21-07-2011, 04:12 PM
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ABSTRACT :-
Over the time there are many changes came into the field of output/display devices. In this field first came the small led displays which can show only the numeric contains. Then came the heavy jumbo CRTs (Cathode Ray Tubes) which are used till now. But the main problem with CRT is they are very heavy & we couldn’t carry them from one place to another the result of this CRT is very nice & clear but they are very heavy & bulky & also required quiet large area then anything else.
Then came the very compact LCDs (Liquefied Crystal Displays). They are very lighter in weight as well as easy to carry from one place to the other. But the main problem with the LCDs is we can get the perfect result in the some particular direction. If we see from any other direction it will not display the perfect display.
To over come this problems of CRTs & LCDs the scientist of Universal Laboratories, Florida, United States & Eastman Kodak Company both started their research work in that direction & the overcome of their efforts is the new generation of display technologies named OLED (Organic Light Emitting Diode) Technology.
In the flat panel display zone unlike traditional Liquid-Crystal Displays OLEDs are self luminous & do not required any kind of backlighting. This eliminates the need for bulky & environmentally undesirable mercury lamps and yields a more thinner ,more compact display.
Unlike other flat panel displays OLED has a wide viewing angle (upto 160 degrees),even in bright light.Their low power consumption(only 2 to 10 volts) provides for maximum efficiency and helps minimize heat and electric interference in electronic devices.
Because of this combination of this features, OLED displays communicate more information in a more engaging way while adding less weight and taking up less space. Their application in numerous devices is not only a future possibility but a current reality.
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 1960, Martin Pope and co-workers at New York University developed ohmic dark-injecting electrode contacts to organic crystals. They further described the necessary energetic requirements (work functions) for hole and electron injecting electrode contacts. These contacts are the basis of charge injection in all modern OLED devices. Pope's group also first observed direct current (DC) electroluminescence under vacuum on a pure single crystal of anthracene and on anthracene crystals doped with tetracene in 1963 using a small area silver electrode at 400V. The proposed mechanism was field-accelerated electron excitation of molecular fluorescence.
Pope's group reported in 1965 that in the absence of an external electric field, the electroluminescence in anthracene crystals is caused by the recombination of a thermalized electron and hole, and that the conducting level of anthracene is higher in energy than the exciton energy level. Also in 1965, W. Helfrich and W. G. Schneider of the National Research Council in Canada produced double injection recombination electroluminescence for the first time in an anthracene single crystal using hole and electron injecting electrodes,the forerunner of modern double injection devices. In the same year, Dow Chemical researchers patented 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 anthracene powder, tetracene, and graphite powder. Their proposed mechanism involved electronic excitation at the contacts between the graphite particles and the anthracene molecules.
Device performance was limited by the poor electrical conductivity of contemporary organic materials. This was overcome by the discovery and development of highly conductive polymers. For more on the history of such materials, see conductive polymers.
Electroluminescence from polymer films was first observed by Roger Partridge at the National Physical Laboratory in the United Kingdom. The device consisted of a film of poly(n-vinylcarbazole) up to 2.2 micrometres thick located between two charge injecting electrodes. The results of the project were patented in 1975 and published in 1983.
The first diode device was reported at Eastman Kodak by Ching W. Tang and Steven Van Slyke in 1987. This device used a novel two-layer structure with separate hole transporting and electron transporting layers such that recombination and light emission occurred in the middle of the organic layer. This resulted in a reduction in operating voltage and improvements in efficiency and led to the current era of OLED research and device production.
Research into polymer electroluminescence culminated in 1990 with J. H. Burroughes et al. at the Cavendish Laboratory in Cambridge reporting a high efficiency green light-emitting polymer based device using 100 nm thick films of poly(p-phenylene vinylene).
INTRODUCTION :-
Organic light emitting diodes (OLEDs) are optoelectronic devices based on small molecules or polymers that emit light when an electric current flows through them. simple OLED consists of a fluorescent organic layer sandwiched between two metal electrodes.Under application of an electric field, electrons and holes are injected from the two electrodes into the organic layer, where they meet and recombine to produce light. They have been developed for applications in flat panel displays that provide visual imagery that is easy to read, vibrant in colors and less consuming of power.
OLEDs are light weight, durable, power efficient and ideal for portable applications. OLEDs have fewer process steps and also use both fewer and low-cost materials than LCD displays. OLEDs can replace the current technology in many applications due to following performance advantages over LCDs.
• Greater brightness
• Faster response time for full motion video
• Fuller viewing angles
• Lighter weight
• Greater environmental durability
• More power efficiency
• Broader operating temperature ranges
• Greater cost-effectivenes
WHAT IS OLED :-
An OLED is a solid state device or electronic device that typically consists of organic thin films sandwiched between two thin film conductive electrodes. When electrical current is applied, a bright light is emitted. OLED use a carbon-based designer molecule that emits light when an electric current passes through it. This is called electrophosphorescence. Even with the layered system, these systems are thin . usually less than 500 nm or about 200 times smaller than a human hair. When used to produce displays. OLED technology produces self-luminous displays that do not require backlighting and hence more energy efficient. These properties result in thin, very compact displays. The displays require very little power, ie, only 2-10 volts.
OLED technology uses substances that emit red, green, blue or white light. Without any other source of illumination, OLED materials present bright, clear video and images that are easy to see at almost any angle. Enhancing organic material helps to control the brightness and colour of light.
