Researchers at Delft University of Technology in the Netherlands have pioneered a method 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.

The capacity for printing silicon ink onto substrates has existed for some time, but necessitated a 350° C thermal annealing step — far too hot for paper and other common surfaces. The researcher’s new method bypasses this step, transforming the liquid silicon directly into polysilicon.

They discuss their research in Applied Physics Letters.

“We coated liquid polysilane directly on paper by doctor-blading, or skimming it by a blade directly in oxygen-free environment,” said Ryoichi Ishihara, the professor who led the research team at Delft University of Technology, with collaborators at the Japan Advanced Institute of Science and Technology in Ishikawa, Japan. “Then we annealed the layer with an excimer-laser [a conventional tool used for manufacturing smartphone displays]. And it worked,” Ishihara said.

The laser blast only lasted a few tens of nanoseconds, leaving the paper completely intact. In testing its conductive performance, Ishihara and his colleagues found that thin-film transistors using the laser-printed layer exhibited mobilities as high as those of conventional poly-silicon conductors.

The most immediate application of this printing capacity is in wearable electronics, as it allows for the production of fast, low-power and flexible transistors at a remarkably low cost. Ishihara believes the future of the project, which involves improving the production process of the thin-film transistors to include additional non-silicon layers, will have applications in biomedical sensors, solar cells, and stretchable electronics.

Abstract of Solution-processed polycrystalline silicon on paper

Printing electronics has led to application areas formerly which were impossible with conventional electronic processes. Solutions are used as inks on top of large areas at room temperatures, allowing the production of fully flexible circuitry. Commonly, research in these inks have focused on organic and metal-oxide ink materials due to their printability, while these materials lack in the electronic performance when compared to silicon electronics. Silicon electronics, on the other hand, only recently has found their way in solution processes. Printing of cyclopentasilane as the silicon ink has been conducted and devices with far superior electric performance have been made when compared to other ink materials. A thermal annealing step of this material, however, was necessary, which prevented its usage on inexpensive substrates with a limited thermal budget. In this work, we introduce a method that allows polycrystalline silicon (poly-Si) production directly from the same liquid silicon ink using excimer laser irradiation. In this way, poly-Si could be formed directly on top of paper even with a single laser pulse. Using this method, poly-Si transistors were created at a maximum temperature of only 150 °C. This method allows silicon device formation on inexpensive, temperature sensitive substrates such as polyethylene terephthalate, polyethylene naphthalate or paper, which leads to applications that require low-cost but high-speed electronics.