Quantum dot leap

As the race continues to produce the first organic light-emitting diode display, researchers at Philips are already developing the next generation of thin, flexible screens, based on the use of quantum dots.

Philips Research is developing quantum dot composites, which it expects to outperform the organic LEDs being produced by companies such as Cambridge Display Technologies and Eastman Kodak.

Quantum dot composite LEDs are more stable, and will offer sharper colours than those based on organic materials, said Dr Rifat Hikmet of Philips Research.

Quantum dots are nano-sized inorganic semiconducting materials that emit light when subjected to an electrical current. As the colour of the light they produce depends on their size, they can be precisely controlled to emit very sharp colours, unlike the broader emissions generated by organic LEDs, said Hikmet.

‘When you combine quantum dots with polymers you get LEDs with very sharp emissions, so if you are developing displays you can get pure colours.’The materials are also much more stable, meaning they can be exposed to light, oxygen and moisture without becoming degraded like organic materials, which have to be sealed for protection.

‘If we can make a cathode material that is stable in air and moisture we won’t have to use any protective material, which tends to take up space, so the displays will be thinner.’

LEDs are often applied to glass substrates to protect them from the elements, making it difficult to produce truly flexible displays. Without the need for this protection, quantum dots can be used with plastic substrates, making thin, flexible displays easier to produce, said Hikmet.

The quantum dots are produced in a chemical solution, where they are grown to exactly the desired size. The by-products of this chemical reaction are then washed away, leaving just the dots. These are mixed randomly into the polymer, to form a light-emitting composite. The material can be printed on to surfaces for cost-effective mass production.

‘In displays red, blue and green pixels are arranged next to each other to produce a colour display. The resolution of such a display could be increased if the dots can be arranged on top of each other, but this is difficult,’ said Hikmet.

The researchers are producing small batches of the materials in the laboratory, but according to Hikmet there is no reason why the process cannot be expanded to make large numbers of dots.

Philips researchers are investigating the use of quantum dots as coloured barcodes which could be attached to molecules for biomedical research purposes, said Hikmet.

The company is also developing miniature LEDs using nanowires made of indium phosphide. By controlling the diameter of these nanowires as they are grown the colour of the light emitted by the LEDs can be precisely tuned.