Although ink-jet (Sumitomo Chemical, Sumation, CDT, etc.), continuous spray (DuPont, Dainippon Screen), roll printing (LG Display, demonstrated for TFT backplanes), and roll-to-roll printing (GE) have gotten most of the attention, there is another, time-tested solution process: slot-coating or extrusion coating, which has been used to apply photoresist, among other things, for decades.

Greg Gibson, CTO of NexTech FAS indicated that the commercial use of slot coating to apply some of the functional coatings used in OLED displays is closer than I would have thought. (NexTech FAS is the operating company for the process equipment companies NexTech Solutions and FAS Holdings Group, which are in the process of merging.)

Slot coaters put down an unpatterned layer of material, so their most obvious application is for OLED lighting and for the unpatterned layers in an OLED display, such as the hole injection layer (HIL) and the electronic transport layer (ETL). Although patterning of multiple organic layers, such as would be required for an RGB display, is difficult, development work is continuing, Gibson said. And patterning of a single layer, as would be required for a color-by-white or monochrome display, is relatively straightforward.

Displays incorporating HIL, ETL, and/or emitting layers applied with slot coaters are likely to appear on the market in 2010, Gibson said. Which layers exactly? Products will incorporate at least two of the three, Gibson answered.

HILs and ETLs are usually apply by thin-film deposition and are very thin. Can slot coaters apply layers that thin? "Our development is to slot-coat very thin layers, and it has taken a combination of equipment and material advancements," Gibson said. "There is a low solids concentration in the wet film, which is fairly thin itself, and then it dries to a thickness of 100 to 200 nm. Although it’s hard to make a direct comparison, that’s in the general range of thin-film deposition. The materials are somewhat different than ink-jet materials because they need to be formulated specifically for slot-coating solution processing."

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