07-04-2009, 11:40 PM
Scientific research in the area of semiconducting organic materials as the active substance in light emitting diodes (LEDs) has increased immensely during the last four decades. Organic semiconductors was first reported in the 60 and then the materials where only considered to be merely a scientific curiosity. (They are named organic because they consist primarily of carbon, hydrogen and oxygen.). However when it was recognized in the eighties that many of them are photoconductive under visible light, industrial interests were attracted. Many major electronic companies, such as Philips and Pioneer, are today investing a considerable amount of money in the science of organic electronic and optoelectronic devices. The major reason for the big attention to these devices is that they possibly could be much more efficient than todays components when it comes to power consumption and produced light. Common light emitters today, Light Emitting Diodes (LEDs) and ordinary light bulbs consume more power than organic diodes do. And the strive to decrease power consumption is always something of matter. Other reasons for the industrial attention are i.e. that eventually organic full color displays will replace todays liquid crystal displays (LCDs) used in laptop computers and may even one day replace our ordinary CRT-screens.
Organic light-emitting devices (OLEDs) operate on the principle of converting electrical energy into light, a phenomenon known as electroluminescence. They exploit the properties of certain organic materials which emit light when an electric current passes through them. In its simplest form, an OLED consists of a layer of this luminescent material sandwiched between two electrodes. When an electric current is passed between the electrodes, through the organic layer, light is emitted with a color that depends on the particular material used. In order to observe the light emitted by an OLED, at least one of the electrodes must be transparent.
When OLEDs are used as pixels in flat panel displays they have some advantages over backlit active-matrix LCD displays - greater viewing angle, lighter weight, and quicker response. Since only the part of the display that is actually lit up consumes power, the most efficient OLEDs available today use less power.
Based on these advantages, OLEDs have been proposed for a wide range of display applications including magnified microdisplays, wearable, head-mounted computers, digital cameras, personal digital assistants, smart pagers, virtual reality games, and mobile phones as well as medical, automotive, and other industrial applications.
OLED Versus LED
Electronically, OLED is similar to old-fashioned LEDs -- put a low voltage across them and they glow. But that's as far as the similarity goes: instead of being made out of semiconducting metals, OLEDs are made from polymers, plastics or other carbon-containing compounds. These can be made very cheaply and turned into devices without all the expensive palaver that goes with semiconductor fabrication.
Light-emitting diodes, based upon semiconductors such as Gallium Arsenide, Gallium Phosphide, and, most recently, Gallium Nitride, have been around since the late '50s. They are mostly used as indicator lamps, although they were used in calculators before liquid crystals, and are used in large advertising signs, where they are valued for very long life and high brightness. Such crystalline LEDs are not inexpensive, and it is very difficult to integrate them into small high-resolution displays.