DuPont Makes Advances in OLED Displays
Peter Singer, Editor-in-Chief -- Semiconductor International, 6/1/2003
DuPont Displays (Wilmington, Del.) recently announced some major advancements in the performance of materials used to make organic light emitting diode (OLED) displays. The company said that its new soluble OLED materials demonstrate high electrical efficiencies and improved lifetimes. Devices have been produced containing green phosphorescent emitters, which show electrical efficiencies of 65 cd/A. Additionally, red devices with 10 cd/A and blue devices with 4 cd/A efficiencies have been demonstrated.
Although high efficiencies associated with vapor-deposited phosphorescent emitters are already well-known, DuPont Displays has focused its research efforts on taking these materials and modifying them in ways to allow them to be patterned by inkjet printing. In December 2002, DuPont Displays and Universal Display Corp. (UDC, Ewing, N.J.), a major innovator of OLED technologies, announced a joint-development agreement to create a new generation of soluble OLED materials and technologies. DuPont Displays is building a state-of-the-art fabrication facility at its Chestnut Run location in Wilmington to prototype devices based on these new materials.
"We are developing materials that exhibit electrical efficiencies very close to the theoretical limit," said Dalen Keys, chief technology officer for DuPont Displays. "The second-generation material sets being developed at the Experimental Station represent a real step-change in performance that will allow DuPont Displays to offer a broad roadmap of products to our OEM partners."
In the DuPont OLED display, which it calls "Olight," thin layers of the organic material are sandwiched between anode and cathode layers (Figure). A relatively modest voltage (typically 2-10 V) applied across the material will cause it to emit light by electroluminescence. In full-color screens, each color "dot" or pixel consists of three individual, single-color subpixels — one each for emitting red, green and blue. An Olight display is a device made up of multiple junctions, arranged in a matrix that allows each junction to be addressed individually. As with LCDs, a thin-film transistor (TFT) at each junction controls the intensity of the light.
According to Tom Miller, president of DuPont Displays' Passive Matrix OLED Business, the LCD industry has gone through a bifurcation, with the active-matrix LCD technology used primarily in very large displays, and passive matrix LCDs — primarily using older super-twisted numeric (STN) technology — relegated to small handheld displays. The problem with STN displays is that they have a restrictive viewing angle and poor contrast ratios, yet they remain popular because of their low cost and very low power consumption.
By comparison, OLED displays can be made as thin as an LCD without backlight; have lower overall power consumption, including the driver electronics, compared with LCDs with backlights; have a good contrast ratio; are visible from wide viewing angles; and have extremely fast response times.
"The big benefit of OLED vs. LCD starts with the fact that OLED is emissive," Miller said. " It doesn't require an external light source in order to make it viewable. And, like field-emissive displays, it has all of the benefits associated with an emissive display in terms of wide viewing angle and high contrast ratio — it's got all of those good qualities people look for in a high-quality display."
Miller said the OLEDs also have a major advantage over other alternative display technologies, such as field-emissive displays (FEDs), in that they are fabricated much like LCDs. "To build a passive-mode OLED display, the first four processing steps are the same as if you were building an LCD. It is still built on top of an ITO glass substrate, it still requires patterning of the ITO, and in the back end — after the OLED material has been deposited — the encapsulation is similar but actually simpler because you don't have to do the fill and seal operation that you do on an LCD. The singulation and test equipment are also very similar to what you would find in a traditional LCD factory." The same is true for active-matrix or TFT-based devices, according to Miller, although a "little tune-up" of the TFTs is required.
Miller said that the flip phone with a sub-display turned out to be an "ideal incubator" for OLED technology. "There might be somewhere on the order of 10 or 15 million — maybe even 25 million — sub-displays using OLED technology that will be shipped during calendar 2003."
For additional information on emerging technologies, go to www.semiconductor.net/emerging.
