Using organic semiconductor materials instead of rigid silicon, it’s possible to make energy-efficient, lightweight, and flexible solar cells and computer displays. But making devices out of organic materials requires investing in completely new equipment, since organic materials are normally destroyed by the harsh chemicals necessary for conventional photolithography.
Now Orthogonal, a company based in Ithaca, NY, is developing materials that will allow organic electronics to be manufactured on the equipment used to make silicon electronics. This should also make it possible to build more-complex organic components. The company has demonstrated four prototype devices, including organic light-emitting diodes, made using new photolithography chemicals that are compatible with organic materials.
Transistors and display pixels that are made from organic materials such as polymers are slower than those made of silicon, but they also require less energy to operate, weigh less, and can be made on flexible backings, making them attractive for use in displays and solar panels. But manufacturing them requires new equipment such as industrial ink-jet printers. “It’s definitely an issue for organic light-emitting diodes and other organic electronics that a lot of the equipment is handmade,” says Paul Semenza, senior vice president at DisplaySearch, a market research company. For manufacturers, buying entirely new equipment is a major expense. “If you could use photolithography to make these devices, potentially you could break that bottleneck,” he says.
Photolithography is the standard method for making silicon electronics, but it is normally incompatible with organic materials because it requires harsh chemicals that cause them to break down. To pattern a surface such as a silicon wafer, the surface must first be coated with a light-sensitive chemical called a photoresist. Light is then shined onto the surface through a patterned mask, and solvent is applied to etch away the exposed areas of the photoresist, leaving behind a pattern.
Orthogonal has licensed a photolithography technique developed by researchers at Cornell University that’s compatible with organic materials. The technique, called orthogonal lithography, was invented by Christopher Ober and George Malliaras, both professors of materials science and engineering at Cornell. Both the photoresist and the solvent that carries it away during the etching process are made from an unusual class of molecules called hydrofluoroethers–compounds that interact with each other but not with semiconducting organic materials. So the solvent and photoresist the Cornell researchers developed won’t degrade the semiconducting layers during the lithography process.
Orthogonal hopes to sell chemicals for making organic electronics using conventional photolithography to companies that make displays and other devices. The company is already working with several display and solar manufacturers to develop products using the manufacturing method.
“Instead of building new plants and developing new processes, we want to enable manufacturers to use equipment and knowledge around a process that already exists,” says Fox Holt, CEO of Orthogonal.
Ober, who developed the photoresist and solvent, says Malliaras has “taken this process and enhanced it to apply to the production of very complex organic transistors, sensors, solar cells, and ring oscillators” (devices that convert direct current into alternating current). Orthogonal is currently showing these four prototypes to potential customers. So far, the process has proven compatible with all the organic semiconducting materials the group has tested. Ober says the solvents are also environmentally benign and easy to work with. While the new solvents may cost more than those used to make silicon electronics, the price of the photoresist should be equivalent. Overall, the process should be cheaper than making silicon electronics.
The performance of devices made using orthogonal lithography is equivalent to organic devices made using other techniques, says Orthogonal CTO John de Franco. The company’s first products, which it hopes to sell in about a year, will be materials for making organic light-emitting diodes. Orthogonal has built prototype pixels using the technique. “Their efficiency is comparable to standard devices, and we’re currently looking at lifetimes,” which can be a problem with organic display materials, says de Franco.
Ober expects orthogonal lithography to make more-advanced organic electronics possible, with layered devices capable of performing more complex operations. When multilayer organic devices are made using printing techniques such as ink-jet printing, each layer disturbs the previous one, he says. The devices are made from inks that consist of a semiconductor in a solvent. But the solvent, which is required to print successive layers, can interact with the semiconductors that have already been printed, with serious consequences for performance. This intermixing between layers isn’t a problem when making multilayer devices using orthogonal lithography because the fluorinated solvents interact only with the photoresist, and not with the organic semiconductor. “Each layer can be placed down without intermixing,” says Ober.
Zhenan Bao, an associate professor of chemical engineering at Stanford University, says the company’s approach is unique. Other groups have focused on developing cheaper manufacturing technologies–if the initial investment in equipment is not factored in, ink-jet printing and other techniques are cheaper. But Bao expects that Orthogonal could get a foothold in the display market. “These are products that are not necessarily going to be cheap and disposable, so lithography may be affordable,” she says. “And using existing infrastructure could be an advantage, especially for display companies.”
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