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ABSTRACT
Organic Light Emitting Diode is a scalable nano level emerging technology in Flat Panel Displays and as a White Light Source with efficient features. This paper focuses on OLED structure, principle aspects, fabrication methodology and different techniques to replace current white light sources like Incandescent bulbs, Fluorescent tubes, and even display techniques like Liquid Crystal Displays, Plasma technologies.
OLEDs can be fabricated using Polymers or by small molecules. OLED matrix displays offer high contrast, wide viewing angle and a broad temperature range at low power consumption.
These are Cheaper, Sharper, Thinner, and Flexible. OLEDs have a potential of being white-light sources that are Bright, power-efficient and long lived, by emitting pleasing white light Ultra-thin, light weight, rugged, and conformable, Inexpensive, portable.
INTRODUCTION
OLED displays are an exciting new display technology that offers improved performance as well as novel applications.
OLEDs offer a decreased manufacturing cost, a brighter, more vibrant display as well as larger viewing angle with low power consumption. Sometimes OLED is referred as to organic light emitting device; mind the difference between device and diode. In the past, prior to standardization, OLED was often also referred to as OEL, which means organic electro-luminescence.
OLED technology will emerge as a leading next-generation technology for displays and lighting. It is already used in mp3 players, cellular phones, digital camera, television screens, computer displays and many other products. It can also be used as a light source for general space illumination.
One of the future visions is to roll out OLEDs or to stick them up like post-it notes. Another vision is the transparent windows which would function like a regular window by day. At night it could be switched on and become a light source.
WHAT IS OLED?
OLED stands for Organic Light Emitting Diode. It is called ORGANIC because the emissive layer consists of organic compounds like Hydrogen, Carbon and Oxygen.
OLEDs are energy conversion devices (electricity-to-light) based on Electroluminescence. Electro-luminescence is light emission from a solid through which an electric current is passed. OLEDs are more energy-efficient than incandescent lamps. The luminous efficiency of light bulbs is about 13 - 20 lm/W but the latest experimental green emitting OLEDs already have luminous efficiency of 76 lm/W, though at low luminance. The development is on track for OLEDs to effectively compete even with fluorescent lamps, which have the luminous efficiency of 50 - 100 lm/W. One big advantage of OLEDs is the ability to tune the light emission to any desired color, and any shade of color or intensity, including white.
So in this present scenario the need for a new technology with both these features combined leaded to invention of OLED.OLED which is a thin, flexible, Bright LED with self luminance which can be used as a display device. The main drawback of LCD display is its Less viewing angle and highly temperature depending which moves us towards a new Technology. Thus OLED promises for faithful replacement of current technology with added flavors like Less Power Consumption and Self Luminance.
HISTORY
The first observations of electroluminescence in organic materials were in the early 1950s by A. Bernanose and co-workers at the Nancy-Université, France. They applied high-voltage alternating current (AC) fields in air to materials such as acridine orange, either deposited on or dissolved in cellulose or cellophane thin films. The proposed mechanism was either direct excitation of the dye molecules or excitation of electrons
In 1979 Chin Tang discovered electroluminescence in the research department of Kodak. During his work with solar cells he observed a blue glow of organic material. In 1987 Tang and Van Slyke introduced the first light emitting diodes from thin organic layers. In 1990 electroluminescence in polymers was discovered.
The first OLEDs were organic light diodes from small molecules coatable in vacuum (small molecule or SM-OLED). The later OLEDs are organic light diodes on the basis of polymers which are applied in liquid phase (Polymer-OLED or PLED). Polymer-LEDs developed by Cambridge Display Technology allow the solution of organic material in liquid. Afterwards, the organic material can be applied by spin coating or in an inkjet printing processing. At present polymer LEDs still lag behind in picture quality but they promise considerably less production costs. The printing process is more flexible.
MATERIALS USED
(Alq3 commonly used in small molecule OLEDs)
Efficient OLEDs using small molecules were first developed by Dr. Ching W. Tang at Eastman Kodak. The term OLED traditionally refers specifically to this type of device, though the term SM-OLED is also in use. Molecule commonly used in OLEDs includes organometallic chalets (for example Alq3, used in the organic light-emitting device reported by Tang), fluorescent and phosphorescent dyes and conjugateddendrimers. A number of materials are used for their charge transport properties, for exampletriphenylamine and derivatives are commonly used as materials for hole transport layers. Fluorescent dyes can be chosen to obtain light emission at different wavelengths, and compounds such as perylene,rubrene and quinacridone derivatives are often used.[29] Alq3 has been used as a green emitter, electron transport material and as a host for yellow and red emitting dyes.
The production of small molecule devices and displays usually involves thermal evaporation in a vacuum. This makes the production process more expensive and of limited use for large-area devices than other processing techniques. However, contrary to polymer-based devices, the
vacuum deposition process enables the formation of well controlled, homogeneous films, and the construction of very complex multi-layer structures. This high flexibility in layer design, enabling distinct charge transport and charge blocking layers to be formed, is the main reason for the high efficiencies of the small molecule OLEDs.
Coherent emission from a laser dye-doped tandem SM-OLED device, excited in the pulsed regime, has been demonstrated. The emission is nearly diffraction limited with a spectral width similar to that of broadband dye lasers.
Polymer light-emitting diodes
(Poly (p-phenylene vinylene), used in the first PLED.)
Polymer light-emitting diodes (PLED), also light-emitting polymers (LEP), involve an electroluminescent conductive that emits light when connected to an external voltage. They are used as a thin film for full colour displays. Polymer OLEDs are quite efficient and require a relatively small amount of power for the amount of light produced.
Polymers are not suitable for vacuum deposition, however they can be processed in solution and spin coating is a common method of depositing thin films of polymers. This method is more suited to forming large-area films than thermal evaporation, although formation of multilayer structures is difficult with this method as application of subsequent layers will tend to dissolve those already present. No vacuum is required, and the emissive materials can also be applied on the substrate by a technique derived from commercial inkjet printing. However, the metal cathode may still need to be deposited by thermal evaporation in vacuum.
Typical polymers used in PLED displays include derivatives of poly(p-phenylene vinylene) and polyfluorene. Substitution of side chains onto the polymer backbone may determine the colour of emitted light or the stability and solubility of the polymer for performance and ease of processing