Plastic diodes are squeezed for more light

17 Jun 2002

Researchers have made an LED from electrically conducting polymers that can convert between 41% and 63% of incoming energy into light - compared with 10% for conventional LEDs. This discovery, reported in the 25th January issue of Nature by Valy Vardeny at the University of Utah, in the US, and his colleagues at Arizona and Bangalore, is quite a breakthrough. Physicists had believed that no more than 25% of incoming energy could ever be emitted as light from an LED.

This upper limit results from the statistical theory describing interactions between the incoming electrons and the holes in the structure. These interactions release light energy but are restricted in number by the spins of the electrons and holes.

However, Vardeny placed his conducting plastics in a magnetic field at an extremely low temperature and then used a laser, rather than electricity, to make them emit light. He also bombarded the materials with microwaves. It was this, believe the scientists, that caused the increase in light production. They suggest that the microwaves randomize the spins of the incoming electrons and the holes so that they combine more quickly, and a further 25% of the interactions can emit light.

A practical alternative to microwaves, says Vardeny, might be to dope light-emitting plastics with compounds that have the same randomizing effect, such as iron compounds. If this were realized it could mean more efficient light emitters for lasers, displays and domestic applications, ranging from room lights to TV screens.