16-01-2012, 01:25 PM
Light-emitting polymers
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ABSTRACT
The seminar is about polymers that can emit light when a voltage is applied to it. The structure comprises of a thin film of semiconducting polymer sandwiched between two electrodes (cathode and anode).When electrons and holes are injected from the electrodes, the recombination of these charge carriers takes place, which leads to emission of light .The band gap, ie. The energy difference between valence band and conduction band determines the wavelength (colour) of the emitted light.
They are usually made by ink jet printing process. In this method red green and blue polymer solutions are jetted into well defined areas on the substrate. This is because, PLEDs are soluble in common organic solvents like toluene and xylene .The film thickness uniformity is obtained by multi-passing (slow) is by heads with drive per nozzle technology .The pixels are controlled by using active or passive matrix.
The advantages include low cost, small size, no viewing angle restrictions, low power requirement, biodegradability etc. They are poised to replace LCDs used in laptops and CRTs used in desktop computers today.
Their future applications include flexible displays which can be folded, wearable displays with interactive features, camouflage etc.
Introduction-Imagine these scenarios
- After watching the breakfast news on TV, you roll up the set like a large handkerchief, and stuff it into your briefcase. On the bus or train journey to your office, you can pull it out and catch up with the latest stock market quotes on CNBC.
- Somewhere in the Kargil sector, a platoon commander of the Indian Army readies for the regular satellite updates that will give him the latest terrain pictures of the border in his sector. He unrolls a plastic-like map and hooks it to the unit's satellite telephone. In seconds, the map is refreshed with the latest high resolution camera images grabbed by an Indian satellite which passed over the region just minutes ago.
. LIGHT EMITTING POLYMER
It is a polymer that emits light when a voltage is applied to it. The structure comprises a thin-film of semiconducting polymer sandwiched between two electrodes (anode and cathode) as shown in fig.1. When electrons and holes are injected from the electrodes, the recombination of these charge carriers takes place, which leads to emission of light that escapes through glass substrate. The bandgap, i.e. energy difference between valence band and conduction band of the semiconducting polymer determines the wavelength (colour) of the emitted light.
CONSTRUCTION
Light-emitting devices consist of active/emitting layers sandwiched between a cathode and an anode. Indium-tin oxides typically used for the anode and aluminum or calcium for the cathode. Fig.2.1(a) shows the structure of a simple single layer device with electrodes and an active layer.
ACTIVE AND PASSIVE MATRIX
Many displays consist of a matrix of pixels, formed at the intersection of rows and columns deposited on a substrate. Each pixel is a light emitting diode such as a PLED, capable of emitting light by being turned on or off, or any state in between. Coloured displays are formed by positioning matrices of red, green and blue pixels very close together. To control the pixels, and so form the image required, either 'passive' or 'active' matrix driver methods are used.
