OLED Displays on the Verge of Commercial Breakthrough
by Robert Jan Visser
This article is an adaptation of a presentation given at the SID Display Applications Conference (DAC) held October 23–25, 2007 in San Francisco. In the presentation, the important contributions that U.S. companies have made to the development of organic-light-emitting-diode (OLED) display technology and the industry were highlighted and used to illustrate important trends in the industry.
The general public, the press, display customers, and the display industry as a whole have been alerted to the superior image quality and possibilities to create super-thin, high-quality, and even flexible displays offered by OLED technology. Just about every OLED display device unveiled recently has created a strong impression, from Sony's OLED TV and Samsung Electronics' 40-in. OLED TV, to beautiful cell-phone displays from Samsung SDI, to the world's thinnest display by Samsung SDI, to very impressive full-color active-matrix flexible displays by LG Display and Samsung SDI. Now that companies such Samsung SDI and CMEL a.o. have shown that many issues that have plagued the industrial production of OLEDs can be overcome, the OLED industry has entered a new stage of additional investments and growth.
No article should start without mentioning the invention of the OLED by Ching Tang and Steven Van Slyke from Kodak. This is and remains the foremost of the U.S. OLED companies, not only because of their invention, but also due to important contributions to new materials, driving, structuring, and production technology over the years.
Currently, there are four main development themes for OLED technology: cost-effective and suitable methods for making RGB displays; active-matrix OLEDs; new materials; higher efficiency and longer lifetimes; and creating very thin and flexible displays. Let's look at each one.
Cost-Effective Methods for Making RGB Displays
Almost all OLED displays produced today use the evaporation of organic small molecules. In order to make a full-color display, the most direct method is to evaporate the different colors one by one using a shadow mask. This has a couple of drawbacks: moving from one position to the next can cause damage and/or contamination and requires very high precision from the thin and a stretched shadow mask and its positioning system. As of now, up-scaling to sizes greater than Gen 4 seems to be very difficult.
A technologically much easier way to make a full-color display is using white pixels, which utilizes a color filter and removes the need for complicated shadow masks. One sacrifices off-course energy efficiency with this approach. Kodak has successfully worked on two aspects of this approach, creating high-efficiency (23.6 cd/A) white materials and white-emitting structures, with good color rendering (>100% NTSC), long lifetimes (180,000 hours for TV), and without a shift in color point during the lifetime.
Because most images contain a lot of white, introducing a fourth white subpixel where the white is unattenuated by the filter (RGBW) improves the energy consumption for most images by 80%.
Printing the different colors is another approach to create a cost-effective way to create full-color displays. When successful, this technology is very well suited for up-scaling but obviously requires solution-processable materials. This approach was pioneered by Seiko-Epson and CDT using polymer materials. Plextronics has developed a process using polymer hole-injection layers called Plexcore® HIL, which offer tunability of the work function and offer greatly improved lifetimes for polymer OLEDs (5,000–10,000 hours at 50,000 nits). DuPont has developed solution-processable small molecules and is making tremendous progress with improving lifetimes (25,000–50,000 hours at 200 nits).
The ink-jet-printing approach requires high precision and reliable ink-jet heads. FujiFilm Dimatix has created a new MEMS-based piezoelectric head with 2-μm placement error and a 2% uniformity which has enabled Litrex and other companies to make large-scale (Gen 8) industrial machines for printing color filters, OLEDs, etc.
AMOLED displays use higher currents than LCDs. This has necessitated the use of low-temperature polysilicon (LTPS). No companies in the U.S. are currently working on large-scale LTPS production. Kodak has made an important contribution to solve the problem of small non-uniformities in the LTPS-TFT performance showing up as a display non-uniformity (Mura). Their compensation algorithm will both increase performance and the AMOLED yield. Leadis Technologies has developed a compensation scheme and driving ICs for differential aging of pixels. The Flexible Display Center at Arizona State University develops AMOLED technology based on a-Si TFTs on flexible substrates for flexible displays.
New Materials: Higher Efficiency and Longer Lifetimes
A key element in the development of the OLED technology has been the creation of new materials with higher efficiency and longer lifetimes. Stephen Forrest from Princeton University and Mark Thompson from USC, together with UDC, created a big breakthrough with the invention of phosphorescent OLEDs (PHOLED™) for which virtually 100% of all the electron–hole recombinations result in the emission of light (compared to a maximum of 25% for fluorescent OLEDs). This has pushed up the efficiencies and lifetimes of displays. Efficiencies well in excess of 100 lm/W have now been reported for green emission. Even blue-emitting PHOLEDs now show high efficiency (21 cd/A) and respectable lifetimes (9000 hours at 500 nits), whereas red (27 cd/A; 200,000 hours) and green (67 cd/A; 250,000 hours) have reached lifetimes at 1000 nits (brightness) that a few years ago were deemed to be impossible for organic materials in such an application.
QDVision is approaching high efficiency and very stable materials in a different way by using inorganic quantum dots in an OLED-like structure to create pixels with a very narrow emission band. By changing the size of the quantum dots, a color gamut that is much wider than that of HDTV can be obtained.
Very Thin and Flexible Displays
OLED displays are notoriously sensitive to water and oxygen. Local oxidation of less than a monolayer of the low-work-function cathodes, which are highly reactive, results in so-called black spots, which render the display useless. Currently, OLED displays are protected on one side by a glass substrate and on the other side by a glass lid that, in most cases, contains a cavity filled with desiccant to absorb all the water that has passed through the glue line need to fix the cover lid.
Vitex Systems has developed a very thin, transparent, flexible barrier coating (Barix coating) with extremely low water-vapor transmission rates (WVTR) of 10-6 gr/m2/day (107 better than a normal plastic film). The thin-film coating can be applied at temperatures in the range of 40°C, which are compatible with organic electronics.
This thin-film coating consists of a multilayer of polymer and inorganic layers. The polymer layers cover and planarize over particles, defects, and display structures; the inorganic layers form the barrier against water. The multiplayer provides redundancy against defects and decouples local defects in the oxide, providing a tortuous path for molecules from the outside to reach the display. While, in the beginning, 4–6 organic/inorganic pairs (dyads) were needed to ensure good and high-yield barrier performance, now 2 dyads suffice, with yields very close to 100%.
Fig. 1: The world's thinnest display made by Samsung SDI. A Vitex Barix coating is used as encapsulation.
This Barix coating can be applied directly on top of the OLED display, replacing the glass lid. Working with many different customers, Vitex has been able to show that the Barix coating can meet telecommunications and automotive applications. Using the Barix coating, Samsung SDI has recently created the world's thinnest display (Fig. 1).
The next step is to then replace the glass substrate by a thin film of plastic covered by a barrier layer and create a flexible display. Vitex has shown that these films covered by the multilayer barrier can meet the barrier performance of a WVTR of 10-6 gr/m2/day, needed to successfully protect OLED displays.
The barrier films do not only enable flexible OLED displays but provide a basis for creating flexible solar cells, flexible batteries, and other non-electronic applications in thermal isolation and medical applications. Using a very similar approach but a different deposition technique, GE has recently obtained good results in using their technology for thin-film encapsulation and the creation of barrier films.
In order to create a flexible, high-resolution full-color OLED display, many more problems need to be solved: low-temperature deposition of the active matrix; lithography on a dimensionally much less stable substrate; handling of the plastic substrates, etc.These topics are being addressed at the Flexible Display Center. They have recently shown excellent capabilities in creating a flexible active-matrix display using an electrophoretic display (E Ink) for showing a first generation of demonstrators.
Add-Vision, a start-up company, is very close to commercializing what would be the first flexible OLED displays on the market. By using a completely printed OLED structure laminated between two of the Vitex barrier films, they have succeeded in making stable flexible segmented displays for cell-phone touch pads, car dashboards, etc. (Fig. 2).
An OLED display is the almost ideal display: best picture quality; thin, fast response; no viewing angle issues; and low power. After years of struggle, OLED displays are now on the verge of a commercial breakthrough. Further reduction in cost, increase in throughput, and scaling to larger sizes will be needed to be price competitive with LCDs. Flexible OLED displays are around the corner and will create a new and unique market opportunity. •
Fig. 2: Add-Vision's flexible OLED displays. A Vitex barrier film is used as the substrate and encapsulation.