What is OLED?

Organic light emitting diodes have been receiving a lot of attention over the world as a new type of display technology. OLEDs have many advantages over conventional display technologies. First, the fabrication process is easy, and devices are thinner and lighter than those fabricated by cathode ray tube (CRT) display technology. Second, there are also some advantages over liquid crystal (LCD) displays: OLEDS can be viewed from different angles and don’t need a backlight. Finally, the drive voltage and power consumption are low. The first commercial OLED display was introduced by Pioneer Electronics as the front panel of a car stereo in 1997.

To enhance the colour or brightness, manufacturers can add complex chains of molecules (polymers) to the carbon-based layers.

Unlike LCDs, which require backlighting, OLED displays are "emissive" devices, meaning they emit light rather than modulate transmitted or reflected light.

Thin organic layers serve these displays as a source of light, which offers significant advantages in relation to conventional technologies:

* brighter and more brilliant picture

* unlimited viewing angle

* low power consumption

* economic production

* fast "response time"

The prerequisites for a breakthrough of this technology in the market, which is estimated in 2010 to be worth over USD 2 billion, are the optimization of certain critical performance data such as lifetime and efficiency. This requires innovations in materials meaning that chemistry will decide about the future and the success of the OLED technology. OLEDs - Organic Light-Emitting Diodes are the light of the future

Video wallpaper - just a millimeter thick - could transform your living room wall into a flat screen and electronic film as thin as a sheet of paper could serve as your screen for the internet, the news, images or games. In future, all of this will be possible thanks to organic light-emitting diodes, so-called OLEDs.

Why are the OLED-Display technology even better than the LCD or plasma technology?

Low power consumption is the reaseon why OLED is a better choice for portable devices. It also makes OLEDs, and a candidate to be the white-light "bulb" of the future Greater brightness.

Light sources based on organic electroluminiscent materials offer the potential to make a high light intensity possible at a low energy consumption on mechanically flexible substrates." said project head Prof. Dr. Karl Leo (IAPP) about the high expectations.

- The Flat screen are brighter, and have a fuller viewing angle. Better durability - OLED-Displays can operate in a temperature range Lighter weight - the screen can be made very thin, and can be 'printed' on flexible surfaces.

OLED-Structure:

Organic light emitting diodes consist of stacks of organic layers (thickness about 100 nm), which are inserted between a cathode and an anode. Usually, the substrate is glass coated with a transparent conductive oxide being the anode, followed by the organic stack, consisting of hole transport and electron transport materials, followed

by the inorganic cathode. Key advantages of the organic luminescence are the chemical variability of the organic light-emitting diodes, allowing virtually any color including white, and the thin film system, allowing large-area and low-cost deposition, and the possibility to use thin and even flexible substrates to realize a novel class of lighting and display solutions not possible for other technologies.

In the OLED technology, two different material groups have to be distinguished: OLED materials with low molecular weight called small-molecule (SM) OLED. SM-OLED were first introduced by the research group led by Dr. Ching Tang at the Kodak Laboratories in 1987. The deposition of SM-OLEDs is based on vacuum thermal evaporation. Polymer based OLED (PLEDs) are based on long polymer organic chains and are deposited by spin-cast or ink-jet principles.

Many electronic appliances are at the threshold of a revolution that began with the discovery of polymeric conductors in the 1970s. Polymeric materials, which have historically been classified exclusively as electrical insulators, are now finding varied applications as both conductors and semiconductors. Expensive ceramic semiconductors that are brittle and difficult to pattern have historically been the driving force of the digital age for the last fifty years. But now a combination of properties exist today in polymers that are making many previously impossible appliances a reality.

Within a very short time organic conductors have been developed with the conductivity of metals such as copper, while organic electronics has evolved photoelectric cells, diodes, light emitting diodes, lasers and transistors. The result is that a class of plastic materials referred to as conjugated polymers are fast displacing traditional materials such as natural polymers (e.g. wood), metals, ceramics and glass in many applications owing to the combination of their physical/mechanical properties (light weight combined with physical strength) and ease of processibility (the ability to mould the shape of plastic materials or extrude into sheet and rod through a die).

What this means is that OLEDs can be deployed in a wide range of electronic devices and can be used extensively throughout any given device. Active components of displays can be polymers, substrates can be polymers, logical electronics can be polymers, and so on. In the years ahead OLEDs will see applications in personal computers, cell phones, televisions, general wide area lighting, signs, billboards, communications and any of a number of information appliances.

The basic OLED cell structure consists of a stack of thin organic layers sandwiched between a transparent anode and a metallic cathode. The organic layers comprise a hole-injection layer, a hole-transport layer, an emissive layer and an electron-transport layer. When an appropriate voltage (typically a few volts) is applied to the cell, the injected positive and negative charges recombine in the emissive layer to produce light (electroluminescence). The structure of the organic layers and the choice of anode and cathode are designed to maximise the recombination process in the emissive layer, thus maximising the light output from the OLED device. Both the electroluminescent efficiency and control of colour output can be significantly enhanced by "doping" the emissive layer with a small amount of highly fluorescent molecules.