Commercializing Electrochromic-Display Technology

Advances in electrochromic technology using nanostructured materials have vastly improved the switching speeds, stability, and performance of display applications.

by David Corr

THE PHENOMENON of electro-chromism has long promised commercial display devices, but several barriers that revealed themselves in the course of device development have prevented this outcome. Principal among these barriers are the poor reversibility of electrochromic (EC) materials in devices and the slow switching response. Instability of the materials over long periods of time and in varied environmental conditions has also impeded prolific commercialization of EC technology within the display industry.

With recent developments in materials, mainly nanomaterials, and the successful commercial demonstration of EC technology in an automotive application, there is now new confidence that the performance and stability problems have been solved and the commercialization of EC-display devices is now possible for a wide range of applications. This possibility is particularly exciting because EC devices are intrinsically easier to manufacture than competing technologies, use fewer discrete components, and offer optical performance that is superior to that of low-cost twisted-nematic liquid-crystal displays (TN-LCDs). To make this technology even more attractive, EC-device manufacturing processes are largely convergent with LCD processes, implying that the initial capital infrastructure required for EC-device manufacturing is already in place within the LCD industry.

A Challenging History

Electrochromism belongs to the family of chromic phenomena that involve the selective absorption of light in the visible range. The stimulus by which an EC material changes color and absorbs visible light is electrical charge. Electrochromism is related to phenomena such as photochromism and thermo-chromism, which are color changes stimulated by light and heat, respectively. This puts electrochromism in a very different category from liquid crystals (LCs), which prevent the passage of light, and light-emitting diodes (LEDs), which emit different wavelengths of light.

Electrochromism has been studied for many years, and several materials exhibit this property. The principal materials used in commercial devices today are tungsten trioxide (WO₃)and compounds based on salts of 4,4′bipyridine [formula (1)], known as the viologens. The viologens have enjoyed significant commercial success, having been employed in millions of auto-dimming automotive mirrors each year.

Several features of the early technology, including stability, switching speed, switching behavior, and power consumption, limited the commercial application of electrochromics. A brief overview of the types of devices incorporating electrochromic materials will make it easier to understand the reasons for these limitations.

Tungsten trioxide, and many other electro-chromic metal oxides, exist in devices as thin films coated onto transparent conducting oxides such as indium tin oxide (ITO). These thin films of electrochromic material require the diffusion of small positive ions into their crystal structure, forming a material referred to as a bronze, which is colored (Fig. 1).

Fig. 1: Inserting small positive ions into tungsten trioxide (WO₃ ) crystals promotes a color change in the material through the formation of a colored complex.

Diffusing the ions into this structure is a slow process, measured, for small devices (less than a square centimeter), in the tens of seconds. This switching speed is clearly inadequate for most display applications. The reverse process of diffusing the ions out of the crystal is usually slower, contributing to an asymmetric switching profile that is also undesirable in display applications.

These devices do, however, show promise of temperature and ultraviolet-light stability, making them arguably more suitable than organic systems for smart-window architectural applications that may require lifetimes of up to 25 years. This goal has yet to be achieved, although some pilot systems have been deployed in niche applications. The metal-oxide systems are also very power efficient, with an ability to retain coloration for years without further charging of the system.

Other display systems, based on inorganic chemistry, involve the rapid deposition of metals from electrolyte solutions within the device. One application of these technologies, based on the deposition of bismuth metal, was attempted by Alpine Polyvision. These devices did not gain widespread commercial use, presumably because of the instability of the system or the narrow capabilities of the system in a commercial setting.

Electrochromic technologies employing organic molecules such as the viologens offer design flexibility, and this has been reflected in the attempted commercialization of several adaptations of viologen technology. The viologens are electron acceptors, and an intensely colored radical is formed when the material accepts electrons (Fig. 2).

Perhaps the best-known example of a high-information-content viologen display was thatreported by IBM in the early 1980s, which had an active-matrix driving scheme with a milli-second switching speed. IBM used a material known as heptyl-viologen, which precipitated from solution when it accepted charge and re-dissolved when it gave up the charge. This phenomenon is known to exhibit irreversible behavior after several million switches; perhaps the stability of the system did not satisfy the requirement for commercial display application.

The viologen systems that have been successfully commercialized are referred to in the industry as solution-based devices. The systems involve the charging of two parallel, spaced electrodes, the reduction (or acceptance of electrons) by solution-phase viologens at the cathode electrode, and the oxidation (or donation of electrons) by another solution-phase species to keep the chemistry in the cell properly balanced.

These solution-based devices have overcome the limitations of earlier systems and have met the strict requirements of the automotive industry, which include an operating-temperature range from –40 to +85°C, a 10-year lifetime, and a high resistance to ultraviolet radiation. Several million are produced each year for internal and external auto-dimming mirrors in automobiles.

The system has been researched for use in displays at Bayer AG, and several display-related patents arose from that work. But it seems that Bayer AG has decided not to commercialize the display-related technology itself. The company appears to have consolidated its electrochromic-patent portfolio and is attempting to license it.

Fig. 2: Viologen molecules become highly colored radicals when charged.

Fig. 3: A generic viologen molecule attached to nanostructured titanium dioxide (TiO₂ ) films (titania) represents the new wave of electrochromic materials. These molecules bind irreversibly to the electrode surface.

Fig. 4: The NanoChromics^™ display device fixes the electrochromic material to the front electrode. It can be driven by 1-V drivers, is bistable, and is highly compatible with existing TN-LCD manufacturing processes.

Increasing Switching Speeds

The limitation of solution-based viologen systems is slow switching speeds due to the excessively slow diffusion of solution-phase molecules toward the electrode in order to be charged. In addition, the architecture of even a simple direct-drive display is complicated by the fact that as they are being addressed, tracks to pixels would be intrinsically colored by viologens along the tracks. This could be overcome by insulating the areas around the pixel, a particularly difficult process step in displays with small feature sizes and comparatively dense pixel arrays.

The solution to slow switching speed arrived with the advent of high-surface-area electrodes utilizing nanomaterials to which many hundreds of viologen monolayers could be directly attached. This replaced the slow diffusion of viologen in solution, and also allowed the pixels to be patterned directly by printing the high-surface-area electrode material directly onto the pixel areas. Ironically, LCD technology – the dominant display technology that hindered electrochromism's early commercialization – has now developed manufacturing technologies that are being put to use in building electrochromic devices.

NanoChromics^™ Technology

The structure of a NanoChromics^™ electro-chromic device incorporates a nanocrystalline, nanoporous film (titania) as the coloring electrode (cathode) onto which a monolayer of electrochromic viologen molecules is anchored (Fig. 3). The difference between these molecules and earlier ones is that they are designed to bind irreversibly to the surface of the electrode.

The molecules exhibit high stability, with the ability to undergo a reversible color change over several million cycles. In a device, the electrochromic effect is mediated by the presence of the counter-electrodes (anodes), which are able to store charge. This endows the device with bistability and, consequently, low power consumption. The cell construction exhibits the classic parallel spaced-apart arrangement of LCD constructions that gives rise to much of the convergence in the manufacturing process (Fig. 4).

The display is operated in dc mode by the application of a negative potential of up to 1.0 V to the cathode, relative to the anode. Upon the application of this potential, the molecules are charged and change to a deep blue color. In open circuit, the separation of charges at the cathode and anode endow the display with a memory that typically persists over tens of hours. The anode material is overlaid with a diffuse white reflector that, in contrast with the deep blue color of the molecules in the colored state, give the device the most paper-like readability of any current display technology.

 NTERA, Ltd.

Fig. 5: This NanoChromics^™ display fabricated on a TN-LCD manufacturing line has a white reflectivity of 50%.

Fig. 6: A comparison of TN-LCD and NanoChromics^™ display manufacturing processes is shown. An Asian TN-LCD fabrication line has produced several hundred NanoChromics^™ displays according to this scheme. Cell-fabrication processes, such as cleaning, glass patterning, sealing, vacuum backfilling (with electrolyte solution), end sealing, and scribe-and-break, are carried out in exactly the same way as for TN-LCDs.

The fact that there exists no high-quality low-cost reflective display suitable for consumer goods is a primary driver of this project. The comparison of cost models between TN-LCDs and NanoChromics^™ displays leads us to believe that the NanoChromics^™ display has significant cost advantages. These advantages derive from the elimination of polarizers, alignment layers, and yield-sensitive process steps such as rubbing and gap control. NanoChromics^™ technology is not as sensitive to cell gap as LCD technology because all coloration of the device occurs in the nano-structured electrode.

Cost, however, is not the only benefit. The highly reflective device that results from the technology can provide manufacturers of display-centric products with the ability to differentiate them simply by the distinct look and feel of the new display (Fig. 5).

Convergent Manufacturing

NTERA, Ltd., the developer of NanoChromics^™ technology, has validated the manufacturing convergence of TN-LCD and NanoChromics^™ display technologies with a partner in Asia, using the partner's TN-LCD fabrication line (Fig. 6). The line has produced several hundred NanoChromics^™ displays, with cell-fabrication processes, such as cleaning, glass patterning, sealing, vacuum backfilling (with electrolyte solution), end sealing, and scribe-and-break, carried out in exactly the same way as that for TN-LCDs. The screen-printing deposition process of the nanocrystalline layers is well known in the LCD industry as the process by which patterned resists and polyimide alignment layers are deposited.

The post-deposition process, which requires a 450°C sintering step, led to the only additional capital expenditure – a batch oven capable of reaching this temperature. Two TN-LCD manufacturing steps that can be eliminated from the NanoChromics^™ display process are the patterning of the back electrode and the lamination of the cell with polarizers.

Another advantage is the simplicity of the driving scheme associated with NanoChromics^™ technology. The displays require only low-voltage (0.5–1.0 V) drivers and may, or may not, be driven utilizing the bistability of the display; either mode retains low power consumption because the display, once colored, requires no additional current flow to maintain the charge. Depending on the application, a range of off-the-shelf drivers already exist that are capable of driving these displays.

Multiplexing is difficult to implement in these displays and is not used in current products. More drivers are therefore required, but their low voltage and easy availability compensate for this. The drivers must be able to operate under high-Z (open circuit) conditions, but this is easily implemented for these displays by schemes adopted from other types of electronic drives.

Performance and Market Appeal

The optical performance of the displays has been measured at the British Standards Institute (BSI). The diffuse reflectivity exceeds 50% and contrast ratios have been measured above 15:1. The contrast ratio increases up to a viewing angle of 45° because the coloration of the device depends on the optical-path length through the front electrode. The contrast drops at larger viewing angles only because of internal reflections in the glass substrate, which may largely be overcome by the use of anti-glare materials. In addition to having a contrast that does not fall off at large viewing angles, the display's contrast ratio remains above 12:1 at illuminance levels of up to 60,000 lux, as verified at BSI.

The bright white background of Nano-Chromics^™ displays is unique among reflective-display technologies. It is proving attractive not only to makers of portable products that require a highly readable paper-like display for product differentiation, but also to white-product manufacturers who desire a truly white display to integrate into the white surfaces they envisage in modern domestic appliances.

The same characteristic, combined with low power consumption and readability in small sizes, is also attractive to retailers who are seeking a better technology for electronic shelf labels. Many retail giants are requiring that displays not be "gray" and therefore incompatible with their bright store designs and product presentations. •