Sony Corp developed an OLED panel that is as thin as 80μm and can be rolled up at a curvature radius of 4mm. The company exhibited it at SID 2010, an international conference on display technologies, in Seattle, the US.
Sony said that the most distinctive technological feature of the panel is the organic semiconductor material "PXX (peri-Xanthenoxanthene) derivative," which was developed by the company. The organic TFT made by using the PXX derivative has a carrier mobility of 0.4cm2/Vs, which is four times higher than that of pentacene, a common organic semiconductor material.
As a result, it became possible to use organic TFTs to form a gate-driver circuit on a resin substrate. Because there is no need to embed a driver chip, the entire panel can be folded.
"When we announced a prototype at SID 2009, it was just bendable," said Makoto Noda, Flexible Display Development Group, Sony Corp. "This time, it is rollable."
However, Sony embedded a driver chip this time because, with the carrier mobility of 0.4cm2/Vs, it is difficult to form a source driver circuit on a resin substrate. As a result, the exhibited OLED panel can be folded only to one side.
The thickness of the panel is 80μm. (1) The resin substrate at the bottom of the panel, (2) organic TFTs and organic EL devices and (3) the resin cover at the top of the panel measure 20μm, 35μm and 25μm, respectively.
As for manufacturing processes, the PXX derivative and the electrode materials are made by a vacuum process while other insulating films are formed by a solution process at a temperature of 180°C or less. The luminance efficiency of the organic EL device was enhanced by using gold (Au), which has a high work function, for the source/drain electrode.
However, the panel can be driven with a common metal material such as copper (Cu) without problems, Noda said. The gate electrode and the gate insulating film are made with commonly-used materials, he said.
Sony also mentioned the properties of the TFT (at the time when the panel is folded) and the durability of the panel. As for the properties of the TFT, when the panel is folded at a curvature radius of 3mm, the change in carrier mobility is within 5% and does not affect the displayed image.
In regard to the durability, when the panel was rolled around a cylinder with a curvature radius of 4mm 100,000 times with its resin cover (screen side) facing up, the properties of the TFT and the display capabilities such as the gamma characteristic and the brightness did not change, Sony said.
At the conference, Sony demonstrated the OLED panel. To get a glimpse of the panel, many engineers crowded around it with cameras. When I interviewed some of them, the most prevalent answer was, "It looks more beautiful than I expected."