18-07-2011, 12:48 PM
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1. INTRODUCTION
Imagine having a high-definition TV that is 80 inches wide and less than a quarter-inch thick, consumes less power than most TVs on the market today and can be rolled up when you're not using it. What if you could have a "heads up" display in your car? How about a display monitor built into your clothing? These devices may be possible in the near future with the help of a technology called organic light-emitting diodes (OLEDs). OLED is a flat display technology, made by placing a series of organic thin films between two conductors. OLED stands for Organic EL (Electro-Luminescence), and is the phenomenon of light emitted by organic materials to which a voltage has been applied
Organic light emitting diodes have been receiving a lot of attention over the world as a new type of display technology. OLED’s 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: greater viewing angle, lighter weight, and quicker response. Unlike LCDs, which require backlighting, OLED displays are "emissive" devices, meaning they emit light rather than modulate transmitted or reflected light. 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 micro displays, wearable, head-mounted computers, digital cameras, personal digital assistants, smart pagers, and mobile hones as well as medical, automotive, and other industrial applications.
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.
2. OLED
2.1 OLED STRUCTURE
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 thin 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.
The basic OLED cell structure consists of a stack of thin organic layers sandwiched between a transparent anode and a metallic cathode. The basic structure of OLED is shown in fig.1. The organic layers comprise a hole-injection layer, a hole-transport layer (HTL), an emissive layer and an electron-transport layer(ETL). 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 maximize the recombination process in the emissive layer, thus maximizing the light output from the OLED device. Both the electroluminescent efficiency and control of color output can be significantly enhanced by "doping" the emissive layer with a small amount of highly fluorescent molecules.
Like an LED, an OLED is a solid-state semiconductor device that is 100 to 500 nanometers thick or about 200 times smaller than a human hair. OLEDs can have either two layers or three layers of organic material
An OLED consists of the following parts:
• Substrate: The substrate which is made of glass, clear plastic, or foil. The substrate is used to support the OLED.
• Anode: The anode is made of natural graphite particles, which is transparent and is the second layer. The anode layer removes electrons from the conductive layer when a current flows into the devices.
• Organic Layers: These layers are made of organic molecules or polymers.
• Conducting layer: The third layer is the conducting layer. This layer is
made of organic plastic molecules. One conducting
polymer used in OLEDs is polyaniline.
•Emissive layer: The fourth layer is emissive layer. This layer is also
also composed of organic plastic molecules, but has
a different function. The emissive layer removes
electrons from the cathode layer which causes light
to be made. One polymer used in the emissive layer
is polyfluorene.
• Cathode: The final layer is the cathode. It may or may not be transparent
depending on the type of OLED. The cathode injects electrons
when a current flows through the device. Tungsten is used.
Fig 1. Structure of OLED
2.2 WORKING PRINCIPLE
A typical OLED is composed of an emissive layer, a conductive layer, a substrate, an anode and cathode terminals. The layers are made of special organic molecules that conduct electricity. Their levels of conductivity range from those of insulators to those of conductors, and so they are called organic semiconductors.
The first, most basic OLEDs consisted of a single organic layer, for example the first light-emitting polymer device synthesized by Burroughs et al involved a single layer of poly (pphenylenevinylene). Multilayer OLEDs can have more than two layers to improve device efficiency. As well as conductive properties, layers may be chosen to aid charge injection at electrodes by providing a more gradual electronic profile, or block a charge from reaching the opposite electrode and being wasted.