Red and green

Researchers at Philips have developed the first electroluminescent material that produces either red or green light depending on the direction of the current that is applied to it.

Normally, electroluminescent materials emit light of a fixed colour that is determined by the energy of the excited state (bandgap) of the material.

Now, however, researchers at Philips and the University of Amsterdam have developed what they claim to be the first electroluminescent material that produces either red or green light depending on the direction of the current that is applied to it.

The material itself is a homogeneous mix of a semiconducting polymer and a metal complex.

To construct a display device from the material, a single layer of the material is sandwiched between electric contacts (electrodes) of different materials, one made of gold and one of indium tin oxide (ITO). When a positive voltage is applied to the ITO electrode (forward bias) only the light-emission process of the metal complex is triggered and the characteristic red emission of the complex is observed.

When the direction of the current is reversed, the light-emission process in the metal complex ceases, and the device emits a green light that corresponds to the bandgap of the polymer.

According to the researchers, the new material will allow simpler colour displays to be manufactured, because, in the case of full-colour displays, only two, rather than three materials are needed.