Organic light-emitting diode

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One of the more useful attributes of an OLED display is its flexibility.

An organic light-emitting diode (OLED) is a thin-film light-emitting diode (LED) in which the emissive layer is an organic compound. These devices promise to be much cheaper to fabricate than inorganic LEDs. When the emissive layer is polymeric, varying amounts of OLEDs can be deposited in arrays on a screen using simple "printing" methods to create a graphical colour display, for use as television screens, computer displays, portable system screens, and in advertising and information board applications. OLED may also be used in lighting devices. OLEDs are available as distributed sources while the inorganic LEDs are point sources of light. Prior to standardization, OLED technology was also referred to as OEL or Organic Electro-Luminescence.

One of the great benefits of an OLED display over the traditional LCD displays found in computer displays is that OLED displays don't require a backlight to function. This means that they draw far less power and they can be used with small portable devices which have mostly been using monochrome low-resolution displays to conserve power. This will also mean that they will be able to last for long periods of time with the same amount of battery charge.

The world's first digital camera with an OLED display was the Kodak LS633 model revealed at the Photo Marketing Association (PMA) trade show in March 2003.

Contents

1 Two main directions

2 How OLEDs work

3 Advantages

4 Disadvantages

5 Long Term Commercial Potential

6 References

7 See also

8 External links

Two main directions

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The largest OLED display prototype as of March 2005, at 21 inches. Compare with below.

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The largest OLED display prototype as of May 2005, at 42 inches. Compare with above.

There are two main directions in OLED, Small Molecules and Polymers.

The first technology was developed by Eastman-Kodak and is usually referred to as "small-molecule" OLED. The production of Small-molecule displays requires vacuum deposition which makes the production process expensive and not so flexible. The term OLED traditionally referrs to this type of device, though some are using the term SM-OLED.

A second technology, developed by Cambridge Display Technologies or CDT, is called LEP or Light-Emitting Polymer, though these devices are better known as Polymer Light Emitting Devices (PLEDs). Although this technology lags the Small-Molecule development by several years (primarily in efficiency and lifetime), it is more promising because of an easier production technique. No vacuum is required, and the emissive materials can be applied on the substrate by a technique derived from commercial inkjet printing. This means that PLED displays can be made in a very flexible and cheap way.

Recently a third hybrid light emitting layer has been developed that uses nonconductive polymers doped with light-emitting, conductive molecules. The polymer is used for its production and mechanical advantages without worrying about optical properties. The small molecules then emit the light and have the same longevity that they have in the Small-Molecule OLEDs.

How OLEDs work

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OLEDs exhibit contrast ratios and brightness levels comparable to CRT and SED display technologies.

OLEDs work on the principle of Electroluminescence. The key to the operation of an OLED is an organic lumophore. Excitons, which consist of a bound, excited electron and hole pairs, (empty state) are generated inside the emissive layer. When the exciton's electron and hole combine, a photon can be emitted. A major challenge in OLEDs is tuning the devices such that holes and electrons meet in the emissive layer in equal quantities. This is difficult because the mobilities of holes are typically much higher than that of electrons in organic compounds. Light emission can only occur when singlet excitons form in the emissive because the materials currently employed are typically fluorophors and cannot emit light from a triplet state. This is a problem because only one in four excitons is a singlet. By incorporating transition metals into small-molecule OLEDs, the triplet and singlet states can be mixed by spin-orbit coupling, which leads to emission from both states, but triplet emission is always red-shifted from the corresponding singlet emission, thus blue light is nearly impossible to achieve from a triplet excited state.

To create the excitons, a thin film of the lumophor is sandwiched between electrodes of differing work functions. Electrons are injected into one side from a metal cathode, while holes are injected in the other from an anode (think of the anode as sucking electrons out). These electrons and holes move into the emissive layer and can meet to form excitons.

Derivatives of PPV, poly(p-phenylene vinylene) and poly(fluorene), are commonly used as polymer lumophors in OLEDs. Indium tin oxide is a common transparent anode, while aluminum or calcium are common cathode materials. Other materials are added in between the cathode/anode and the emissive layer to enhance the efficiency by facilitating or hindering hole or electron injection. You may find more materials (http://www.odcad.com/html/organicdevice_appearance1.HTM) for this technology.

Advantages

The radically different manufacturing process of OLEDs lends itself to many advantages over traditional flat panel displays. Since OLEDs can be printed onto a substrate using traditional inkjet technology they can have a significantly lower cost than LCDs or plasma displays. A more scalable manufacturing process enables the possibility of much larger displays. Unlike LCDs which employ a back-light and are incapable of showing true black, an off OLED element produces no light allowing for infinite contrast ratios. The range of colors, brightness, and viewing angle possible with OLEDs is greater than that of LCDs or plasma displays.

Without the need of a backlight, OLEDs use less than half the power of LCD displays and are well-suited to mobile applications such as cell phones and digital cameras.

The fact that OLEDs can be printed onto flexible substrates opens the door to new applications such as roll-up displays or displays embedded in clothing.

Disadvantages

The biggest technical problem left to overcome now is lifetime. Red and green OLED elements already have life-times of well over 20,000 hours but blue OLED life-times lag significantly behind at 1,000 hours.

According to Kodak, which is developing small molecule OLED, lifetime problems are not so significant for that type of OLED, mainly as a result of doping the base material of the OLEDs, which, they claim, has led to much better device performance both electrically and optically. Universal Display for example have produced a blue OLED that has a lifetime of 10,000 hours. There are still a number of problems to overcome though, and one of these is intrusion of water into displays which damages and destroys the organics, as well as outcoupling, which can result in the loss of much of the light in waveguided modes within the substrates.

In May 2005 Cambridge Display Technology announced a blue OLED with a lifetime of over 100,000 hours.

Commercial development of the technology is also hampered by intellectual property issues since even the basics of OLED technology is heavily patented by Kodak and other firms, requiring outside research teams to acquire a license.

Long Term Commercial Potential

Many proponents and investors in the burgeoning field of OLED research and development are so optimistic regarding the technology because it offers the potential to revolutionize the flat-panel display (FPD) industry, and therefore change how and where people can watch television or use computers.

Within a decade, OLED screens could find themselves in applications as diverse as heads-up displays in helicopters, windshield displays inside high-end sports cars, or even as a replacement for lightbulbs in modern houses. More speculative uses include ideas as varied as clothing that incorporates flexible OLED screens in order to change its color at the click of a button, or high definition virtual reality rooms where OLED screens cover every surface.

According to data compiled by the Society for Information Display: in 2003, the world OLED market was only $251 million. As of 2004, the world-wide OLED market was approximately $408 million. By 2008, experts are unsure exactly how fast it will have grown - conservative estimates are as low as $3 billion while other industry analysts feel it could reach as high as $8 billion. If such a high threshold is reached, it will have deep economic repercussions for developers and vendors of LCD displays and CRT displays.

References

Howard, Webster E. (Feb. 2004). Better Displays with Organic Films. Scientific American, p. 76.
Shinar, Joseph (Ed.) (2004). Organic Light-Emitting Devices: A Survey. NY: Springer-Verlag. ISBN 0-387-95343-4.