Excimer Lasers Print Silicon on Paper

For years, scientists have sought an alternative to silicon that possesses the same electrical properties while offering "superior printability." A replacement may not be necessary, though, as laser-printed silicon could be the answer.

Researchers at Delft University of Technology, in collaboration with the Japan Advanced Institute of Science and Technology, have developed a technique that allows silicon in the polycrystalline form used in circuitry to be produced directly on a substrate from liquid silicon ink with a single laser pulse. Previous research has involved various organic and metal-oxide ink materials, but these often lack silicon's favorable electronic properties.

The researchers coated liquid polysilane onto paper by "doctor-blading, or skimming it by a blade directly in oxygen-free environment." Next they annealed the layer with a conventional excimer laser, a type of laser often used in manufacturing smartphone displays.

"And it worked," said Delft professor Ryoichi Ishihara. "It was very simple."

A high-mobility polysilicon layer was directly formed on paper by coating liquid silicon and annealing it with pulsed laser light. Courtesy of R. Ishihara and M. Trifunovic/TUDelft.

The laser pulse lasted just a few tens of nanoseconds, he said, and left the paper completely intact. Testing the conductive performance, researchers found that thin-film transistors using the laser-printed layer exhibited mobilities as high as those of conventional polysilicon conductors.

The new method also transforms the liquid silicon into polysilicon, bypassing the need for the conventional 350 °C thermal annealing step that has proved too hot for many flexible surfaces.

Numerous applications could be possible thanks to the researchers' findings.

"The process can be expanded to biomedical sensor and solar-cell areas," Ishihara said. "And [it] will also realize stretchable — and even edible — electronics."

Other applications include wearable electronics, as the new technique allows for production of fast, low-power, flexible transistors at low cost. Ishihara said he believes the future of the project, which also involves "improving the production process of the thin-film transistors to include additional non-silicon layers," could expand even further into other electronics.

The research was published in Applied Physics Letters (doi: 10.1063/1.4916998).

For more information, visit www.tudelft.nl.