TOKYO With new applications like electronic books on the horizon, LCD makers are being called on to cut power consumption while providing better-looking images. A number of vendors, including Toshiba and Sharp, are taking a reflective tack, developing LCDs that efficiently utilize ambient light as their lighting source. Sanyo is using a hybrid approach.
Reflective active-matrix (AM) LCDs with color filters are already appearing in such products as the color version of Nintendo's Gameboy. But color filters severely degrade efficiency and also cut effective resolution by one third. At the recent International Display Workshop in Kobe, Japan, Sharp and Toshiba discussed reflective AM LCDs that generate color by means of multiple layers rather than filters, promising greater brightness per unit of power or extended battery life.
Toshiba Corp. first demonstrated a triple-layer reflective LCD in the spring of 1997, a 3.4-inch-diagonal, 1/8-VGA display containing yellow, cyan and magenta guest-host LCD layers. Kazuyuki Sunohara, research scientist at Toshiba Materials and Devices Research Laboratories, admitted that precision alignment of the three layers was initially difficult and expensive. And once assembled, the resulting device suffered from parallax, with the dots out of alignment.
The company has since revamped its trilayer scheme and solved both problems with a new structure it calls "stacked films of encapsulated liquid crystal," Sunohara said. In the original structure, each layer had its own AM array of thin-film transistors (TFTs). The new structure uses only one TFT array substrate on the bottom layer, with three TFTs per pixel to drive each of three colors. This makes alignment as simple as making a conventional AM LCD, said Sunohara.
Toshiba researchers have built a one-pixel device with reflectivity of 60 percent and contrast ratio of 7:1, he said.
Sharp Laboratories of Europe Ltd. is likewise pursuing a layered guest-host reflective LCD, but its R&D revolves around a two-layer device using red, green and blue dots. According to Henning Molsen, senior researcher at the lab, this technique "has less problem of parallax at high resolution and it is slimmer, more lightweight and easier to align and manufacture." And yet, he admitted, maximum reflectivity is theoretically less than that of a three-layer device.
Atop a TFT substrate with a reflective layer, Sharp places a guest-host LC layer, a 6-micron-thick polyethylene telephthalate polymer layer with ITO electrodes on both sides, a second guest-host LC layer and a top transmissive TFT substrate. In the device, each pixel is divided into six dots, three driven by the top TFT array, three by the bottom TFT array. Molsen said he and his colleagues have fabricated a 20 x 20-mm test cell with 40 percent reflectivity and a contrast ratio of 5:1.
As Sharp and Toshiba pursue reflective LCD structures, Sanyo has begun sampling a hybrid LCD that uses a hybrid collector backlight (HCBL) to operate both as a very low-power reflective display and a transmissive backlit display, depending on ambient lighting conditions. In relatively dim light, this LCD behaves like a conventional transmissive LCD, using a cold-cathode fluorescent lamp for illumination. But in a bright room or in sunlight, the hybrid collector backlight captures and reflects ambient light to illuminate the LCD.
"We applied compact-disk molding technology to form the light-collecting and conducting panel," said Kiyoshi Yoneda, general manager of Sanyo Semiconductor Business Headquarters. "We can mold the HCBL into one unit, so the production cost won't go high-just about 10 percent more than existing backlights."
Sanyo has started sampling HCBL-equipped LCDs, specifically 2-inch-diagonal AM LCDs using low-temperature polysilicon TFTs. The company intends to use the new lighting scheme for larger panels in the future. When the backlight is active, the 2-inch LCD provides 230 nits of brightness and consumes 1.03 W. When it is illuminated by reflected light, that dips to just 0.33 W, and under 100,000-lux ambient lighting conditions, it delivers a bright 400-nit image.