POLYMER LIGHT EMITTING DIODES (PLEDs)
Ø Polymer LED (light-emitting diode) - sometimes called light-emitting polymer or polyLED - is a technology based on the use of polymer as the semiconductor material in LEDs.
Ø Polymers are chemical substances that consist of large molecules that are themselves, made from many smaller and simpler molecules.
Ø Polymers are largely identified with insulation.
Research institutes have come up with ideas to integrate polymers into main stream electronics as it has been established that certain polymers have typical conductive and light emitting properties.
PLED-A Review
Ø The making of the first PLED is credited to the Cavendish Laboratory of Cambridge University in 1989.
Ø Polyphenylene vinylene (PPV) was found to emit yellow-green light when sandwiched between a pair of electrodes. The initial device efficiencies were very low .
Ø PLEDs today have a number of intrinsic advantages over liquid crystal devices :
v PLED is an emissive technology
v does not require additional elements such as backlights, filters and polarizers.
v PLED technology is very energy efficient.
Ø Now a breakthrough in blue-emitting polymer material has solved the problem to create full-color organic-LED (OLED) displays. This has resulted in the possibility of a new class of light-emitting plastic displays that promise to be simultaneously lightweight and low in profile, power and cost.
Working of PLEDs
Ø An amorphous film of the PLED material is sandwiched between two metallic electrodes forming the anode and cathode on a transparent substrate
Ø Electronic charges are injected into the polymer from the electrodes: electrons from the cathode, and 'holes' from the anode
Ø The electrons and holes 'capture each other' through electrostatic interaction
Ø Radiative recombination of electron and hole generates light
Ø The wavelength of this emitted light depends on the band gap of the polymer used.
Ø A transparent electrode with a large work function. Indium Tin Oxide (ITO) is commonly used.
Ø A layer of PLED material less than 100nm thick. PPV and its derivatives are commonly used.
Ø A metallic electrode with a low work function, typically calcium.
Features and Benefits of PLEDs
Ø Features include:
v Perception
v Power
v Temperature behaviour
Ø Its benefits are mostly in the following areas:
v Processability, patternable, light emitting, thin films, low voltage, solid state devices, light weight, fast swithching speed and formable substrates
Applications
The world has already seen the first commercial products and prototypes which demonstrate some of the potential of polymer light emitting diode technology.
Ø Philips™ thin-film PolyLED technology will enable the production of full-color displays less than 1 mm thick. Combined with a large viewing angle, high brightness and contrast, and full video capability, PolyLED displays are ideal for the next generation of information displays.
Ø The Sensotec Philishave is the first ever product equipped with a display based on superior PolyLED technology and is prominently featured in the latest James Bond movie, Die Another Day
Ø The Kodak display AM550L, with PolyLEDS features 165-degree viewing on a 2.2-inch screen that is up to 107 percent larger than the LCDs on most cameras
Ø New 639 mobile telephone from Philips, using 'Magic Mirror' technology. The clam shell design provides a mirror for checking personal grooming when it is closed. Incoming calls are indicated by the display showing through the mirror - a highly effective use of a secondary display.
Ø Other new fields of applications, such as light emitting advertisement, corporate identity signs, greeting cards, gadgets etc.
Ø These are just a brief look at the advancement of PLEDs, in the near future the PLEDs will completely replace conventional display panels, lighting instruments. The PolyLED has three major advantages over its silicon counterpart: it has a high contrast, a high brightness and requires much less power. In addition, the production of the PolyLED has fewer restrictions in terms of size. This offers the prospect of the ultimate use of the PolyLED: a cheap, lightweight, roll-down screen for a computer, a TV or a laptop, made entirely of plastic.
