To make a display using a today’s techniques, an array of transistors called a backplane is first covered with a stencil called a shadow mask, which has small holes where the pixels will be. The backplane is then placed inside a high-vacuum chamber with a crucible filled with the light-emitting organic molecules in powder form. This process is repeated for each of the red, blue, and green molecules that make up the display’s pixels. When the temperature is raised, the organic molecules sublime into a gas and coat every surface inside the chamber. The difficulty of aligning the stencil limits the area of OLED pixels that can be made at once. Clogging problems limit how small the pixels can be; this in turn limits the resolution of the resulting displays.
Making the molecules into an ink and printing them with an ink jet also has limitations, says Madigan, because an already-printed blue spot will be dissolved by the solvents in a subsequently printed red spot, for example, leading to a deformed pixel.
Kateeva’s equipment uses a printing nozzle first developed by Bulovic’s group at MIT to deposit OLED pixels on a backplane. The Kateeva nozzle has two parts stacked on top of each other. The first is an ink-jet-like printhead that dispenses OLED ink into the pores of an underlying thermal jet. The thermal jet is a silicon chip full of holes that suck up the ink like a sponge. A metal heating element surrounding the pores generates enough heat to evaporate the solvents in the ink, leaving behind only the organic molecules. A second blast of heat turns the chemicals into a gas to deposit them on the surface.
The company is testing a prototype printing machine that can make displays over an area of .6 by .7 meters. The company’s first production machines will print over areas 1.8 by 1.5 meters–smaller than the industry standard for LCDs, but larger than what’s currently used for OLED displays. At this size, says Madigan, “you start getting good economies of scale.” Madigan says Kateeva is in talks with leading display manufacturers, who will test the company’s equipment and inks in 2011.