Researchers around the world striving to develop large flat TV screens
that could hang on the wall may soon be joined by organic chemists.
A team of polymer scientists from Austria have succeeded in obtaining
blue light from light-emitting diodes made from an organic polymer. LEDs,
the familiar red lights on stereos and other devices, are the building blocks
of large flat displays. This, combined with the work of researchers at the
University of Cambridge who obtained yellow-green light from a similar polymer
in 1990, makes full-colour polymer displays possible.
Conventional LEDs are made from inorganic materials using processes
similar to those used to make silicon chips and are delicate and expensive
to produce. Polymer LEDs would be tougher and cheaper.
Although conventional LEDs are available in many colours, diodes that
emit blue light have, until very recently, been very expensive and inefficient.
The group of researchers, working at the Technical University of Graz
in Austria and led by Gunther Leising, made its blue LED from the semiconducting
organic polymer poly(p-phenylene), or PPP (Advanced Materials, vol 4, p
The ability of a material to emit light of particular colours depends
on the difference between the energy level in the material where electrons
usually reside and the next energy level up. When electrons are given extra
energy they jump up to the higher level across this ‘bandgap’. They then
spontaneously jump back down releasing their extra energy as a photon of
The colour of the light depends on the size of the bandgap. PPP was
chosen because its bandgap, of 2.7 electron volts, is just the right size
to produce blue light.
The Austrian group made their LED by drawing out strands of a precursor
to PPP from solution, spinning it into a thin layer and depositing it onto
a glass substrate coated with a conducting layer of indium tin oxide. They
then convert the precursor to PPP by heating it in a vacuum at 340 °C.
Finally, they deposit an aluminium electrode onto the surface of the PPP
film so that the polymer is sandwiched between two conductive layers. When
they apply a voltage of 12 volts between the conducting layers, blue light
emerges out of the side of the sandwich.
The structure of the LED is similar to the polymer LED reported by the
Cambridge researchers, led by Richard Friend, which was based on the polymer
poly(p-phenylene vinylene), or PPV. Both PPP and PPV belong to the class
of materials known as conjugated polymer molecules which behave like semiconductors
in one direction only because electrons from double bonds in the polymer
have some freedom of movement along the polymer chains.
The advantage of polymer LEDs over present devices is that they can
be easily coated onto a cheap substrate material, and processed using only
moderate temperatures. In contrast, the manufacture of blue LEDs based on
silicon carbide is costly and time-consuming.
The efficiency of polymer LEDs, in terms of light produced from electrical
input, is low, at 0.01 per cent, when compared to recently developed inorganic
LEDs. Last month, two Japanese groups reported improved blue LEDs based
on inorganic semiconductors. Isamu Akasaki of Nagoya University developed
LEDs made from gallium nitride (Technology, 14 December, 1991). They require
4 to 5 volts at room temperature, and convert up to 0.8 per cent of electrical
power into light – 80 times as efficient as existing blue LEDs, both organic
The Matsushita Opto-Electronics Research Laboratory in Osaka also reported
a new way to make blue LEDs from inorganic zinc compounds. Earlier devices
made of zinc sulphide were very inefficient be-cause of their low electrical
conductivity. Toshiya Yokogawa and Tadashi Narusawa used many alternating
layers of zinc telluride and zinc sulphide, each 2 nanometres thick. This
combined the larger bandgap of zinc sulphide with the better conductivity
of zinc telluride.