Advancements in OLEDs and e-paper technologies were some of the most talked-about developments at Display Week 2007.

by Paul Drzaic

IT WOULD HAVE BEEN EASY to spend the entire week at Display Week 2007 (The SID International Symposium, Seminar, and Exhibition) focusing on mainstream display applications such as liquid-crystal-display (LCD) and plasma television, LCDs for mobile applications, and the like. For many, though, the fun of the week comes from sampling the emerging display technologies that could become mainstream products in the coming years. Two technologies in particular garnered significant attention both on the exhibit floor and during the Symposium: organic-light-emitting-diode (OLED) displays and reflective displays (now often re-branded as electronic paper). Interestingly, several offerings in both these areas were curved or even flexible, demonstrating another area of potential product differentiation.

OLED Displays: So Much Progress

OLED proponents at Display Week 2007 continued to show innovations on a number of fronts, including emitting materials, backplanes, and manufacturing methods. Many of these innovations appear headed toward challenging the dominance of LCDs in medium-sized displays, along with OLED's current encroachment in small-format displays. Higher performance, longer lifetimes, and improved manufacturability were common themes across much of the news announced during Display Week.

Several fabulous active-matrix displays were on display throughout the week, some drawing large crowds in the exhibition, others available for short periods during author interviews. During the author interview for paper 53.2 on Friday, May 25, Sonydemonstrated a 27.3-in. 10-mm-thick small-molecule OLED display. This top-emission display incorporated a microcrystalline-silicon backplane and was fabricated using a laser-transfer process to generate patterned emitters (Fig. 1).

On the show floor, the LG.Philips LCD/ Universal Display Corp. (UDC) flexible active-matrix OLED (AMOLED) attracted significant crowds. This 4-in. module was constructed using amorphous-silicon (a-Si) on a steel backplane and was exhibited as a wearable display on an armband [Fig. 2(a)]. Sony also drew significant attention with its paper (63.2) on a highly flexible 80-ppi OLED display driven using an organic TFT backplane on plastic. Videos of the display showed that it could be flexed considerably during operation, made possible through its all-organic construction [Fig. 2(b)].

 Sony Corp.

Fig. 1: Sony's 27.3-in. OLED display is constructed by using a μ-Si backplane and a laser transfer process of small-molecule emitters.

On the polymer-LED side, during the author interviews, Toshiba-Matsushita Display Technology Co., Ltd. (paper 13.4) showed a 20.8-in. WXGA top-emitting display that incorporated a micro-patterned reflective layer to improve light extraction (Fig. 3). On the exhibit floor, Casio and Cambridge Display Technology (CDT) each exhibited a 6-in. low-temperature polysilicon (LTPS) based display, showing great color at an impressive 160 ppi, with printed polymer emitters.

While polymer-LED developers have tradi-tionally taken the position that they provide the only means for the cost-effective manufacture of OLED displays, several companies demonstrated innovative fabrication processes using small-molecule OLEDs.Kodak exhibited its newly developed Gen 5 small-molecule deposition head, which it claims cuts material utilization and improves throughput sufficiently well to cut fabrication costs by as much as 40% compared to a current Gen 4 tool. Samsung described its work on a Gen-4-compatible laser-transfer process; the Sony 27.3-in. OLED

display described earlier also was fabricated using laser transfer. DuPont showed improved color gamut and lifetime in its solution-coated small-molecule materials, showing off this innovation in several 4.3-in. WQVGA (128 ppi) displays in its exhibition booth. These innovations all certainly tighten the race between the small-molecule and polymer systems and provide potential display manufacturers an increased number of choices to consider.

Fig. 2: Shown is (a) LG.Philips LCD/Universal Display Corp.'s OLED display (a-Si on steel) and (b) Sony's flexible OLED display (organic TFT on plastic film).

Lifetime and performance of OLED materials continue to improve as well. Novaled described two red-emission systems that it calculates can provide approximately 1 million hours (100 years!) of lifetime at 1000-cd/m² luminance levels. Konica Minolta (paper 19.1) described a white OLED with more than 64-lum/W efficiency at 1000-cd/m² luminance levels. In a poster presentation (P-176),QD Vision described quantum-dot emitters with sufficiently narrow emission line widths [<25-nm full width at half-maximum (FWHM)] to compete with the color gamut of inorganic LED backlights without using color filters. These achievements should remind us that OLEDs have great potential for general lighting and backlighting as well as forming a primary display medium.

Overall, the pace of innovation and investment in active-matrix OLED (AMOLED) displays continues to increase. It is remarkable that many of the companies that produce today's dominant LCD technologies are stepping up to develop this competing technology in display sizes that today are ruled by LCDs.

Electronic Paper Presses Ahead

While reflective displays have always had some level of presence at Display Week, the 2007 conference was extraordinary both in the areas of technical innovation and in product and manufacturing announcements.

Electrophoretic displays (EPDs) were prominent throughout the exhibition floor, with many companies showing displays built using E Ink Vizplex imaging film. Prime View International (PVI) announced its plans to manufacture flexible electrophoretic displays using backplanes that originate in a conventional a-Si fab and then are transferred onto a flexible backing. In other first-ever-seen demonstrations, LG.Philips LCD introduced its 14.1-in. color AMEPD built using an a-Si backplane on stainless steel (Fig. 4).Samsung Electronics took the plastic route, building a 14.3-in. black-and-white AMEPD using a-Si on a plastic backplane (paper 58.5). Samsung showed what is easily the world's largest high-resolution AMEPD panel, a 40-in. display built using E Ink Vizplex film.

At the E Ink booth, several tantalizing enhancements in its core electrophoretic materials were visible. Most surprising was the demonstration of a color AMEPD panel with moving images, running at 15 fps. While the moving images were not of the same quality as their static cousins, they were suitable for watching short video clips. The E Ink booth also showed panels with improved color gamut and reflectivity (Fig. 5).

Not all electronic paper came from AMEPD technology, though. Bridgestone showed a number of color A4 passively addressed signs using their air-based EPD (QRLP) technology (Fig. 6). A flexible version of these displays was also demonstrated, using a specially thinned (and flexible) 35-μm-thick VLSI driver integrated into the plastic panel (paper 45.1). Nemoptic exhibited an A4-sized version of its bistable, passively addressed liquid-crystal-based BiNem display (Fig. 7).

Space limitations only allows brief mention of some of the diversity of approaches shown in reflective displays. Qualcommannounced that it was ready to lauch it's first microelectromechanical systems (MEMS) based product, a display used in the Ubixon Bluetooth stereo headset. ZBD (paper 40.1) demonstrated a highly shock-resistant bistable ePop (electronic point-of-purchase) platform for shelf labels. Kent Displays showed a number of indicators and signs using its bistable cholesteric technology in its booth and described highly flexible displays in an invited Symposium presentation (paper 6.1). Liquavista announced its first line of products: custom-colorized direct-drive display modules for watches and other small applications. SiPix Imaging demonstrated a number of EPD-based modules as well, with demonstrations of flexible displays for smart-card applications. Sharp (paper 40.5) showed a reflective polymer-dispersed liquid-crystal (PDLC) module built using a retro-reflective optical element: it had a surprisingly good appearance when demonstrated at the author interviews.

Finally, basic development was reported in the general area of flexible backplanes, with LG Philips, Seiko Epson, ITRI, Philips Research, and Samsung Electronics all reporting new work in this area at the Friday Symposium sessions. A focus on manufacturability is now present in this area. While many companies continue to press with organic thin-film-transistor (TFT) backplanes, several others are developing innovative strategies to use conventional a-Si fabs to fabricate flexible backplanes. The increasing amount of work in this area indicates that these companies are taking the promise of flexible displays quite seriously and that mainstream applications for these devices are on the horizon. •