Electronic Design

OLED Displays Bring Much-Needed Light To The End Of The Economic Tunnel

A growing number of opportunities in flexible, micro, TV, and transparent displays as well as high-efficiency lighting is paving the way for many large niche applications.

Organic light-emitting diode (OLED) technology continues to buck the industry’s current economic struggles, carving out lucrative applications in numerous display and lighting applications. And indications show that active-matrix (AM) OLEDs rather than passive-matrix (PM) OLEDs will eventually dominate this space.

DisplaySearch forecasts that OLED display revenues will reach $6 billion by 2015, up from $591 million in 2008, with a compound annual growth rate (CAGR) of 40%. OLED TVs will become the largest segment by then, totaling $2.6 billion. Mobile phone displays, which are largely of the PM OLED variety today, will account for $1.9 billion (Fig. 1a).

The market research company also says that while PM OLED displays will grow in the number of unit shipments by 2015, their revenues will stay flat. Meanwhile, the number of AM OLED unit shipments will triple this year and surpass those of PM OLEDs in 2011 (Fig. 1b).

DisplaySearch states that there’s a significant oversupply of PM OLEDs. Further, many companies that set up large PM OLED production lines are finding that those lines are now underutilized due to a limited number of applications and competition from liquid-crystal displays (LCDs). Compared to LCDs, PM OLED displays are hampered by their inability to be produced cost-effectively in large panel sizes like LCDs.

“AM OLEDs are making up for the slowdown in PM OLEDs over the past year,” says Jennifer Colegrove, DisplaySearch’s director of display technologies. “Going forward, it will be important for OLEDs to find a niche market where it will be difficult for LCDs to compete, such as flexible or transparent displays or lighting. OLED developers should also look for opportunities to combine their technology with other hot technologies such as touchscreen.”

Production capacity for AM OLED displays, on the other hand, is ramping up for an expected large demand. Compared to LCDs, AM OLED displays offer a thinner form factor; a wider viewing angle; a faster response time; lower power dissipation; a better color gamut and reproduction; higher contrast ratios; and a wider operating-temperature range.

However, companies still must work out how to scale to larger panels and provide higher operating lifetimes. Also needed are more efficient and longer-life blue OLEDs. To address these issues, designers are turning to amorphous silicon, improved materials, thin-film transistor (TFT) and metal-oxide driver circuitry, and better processing methods that can provide higher yields for TFT backplanes.

An AM OLED pixel turns on and off more than three times faster than the speed of pixels in a conventional motion-picture film. AM OLEDs, which have faster response times and consume less power than PM OLEDs, are ideal for fluid, full-motion video and graphics. They’re better suited to large-screen monitors and TVs, electronic signs, and billboards.

“The power-efficiency benefits of AM OLEDs are much better than those of PM OLEDs,” says Janice Mahon, vice president for technology commercialization at Universal Display Corp. (UDC).

A SIMPLER STRUCTURE
A PM OLED is structurally simpler than an AM OLED and thus less expensive to produce. PM OLEDs can be patterned using conventional fabrication techniques. The entire panel fabrication process can easily adapt to large-area and high-throughput manufacturing. PM OLEDs are well suited for low-cost and low-information-content small display panels with diagonals of 1.6 to 4 in., like those found in mobile phones, MP3 players, and cameras.

Despite the PM OLED’s attributes, it’s AM OLEDs that are becoming the rage. Nearly all major OLED display manufacturers, including Sony, RiT Display Corp., Univision, Nippon Seiki, MicroEmissive Displays, Truly Semiconductor Ltd., Samsung SDI, Taiwan-based Chei Mei EL (CMEL), Pioneer, eMagin, Wintek, and LG Display, have or will adopt the technology.

AM OLEDs are finding homes in highend 3G and 4G mobile phones from Nokia, Sanyo, and Toshiba. Other landing spots include digital cameras, digital photo frames, and portable media players, as well as handheld and free-standing TVs.

Driving OLEDs, particularly AM OLEDs, can be challenging. That’s because unlike LCDs, OLEDs are current-driven. Thus, any variation in a thin-film-transistor (TFT) driving circuit’s performance affects the OLED display’s luminance.

Kodak developed its global mura correction (GMC) technology to deal with AM OLED driving performance variations. (“Mura” is the Japanese word for “error.”) GMC is incorporated in an external driver IC that detects and compensates for mura problems.

Ignis Innovations Inc., which develops and licenses complete backplane and driver technologies for AM OLEDs, offers two technology platforms: AdMo and MaxLife. Admo is aimed at handheld and ultra-mobile devices, while MaxLife targets monitors, desktops, and TVs. Both platforms come in amorphous and polysilicon versions to correct for image-sticking and mura artifacts. At this year’s International Consumer Electronics Show (CES), Ignis demonstrated a 2.2-in. QVGA OLED display driven by the company’s technology.

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That’s not to say PM OLED technology has stood around idle, though. Recently, TDK demonstrated a 3-in. diagonal PM OLED display for mobile phones that features QVGA (320- by 240-pixel) and wide QVGA (W-QVGA) resolution. The display uses Dialog Semiconductor’s first driver IC based on its Smart- Xtend technology.

SmartXtend uses a multi-line addressing scheme with accurate dynamic current matching. This reduces peak currents by as much as 30% through each diode in a driving matrix compared to conventional driving methods.

Then there’s Cambridge Display Technology’s total matrix addressing. It blends the best characteristics of both passive- and active-matrix addressing at little or no penalty. CDT is working on bringing the technology to market.

TVS AND MOBILE PHONES
When Sony introduced its 11-in. diagonal XEL- 1 AM OLED TV a couple of years ago, a largerscreen- size OLED didn’t seem far behind. But so far, such screens haven’t materialized due to their high manufacturing cost, even though the XEL-1 is available. At this year’s CES, however, Sony CEO Howard Stringer said in his keynote address that “Sony’s next step is an OLED TV with a diagonal of 20 to 30 in.”

Four years ago, Samsung showed off a working prototype of the first 40-in. AM OLED for flat-panel TVs. And in early 2007, Sony demonstrated prototypes of a 27-in. AM OLED TV. Neither has materialized on the commercial market.

One obstacle in developing OLED TVs with mediumto large-sized diagonals involves producing blue OLEDs with acceptable quantum efficiency levels and lifetimes. Blue, the most energy-intensive of the three primary colors of red, green, and blue, is mixed in with them to make up a full array of colors.

UDC developed a patented high-performance phosphorescent OLED (PHOLED) technology, UniversalPHOLED, which offers up to four times the efficiency of conventional OLEDs. The technology can be found in mobile phones, multimedia players, and other display devices. It also plays a critical role in the development of novel OLED lighting. “We continue to develop better blue OLEDs as well as red and green ones,” says UDC’s Mahon.

Arizona State University (ASU) demonstrated a 4-in. diagonal flexible QVGA AM PHOLED display. It was integrated on an amorphous silicon TFT backplane and fabricated on a 180°C process using DuPont Teijin heat-stabilized polyethylene-naphthalate (PEN) polyester material. The display was developed at the Flexible Display Center, of which ASU and UDC are founding members.

Materials engineers at the University of Florida in Gainesville developed very high-efficiency blue OLEDs using phosphor materials. These phosphorescent PHOLED materials have achieved peak power efficiency that’s more than 300% greater than conventional devices, the engineers say.

At a turn-on potential of 3.2 V, the technology improved from 8 ±1 lm/W to 25 ±2 lm/W at a luminance of 100 cd/m2. An efficiency of 20 lm/W at 1000 cd/m2 was also achieved. A maximum quantum efficiency of 17 ±1% was a five-point improvement, while the electroluminescence spectra of the p-i-n photodiodes used was nearly identical to that of conventional PHOLEDs.

Researchers at Korea’s Seoul National University are also working on improving PHOLEDs. They’ve developed a novel architecture incorporating an exciton-blocking layer that stabilizes PHOLED efficiency at high levels of luminance, making them more useful for large displays and solid-state lighting applications.

Samsung is working on a 14.1-in. diagonal AM OLED display for TVs and laptop computers that may be on the market this year. Meanwhile, Samsung’s Omnia HD is the latest mobile phone to use an AM OLED (Fig. 2). The phone features a 3.7-in. diagonal display with resolution of 360 by 640 pixels, a high-speed downlink packet access (HSDPA) touchscreen, and an 8-Mpixel camera that can record high-definition (HD) video.

AM OLEDs have been achieving greater success in penetrating high-definition 3G and 4G mobile phones, an area dominated by PM OLEDs, primarily in earlier-generation mobile phones. Various market estimates place the OLED display market for mobile phones at about $1.5 billion to $2 billion by 2015, second only to OLED TVs, which are estimated to reach $2 billion to $3 billion by then. OLEDs of both types are also seeing applications in digital and video cameras, near-eye viewers, micro-displays, games, notebook PCs, MP3 players, portable DVD players, and digital picture frames.

LIGHT-EMITTING POLYMERS
One relatively simple OLED structure, the polymer OLED (P-OLED), uses a light-emitting polymer (LEP) material. Discovered at the Cavendish laboratory in Cambridge University in 1989, it can be manufactured inexpensively on a solution process like ink-jet printing or spin coating due to its simple structure.

Such a process is much less complex and less expensive than a conventional vacuum-deposition process. Work is ongoing to improve P-OLED’s already low efficiency levels. Companies like Add-Vision, Casio, and Panasonic are actively looking at P-OLED technology and might produce displays commercially this year or next.

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Add-Vision’s printed P-OLED technology offers many of the attractive characteristics of mainstream OLEDs, but is much easier and cheaper to manufacture. The company developed a doped P-OLED LEP ink that’s specially formulated and doped with additives and transport materials.

The additives enable low-voltage charge injection and high-efficiency radiative recombination in the luminescent polymer without the need for unstable, vacuum deposited metal electrodes. Furthermore, the film is less sensitive to high-throughput variations that are common in vacuumdeposition approaches.

The Add-Vision structure uses air-stable printable cathode materials that enable low-voltage operation, high power efficiency, and uniform emission. Simultaneously, these materials are printable in air and compatible with the low-temperature handling of flexible substrates (Fig. 3).

FLEXIBLE OLED DISPLAYS IN VOGUE
An exciting trend is the AM OLED’s move to flexible displays. “Consumers are attracted to flexible displays and devices not only because they are innovative in design, but also because they provide enhanced functionality,” says Chris Schreiner, senior user experience analyst at Strategy Analytics.

UDC demonstrated a 4-in. diagonal prototype using AM OLEDs. Designed for the military and funded by the U.S. Army’s Communication Electronics Research Center, the device was fabricated in collaboration with LG Display and L-3 Communications’ Display Systems Division as a complement to the work ongoing at ASU (Fig. 4). UDC is also working on a touchscreen version of the display.

At this year’s CES, Sony demonstrated a flexible AM OLED display prototype called Flex OLED, in the form of a Sony Walkman bracelet. Sony believes the product could be a future electronic ink (e-ink) reader. Samsung is also bullish on flexible OLED displays, saying it will double production of OLEDs this year and next year. It has made 2 million OLED panels so far and indicated that it will be able to commercialize such a display by next year.

TURNING ON TO TOLEDS
Transparent OLEDs (TOLEDs), which can be PM or AM devices, produce a white light output. They only have transparent components—the substrate, cathode, and anode. When turned off, they’re up to 85% as transparent as their substrate, as shown by a 12-cm prototype TOLED panel from Philips (Fig. 5).

When a TOLED is turned on, it allows light to pass in both directions. This is useful for heads-up displays, light partitions (nearly invisible by day and a pleasant diffused light at night), mood lighting, light canopies, automotive windshields for navigation and warning systems, and architectural windows. Companies developing TOLEDs include Osram Opto Semiconductors, Philips, and UDC. Prototype TOLEDs are expected to emerge within a year or two.

Using OLED technology for lighting is a major goal of the U.S. Department of Energy (DoE), which is funding the industry to develop energy-efficient products for myriad lighting applications. The DoE estimates that by 2016, white-light OLEDs could save well over $20 billion in electricity costs worldwide and more than 9 million metric tons of carbon emissions from the U.S. alone.

Last year, UDC achieved a record power efficacy rating of 102 lm/W at 1000 cd/ m2 from a white light source, using the aforementioned UniversalPHOLED technology. The DoE last year awarded the company $1.9 million to accelerate the development of white OLED lighting products. UDC also licensed its UniversalPHOLED technology to Konica Minolta for use in Konica’s white OLED lighting products.

An integrated research project launched by the European Union (EU), dubbed OLED100, will look to achieve OLED efficiencies of 100 lm/W and lifetimes of more than 100,000 hours from a chip unit area of 100 by 100 cm at cost of 100 €/m2 or less. This project follows the EU’s successful Organic LEDs for Lighting Applications (OLLA) program, whose membership includes leading European electronics manufacturers as well as research institutions and universities.

One such member, Osram Semiconductor AG, last year demonstrated a 10- by 10-cm OLED tile with a light output of 30 to 50 lm/W at 1000 cd/m2 and 5000 hours of lifetime. This intermediate step in Osram’s work proves that OLED whitelight output can be scaled upward from the lower levels achieved previously. Osram employed a basic vacuum-deposition technology that uses small molecules instead of printable polymers (Fig. 6). That output was later increased to 60 lms/W thanks to a joint effort with BASF AG.

Novaled AG achieved light power efficiency levels of 35 lm/W at 1000 cd/m2 and lifetimes of 100,000 hours using a white stacked structure that connects individual red, green, and blue emission units or multiple white emission units in series. The p-i-n structure comprises proprietary materials (Fig. 7). The emission color can easily be tuned to the equal-energy white for display applications by selecting the emitting materials and varying the thickness of the transport layer.

TAGS: Components
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