New bendy LEDs don't crack under the strain.

Electrodes made from conducting polymers make LEDs more flexible, according to a team from Imperial College London, UK.

Polymer LEDs consist of layers of light-emitting polymer sandwiched between two electrodes. Although various studies have tried to build polymer LEDs on flexible substrates, the electrode materials cannot always cope with being bent. One of the most commonly used cathode materials, indium tin oxide (ITO), is quite brittle, and tends to crack when the LED is flexed. ITO films are usually made by evaporation at high temperatures, but when working with plastic substrates the temperature has to be kept much lower to avoid melting the plastic, and ITO films made at these lower temperatures tend to be of low quality.

The Imperial team replaced the ITO cathode with a conducting polymer layer, made from poly(3,4-ethylenedioxythiophene) and polystyrenesulfonate. They built their LEDs on overhead projector transparencies, and succeeded in making bright yellow LEDs that work even when tightly rolled up into a tube.

'The main challenge now is to drive up the conductivity of the polymer anode to the point where it actually starts to compete with ITO. This won't be easy but we have a few ideas up our sleeves,' said John deMello, who led the research. 'The other issue is LED lifetime. The organic layers are extremely air-sensitive so finding an effective way of protecting the devices inside flexible, transparent packaging is going to be absolutely crucial.'

Commercial applications are already on their way for these LEDs, added deMello. 'The technology has recently been licensed to a start-up company called Molecular Vision, who are using it to develop a new line of disposable diagnostic devices for health care testing. They're combining the LEDs with flexible photo-detectors (also developed at Imperial) to create ultra-miniaturised low cost sensors for the analysis of blood and urine.'

Clare Boothby