RGB Color Patterning for AMOLED TVs
RGB color patterning is one of the key technologies for manufacturing large-sized AMOLED TVs. Two competing approaches are currently being used to realize RGB subpixels. One requires a more difficult manufacturing process but has better color purity; the other is easier to manufacture but requires optimization in algorithms to overcome some weaknesses.
by Jang Hyuk Kwon
ALL COLOR TVs sold today realize color images through combinations of pixels that, in turn, consist of RGB subpixels. Liquid-crystal-display (LCD) TV uses backlighting as the light source and color filters for the RGB subpixels, which are patterned by photolithography or ink-jet printing. Since organic light-emitting diodes (OLEDs) are self-emissive, we can realize RGB subpixels by direct deposition of RGB emission materials for each subpixel (RGB side-by-side) or by filtering white-OLED light through RGB color filters (WOLED + CF). Currently, there is much debate with regard to which approach will be the winner in the large-sized OLED-TV market. This article describes the pros and cons of each technology and presents an outlook for future OLED technology development.
RGB Side-by-Side Patterning by Shadow Mask
Currently, the active-matrix organic light-emitting-diode (AMOLED) pixels for mobile applications are manufactured by evaporation of small-molecule light-emitting materials through metal shadow masks. When one-color material is being deposited, the other color areas are blocked by this mask. Figure 1 schematically shows the shadow-mask technology for red, green, and blue pixel patterning of AMOLEDs.
Fig. 1: A typical shadow-mask technology for pixel patterning shows green and blue pixels being blocked. The table at bottom shows the dimensional capability of the shadow-mask process to date.
Current shadow-mask technology is already mature and does not present any serious difficulties when used on substrates up to 750 mm × 650 mm (which represents ¼ of a Gen 5.5 substrate of 1500 mm × 1300 mm). However, this technology cannot be used for Gen 8 substrates for AMOLED-TV production because glass and mask sagging become critical issues.
A new process technology called Small Mask Scanning (SMS) has been developed by Samsung for large-area TV manufacturing. In order to circumvent the sagging problem of the substrate and the mask, the glass substrate is moved during pixel deposition while both the small-area shadow masks and the linear evaporation sources are kept stationary (see Fig. 2). The merits of the SMS process are (1) practically, the same small-molecule materials used for mobile application can be used for TVs, and therefore good synergy exists between the two applications, and (2) the benefits of low power and long lifetime and the color purity of the RGB side-by-side pixel configuration are maintained.
Fig. 2: The OLED RGB process using SMS (left) is compared with the WOLED + CF process (right) in terms of pixel structure, methodology, and pros and cons.
One major drawback is that achieving pattern accuracy becomes more difficult for larger substrates (such as Gen 8 and above) and higher resolution [ultra-definition (UD) and above], which are both necessary for next-generation TVs. To be more specific, any misalignment will result in color mixing and non-uniformity of the subpixels. Mask window variation with multiple patterning processes can also be a serious problem for real mass production. Further technology development is needed in order to increase manufacturing yield as well as to improve scalability in substrate size and high resolution.
White OLEDs with Color Filters
The approach of white OLEDs with color filters (WOLED + CF) has received a great deal of attention because of its simpler process and attendant benefit of using the existing color-filter infrastructure.1 This method can easily overcome the inherent complexity of RGB side-by-side patterning with the shadow masks. White emission can be achieved by mixing three primary colors (red, green, and blue) or two complementary colors (yellow and blue) in the emissive layers. Generally, WOLEDs consisting of small organic molecules have a multilayer structure with two or more emitting layers in a simple stack or tandem OLED structure.
Figure 3 shows several structures developed to realize white-OLED devices. Among these, the tandem OLED structure has merits in terms of longer lifetime and higher efficiency; however, the manufacturing process is a bit more complicated. The tandem device with two emissive layers as shown in Fig. 3 can greatly reduce current level to achieve the required brightness value of each pixel and more than doubling lifetime. The tandem device can also easily achieve very high efficiency because both efficiency values of each single cell are combined.1 In the beginning of 2013, LG Electronics announced commercial production of 55-in. AMOLED displays that use the tandem WOLED + CF approach. (In the CES Review in this issue of ID, author Steve Sechrist notes that LG planned to begin shipping these units in Korea in February of 2013.)
Fig. 3: Above are shown the merits and demerits of various WOLED structures (EML = emissive layer).
One problem with the WOLED + CF approach is that light-output efficiency is reduced to 30% of the efficiency of RGB side-by-side because of the light absorption induced by the color-filter pigments. This problem can be minimized if the RGBW four-pixel color system is employed. Theoretically, the RGBW system can have about ~50% light output due to the mixing of white colors for bright images and thereby compensate this for the light loss.2 However, there is a trade-off between the reduction of power consumption and the degradation of color purity. More circuitry to drive the white pixel is needed and light output efficiency to each RGB subpixel depends on the white spectrum of WOLEDs. Therefore, the light output efficiency of actual displays may not reach this ~50% theoretical level. Another issue lies in the color variation on viewing angle because thick organic layers in the tandem structure result in several possible optical paths, which can distort color. Overall, a more than two-times-higher power consumption and a much shorter lifetime can be a real concern in AMOLED TVs using the WOLED + CF system. However, a combination of new OLED materials and optimum color-rendering algorithms may be able to solve the lifetime, power consumption, and color characteristics issues, as discussed in Ref. 2.
Future Prospects in Color Patterning
Several companies are trying to develop alternative technologies to replace the current shadow-mask process used for large-area TV applications. Solution-process printing technologies such as ink-jet printing3 and nozzle printing4,5 have been proposed as pixel-patterning methods. These processes will be the ultimate objective for pixel-patterning methods for AMOLED displays since using them is fairly simple and does not require any vacuum equipment.
Several material companies are developing soluble materials for these solution processes. However, it is not an easy task to develop good soluble materials because OLED lifetime is very sensitive to impurities, film quality, and environmental conditions. (One such possible solution is discussed in the Industry News story, “Merck to Use Epson Ink Technology for Large OLED Displays,” in the January/February issue.) Among the RGB materials, blue has the shortest relative life-time. Device lifetimes of red and green OLED devices with solution printing processes are sufficiently extended to be of practical use. Consequently, a process involving blue thermal deposition of the entire active area with an open mask after the solution printing of red and green subpixels is being developed to overcome these lifetime issues. Recently, significant progress has been made in the development of soluble materials and the process for AMOLED fabrication. Hence, it is expected that solution printing processes will be available in the near future.
Laser-printing technologies have also been proposed as pixel-patterning methods, suitable for large-area displays. Laser-induced thermal-imaging (LITI) technology6 was first reported by Samsung. This technology uses donor films with a laser-light-absorbing layer and a transfer organic layer. Laser light is converted into thermal energy as it shines on the light-absorbing layer. Subsequently, the transfer layer melts and transfers to the substrate. The company is focusing on this method only for small-sized high-resolution display applications. This process is very sensitive to particles because any contaminated particles can fix onto the substrate during the transfer process. Small-sized applications are better with regard to minimizing yield loss.
Laser-induced pattern-wise sublimation (LIPS) technology7 has also been reported as an alternative to LITI technology. Both technologies are fundamentally very different. In LIPS technology, emissive organic layers on the light-absorbing metal layer are deposited on glass and then attached to two substrates, one organic deposited substrate and one active-matrix backplane under vacuum. Finally, a laser scans the organic layer to form a pattern through sublimation transfer from the organic layer to the active-matrix backplane. Sony reported this process and demonstrated a 25-in. AMOLED panel at Display Week 2007. In this process, similar thermal evaporation temperatures for host and dopant molecules are required. Patterning of phosphorescent materials using this methodology is difficult because phosphorescent materials have much higher evaporation temperatures originated by high molecular weights. To date, no additional progress has been shown with this process.
Refinements Needed for Progress in Both Processes
Currently, the two main patterning processes, SMS and WOLED + CF, are being intensively developed for commercialization. Gen 8 lines for both technologies have been already invested in for the production evaluation of 55-in. AMOLED-TV applications. Both technologies are rapidly improving. Additional possible processes such as solution printing and LIPS are also being investigated. In the near future, significant progress in solution printing processes in particular is expected.
References
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