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The large-scale road to the OLED panel reveals the dawn (1): LG actively promotes, (2): lighting use is another driving force, (3): large-scale and low-cost technology is steadily advancing, (4): TFT Multiple alternate technologies
White OLED emerged
Another key to large screens is the film formation process of the light-emitting elements. The processes currently under study can be broadly classified into two categories: the manner in which RGB luminescent materials are applied; the combination of white materials and RGB trichromatic color filters (Table 2).
Table 2: Main film forming methods of OLED elements
The small and medium-sized OLED panels currently being mass-produced are manufactured by mask-coating low-molecular luminescent materials. However, it has been suggested that the method of using a shadow mask is difficult to ensure the positioning accuracy of sub-pixels, and there is a limit when applied to the fourth-generation glass substrate. Moreover, its low material utilization rate leads to increased costs.
Therefore, as a method for facilitating the enlargement of the panel, a combination of a white luminescent material and a color filter is beginning to emerge. In addition to Kodak, which has long adopted this approach, LG displays, which are scheduled to acquire Kodak's OLED business, are also undergoing trial production (Figure 9).
Figure 9: White OLED + color filter regained favor For large-scale, Kodak and LG Display are developing OLED panels that combine white OLED materials with color filters. Compared with the film formation of each of the RGB three-color OLED layers, it is characterized in that it is easy to reduce the cost.
In general, the use of a color filter does not require the application of a luminescent material, and thus it is most easy to increase the size of the glass substrate. However, expanding the range of color expression requires increasing the thickness of the RGB three-color material film. Moreover, color filters increase the amount of light absorption and detract from the panel brightness index of one of the advantages of OLEDs. In this case, in order to obtain the same brightness as the dispensing method, it is necessary to increase the brightness of the white luminescent material, which not only increases power consumption but also shortens the life. In other words, the expansion of color performance range and power consumption are contradictory relationships.
Kodak's 8.1-inch product solves the above problems while expanding the color performance range of color filters. The color performance of the prototype is as high as 100% for NTSC. For power consumption, Kodak's village is full of confidence: "If you use the latest white materials, the power consumption of the 8.1-inch panel is less than 2W. The power consumption of the same size LCD panel is about 2 ~ 4W, which can compete with the product." Further, since the configuration of the element is a bottom emission type in which light is emitted from the TFT side, the color filter needs to be formed on the TFT substrate.
The improvement in these characteristics is mainly due to two reasons. First, the lighting efficiency of white materials has been greatly improved by the development of lighting applications. The white material's current conversion efficiency of Kodak is about 50 cd/A. “Since 2007, the annual increase has been 50%†(Kodadar Village).
Another reason is the use of the "W-RGBW" independent sub-pixel arrangement and improved drive methods (Figure 10). Compared with the traditional RGB three-color color filter, this method can achieve both low power consumption and color performance.
Specifically, in this method, four sub-pixels of RGB and W (white) are used as one pixel to display a color picture. Since there is no filter in the W section, the power consumption is lower than the conventional method when displaying a full white screen. Moreover, when displaying a specific color, it is only necessary to use two of the three colors of W and RGB. The rest of the color can be non-illuminated, helping to extend life.
Figure 10: Improving problems with RGBW four-color color filters Kodak developed an OLED panel (a) using white OLED material and RGBW four-color color filters. Compared with the ordinary RGB three-color color filter, this method can achieve both low power consumption and color expression (b). This site is based on the company's information.
The printing process has also made progress
Another method that can be applied to large screens is to use a printing process. Seiko Epson has successfully prototyped 14-inch panels for inkjet printed polymer OLED materials (Figure 11). This panel uses a polymer luminescent material from Sumitomo Chemical. By appropriately adjusting the amount of ink ejected from the nozzle, the film thickness deviation of ±3% or more in the past is reduced to 0.2% or less.
Figure 11: Development of printing techniques is also progressing Seiko Epson has developed a technology for making OLED layers using inkjet printing. The 14-inch prototype has a resolution of 60 ppi, which is equivalent to a 37-inch full HD panel. This site is based on the company's information.
In each OLED element layer of the prototype panel, an inkjet method is employed for five layers of an RGB three-color light-emitting layer, an intermediate layer, and a hole transport layer. The resolution is 60ppi, which is equivalent to a 37-inch full HD (1920 x 1080 pixels) panel. "The problem of film formation of OLED elements has been basically solved" (Seiko Epson).
What has not been solved is the problem of low performance of blue luminescent materials. Now, “Blue material candidates with the range of color requirements for TVs have emerged, and methods to extend the life of materials are under study. We hope to start mass production after 2011†(Sumitomo Chemical Representative Director Executive Director Technology and Director of Business Planning (Technology and Research and Development) Business Promotion Director Nakagawa Kiyohiko).
In addition, DuPont Display and Great Japan Screen are developing printing processes using low molecular luminescent materials. A 4.3-inch OLED panel was prototyped using a "multi-nozzle printer" technology in which the ink flowed linearly. The brightness unevenness is 6% (±3%). (Full text reporter: Saeki Shinya) 