The current state of OLED research

The number of organic LED suppliers grew from six in 2003 to 21 in 2004, with Samsung SDI Co. Ltd. the leader in both shipments and revenue, according to market research firm DisplaySearch of Austin, Texas. Samsung presented a slew of technical papers at SID and showed off a 40-inch diagonal HDTV prototype that it called the first single-sheet, 40-inch active-matrix display with a wide-screen WXGA (1,280 x 800-pixel) format.

OLED emissive display technology can be used in passive- and active-matrix displays. For the most part, passive-matrix OLEDs target mobile-phone subdisplays, car audio displays and MP3 players. Volume shipments of active-matrix OLEDs are low due to low backplane yields, but that hurdle should be overcome this year, leading to higher volumes in 2006, according to DisplaySearch.

Progress in OLED technology is as much about materials and design as about successful manufacturing techniques. "OLED Manufacturing Equipment," a newly released report by market research firm iSuppli Corp. (El Segundo, Calif.), provides a searchable database of some 50 OLED equipment makers and their products. It also profiles key deposition equipment suppliers and surveys panel makers about how well equipment functions in the field.

"We set out to analyze the state of OLED manufacturing and how it affects the development of the market," said Kimberly Allen, director of display technology and strategy for iSuppli.

Three companies — Universal Display Corp. (UDC; Ewing, N.J.), Cambridge Display Technologies Ltd. (Cambridge, United Kingdom) and Eastman Kodak (Rochester, N.Y.) — hold a majority of OLED patents.

At lat year's SID, UDC had announced development of what it said is the first high-resolution, active-matrix transparent OLED (AM-TOLED). The display uses the company's phosphorescent OLED and transparent OLED technologies, along with amorphous silicon backplane technology developed at Kyung Hee University (Seoul, South Korea). UDC envisions full-motion video displays providing transparency when turned off in such applications as architectural vision glass, entertainment, medical and industrial products, and helmet shields for the military.

The company also demonstrated a 120 x 160-pixel quarter QVGA, 200-dot/inch monochrome AM-TOLED.

Meanwhile, researchers from Samsung SDI reported on using AM-OLEDs for large HDTV screens. According to Samsung, AM-OLEDs offer many advantages over LCD or plasma technologies for HDTV, promising thinner, lightweight displays with a wider viewing angle, faster video speed, higher brightness and lower power consumption than LCDs at full-color operation.

Samsung SDI had also demonstrated its 40-inch AM-OLED prototype at SID and said it is gearing up for mass production of AM-OLEDs next year. It recently concluded a patent license agreement with UDC to use the latter's OLED phosphorescent material and related technologies. The phosphorescent material can deliver bright and crisp images with low power, thanks to an efficiency that's about four times higher than conservative fluorescent materials, the company said.

Also, another research group from Samsung Electronics, together with Dupont Display (Santa Barbara, Calif.), described collaborative work on what the team called the "largest full-color, amorphous-silicon, TFT-based, solution-processed active-matrix OLED display." By combining an amorphous-silicon thin-film-transistor architecture and solution-processed OLED emitters, the researchers believe they have achieved the most cost-effective method to date for producing large OLEDs in volume.

"At 14.1 inches, we have broken the 10-inch size barrier and have taken a definitive step toward realizing the mass production of OLED televisions," the teams reported.

Their TFT backplane architecture is identical to that used for LCD backplanes through the first five masks. An optional, sixth mask is used to create the wall layer to contain the OLED material within the pixel area. The commonality between LCD and OLED backplane fabrication greatly reduces costs and mitigates scaling risks through the use of existing fabrication capacity, the team said.

One remaining hurdle for large AM-OLEDs is operating life. While advances in metal and glass encapsulation technologies have extended the life of AM-OLEDs in mobile displays, the goal of at least a 40,000- to 50,000-hour lifetime for HDTV panels remains elusive.

Smaller displays
Progress in smaller OLEDs was reported by AU Optronics Corp. (Hsinchu, Taiwan), where researchers have enabled the display of different images on both sides of a 1.5-inch full-color AM-OLED panel controlled by a single IC driver. The double-sided display has high potential in mobile applications, especially in cell phones, the researchers said.

While other dual-emission AM-OLEDs present images on both sides of the display, those are mirror images to each other, which limits such displays' use in commercial applications wishing to display two independent images.

Today's cell phones with dual displays usually use LCDs rather than OLEDs, with a TFT-LCD as the main display and a color STN-LCD as a subdisplay; some use TFT-LCDs for both the main and subdisplays. In effect, the module on today's cell phones has two LCD displays physically assembled back-to-back. This makes the module thick, heavy and costly.

The 1.5-inch double-sided AM-OLED developed at AU Optronics has only one backplane, fabricated by putting a top-emitting OLED and its bottom-emitting counterpart on the same substrate. The original subpixel area is divided into top- and bottom-emission regions. By individually driving the top- and bottom-emitting subpixels, two images can be displayed on the panel.

To show the different images simultaneously, an image mixer integrates the pictures and then transfers the mixed image signals to an image processor. The processor marks the mixed image signals by individual scan addresses. The signals are then sent to the IC driver according to the individual scan addresses of subpixels, without any image interference, AU said.

Paint it white
White OLEDs are coming, too. High-efficiency white OLEDs are desirable as an alternative to full-color active-matrix OLEDs because they can be coupled with color filters to circumvent the problematic shadow mask for RGB pixelation in production and can help achieve higher display resolution, researchers said.

At National Chiao Tung University (Hsinchu, Taiwan), researchers developed efficient top-emitting OLEDs with broad white-light emission. The top-emitting OLEDs have a modified anode and cathode. The researchers effectively removed the ITO layer, shortening the optical length of the device and alleviating the adverse microcavity effect from which a broad emission of white light is obtained.

Researchers at Eastman Kodak are similarly pursuing white OLEDs. At SID, they described how a highly efficient and stable white OLED enables low power consumption as well as stable emission without use of a color filter. In recent weeks, Kodak has broken its ties with Sanyo, and firmed up relations with LG.Philips.

White OLEDs hold appeal for lighting as well as for displays. At GE Global Research (Niskayuna, N.Y.), researchers are working on large-area white OLED panels that emit 1,200 lumens of illumination-quality light. The researchers explained at SID that general diffuse lighting applications require large OLEDs (greater than 2 feet) in order to provide sufficient illumination and that they must be cost-competitive with today's lighting products.

The researchers used a downconversion approach. A blue-emitting OLED is coupled with one or more luminescent layers, which absorb a fraction of the blue light and re-emit in the green and red spectral ranges. By varying the thickness of the downconversion layers, the composite emission spectrum can be varied to maximize performance for lighting or display apps while keeping the properties of the underlying blue OLED constant.

The architecture divides the device area into smaller emitting elements connected monolithically in series. The required current is controlled by the individual element area rather than by the total emitting area of the device.

The GE team built large-area polyfluoren-based blue OLEDs on 6 x 6-inch glass substrates using the monolithic series architecture. The total area was divided into 144 emitting elements, measuring 1.2-cm2. The structure is configured as 12 rows connected electrically in parallel, with each row containing 12 elements connected in series. A 4-square-foot white-light demonstration panel was made by tiling 16 of these white-OLED devices together on a common substrate.

The resulting panel has an average luminance of 1,000 cd/m2. The efficacy and total lumen output equate to that of an 80-watt incandescent bulb, and the color quality for illumination exceeds that of fluorescent bulbs, the researchers said.