A light-emitting diode (LED) is a two-lead semiconductor light source. It is a basic pn-junction diode, which emits light when activated.[7] When a fitting voltage is applied to the leads, electrons are able to recombine with electron holes 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 band gap of the semiconductor.With the development of high-efficiency and high-power LEDs, it has become possible to use LEDs in lighting and illumination. Replacement light bulbs have been made, as well as dedicated fixtures and LED lamps.On October 7, 2014, the Nobel Prize in Physics was awarded to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura for "the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources" or, less formally, LED lampsRed-AlGaAsBlue-InGaNGreen-AlGaPWhen we talk about LED Displays, a distinction has to be made between true LED Displays and LCD Displays with LED backlighting which also goes by the moniker LED TV/Screen.

An LED-backlit LCD display is a flat panel display which uses LED backlighting instead of the cold cathode fluorescent (CCFL) backlighting used by most other LCDs.[1] LED-backlit LCD TVs use the same TFT LCD (thin film transistor liquid crystal display) technologies as CCFL-backlit LCD TVs. Picture quality is primarily based on TFT LCD technology, independent of backlight type.Three types of LED may be used:Edge-lit LEDs - in which the LEDs are formed around the rim of the screen, using a special diffusion panel to spread the light evenly behind the screen (the most common use)LED backlighting (Full array)- behind the screen, whose brightness is not controlled individuallyDynamic “local dimming” backlight - LEDs controlled individually (or in clusters) to control the level of light/color intensity in a given part of the screen.Advantages:reduced energy consumption, better contrast and brightness, greater color range, more rapid response to changes in scene and more accurate image rendering.Because LEDs can be switched on and off faster than CCFLs and can offer a higher light output, it is theoretically possible to offer very high contrast ratios. They can produce deep blacks (LEDs off) and high brightness (LEDs on). However, measurements made from pure-black and pure-white outputs are complicated by the fact that edge-LED lighting does not allow these outputs to be reproduced simultaneously on screen.No Mercury is used in manafacturing.

LED TVs provide a better picture for two basic reasons. First, LED TVs work with a color wheel or distinct RGB-colored lights (red, green, blue) to produce more realistic and sharper colors. Second, light emitting diodes can be dimmed. The dimming capability on the back lighting in an LED TV allows the picture to display with a truer black by darkening the lights and blocking more light from passing through the panel. This capability is not present on edge-lit LED TVs; however, edge-lit LED TVs can display a truer white than the fluorescent LED TVs.Because all these LCD TVs are thin-screen, each has particular angle-viewing and anti-glare issues. The backlit TVs provide better, cleaner angle viewing than the edge-lit LED TV. However, the backlit LED TV will

usually have better angle viewing than the standard LCD TV. Both LED and LCD TVs have good reputations for their playback and gaming quality.Used to make TFT/IPS displays for smartphones etc

A Full LED Display is a flat panel display, which uses an array of light-emitting diodes as a video display. An LED panel is a small display, or a component of a larger display. They are typically used outdoors in store signs and billboards, and in recent years have also become commonly used in destination signs on public transport vehicles or even as part of transparent glass area. LED panels are sometimes used as form of lighting, for the purpose of general illumination, task lighting, or even stage lighting rather than display.

A cluster of red, green, and blue diodes is driven together to form a full-color pixel, usually square in shape. These pixels are spaced evenly apart and are measured from center to center for absolute pixel resolution.These are used as monochromatic dot matrix displays to display information.For Example,destination displays in MTC buses,numbers in digital clocks etc.

Full LED displays paved way for a new display technology found in phones called OLED-Organic LED. Here the  emissive electroluminescent layer is a film of organic compound which emits light in response to an electric current.

A typical OLED is composed of a layer of organic materials situated between two electrodes, the anode andcathode, all deposited on a substrate. The organic molecules are electrically conductive as a result ofdelocalization of pi electrons caused by conjugation over part or all of the molecule. These materials have conductivity levels ranging from insulators to conductors, and are therefore considered organic semiconductors. Valence band-Highest Occupied molecular orbitalconduction band-lowest unoccupied M.o

The battery or power supply of the device containing the OLED applies a voltage across the OLED.

1.An electrical current flows from the cathode to the anode through the organic layers (an electrical current is a flow of electrons). The cathode gives electrons to the emissive layer of organic molecules. The anode removes electrons from the conductive layer of organic molecules. (This is the equivalent to giving electron holes to the conductive layer.)

2.At the boundary between the emissive and the conductive layers, electrons find electron holes. When an electron finds an electron hole, the electron fills the hole (it falls into an energy level of the atom that's missing an electron). When this happens, the electron gives up energy in the form of a photon of light (see How Light Works).

3.The OLED emits light.

4.The color of the light depends on the type of organic molecule in the emissive layer. Manufacturers place several types of organic films on the same OLED to make color displays.

5.The intensity or brightness of the light depends on the amount of electrical current applied: the more current, the brighter the light.

Passive-matrix OLED (PMOLED)PMOLEDs have strips of cathode, organic layers and strips of anode. The anode strips are arranged perpendicular to the cathode strips. The intersections of the cathode and anode make up the pixels where light is emitted. External circuitry applies current to selected strips of anode and cathode, determining which pixels get turned on and which pixels remain off. Again, the brightness of each pixel is proportional to the amount of applied current.

PMOLEDs are easy to make, but they consume more power than other types of OLED, mainly due to the power needed for the external circuitryEven with the external circuitry, passive-matrix OLEDs consume less battery power than the LCDs that currently power these devices.

Active-matrix OLED (AMOLED)AMOLEDs have full layers of cathode, organic molecules and anode, but the anode layer overlays a thin film transistor (TFT) array that forms a matrix. The TFT array itself is the circuitry that determines which pixels get turned on to form an image.

AMOLEDs consume less power than PMOLEDs because the TFT array requires less power than external circuitry, so they are efficient for large displays. AMOLEDs also have faster refresh rates suitable for video. The best uses for AMOLEDs are computer monitors, large-screen TVs and electronic signs or billboards.

OLED tech can be used to make foldable displays. They do not require backlighting and can be made ultra-thin. OLED TVs from panasonic,Sony etc are in the market as cutting edge pieces of craft for a hand and a leg.

Advantages Lower cost in the future

OLEDs can be printed onto any suitable substrate by an inkjet printer or even by screen printing,[60] theoretically making them cheaper to produce than LCD or plasma displays

Lightweight and flexible plastic substratesWider viewing angles and improved brightness

Better power efficiency and thickness Response time

Disads Lifespan-very poor.due to degradation of the organic molecules or polymers