OLEDs see action in mobile future
Researchers looking into reflective and emissive display technologies hope to turn their fascination with organic light-emitting diodes into real applications for mobile CE products. Their progress was detailed at the Society for Information Display conference held in San Francisco in June.
The OLED market will reach $1.23 billion this year, said forecaster DisplaySearch, with the CAGR for OLEDs at 57.1 percent—the highest among all flat-panel displays.
In an invited talk, Seiko-Epson presenters even displayed a chart showing that OLEDs lead in every performance parameter except lifetime when compared with other displays. If researchers succeed in overcoming this last challenge, OLEDs might be the ideal display not only for portable devices, but also for large-panel TVs.
While most of today's OLED products are passive-matrix displays, several characteristics of active-matrix (AM) OLEDs—such as high color saturation, fast response time, thin form factor and high contrast ratio—make them good candidates for next-generation full-color displays. They can also be manufactured for volume production using the currently available shadow mask process, which ensures that OLEDs and AMOLEDs can emit RGB light without further conversion or filtering, so there is virtually no energy loss. Additional advantages are long lifetime and lower cost.
AMOLEDs carry some disadvantages, however. The high-precision shadow mask required in their production has a physical limitation: Mask openings can be no smaller than 50μ. And while high resolution is a general requirement for small displays, this hampers the use of the shadow-mask process in high-resolution AMOLED manufacturing.
To overcome the resolution constraint, AU Optronics Corp. researchers developed an innovative pixel arrangement of RGB subpixels. With the new arrangement, two subpixels of the same color can be deposited through the same opening of the shadow mask. Thus, the size of the shadow mask's opening can be doubled in both length and width. The area of each opening can be up to four times that of a conventional design.
In the search for a robust and lightweight display, researchers at Samsung SDI Co. Ltd have focused on such flexible substrates as stainless steel foil and plastic. Since the melting point of stainless steel foil is similar to that of silicon, it is possible to exploit a high-temperature process to crystallize the amorphous silicon precursor. An additional advantage of the stainless steel foil substrate is its low water and oxygen permeability, which is comparable to that of glass.
In addition to being thin and lightweight, flexible substrates can allow fabrication of curved, bendable and even rollable displays for use in a car, a book or newspaper, or clothing.
In another development, Samsung SDI has successfully created a novel top-emitting full-color OLED display on a stainless steel foil substrate. The result was an ultralightweight display with an overall thickness of less than 0.2mm.
Another key challenge in the development of high-resolution AMOLEDs centers on color-patterning techniques. The ability of the conventional fine-metal mask (FMM) process is limited when it comes to mass-producing high-resolution AMOLED panels.
Researchers at Toppoly Optoelectronics Corp. have used a white OLED in combination with the company's color-filter-on-array technology to achieve high resolution for a large panel without using an FMM process. Using this technique, Toppoly realized a 7-inch AMOLED panel that also had high color saturation. The company also reduced the number of shadow masks needed to make a full-color device from three to two, allowing the design rule on FMMs to be relaxed by one-third.
Meanwhile, Dupont has developed new OLED manufacturing process prints with "ink solutions" that are cheaper and more scalable than the physical mask process. Its process uses small-molecule OLED materials to combine high performance with long lifetime. The company's process also costs substantially less and is more scalable to larger display sizes than such current industry processes as vapor deposition.
- Nicholas Mokhoff