Advanced Displays Bring Mobiles to Life

Mobile displays have become a focal point of design activity in the wireless-handset, PDA and mobile-computing sector. With wireless data services arriving and streaming media being seen as a key application, these displays must be reworked to provide high contrast and faster response times while still meeting the tight power and space budgets of today's mobile devices. To meet these heavy demands, manufacturers are working on revolutionary technologies, such as organic light-emitting diodes (OLEDs), that promise higher contrast, lower power and faster response times for video applications. At the same time, many of these manufacturers are evolving strategies that will enable existing supertwist-nematic (STN) liquid-crystal displays to support enhanced services and deliver brighter contrast without consuming more power and adding tremendous cost to manufacturing.

"As mobile products become entertainment terminals, one of the biggest criteria will be the performance of the display," said Stewart Hough, vice president of business development at Cambridge Display Technology (CDT). The challenge is picking the right display technology for that mobile device. With so many options to choose from, that's a daunting task, but some due diligence can minimize the risk.

OLED ole
Of all the new technologies coming to market, OLEDs have clearly captured the most attention in the mobile-communication space. Unlike STN displays, which require a light source to operate, OLEDs are emissive, generating their own red, green and blue light. Those are then mixed proportionately in an additive process to synthesize a range of colors on the display.

OLEDs come in two flavors—small molecule and polymer. Small-molecule OLEDs are constructed with molecules weighing roughly 100 to 400 atomic units. A shadow mask is used to manufacture these displays; the molecules are deposited onto a glass substrate by means of an evaporation/vacuum process, said David Williams, vice president and CTO of Kodak's Display Products division.

Polymer-based OLEDs, often called PLEDs or light-emitting polymers, take a different approach (Figure 1). As opposed to using a small molecule, these displays are developed by grouping together a chain of molecules that delivers an atomic weight of 100,000 to 500,000 atomic units, CDT's Hough said. Additionally, instead of a vacuum process, polymers are deposited on the glass using an inkjet printing process.

Right now, manufacturers are split on which OLED scheme will be the best to use. Many companies, including Philips, Seiko and CDT, are banking on the polymer technique. By avoiding the vacuum process and using the inkjet placement, Philips believes it can produce a more cost-effective method for developing OLEDs, said Peter Hopper, chief executive officer of Philips' Mobile Display Systems unit.

But according to Williams, arguments that the vacuum process is too costly don't stand up. Kodak will be commencing mass-production of small-molecule displays next year, he said said.

Williams also pointed out that PLED displays come with their own set of challenges, citing in particular the production of the color blue. "They haven't developed blue polymers that are stable yet."

Hough conceded that CDT is expecting to hit a maximum 100,000-hour lifetime on its red and green polymers—but only 5,000 hours for blue polymers.

Though philosophies may differ, "OLEDs are bright, deliver high contrast and are easily readable," said Bob Melcher, CTO at Three-Five Systems.

On the contrast front, Kodak's Williams said existing LCD technologies deliver a contrast of roughly 20:1. OLEDs, on the other hand, can deliver a contrast of 1,000:1 in dim-light conditions, he said. Viewing angle is also a big win. Typically, STN developers quote brightness at a 90 degrees viewing angle. "OLED's brightness stays the same at all angles," Williams said.

Aside from brightness and viewing angle, a fast switching speed allows OLEDs to effectively handle video, a major selling point. According to Paul Semenza, executive vice president at research firm iSupply/Stanford Re-sources, OLEDs can deliver a switching time in the microsecond range. But building drivers that can take advantage of this response time is a real challenge.

Power consumption is also being touted as a big win for OLEDs/PLEDs. Unlike STN or thin-film transistor (TFT) displays, they do not require all pixels to be lit, thereby saving on power, Melcher said. But not everyone is convinced that OLEDs will have substantial power saving.

A white paper from Philips states that PLEDs consume 70 milliwatts when handling text messages and 200 mW for video signals, while backlit LCDs drew approximately 150 mW when displaying both video signals and text. The paper also showed that in a typical standby mode, the PLEDs drew almost three times more power than LCDs: 3 mW, versus 1 mW for LCDs (Figure 2).

Despite these numbers, designers should not rule out the possibility that OLED developers will actually deliver on their promise of a lower-power solution, industry watchers say.

Other OLED issues revolve around pricing and timing. To be successful in the long run, they must compete with STN and TFT displays in terms of cost, Hopper said. And that could make the mass adoption of the technology at least four years away, Hopper added.

Timing is another barrier for OLEDs. While Motorola Inc. has already rolled out an OLED display in a GSM phone, most put the delivery of OLEDs to the mass market out until at least late 2003 or early 2004, if not longer. That long lead time could give STN manufacturers the time they need to advance to a point where OLEDs become less attractive.

A plastic approach
Interest is also beginning to stir around the development of organic transistor technologies that will allow developers to build displays using a low-cost plastic substrate (Figure 3).

Transistors on plastic substrates enables the development of flexible displays and, potentially, even foldable ones, said Michael Kane, distinguished member of the technical staff at Sarnoff.

Furthermore, according to Kane, depositing transistors on plastic can reduce manufacturing costs by eliminating expensive vacuum-processing steps. Material costs also go down. For example, Sarnoff has developed a tiny active-matrix display using Mylar, the same variety of plastics used to produce soda bottles. However, as Kane points out, to make transistors for today's displays, manufacturers must heat glass to 250 degrees C. which can curl a plastic substrate.

To solve this problem, Sarnoff is working on a transistor technology that can be implemented in a low-temperature process, thus allowing it to be deposited on glass. So far, the company has been able to use this process to implement all of the layers, except the organic-semiconductor layer, Kane said. Sarnoff is searching for new compounds, such as poly hexylthyiophene, to solve those problems. Kane said Sarnoff has proven that this technology can produce a transistor on glass if it's deposited using an eye dropper—a technique that clearly is not feasible in real-world applications.

Sarnoff is still working on poly hexylthyiophene and other compounds, but doesn't expect to see organic-transistor technology on the market for three to five years, said Kane.

If Sarnoff can find an optimal compound, however, organic transistors may provide a nice complement to polymer LED technologies. According to Kane, PLED developers have struggled with depositing transistors through the inkjet process. The answer could lie in marrying PLEDs with organic transistors.

LCDs push ahead
While most manufacturers focus their attention on revolutionary solutions like OLEDs, they are also spending a great deal of time looking at ways to push existing display technologies to new plateaus. Philips is one company focusing a lot of attention on enhancing the performance of existing color STN displays, Hopper said. One of the solutions the company is touting is a color-adjusting technology (CAT), which improves the color quality of STN displays.

Over the past few years, designers have turned to transflective displays-those that merge reflective and transmissive technologies. However, these typically require a trade-off between color saturation and reflectance.

In an LCD, a single-color film, or filter, covers each pixel. Thick filters are used for transmissive displays and thin ones for reflective, because in the latter displays the light passes the filter twice. But since transflective displays house both reflective and transmissive technologies, designers must be careful about which filter technology to choose. If the thin filter is used, color saturation will decrease, creating a washed-out look. If a thick filter is chosen, poor reflectance will result in a dark picture.

CAT technology helps bridge this gap. Through CAT, a corner of the pixel filter is made thinner than the rest, allowing light to pass through the filter unimpeded and without adding color. Color saturation is improved through the thicker color filter, while reflectance is equally enhanced as a result of the filter's partial-reflectance "window." CAT technology will also be customizable: Users will be able to adjust color setting based on viewing angles and other requirements, Hopper said.

Philips is also addressing the reflective brightness of its color STN displays through an integrated diffusing reflector technology (IDR) [Figure 4]. Using an IDR, Philips can process the rear-pixel electrodes to improve and optimize the reflective brightness of the display, according to Hopper.

For its part, Seiko also is developing technologies that will increase the overall luminescence of current color STN displays. "We have developed patented polarizers that increase luminescence, reduce backlighting and increase contrast on STN displays," said Mike McLachlan, director of marketing at Seiko. "These polarizers increase brightness by 20 to 30 percent and contrast by a minimum of 25 percent."

Seiko is also looking for ways to increase the video performance with smart addressing, which will allow engineers to drive a color STN display at 60 nanoseconds, allowing full-motion video. "These displays will not deliver the greatest video performance," McLachlan said. "But for a lot of people this video performance will be good enough."

Integrating drivers
Along with evolving the physical display, efforts are under way to provide higher levels of integration across the entire display subsystem. In particular, many members of the display-development community are eyeing low-temperature polysilicon (LTPS) transistors as a means for integrating the display driver and other surrounding electronics onto the display substrate itself.

According to iSupply's Semenza, the LTPS process employs the very same steps that are involved in developing amorphous silicon, which is the base technology for most of today's TFT displays. The big difference, however, is that LTPS requires recrystallization, which entails imparting energy into the TFT to change the structure of the silicon. "The primary benefit of this step is that the resulting polysilicon TFTs have much higher electron mobility, allowing them to switch faster and be smaller," Semenza said. "This, in turn, allows more functions, such as the driver circuitry, to be built onto the display glass."

The integration of driver circuitry also may lead to more durability by reducing the number of connections to the display glass, thus increasing resistance to damage from shock or other external damage, Semenza said.

Related Articles

1. "Components Boost Display Options"; www.commsdesign.com/story/OEG20010214S0058.
2. "Organic LEDs Gear Up for Portables Push"; www.commsdesign.com/story/OEG20020524S0085.

About the Author
Robert Keenan (rkeenan@cmp.com) is the editor in chief of www.CommsDesign.com, the online home of Communication Systems Design.