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 PLOYMERS
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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