Organic light-emitting diode (OLED) displays are more energy-efficient and provide a better picture than liquid-crystal displays (LCDs), but they haven’t gained much of a market foothold because they’re far more expensive. A recently introduced OLED TV sold by LG in South Korea costs over $2,500, for example.
A startup in Menlo Park, CA, hopes to bring down the cost of these high-performance displays by making equipment for printing them on a large scale. Kateeva is testing a prototype large-area OLED printer that it will send to display manufacturers for testing next year. According to the company, its equipment can be used to print OLED displays for 60 percent of the cost of LCDs.
OLED displays are now found in a few products that take advantage of the picture quality, such as a high-end 11-inch flat-panel television made by Sony. Some portable electronics, including Google’s Nexus One phone, also use OLEDs because the relatively low-power screen extends battery life.
All the OLED displays on the market are manufactured using an expensive, small-scale technique called shadow-mask evaporation to lay down the light-emitting organic molecules that make up the pixels. Companies have looked into alternatives that are compatible with large-area manufacturing, such as ink-jet printing, but all the processes entail compromises on the performance and lifetime of the display. Kateeva’s technique combines features of shadow-mask printing and ink-jet printing to make high-quality OLED pixels over a large area. The company plans to sell printing equipment and OLED inks made of light-emitting small molecules.
From a technology perspective, “OLEDs do have a leg up” on liquid-crystal displays, says Vladimir Bulovic, professor of electrical engineering and computer science at MIT and a scientific advisor to Kateeva. LCDs use an array of liquid crystals to filter light from a white backlight. They have a relatively low contrast ratio–making a pixel truly black is impossible because some light always leaks through.
OLED displays are made up of layers of organic molecules sandwiched between two electrodes. The organic molecules in each pixel emit light when they’re electrically stimulated. Because the pixels in an OLED produce their own light and that light can be turned off, they produce a better image, and they use less energy. In the lab, OLEDs use 30 percent of the power that state-of-the-art LCDs do.
Where OLED displays fall short is in manufacturing. LCDs have been around since the 1970s, and manufacturing processes have been honed to make them cheaply at a large scale. LCDs are fabricated over very large areas, as big as about nine square meters, then sliced into individual screens, for economies of scale that keep costs low. With the industry-standard shadow-mask printing for making OLED displays, says Conor Madigan, CEO and cofounder of Kateeva, “it’s painful to go larger than .6 by .7 meters.”
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.