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Polymer yields two-color switch

09 Jan 2003

Philips researchers make an electroluminescent switch that could simplify the manufacture of color displays.

Scientists in The Netherlands have fabricated an electroluminescent device that can reversibly switch between red and green emission (Nature 421 54).

As well as providing a very simple way to make color switches for signalling, the University of Amsterdam-Philips collaboration believes that the device will greatly simplify future color displays because the two-color element is the first to be based on a single-layer structure.

"[Only] two - instead of three - materials are needed, meaning a significant simplification in manufacturing," said Klemens Brunner from Philips. "So you can build up a display using two pixels instead of three."

The key to the new device is the material sandwiched between its two electrodes. "It is the combination of the semiconducting polymer with a di-nuclear ruthenium complex that does the trick," explained Brunner.

To generate the two different colors, two different electrodes are needed so the polymer is sandwiched between one made from gold and one from indium tin oxide (ITO).

Under forward bias, the doped polymer emits red light (centered at 620 nm) corresponding to the excited state of the ruthenium complex. Under reverse bias, the ruthenium complex acts as an electron bridge and this populates the excited state of the polymer instead, which gives a green emission centered at 510 nm.

Both these colors are saturated, meaning that no green emission is seen in the red state and vice versa.

Brunner told Optics.org that the team is currently working on material systems showing a similar switching effect between two other colors.

He estimates that commercial devices based on the new material system could be seen in as little as two years. As well as displays applications, he believes that the enhanced brightness and improved color saturation of the new material could see it find uses in solid-state lighting.

Michael Hatcher is technology editor of Opto and Laser Europe magazine.

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