-based EC were combined to realize full-color ECD. The white reflectivity was 70 % at 550 nm, and multilayered ECD showed 27 % of color reproducibility compared with the standard color chart. Active matrix multilayered ECD panel using 3.5″ QVGA LTPS-TFT successfully expressed full-color image as shown in Fig. 9. A possibility of flexible multilayered ECD was also pointed out. This strongly supports that EC can be a technology for the real full-color e-paper.

Fig. 9

Demonstrated image of active matrix 3.5″ QVGA LTPS-TFT multilayered ECD panel (Yashiro et al. 2011)

Conducting Polymers

The use of conducting polymers has been a focal prospect for film formation, multicolor property, and EC efficiency. The coloration efficiencies of the well-known conducting polymers, poly(bi-thiophene) and poly(3-methylthiophene), are 100 cm²/C and 240 cm²/C, respectively. The potential of poly(3,4-alkylenedioxythiophene) molecules, typified by poly(3,4-ethylenedioxythiophene) (PEDOT), and copolymers of the monomers of these molecules with other conjugate monomers has been recognized in conducting polymer systems in recent years. PEDOT possesses an ethylenedioxy group in positions 3 and 4 of the thiophene ring. Improved conductivity and redox switching stability have been achieved with PEDOT, derived from high electron density and reduced distortion of the thiophene ring due to the substitutional effect of the ethylenedioxy groups. Various colorations, including RGB, may be accessed using the copolymers of the alkylenedioxythiophene system by judicious variation of the combination of monomers. Certain copolymers of this category exhibit coloration efficiencies higher than 1,000 cm²/C (Gaupp et al. 2002). In addition, conducting polymers exhibit quick response and good memory property and, thus, may be advantageous for electric power saving. Recent remarkable progress in EC of conducting polymers such as color variation, quick response, and so on is reported by academic scientists (Bulloch et al. 2014; Liu et al. 2011; Beaujuge and Reynolds 2010; Amb et al. 2011) On the other hand, the conductivity and narrow band gap of conducting polymers are debated to the realization of the fully transparent state suitable for e-paper and display. Generally, transparent polymers are insulating, and most conducting polymers show some color in certain of their neutral and redox states. Therefore, conducting polymers may be disadvantageous to realize clear full-color representation.

Electrodeposition (Electroplating)

Electrodeposition can be described as an aspect of EC. A well-known electrodeposition process is the electroreduction of metal ions, commonly known as metal plating . The Zn electrodeposition system, in which electroreduction of Zn²⁺ is performed in solution, was reported in the 1920s. Subsequently, black and white presentation by electrodeposition of Bi was reported in 1995. The Bi electrodeposition system exhibits good contrast because colorless Bi³⁺ accentuates the white color of the white reflector. The EC efficiency for this Bi electrodeposition system is 75 cm² C⁻¹ at 700 nm, with high white and black contrast ratio of 25:1 because colorless Bi³⁺ ions do not dull the color of white coating layer. It was reported that addition of Cu²⁺ to this system improved the reversibility of the electrodeposition and elution processes. A cell has been fabricated by sandwiching an electrolyte containing Bi³⁺ and Cu²⁺ between an ITO working electrode and a polyethylene/carbon counter electrode. Although degradation of the ITO electrode occurred with the use of the aqueous solution system due to the low pH of 1.5, degradation could be reduced by adjustment of the driving voltage and driving mode. Interest in the Bi system is still active; 7 segment Bi-electrodeposition-based ECD was reported in 1999 (Ziegler 1999), and good long-term switching stability of Bi in a nonaqueous solvent system utilizing the EC of Bi was independently demonstrated (Imamura et al. 2009) Interestingly, a combination of the Bi system and cellulose was reported to realize paperlike feeling in the display (Nakashima et al. 2010).

Electrodeposition of silver ion (Ag⁺) has also attracted many interests and has been reported in 1962. An Ag deposition-based EC display was also demonstrated. In 2002, a black and white presentation cell utilizing the electroreduction of silver ions was proposed with the aim to realize black and white color e-paper (Fig. 10; Shinozaki 2002). This cell was fabricated by sandwiching a white-colored electrolyte layer consisting of TiO₂ and a gel electrolyte containing a silver salt between a pair of electrodes. Reflectance of the white color based on TiO₂ was higher than 70 % because the silver ion is colorless, and the contrast ratio was over 20:1. This cell exhibited a low driving voltage of less than 3.0 V and a quick response of less than 100 ms. Improvement of the cell stability was attributed to the use of an ion-conducting polymer as the electrolyte.

Fig. 10

(a) Structure of Ag⁺ electrodeposition-based EC display. (b) Passive matrix display (50 dpi, 160 × 120 pixels). Each square in the upper image consists of 10 × 10 pixels (Shinozaki 2002)

Interesting results were reported in Ag electrodeposition in 2012 (Araki et al. 2012). Although black color was generally generated by the electrodeposition of silver ions, an EC device based on silver electrodeposition that achieved three reversible optical changes (transparent, silver mirror, and black) in a single cell was successfully demonstrated (Fig. 11). The driving principle of this EC device is the exploitation of Ag nanoparticle deposition on two different transparent electrodes: a flat indium tin oxide (ITO) electrode and a rough ITO particle-modified electrode. The EC material, consisting of a gel electrolyte in which Ag⁺ is dissolved, is sandwiched between the two electrodes. The default state of this device is transparent, whereas by applying a negative voltage to either one of the electrodes Ag is electrodeposited on the electrode surface. Following Ag deposition on the flat ITO electrode, the device becomes mirrored. Conversely, when Ag deposition occurs on the ITO particle-modified electrode, which has a rough surface, the device turns black. This can be applicable to smart window systems as well as displays.

Fig. 11

Digital camera images of Ag electrodeposition-based EC cell. Upper left, side view: mirror state (−2.5 V application), transparent state (before voltage application), black state (+2.5 V application)

Although progress toward multichromatic representation in full-color EC displays has been reported as described above, control of the multichromatic state using inorganic EC devices has rarely been reported. The optical state related to its color of the metal electrodeposit on the electrode depends on the metal particle size, shape, and coalescence between particles. If the size and shape of the Ag nanoparticles electrodeposited can be controlled uniformly and homogeneously, localized surface plasmon resonance (LSPR) band of the Ag nanoparticles can be used as a tool for controlling multiple chromatic states, because Ag nanoparticles is known to show various colors based on their LSPR. On this basis, by changing voltage application method to Ag electrodeposition-based EC cell, a reversible multicolor changing phenomenon, from transparent to magenta, cyan, and yellow in addition to mirror and black states, based on controlling the size of Ag nanoparticles during electrochemical deposition, was successfully demonstrated (Fig. 12; Tsuboi et al. 2013a, b, 2014). The multifunctionality of this LSPR-based EC display device could make it suitable for use in information displays and light-modulating devices such as electronic paper, digital signage, and smart windows.

Fig. 12

Digital camera images of Ag electrodeposition-based EC cell exhibiting BCMY and mirror states (6 optical states). The cell structure is a flat ITO electrode/Ag-based electrolyte/ITO particle-modified electrode. Constant voltage application to flat ITO electrode (mirror) and to ITO particle-modified electrode (black) and voltage-step application to flat ITO electrode (magenta to cyan) and to ITO particle-modified electrode (yellow)

Summary

This chapter briefly reviews electrochromic display technology including cell structure, materials, and recent progress. The application of EC was mainly focused on light-modulating system such as dimming window and car mirror although display application was reported over 25 years ago. However, growing interests to full-color e-paper and energy-saving reflective display encourage research of EC and extend its possibility. Other several interesting materials and phenomena, rather than display application, which are not covered in this chapter, are also emerging. These activate research field and are believed to have a potential to generate unexpected attractive applications in the near future. We wish this chapter will be of some help to awaken interest for further investigations and applications of EC.