Flexible OLEDs - The state of play

Were we not promised super-thin bendable, flexible and rollable organic LED (OLED) displays?

Phones like Samsung's Galaxy and HTC's Desire have shown us that the rigid versions can deliver stunning images. So where are those flexies?

It turns out that they are also here, at least in proof-of-concept form, and they could even be on the shelves next year.

Sony has shown flexible OLED displays at the Society for Information Display conference more than once in the past few years, and Samsung has demonstrated flexible displays and said it will have products on the market in 2014, with rumours suggesting the Samsung is bringing this forward to 2012.

This said, significant hurdles remain in all four parts of a flexible OLED display: active matrix, substrate, OLED and barrier layers.

Like LCDs, a video-speed OLED display needs a matrix of drive transistors, thin-film transistors (TFTs), and flexible OLED displays will need flexible transistors.

Because OLED is a current-driven technology its transistors have to switch a non-trivial current, compared with LCD TFTs that are only required to change the voltage on what is essentially a capacitor.

Dr Jan Genoe is head of the Polymer and Molecular Electronics (PME) group at Belgian research lab Imec. His group works with the Netherlands' lab TNO on backplanes for rollable AMOLED displays at their Holst Centre in Eindhoven.

Both teams have been in the European Flame (flexible organic active matrix OLED displays for nomadic applications) project.

"Our main goal is transistors on flexible foil for making backplanes for OLED and other flexible displays," he told Electronics Weekly. "Our focus is truly rollable displays that can be rolled to 7mm diameter 10,000 times."

For LCDs on glass, amorphous silicon TFTs have been the order of the day or, for better quality displays, higher mobility polycrystalline transistors made by lasering amorphous silicon.

However, as far as anyone can tell, amorphous silicon has reached its slightly-disappointing full potential and will not be good enough for -current-driven displays.

The higher-mobility polycrystalline materials also have limitations in current-driven technologies because different crystals have different characteristics leading to variations between neighbouring transistors. With LCDs, this will show up as a slight difference in pixel speed, which is irrelevant. "One really crucial thing for displays is uniformity because eyes are very sensitive to slight differences," said Genoe. "Voltage-driven displays are very forgiving of transistor-to-transistor variations. In a current-driven display you will see the small differences."

So OLEDs need materials with higher mobility than amorphous silicon, and better uniformity than polycrystalline silicon.

"Our main focus is organic or oxide transistors on plastic foil," said Genoe, "and nowadays there is a shift to oxide transistors."

Oxide transistors are from a class of devices made from compounds that include metal and a non-metal atoms like oxygen. Copper oxide transistors are one of the earliest examples.