AMOLEDs: A Bright and Flexible Future

Hank Hogan, Contributing Editor, hank.hogan@photonics.com

They’re bright and getting bigger, and they can be flexible as well. AMOLED (active-matrix organic light-emitting diode) displays promise devices that offer vibrant color, fit in the palm of the hand or across the wall of a room, and come in sheets that can be rolled out – or up – as needed. However, problems with size, lifetime and electronics have to be addressed, along with other issues.

Jennifer Colegrove, vice president of emerging display technology at the Santa Clara, Calif.-based market analysis firm DisplaySearch, forecasts substantial growth in AMOLED displays over the next few years. In 2010, total sales topped $1 billion for the first time. By 2017, the company predicts, the market will swell 15-fold.

At right: Small-form-factor screens, such as on these smart phones, are currently the main market for AMOLED displays. Courtesy of Samsung Mobile Devices.

“The market growth will be pretty healthy in the next several years, given all of the investment and capacity expansion already under way,” Colegrove said.

Today, AMOLED sales are almost entirely in cell phones and other small-form-factor displays. Toward the end of the decade, televisions will represent a substantial part of the market, although small sizes will still lead in active-matrix OLED displays. As the name implies, such displays use an electronic array to turn specific OLED pixels on or off, requiring a backplane that spans the device.

At left: Larger-size AMOLED displays are on the horizon, as evidenced by this 14-in. AMOLED TV. Courtesy of AU Optronics.

That transition to a market with a significant fraction of larger sizes assumes that a variety of problems are solved. Some are technical in nature, such as the fact that OLED materials, particularly blue pixels, today have half the lifetime of that of LCDs or other competing technologies. That is, blue OLED pixels reach the point of 50 percent of their original brightness in much less time than LCD pixels do. This is not a problem for handsets because people change devices every few years, but it is an issue for monitors and televisions.

Another technical hurdle is that OLEDs, unlike LCDs, are driven by current, not voltage. Thus, AMOLEDs require advanced backplane technologies.

On the plus side, AMOLED displays are emissive and don’t require a backlight, unlike LCDs. They also offer faster switching than LCDs – an advantage in some 3-D TV implementations and other applications.

At right and below left: Crisp images show the benefits of AMOLED displays, which are emissive and thus don’t require a backlight. Courtesy of Samsung Mobile Devices.

Today a large percentage of AMOLED display sales are by a single company, Seoul, South Korea-based Samsung Mobile Devices. This situation is partly the result of Samsung’s technological expertise, as evidenced by its 2010 introduction of Super AMOLED. This year, the technology will allow a display to be combined with a touch screen, eliminating brightness-decreasing layers and yielding various advantages.

“Super AMOLED is 10 percent thinner. Outdoor readability has been improved. It consumes less power, about 20 percent less,” said Samsung Mobile Display spokesperson Hojung Kim.

To support demand for its devices, the company is expanding factories and investing $2.1 billion in its next-generation manufacturing facility. Such expenditures are indicative of the amount of money required to get into the AMOLED market.

This single-manufacturer dominance may be changing, with an expansion of AMOLED capacity under way by established players such as Seoul-based LG Display. Newcomers are also entering – or re-entering – the market. For example, AU Optronics Corp. of Hsinchu, Taiwan, plans to mass produce displays for the mobile market, beginning in the latter half of this year. This comes after the company exited the market a few years ago. According to Yusin Lin, the company’s OLED technology division director, AU Optronics is developing AMOLEDs for both tablet and television applications, but the initial focus will be on handsets and other small-form-factor displays.

A 2.4-in. transparent AMOLED in-cell touch panel. Courtesy of AU Optronics.

“At the moment, the demand for small- and medium-sized AMOLED panels is enormous, even with the supply from Samsung and others,” Lin said.

The company also is a supplier of active-matrix LCD flat panels, using low-temperature polysilicon thin-film transistors for the backplane. Despite the differences in drive requirements between LCDs and OLEDs, Lin said that the company uses this technology for both.

Future backplanes, though, may demand a different solution. AMOLEDs could be used in a flexible display – one that could be rolled up to fit into a pocket and then unfurled when needed. That requires a flexible backplane, a problem for silicon unless it is very thin, segmented into small pieces, or both. AU Optronics has plans to introduce a flexible display, Lin said, and the company is looking at backplanes based on silicon and organic transistors.

Such a highly flexible, lightweight and mobile display is the goal of a European multi-institution research program dubbed FLAME, or flexible organic active matrix OLED displays for nomadic applications. According to program head Paul Heremans, who is director of large-area electronics at the Leuven, Belgium-based research consortium IMEC, there should be a working demonstrator of the concept by the middle of this year. It will consist of a stack of organic transistors, OLED materials and protective encapsulation, all sitting atop a flexible substrate 100 µm or thinner.

Heremans noted that organic transistors have been shown to be suitable drive current producers and that the process flow works. The main challenge lies in processing the dielectric layer between the transistors and the OLEDs. Organic transistors, he said, are fragile, and their performance easily degrades when layers are processed on top of them – something that must be done to the dielectric due to electrical connectivity requirements.

“You need via holes, and that’s because the OLED requires current drive, not voltage drive. Voltage drive you can couple through capacitance,” Heremans said.

Although he expects the proof-of-principle demonstrator to be successful, more work will have to be done before a commercial display can be built. For instance, the reliability of the display will have to be verified, particularly after it has been rolled up and spread flat numerous times. Likewise, the ability of the screen to be economically scaled to large sizes will have to be determined.

When it comes to improving the economics of large-format manufacturing, DuPont has its own solution. John Richard, global business manager for Santa Barbara, Calif.-based DuPont OLEDs, noted that a high-definition television contains millions of pixels, with only a handful of defects allowed. Currently, small displays are created by evaporating OLED material through a metal mask, with pinholes aligned to where the pixels will be. It’s difficult to scale up this approach to larger sizes, due to both display quality and cost issues, Richard said.

DuPont, which produces small-molecule solution-processable OLED material, has spent years coming up with a different solution. Last May, the company announced a commercial-grade device with 16 nozzles that precisely pattern the pixels on a substrate, putting down five groups of three-color RGB sets at a time until the substrate is covered.

“This printer runs up to five meters per second and accelerates at up to 10 g’s. It’s incredible how rapidly it can print a display,” Richard said.

For a 730 x 920-mm Gen4 glass size, he said, the printing time runs as short as two minutes, adding that there is no fundamental engineering limitation or cost constraint that prevents scaling up to handle Gen8 glass size.

A newly developed commercial printer from DuPont could cut the cost of manufacturing large AMOLED displays. Courtesy of DuPont.

Nominally 2200 x 2500 mm, Gen8 glass is big enough to produce six 55-in. television screens, and the sheet would have to be segmented this way for the current DuPont device to print a display on it. Richard noted that the advantage of the technology is greatest for larger displays, such as tablets, monitors and televisions. Thus, the current generation of phone-size AMOLEDs would see little or no benefit.

The company plans to license the technology to display makers. Richard characterized the market response to DuPont’s technology as very positive. However, there are not yet any products on the market produced with the new technique, in part because building a complete display requires integrating the OLED materials, printing process and electronics.

Other companies are offering their own solutions, which illustrate the interaction between the various display components. A case in point comes courtesy of Universal Display Corp. of Ewing, N.J.

“Our phosphorescent OLED technology and materials are responsible for reducing the power consumption of an OLED by a factor of four,” said Janice Mahon, vice president of technology commercialization at Universal Display. She was comparing the company’s technology to fluorescence-based small-molecule and polymer-based OLED materials.

Shown at right is a flexible display that can be rolled up. It is built using OLEDs and organic transistors.

Bottom, geometric patterns of OLED pixels driven by pentacene transistors; bottom, backplane of pentacene transistors, including interlayer to OLEDs and patterned reflective OLED anodes. Courtesy of Paul Heremans, IMEC.

In addition to selling OLED material, Universal Display licenses its technologies to its AMOLED customers. Mahon predicted that the next areas for AMOLED would be larger sizes and flexible displays.

For futuristic flexible displays that can be used in windshields and elsewhere, an effort at Purdue University in West Lafayette, Ind., points to one possible AMOLED display transistor technology. Researchers there, along with others from Northwestern University and the University of Southern California, three years ago demonstrated an AMOLED display based on a nanowire transistor. The project has not yet led to a commercial device. However, it does illustrate what will be required to create an AMOLED display that takes advantage of what the technology offers.

David Janes, professor of electrical and computer engineering at Purdue, said: “If you want to have a display that’s both flexible and transparent, then your electronics either have to be minimal in size, or they have to be transparent.”