FLAT-PANEL DISPLAYS: Organic electronics support rollable display

At the Society for Information Display (SID) International Symposium 2005 (May 22−27; Boston, MA), researchers from a Philips spin-off company demonstrated an active-matrix display that can be rolled into a smaller package when not in use. Currently, 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 in the SID exhibition demonstrated the quarter-VGA (QVGA; 320 × 240 pixel) monochrome display that measures 0.5 in. high when rolled, but almost a 5-in. diagonal when opened (see figure). 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.

Electrophoretic display

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

At the conference, E-Ink’s electrophoretic front-plane technology won SID’s “Display material or component of the year” Gold award. The technology solves several problems with electrophoretic displays and producing four-gray-level displays.

Darren Bischoff of E-Ink explains that the 50-μm display layer contains oppositely charged white (titanium dioxide) and black particles. The particles are trapped in a transparent fluid within a microscopic capsule. Depending on which submicron particles are closest to the surface, light is scattered back (white state) or absorbed (black state). Because the electrophoretic effect is multistable, 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.

Organic electronics

The film consists of electrophoretic microcapsules in a polyethylene terephthalate polymer binder, coated onto a polyester/ indium-tin-oxide sheet, which is then hot-roll-laminated onto the backplane. Gelinck explained that using organic semiconductors allows the use of low-cost spin coating, and the decreased maximum processing temperature increases the choice in plastic substrates. In this case, the group used polyethylene naphthalate on a 25-μm-thick freestanding foil, which is laminated during processing to a reusable rigid support.

Patterning the thin-film transistors (TFTs) for the active-matrix display was done with a four-mask photolithographic process. The group was able to achieve a typical registration better than 2.5 μm over a 150-mm-diameter wafer. The TFT channel length was 5 μm and the width was 140 μm.

The group used an inverted staggered-electrode device architecture with gold electrodes. The devices have an average field effect mobility of 0.01 cm²/Vs at a gate bias of -20 V, a sharp on/off transition with about six orders of magnitude on the on/off current ratio, and a threshold voltage close to 0 V.

Because the front plane 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 requires the maximum-length pulse. Intermediate shades of gray are achieved by applying a shorter pulse (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 two months at ambient conditions the contrast degradation is 10%, while the field-effect mobility is reduced by 30%. Whether the display is used or not, the electronics degrade over time.

“In general, the materials used to organic semiconductors are relatively stable under ambient conditions,” says Zhenan Bao of Stanford University (Palo Alto, CA), “but encapsulation is needed because organic TFTs change their electrical characteristics with a small change in humidity.”

Hans Driessen of Philips notes that a reference design kit will be ready by the end of the year and the earliest commercial products aren’t likely to appear until 2007. Meanwhile, however, Gelinck and others are investigating field-shielding pixels to increase the optical aperture of the display from 79% to 95%.

REFERENCE

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