FLEXIBLE DISPLAYS: Roll Up And Carry

Display screens so light and flexible that you could roll them up and carry them anywhere! This is no science fiction. Flexible displays are close to reality. With companies like Kodak, Philips, Toshiba, Sony, DuPont, Bell Labs, E Ink, Universal, FlexICs, Linkoping University and Volkswagen successfully demonstrating prototypes or concepts of flexible displays, commercial application of this technology is expected by 2007.

Flexible Displays (FD) were first envisioned many decades ago, but their demonstration started quite recently. Companies have been exploring various substrates (prime materials) like glass, plastic and metal that can be efficiently used to make FDs rugged, light and thin. 

HOW AN FD WORKS 

In a flexible LCD, the glass substrate is replaced by a polymer substrate. This increases its flexibility, so that besides the conventional flat positioning of displays, its curved arrangement becomes possible. Additionally, the use of polymer substrates reduces the weight and gives increased fracture resistance, particularly important for applications in mobile products. Another feature of these FDs is their brightness and reflectivity. The first feature enables the fabrication of displays, which preserve the displayed information in the switched-off state. With the second feature, the necessity for back light is omitted and the device can be read out under ambient light conditions. To make FDs work, there must be flexible substrates and
backplanes.

Substances like flexible glass, plastic as a substrate for displays and circuits, paper as a medium for lighting, cloth as a host circuit and cloth as a lighting medium, are being investigated to produce the best
FDs. 

APPLICATIONS OF AN FD 

Researchers are impressed about the striking appearance and the thin profile that can be achieved using various FD technologies. The thin profile designed for appealing products based on FDs can be used in cell phones, PDAs, audio systems, wearable labels, badges, video games and very large screens. 

FDs can even be used in low-cost applications sucg as electronic labels in the transport industry (baggage-tags and parcel-tags) or as price-tags. The display information can be switched and read-out via transponder techniques. Other applications include electrophoretic dynamic signage, passive LCD curved watch display, ebooks, epaper and roll-up screens. 

Some of the display technologies currently being explored in the development of flexible displays are LCD, OLED, Electrophoretic and Electrochromic. Of these, OLED seems to have made a headstart in the flexible display research; most companies have used OLED-based displays. 

WHY IS OLED POPULAR 

Scientists, researchers and major companies believe that OLED is the ‘Next Big Thing’ in display technology. Some even predict that OLEDs will eventually replace active-matrix LCD panels. 

An OLED is an electronic device made by placing a series of organic thin films between two conductors. When electrical current is applied, a bright light is emitted. This process is called electrophosphorescenc. Even with the layered system, these systems are very thin, usually less than 500 nm (0.5 thousandths of a millimeter). 

OLED technology produces self-luminous displays that do not require backlighting. These properties result in thin and very compact displays, that have a wide viewing angle, up to 160 degrees. Also, they require very little power–only 2-10 volts. 

OLED displays have other advantages over LCDs as well–increased brightness, faster response time for full motion video, lighter weight, greater durability and broader operating temperature ranges. They come in two flavors–passive or active matrix. The former is targeted at high-resolution displays, whereas the latter is targeted at ‘low-information’ displays. 

More than 50 companies developing OLEDs include Philips, Cambridge Display Technology, Universal Display Corporation, Dupont Displays - Uniax Polymer Electronics, Siemens and Kodak have demonstrated products and have setups to deliver full OLED-based system solutions. 

DEVELOPMENT ISSUES IN FDs 

There are many issues in the development of commercial viable flexible display. However, the rewards of FDs can be as large as the success seen in traditional electronics today. Auxiliary barrier, optical layers, manufacturing process, choosing a flexible substrate: light weight and thin, toughness, curved display one-time bend, design freedom, flexible or roll-up displays are some of the problems that are currently being worked upon by researchers. 

NELSON JOHNY

OLED EXPLAINED

The world´s first matrix display that keeps functioning while being bent. It uses a bistable liquid crystal effect and a polymer-base substrate. Photo courtesy: Philips Research

Measuring the electro-optical properties of a flexible LCD. Photo courtesy: Philips Research

What is an OLED?

Organic Light Emitting Displays (OLED) is a new technology that uses organic materials to produce light. When thin layers of this material are sandwiched between appropriate anode and cathode layers, a relatively modest voltage (typically 2 - 10 Volts) applied across the material will cause it to emit light in a process called
electroluminescence.

What is an OLED display?

An OLED display is a device made up of multiple OLED junctions, arranged in a matrix that allows each junction to be addressed individually.

What are the different types of OLEDs?

There are two essential types of OLEDs: small molecule OLEDs, (SMOLED), and Polymer OLEDs
(poly-OLED).

SMOLEDs use organic materials with a relatively small molecular structure, which must be applied using sophisticated vacuum vapor deposition techniques. Poly-OLEDs use organic materials with much larger molecular structures, which allows simpler spin coating or printing techniques to be applied the organic layer.

How are OLEDs different from LCDs?

OLEDs are emissive displays; they generate their own light. LCD´s are passive displays, and simply transmit or block an external light source to form an image.

Why is an emissive OLED display better?

Emissive OLEDs displays do not need ambient light or a backlight. This eliminates the cost, space, weight and power consumption of a backlighting system. These displays offer a much wider viewing angle of 160 degrees.

What colors can OLEDs produce?

For single color displays, it comes in many colors including the primaries–red, green, blue. For full color displays, a matrix made up of tiny triads of OLED junctions, with red, green and blue pixels, is used to produce thousands of different colors by varying the intensity of these three primary colors.

Does this work in the same way as a color LCD monitor?

Partially. Color LCDs take white light, and pass it through a color filter to create the red, green and blue pixels. The LCD pixels simply control how much of each of these colors is allowed to pass through. Unfortunately, the LCD pixel and color filter absorbs a significant amount of light in this process.

SOURCE: DuPont