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Optoelectronics Research Group
www-oe.phy.cam.ac.uk
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Look around you and you will find a diverse range of things, from cling film to
artificial limbs, made from plastics. Plastics themselves are made from long chain
organic molecules (fig. 1a) consisting of hydrogen and carbon arranged in different
ways. Most of these well-known plastics are insulators and can be used as a shielding
on electric cables to protect you from electric shock.
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a) Polythene or Poly(ethane)
b) Poly (Phenylenevinylene) (PPV)
c) Poly (3-Hexylthiophene) (P3HT)
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Figure 1. Polythene is used in plastic bags whereas PPV is commonly used in
organic LEDs and P3HT is used in organic transistors.
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A newly discovered group of polymers have electrical properties similar to silicon,
which is a semiconductor. The polymer chains have alternating double and single
carbon bonds (fig. 1b, fig. 1c) and the materials made from them can conduct
electricity under certain controllable conditions. Research in the Optoelectronics
group is to understand how these polymers conduct electricity and how they can be
used as LEDs (light emitting diodes), photovoltaics (solar cells) and transistors.
LEDs are extremely common and are used as tiny, low current lamps; for example as
the 'on' indicator on electrical appliances. They emit light by a process in which
electrical energy is transferred into light energy without any heating effect, as in
the case of tungsten filament lamps. Most LEDs are made from inorganic silicon-like
materials.
However, polymer LEDs are easier to make than conventional LEDs. They have all the
advantages associated with plastics such as cheapness. An example of a light emitting
polymer is PPV shown in (fig. 1b). Polymer LEDs can be made which emit red, green and
blue light, which leads naturally to making full colour flat TV screens (fig. 2) and
back lit displays for mobile phones.
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(fig. 2) A green and black plastic TV screen
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(fig. 3) Prototype security tag
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Solar cells are found in solar powered calculators, clocks and satellites, where
the energy from the sun is used to generate electricity. In an LED, electrical
energy is transferred into light energy. By running an LED 'backwards' then light
energy can be transferred into electrical energy in a solar cell. Light absorbed
by the polymer generates positive and negative charges that can be collected at
electrodes and the solar cell will then act like a battery. The problem is that
positive and negative charges like to stick together but they can be pulled apart
by making the solar cell from two different polymers, one of which attracts negative
charges and the other positive charges. Conventional solar cells are made from
silicon which is costly to process, but polymer solar cells can be produced very
cheaply and large arrays will be able to bend flexibly around objects, such as round
a house.
An FET is an electronic switch. When a voltage is applied to one terminal, a charge
flows between two other terminals. The FET is the basis of all modern microchips and
a modern computer has several million FETs processing the information. Using polymer
FETs, small flexible microchips can be produced at next to no cost. The best polymer
to use in organic FETs is shown in (fig. 1c). It operates about 1000 times slower
than FETs made of silicon. However, because it is soluble and can easily be processed
into thin films, it may be useful in low performance, high volume applications such
as automatic radio identification tags (fig. 3).
Imagine going into a supermarket and filling your trolley without having to go out
through a checkout. Your bill would then be sent at a later date. Radio waves emitted
by the EXIT would power a plastic transistor circuit a few centimetres across which
was printed onto every product. Each circuit would alter the radio waves, which would
be detected by a nearby aerial. This is similar to security tags already used in
shops. The tags however, would be more sophisticated and would be able to identify
the product. Shoplifting would be more difficult!
Watch out for polymer displays in mobile phones and lap top computers in the next
few years. Maybe one day we will be able to watch TV on a polymer screen and then
roll it up like a poster when the programme is finished! Plastics have a very exciting
future.
(Try searching for plastic LEDs or plastic FETs)
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