Imagine a future where everything—your bed, your wallpaper, the box your cereal comes in—is capable of connecting to the Internet and the windows and walls of skyscrapers harvest energy from the sun. It’s a scenario many technologists talk about, but it would certainly strain today’s infrastructure for building silicon-based electronics. The future many depend on a more old-fashioned production process—roll-to-roll printing.
That, at least is what scientists and engineers argued at a session on the future of roll-to-roll processing at the Material Research Society’s fall meeting in Boston last month. Existing fabs and foundries produce about 20 billion silicon chips a year, far fewer than would be needed if the Internet of Everything is to become a reality, Donald Lupo, a professor of electronics and communications engineering at Tampere University of Technology in Finland, said at the meeting. Estimates hold that 50 to 200 billion objects will be connected to the Internet within five years, some with multiple devices on them, and many more coming in subsequent years, he added.
“A trillion sensors means a lot of new fabs you’re going to need,” Lupo said. “We think that printing and coating is a promising option.”
He’s part of a team working on a four-year project to develop some of the components that giant presses could print onto flexible substrates, such as plastic. Those include devices that could harvest energy from light, radio waves, or motion. To store that energy, the team wants to replace batteries, which would contribute too much toxic waste to the environment, with printable supercapacitors. They’re developing various inks to print electrodes, made of carbon nanotubes, activated carbon, graphene, or graphene-polymer composites, as well as non-toxic hydrogel electrolytes. Hydrogels could have the same ionic conductivity as salt water but are easier to manufacture than a fluid-filled device and are not subject to leakage.
The way to build all this low-power, high-speed circuitry for sensors, processors, and wireless communications, Lupo said, is to combine standard printing processes, which create devices over large areas inexpensively, with atomic layer deposition, a precision chemical process that builds up thin films layer by layer and allows the fabrication of complex electronic devices.
Heliatek, a start-up in Dresden developing printable thin-film solar cells, also wants to improve on roll-to-roll printing, in this case by combining it with vacuum deposition. In a vacuum, their machine processes rolls of PET, a common plastic, onto which they deposit layers of organic photovoltaics and indium tin oxide, which acts as a transparent electrode. They perform all the laser patterning necessary to inscribe the circuitry on the solar cells while the material is still in vacuum, eventually producing rolls of photovoltaic foils that can be applied to the facades and windows of buildings.
The machines the company uses are expensive, and its product is not yet cost competitive, admits Martin Pfeiffer, Heliatek’s chief technology officer. But as production scales up and processes improve, he expects the foils to hit a target of €30 (US $33) per square meter. And with so many buildings in the world, he says, the potential market is huge. “It’s almost unlimited.”
There are many other methods for creating high-performance circuitry and devices on large rolls of plastic or paper. Zhichao Zhang of the University of Hong Kong told of using a paraffin stamp to place an organic crystals on top of a dielectric, creating an organic field-effect transistor. Dimitra Georgiadou of Imperial College London described adhesion lithography, which deposits layers of different metals and organics, then uses an adhesive material–essentially a piece of Scotch tape—to peel away the excess, leaving behind electrodes with gaps between them of only 10 to 30 nm, which improves their electrical efficiency. Darren Lipomi from the University of California, San Diego is investigating blends of polymers and fullerenes—balls of carbon atoms—in an effort to produce the right combination of electronic performance and mechanical stability in thin films for solar cells.