Organic electronics support rollable display

BOSTON, MA - At the Society for Information Display meeting in late May, researchers from a Philips spin-off company demonstrated an active-matrix display that can be rolled into a smaller package when not in use. At the moment, users of electronic mobile displays have the choice of (comparatively bulky and heavy) laptops or the small, relatively low-resolution displays in PDAs and cell phones. The makers of the rollable device hope to bridge the gap with a low-power device small enough to be stored in a pocket but that can be enlarged during use.

Gerwin Gelinck of Polymer Vision/Philips Research (Eindhoven, the Netherlands) presented an invited paper about a rollable active matrix display that combines an electrophoretic capsule bistable display and organic electronic backplane.¹ A product concept shown in the SID exhibition shows the quarter VGA (QVGA, 320 × 240 pixel) monochrome display that measures a half-inch diagonal when rolled, but almost a 5-inch diagonal when opened. The display is 100 µm thick, has 300 × 300 µm pixels, can be rolled to a radius of 0.75 cm, and weighs 1.6 g.

For the front-plane of the display, the researchers chose to use electrophoretic film from E-Ink Corp (Cambridge, MA), in part because this technology doesn’t degrade when rolled. With liquid crystal displays, the cell gap is crucial to the display characteristics, so rolling the display creates artifacts on screen.

Darren Bischoff of E-Ink Corp explains that the 50-µm display layer contains oppositely charged white (TiO₂) and black particles. The particles are trapped in a transparent fluid within a microscopic capsule. Depending on which sub-micron particles are closest to the surface, light is scattered back (white state) or absorbed (black state). Because the electrophoretic effect is multi-stable, no power is required to maintain an image. The contrast is better than the average LCD.

“If you have enough ambient light to read a newspaper,” says Bischoff, “you can read this.” The technology has been commercialized in a rigid e-book display by Sony, and will be part of a recently announced Seiko watch.

Because the frontplane is bistable, updating the image takes into account the initial as well as final state of a pixel. A pixel that doesn’t change state needs no pulse, while one that changes from low to high reflectance required the maximum length pulse. Intermediate shades of gray were achieved by applying a shorter pulse (although a lower-voltage pulse would also work). The TFT backplane is addressed at a frame rate of 50 Hz, and the display’s switching speed is about 0.5 s. The display’s white state has a reflectivity of 35%. The optical contrast ratio is about 10.

The lifetime of the display remains a question. The display shown at SID can be bent to a radius of 0.75 cm more than 600 times without image degradation. But after 2 months at ambient conditions the contrast degradation is 10%, while the field-effect mobility is reduced with 30%. The electronics degrade over time, whether the display is used or not.

“In general, the materials used to organic semiconductors are relatively stable under ambient conditions,” says Zhenan Bao of Stanford University (CA), “but encapsulation is needed since organic TFTs change their electrical characteristics with small change in humidity.” Whether the encapsulation of the Philips display resists damage from rolling is another question.

On the show floor, Hans Driessen of Philips admitted that the e-reader isn’t ready to be commercialized. By the end of the year, the group will have a reference design kit ready, and the earliest commercial products aren’t likely to appear until 2007.

REFERENCE

1. G. H. Gelinck et al, Invited paper 3.1, Society for Information Display 05, May 22-27, 2005.