Organic Light Emitting Diode

Content :

1 History 2 Construction 3 Working principle 4 Advantages 5 Disadvantages 6 Manufacturers and commercial uses

Introduction

Before going into deep of Organic Light Emitting Diode, we will have short glimpse over the basics of Light Emitting Diode means how it is prepared and how it work.

Bascially Light Emitting Diode is prepared using Semiconductor Now thinking about Semiconductor, its has electrical conductivity between that of a conductor and an insulator. Semiconductors differ from metals in their characteristic property of decreasing electrical resistivity with increasing temperature. Semiconductors can also display properties of passing current more easily in one direction than the other, and sensitivity to light.

semiconductors are very useful devices for amplification of signals, switching, and energy conversion.

Light Emitting Diode

Now this Semiconductor is a main content of Light Emitting Diode. Light-Emitting diode (LED) is a semiconductor light source.

Working of LED

When a light-emitting diode is forward-biased i.e switched on, electrons are able to recombine with electron holes i.e the absence of an electron from an otherwise full within the device, releasing energy in the form of photons.

This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. Electroluminescence-is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field. Discovered in 1907 by the British experimenter H. J. Round of Marconi Labs, using a crystal of silicon carbide and a cat's-whisker detector.

Functional Diagram of LED

History

The first observations of Electroluminescence in organic materials were in the early 1950 by A. Bernanose and co-workers at the Nancy-University, France. By applying high-voltage alternating current (AC) fields in air to materials such as acridine orange i.e (nucleic acid), either deposited on or dissolved in cellulose or cellophane thin films. In 1960, Martin Pope and co-workers at New York University developed ohmic dark-injecting electrode contacts to organic crystals.

Pope's group also first observed direct current (DC) Electroluminescence under vacuum on a pure single crystal of anthracene (polycyclic aromatic hydrocarbon with 3 Benzene rings) and on anthracene crystals doped with tetracene in 1963 using a small area silver electrode at 400 V.

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. Electroluminescence from polymer films was first observed by Roger Partridge at the National Physical Laboratory in the United Kingdom

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. Resulting in voltage and improvements in efficiency and led to the current era of OLED research and device production.

Construction

A typical OLED is composed of a layer of organic materials between two electrodes, the anode and cathode. The most basic polymer OLEDs consisted of a single organic layer, which involved a single layer of poly(p-phenylene vinylene). Multilayer OLEDs can be fabricated with two or more layers in order to improve device efficiency & conductive properties. Many modern OLEDs incorporate a simple bilayer structure, consisting of a conductive layer and an emissive layer

Schematic of a Bilayer OLED

1. Cathode (−), 2. Emissive Layer, 3. Emission of radiation, 4. Conductive Layer, 5. Anode (+)

Working principle

Voltage is applied across the OLED such that the anode is positive w.r.t the cathode. A current of electrons flows through the device from cathode to anode, as electrons are injected into the LUMO of the organic layer at the cathode and withdrawn from the HOMO at the anode. Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exciton, a bound state of the electron and hole.

In organic semiconductors holes are generally more mobile than electrons, results in a relaxation of the energy levels of the electron,accompanied by emission of radiation whose frequency is in the visible region. The frequency of this radiation depends on the band gap i.e (energy gap where no e- exixt) of the material.

Advantages

Lower cost in the future OLEDs can be printed onto any suitable substrate by an inkjet printer or even by screen printing,[52] theoretically making them cheaper to produce than LCD or plasma displays.

Light weight & flexible plastic substrates By fabricating possiblity of flexible organic light-emitting diodes being fabricated or other new applications such as roll-up displays embedded in fabrics or clothing. Better power efficiency OLED element does not produce light or consume power. And response time is also higher than standard LCD screens.

Disadvantages

Current costs OLED manufacture currently its extremely expensive. Lifespan The biggest technical problem for OLEDs was the limited lifetime of the organic materials. Water damage Water can damage the organic materials so sealing processes are important for practical manufacturing.

Outdoor performance The metallic cathode in an OLED acts as a mirror, with reflectance approaching 80%, leading to poor readability in bright ambient light such as outdoors. Power consumption OLED will consume around 40% of the power of an LCD displaying an image which is primarily black, for the majority of images it will consume 60–80% of the power of an LCD.

Manufacturers and commercial uses

OLED technology is used in commercial applications such as displays for mobile phones and portable digital media players, car radios and digital cameras. OLEDs have been used in most Motorola and Samsung colour cell phones, as well as some HTC, LG and Sony Ericsson models. Nokia has also introduced some OLED products including the N85 and the N86 8MP, both of which feature an AMOLED display.

The Google and HTC Nexus One smartphone includes an AMOLED screen, as does HTC's own Desire and Legend phones.

Magnified image of the AMOLED screen on the Google Nexus One smartphone

Samsung, South Korea's largest conglomerate, was the world's largest OLED manufacturer, producing 40% of the OLED displays made in the world. 2005 the world's largest OLED TV at the time, at 21 inches consisting of of 6.22 million pixels. Sony demonstrated a 0.2 mm thick 3.5 inch display with a resolution of 320×200 pixels and a 0.3 mm thick 11 inch display with 960×540 pixels resolution, one-tenth the thickness of the XEL-1

Sony XEL-1, the world's first OLED TV

On June 25, 2012, Sony and Panasonic announced a joint venture for creating low cost mass production OLED televisions by 2013. On December 26, 2011, LG officially announced the "world's largest 55" OLED panel" . Mitsubishi installed a 6-meter OLED 'sphere' in Tokyo's Science Museum