PerspectiveELECTRONICS

Toward Paperlike Displays

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Science  23 Feb 2001:
Vol. 291, Issue 5508, pp. 1502-1503
DOI: 10.1126/science.291.5508.1502

Low-cost electronic displays that look and feel like conventional printed paper will dramatically change the way we use and interact with laptop computers, personal digital assistants, and cellular telephones. They will also alter our notions of newspapers, magazines, greeting cards, and even cereal boxes, bumper stickers, and wallpaper.

Such “electronic paper” displays are radically different from traditional electronic systems, which rely on cathode ray vacuum tubes or liquid crystals with silicon-based circuitry on plates of glass. Electronic paper is a thin, high-contrast, reflective display that can be flexed, bent, rolled-up, and folded. A portable computer that uses this technology will resemble a pad of paper more closely than a standard laptop. Future printed paper products will retain the attractive appearance of conventional ink on paper but will be reconfigurable and reusable. A newspaper, for example, will consist of one or several sheets of electronic paper onto which information content, including animated images, will be downloaded through the wireless internet.

The technologies required for paperlike displays are just beginning to emerge from research laboratories in the form of realistic prototypes. The first sheets of electronic paper were recently demonstrated by Bell Labs and E Ink Corporation at the Fall 2000 meeting of the Materials Research Society (1). They incorporate the three new technologies that are essential for these types of systems: electronic “inks” for the optical component of the display, mechanically flexible transistors and circuits to drive the ink, and low-cost fabrication techniques to produce the circuits. The Bell Labs/E Ink displays use thin rubber-stamped plastic circuits and electrophoretic inks.

Two types of electronic inks are currently under development for commercial use in large-scale signs. One uses microencapsulated suspensions of charged white particles in a black fluid (2); the other relies on tiny rotatable balls that are white on one side and black on the other (3). Both are well-suited for electronic paper: They are thin (0.1 to 0.2 mm) and power efficient, they can support high-resolution images (more than 200 pixels per inch), and their contrast is better than newsprint.

To produce electronic paper displays, these “inks” must be laminated onto sheets of active matrix drive circuitry. These types of circuits use transistors at each pixel location to control the electric fields that determine the color of the “ink.” They must be mechanically flexible and should be built on thin, low-cost plastic substrates. Meeting these requirements requires materials and patterning techniques that are completely different from those currently used in the microelectronics industry. Most plastics, for example, cannot survive the high-temperature deposition steps that are common for conventional silicon-based circuits. Also, plastic sheets typically have surfaces that are rougher and more uneven than traditional silicon or glass substrates. These and other characteristics make them difficult to process in traditional ways.

Over the last several years, substantial progress has been made toward materials and patterning methods for flexible electronics on plastic. Several classes of semiconductors can now be deposited on plastics at low temperatures: inorganics formed from solution or cast from colloidal suspensions, hybrid inorganic-organic materials, small molecule organics, and even polymers (4). A few of them seem to be useful for electronic paper. They have recently been incorporated into small laboratory demonstrator circuits fabricated on plastic (5, 6) with low-cost fabrication methods, such as ink-jet printing and a high-resolution form of rubber stamping known as microcontact printing (7). A photochemical patterning process has also been developed for devices that use certain types of photosensitive plastics (8).

Our recent work combines organic semiconductors with rubber-stamped circuit elements on thin sheets of plastic to produce high-quality, large-area circuits for displays (9). A typical system consists of a square array of several hundred suitably interconnected organic transistors with micrometer feature sizes distributed over areas of 6 inches by 6 inches. The circuits incorporate five layers of material, patterned in registry with one another and processed entirely outside a clean-room environment. The compatibility of the stamping method with high-speed, continuous reel-to-reel printing approaches, the large area coverage, and the good performance of the transistors are all important features of these flexible circuits.

The figure shows a photograph of the Bell Labs electronic paper display and an artist's impression of the different components of this system. It uses rubber-stamped plastic circuits and microencapsulated electrophoretic inks. The entire device is less than 1 mm thick and weighs about 20% as much as a liquid crystal display of similar size. The exploded view (not to scale) illustrates the layout of a unit cell. Each pixel is associated with an organic transistor that acts as a voltage-regulated switch to control the color of the ink.

The nuts and bolts of electronic paper.

The exploded view shows the elements in a unit cell (not to scale). Arrays of rubber-stamped plastic transistors (inset on the left; blue, organic semiconductor; gold, source/drain electrodes; gray, gate electrode) control the color of a layer of microencapsulated electronic ink (inset on the right).

The ability of the stamping method to form micrometer-sized features on plastic substrates is critically important for this circuit. It enables the transistors to achieve the necessary switching speed, even with semiconductors that have modest electrical performance. Furthermore, it allows the same circuit design to be extended to high-resolution displays with large numbers of pixels. This scalability and the gradual emergence of other suitable materials and processing techniques point to a bright future for flexible electronic systems. These technologies will enable not only electronic paper displays and other applications that we can anticipate today (such as low-cost identification tags) but also completely new and unexpected devices that will change the way that we think about consumer electronics.

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