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Illumination & Displays

Flexible electronic flat-panel displays find novel uses

Reflective displays can be used for the nontraditional application of switchable-color exterior cases for consumer-electronic devices such as cell phones.
30 December 2008, SPIE Newsroom. DOI: 10.1117/2.1200812.1385

Electronic flat-panel displays are used in more applications and markets than ever before. Common applications include laptops, cell phones, and televisions. The flat-panel-display industry is excited by the opportunity to gain further widespread use by developing flexible displays. In particular, there is a need for displays that are readable in sunlight, consume very low power, and have slim form factors. Those attributes are most interesting in the electronic-book, cell-phone, and general consumer-electronic-device markets. New markets not yet imagined will be created by the paradigm shift generated by flexible-display applications.

Numerous companies are developing flexible displays on the basis of different electrooptic approaches, such as liquid-crystal, electrophoretic, and electrowetting technologies. Typical flat-panel displays are difficult to make thin and flexible because the backlights used for illumination contain several films and components that—once stacked up—result in an inflexible form factor. The most frequently developed electrooptic technologies for flexible displays are therefore reflective.

We have developed flexible displays using the cholesteric liquid-crystal 1,2 technology, trademarked as Reflex. They have inherent reflective color and grayscale, and require no power to maintain a static image. The simple structure and inherent properties make a reflective flexible full-color display possible, which is also simple to manufacture compared with competing technologies. Reflex displays are based on bistable cholesteric liquid crystals that are encapsulated using polymerization-induced phase separation. Encapsulation of cholesteric materials prevents liquid-crystal flow during bending, curving, and flexing.3

One of the most compelling applications being developed is the Reflex electronic skin,4 which uses a stacked configuration of ultrathin substrates (see Figure 1) with a total thickness of about 60μm. Figure 2 shows a cross-sectional view of the electronic film. The electrode material is conducting polymer or PEDOT/PSS (poly(3,4)ethylenedioxythiophene/polystyrenesulfore acid), which is coated roll-to-roll on a web that is several thousand feet long. The electrodes are not patterned, thus simplifying manufacturing and decreasing cost. These skins can be applied to consumer-electronic devices such as digital cameras, mobile phones, and digital music players, allowing the consumer to enjoy personalization through switchable color. Surprisingly, the material can be cut to custom shapes and bent to tight radii. The colors can be selected via simple drive pulses. Since power is only used during a color change, the battery drain is extremely small. Figure 3 shows a conceptual mobile-phone demonstration device with an integrated electronic skin switched to various colors. Significantly, there are tight curvatures on the sides, a hole in the middle for a subdisplay, and a uniquely cut shape including inside corners.

Figure 1. Flexible cholesteric display using Reflex technology.

Figure 2. Cross-sectional view of electronic-skin display.

Figure 3. Demonstration device showing a conceptual phone using integrated electronic skin switched to several different colors.

Reflex displays are ideal for manufacturing roll-to-roll because of the simple yet elegant structure of two flexible substrates, a liquid-crystal/polymer dispersion, and a light-absorbing backcoat. This production process offers the promise of low cost and high volume because of a reduction in handling and continuous web transport of displays at a rapid rate.

The electronic skins are manufactured and developed for typical mobile-phone environmental specifications. The material is rugged enough to withstand a wide gamut of environmental tests including high heat, high humidity, and thermal cycling. The integrated skin has a hard clear cover so it is mechanically robust. The driving is performed through a simple application-specific integrated circuit with built-in driver and controller functionality. The simplicity of the integration and compact design allows customers to integrate the device with little effort.

Figure 4 shows the reflection spectra for a typical electronic skin showing red, green, blue, white, and black reflected colors. The reflectivity is measured using an integrating sphere that includes the specular component. The display has a high reflectivity because it consists of primary colors, each in a single stacked layer. The entire display area is thus used to reflect each color. In conventional LCDs the primary colors are spatially arranged in lines such that each primary color encompasses only one third of the display area. In addition, the area is not shared between an active part of the pixel and an inactive interpixel region as in conventional LCDs and other electronic-paper displays. Instead, these electronic skins have a 100% aperture ratio, making them ideally suited for color-reflective applications.

Figure 4. Reflection spectra for a typical electronic skin.

Because Reflex electronic-skin displays can be curved and cut into any shape—even holes can be cut through them—they can be used on any device (including cell phones, computers, and MP3 players) to uniformly change colors. The displays are attractive for numerous applications because of the low power consumption and paperlike reflective color. The electronic-skin displays are—for the first time—manufactured roll-to-roll, making future volume production possible. The optical properties, environmental ruggedness, and ability to conform the display in the final device are being improved by incorporating new materials and enhanced processes. As the possible applications for flexible, reflective displays grow, new manufacturing methods for these displays also become important.

We thank the large team at Kent Displays for their support.

Erica Montbach, Asad Khan
Technology Development
Kent Displays Inc.
Kent, OH

Erica Montbach is technology-development manager. She is responsible for flexible-display development and innovation. She holds a BA in physics from the College of Wooster, an MS in physics from Colorado State University, and a PhD in chemical physics from the Liquid Crystal Institute at Kent State University.

Asad Khan is vice president of technology development. He has a BA in physics from the College of Wooster, an MS in physics from Kent State University, and a PhD in chemical physics from the Liquid Crystal Institute at Kent State University.

Donald Davis 
Process Development
Kent Displays Inc.
Kent, OH

Donald Davis is process-development manager responsible for identifying and developing processes specific to flexible displays manufactured in a roll-to-roll process. He has BA and MS degrees in physics from Thiel College and Oklahoma State University, respectively.

Nick Miller
Product Development
Kent Displays Inc.
Kent, OH

Nick Miller is the product-development manager responsible for developing and overseeing the electronic-skin product line. He has a BS in electrical engineering from Akron University.

J. William Doane
Chief technology officer
Kent Displays Inc.
Kent, OH

J. William Doane is the cofounder of Kent Displays. He is director emeritus of the Liquid Crystal Institute at Kent State University.