Imperial College researchers move closer to electrically driven plastic laser diodes

May 27, 2008--Although many devices based on organic-molecule-based semiconductors or plastics have been developed such as organic light emitting diodes (OLEDs) for displays and lighting, field effect transistors for electrical circuits, and photodiodes for solar energy conversion and light detection, plastic laser diodes remain the only major device type not yet demonstrated until now.

One of the main stumbling blocks is that it was widely considered that plastic semiconductor laser diodes would be impossible to produce because scientists had not found or developed any plastics that could sustain a large enough current whilst also supporting the efficient light emission needed to produce a laser beam. But this was before researchers at Imperial College (London, England) published their findings regarding a new plastic material in Nature Materials in April. The plastics studied, synthesised by the Sumitomo Chemical Company in Japan, are closely related to PFO, an archetype blue-light emitting material. By making subtle changes in the plastic's chemical structure the researchers produced a material that transports charges 200 times better than before, without compromising its ability to efficiently emit light--indeed the generation of laser light was actually improved.

Professor Donal Bradley, lead author of the new study and head of Imperial's Department of Physics said, "This study is a real breakthrough. In the past designing polymers for electronic and optoelectronic devices often involved maximising one key property in a material at a time. When people tried to develop plastic semiconductors for laser diode use, they found that optimising the material's charge transporting properties had a detrimental effect on its ability to efficiently emit light, and vice versa."

"The modifications made to the PFO structure have allowed us to convincingly overcome this perceived incompatibility and they suggest that plastic laser diodes might now be a realistic possibility," added co-author Paul Stavrinou. Bradley, Stavrinou and their colleagues point out that plastic laser diodes could operate across a much more substantial wavelength range spanning the near ultraviolet to the near infrared.

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