Plastic LEDs break telecommunications barrier

Researchers in Israel report in today’s Science that they can get polymers to emit near-IR radiation by incorporating tiny nanocrystals in the polymers.

Polymers that emit visible light have stirred excitement with the prospect of inexpensive, flexible products. The optical telecommunications market, however, seemed closed to these new light-emitting polymers because the plastics could not emit efficiently in the near-infrared (near IR) band where the optical fibres that carry the communications are most transparent.

In today’s Science, Dr. Nir Tessler and his team at the Technion-Israel Institute of Technology, together with Uri Banin and his team at Hebrew University in Jerusalem, announce a way to get polymers to emit near-IR radiation by incorporating tiny nanocrystals in the polymers.

Once commercialised, such nanocrystal polymers could potentially cut the costs of the hundreds of millions of telecommunications terminals needed to bring fibre optic communications to individual homes.

Polymer light-emitting diodes (LEDs) are said to be much cheaper to make than conventional solid state LEDs and lasers. In the conventional devices, materials are laid down in a vacuum and exposed to light through a patterned mask. Then part of the layer is removed by acid in a complex, multi-step process. But because polymers are soluble in various solvents (organic solvents, water, and acetone), polymer layers can be sprayed onto materials with ink-jet printers, forming devices as the solvent evaporates in a much simpler and cheaper method. Visible light-emitting polymers already are being incorporated into products ranging from flat panel displays to infant mobiles.

Optical telecommunications, however, require near-IR radiation, for in this bandwidth optical fibres are almost entirely transparent.

‘Researchers thought that polymers could not efficiently produce near-IR radiation,’ said Dr. Tessler, who was among the pioneers at Cambridge University in England who found that polymers could be used to emit laser light. ‘The problem is that the softness and flexibility of polymers are associated with vibrations (motion of atoms) on the molecular scale. These vibrations bled off most of the energy of the radiation.’

Consequently, polymers or polymer-erbium composites produce near-IR radiation at a very low efficiency – around 0.01 per cent – far below that needed for economical operation.

Dr. Tessler’s solution to this problem is to incorporate nanocrystals of solid semiconductors into the polymer.

‘The polymer is used to create the necessary device structures and the nanocrystals act as near-IR emission centres,’ Dr. Tessler said. ‘On top of that the polymer conducts the electricity to the nanocrystals, where the near-IR radiation is emitted.’

The nanocrystals are reportedly designed to have a shell of zinc selenium, which isolates a core of indium arsenide from the vibrations of the polymer, acting as a nano-scale shield. In this way, when the nanocrystals are made to emit near-IR radiation, the energy is not linked to the polymer vibrations and hundreds of times more radiation is produced.

The Israeli team has demonstrated an efficiency of two to three per cent at a wavelength of 1.3 microns, within the range of interest in fibre optic telecommunications. Since the nanocrystals are dissolved within the polymer, the polymer-plus-nanocrystal solution can be ink-jet printed just as other polymers can.

The Technion and Hebrew University research groups are now developing different nanocrystal-polymer combinations to increase efficiency another 10-fold to the range of 20-30%.