Friday, June 24, 2011

Organic Light Emitting Diode (OLED)

An Organic Light Emitting Diode (OLED) is a futuristic technology, which represents great combination of thin, proficient and bright display. OLED monitors are made up of LED (light emitting diode) in which light is emitted in response to an electric current through film of organic compounds i.e. emissive electroluminescent layer.

For the functioning of flat OLED monitors, no back light is required. Thus, the OLED monitors illustrate thinner and lighter deep levels. Likewise, in dark room where ambient light is low, a higher contrast ratio is achieved by OLED screen.

History of Organic Light Emitting Diode (OLED)

In early 1950s, A. Bernanose and co-workers at the Nancy-Université, France observed electroluminescence in organic materials. High voltage alternating current was applied in air to acride orange, which is either dissolved in or deposited on cellphase or cellulose thin films.

An ohmic dark-injecting electrode contact to organic crystals was developed by Martin Pope and co-workers at New York University in 1960. Further they described the basis of charge injection in all modern OLED devices. On a anthracene crystals and pure single crystal of anthracene, pope group observed direct current electroluminescence under vacuum in 1963.

In 1965, for the first time, double injection recombination electroluminescence was produced in an electron injecting electrodes and anthracene single crystal using hole, the forerunner of modern double injection devices by W. Helfrich and W. G. Schneider of the National Research Council in Canada.

A method of preparing electroluminescent cells using high voltage (500–1500 V) AC-driven (100–3000 Hz) electrically-insulated one millimetre thin layers of a melted phosphor consisting of ground graphic powder, anthracene powder and tetracene was initiated in the same year by Dow Chemical researchers. Earlier, its performance was limited but, later on with the discovery and development of higher conductive polymers, it overcomes all its drawbacks.

At National Physical Laboratory (UK), Roger Partridge observed the Electroluminescence from polymer films and was published in 1983.
In 1987, Steven Van Slyke and Ching W. Tang reported the first diode device at Eastman Kodak. Further, in 1990 with J. H. Burroughes et al. at the Cavendish Laboratory in Cambridge research into polymer electroluminescence was finished resulting a high efficiency green light-emitting polymer.

Working Of OLED Computers

Anode and cathode are two electrodes in which OLED is situated. OLED is composed of a layer of organic materials that are electrically conductive as an effect of delocalization of pi electrons. These materials are considered as an organic semiconductor as it comprises of conductive levels from insulators to conductors.

A single organic layer is available under the basic polymer OLEDs. Thus, to improve device efficiency, multiple OLEDS can be used with two or more layers. Along with this, a simple bilayer structure is included by many modern OLEDs, which incorporate an emissive layer and a conductive layer.

Across the OLED, a voltage is applied during operation, which results positive (anode) with respect to the cathode. With this process, HOMO (higher occupied molecule orbitals) is injected into the electron holes. Here the holes and the electrons bring towards each other through electrostatic forces to form an exciton (a bound state of the electron and hole). By emission of radiation whose frequency is in the visible region, the excited state can result to delay with relaxation of energy levels of the electron.

Depending on how the spins of the hole and electron have been combined, an exciton may either be in single state or a triple state. Thus, each singlet exciton forms the three triplet exciton. Limiting the internal efficiency and raising the timescale of the transition of fluorescent device, spin forbidden will be delayed from triple states (phosphorescence). So, improving the internal efficiency and attaining emission from both singlet and triplet state will facilitate intersystem crossing between both states with Phosphorescent organic light-emitting diodes, which make use of spin–orbit interactions.

Moreover, injection of holes into the HOMO level of the organic layer are promoted bu Indium tin oxide (anode material), which is transparent to visible light. Whereas, injection of electrons into the LUMO (lower unoccupied molecule orbitals) of the organic layer are used promoted by metals like calcium and barium (cathode). Capping of layer of aluminum is required to avoid degradation for which such metals are reactive.

The current that passes through the single carrier device is composed of only one type of charge carrier, so the recombination does not take place and light is not emitted.

Types Of flat screen OLED monitors
  • Passive Matrix (PMOLED): They are simple and economical to build but, are restricted to size, refresh rate and resources.
  • Active Matrix (AMOLED): A thin-film transistor backplane is required by active matric to switch each pixel on/off. It is difficult to create, expensive, can represent larger displays and is power efficient.
  • Transparent OLED: It includes only transparent components like substrate, cathode and anode, which can either be active or passive matrix. Light pass in both directions, when OLED display is turned on whereas, light pass up to 85 percent as transparent as their substrate, when turn off.
  • Top-emitting OLED: It has either opaque or reflective substrate, which is best suited to active-matrix.
  • White OLED: It produce bigger white light, which is more energy efficient and uniform. It consumes less energy and thus, replaces fluorescent lights used in home.
Components Of OLED
OLED is a thin and small semiconductor device that has two or three layers of organic material. Following parts are included in the OLED such as:
  • Substrate (clear plastic, glass, foil)
  • Anode: when a current flows through the device, it automatically removes electrons.
  • Organic layer: It is made up of conducting layer (organic plastic molecules) and emissive layer (polymers).
  • Cathode: Electrons are injected, when current flows through device.
Advantages Of OLED Computer
  • Lighter, thinner and efficient
  • Enhanced image quality with superior brightness and contrast
  • Power consuming
  • Superior viewing angles
  • Flexible and transparent
Disadvantages Of OLED Flat Screen Monitor
  • Screen burn-in
  • Poor readability in bright ambient light such as outdoors
  • Organic materials of the displays can be damaged by water
  • Color balance issues

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