A polysilicon layer was coated onto paper with the aid of a pulsed laser light (credit: R. Ishihara, M. Trifunovic/TUDelft)
A polysilicon layer was coated onto paper with the aid of a pulsed laser light (credit: R. Ishihara, M. Trifunovic/TUDelft)

A route to low-power, fast, wearable electronics at a remarkably low cost has been designed by researchers in the Netherlands and Japan. They claim that using lasers to print silicon onto paper could enable the large-scale manufacture of effective flexible, wearable electronics containing wireless sensors. Stretchable and even edible electronics may also be possible.

The quest to develop wearable electronics has mostly focused on organic and metal-oxide based inks, due to their ability to be printed onto a wide range of flexible materials easily. However, the electronic performance of these inks is not as good as that of silicon electronics. “The printed organic semiconductor's mobility for holes is too low and that for electrons is even lower. The printed metal oxide semiconductor has got higher electron mobility but its hole mobility is almost zero,” explains Ryoichi Ishihara, who led the research team based at Delft University of Technology and the Japan Advanced Institute of Science and Technology in Ishikawa. High quality polycrystalline silicon provides high mobilities for both electrons and holes, he says. This is essential for the configuration of highly reliability, fast and low-power circuits.

The ability to print the electronics is the key to keeping costs down. Silicon integrated circuit chips made using the traditional route could be mounted onto low-cost substrates, but the robotics used to make these – while constantly reducing in price – will always impede the costs dropping as low as that for printing.

Routes to printing silicon ink have been designed previously, but the high temperatures (350ºC) required in the annealing step rule out their use on many flexible materials including paper, polyethylene terephthalate and polyethylene naphthalate. “We were able to reduce the formation temperature of polysilicon significantly (to 150ºC) in the solution process of Si, so inexpensive substrates with a low thermal budget such as paper can now be used,” Ishihara says. This work is published in Applied Physics Letters [Trifunovic, M. et al, Appl. Phys. Lett., (2015) doi: 10.1063/1.4916998].

The team first skimmed the liquid polysilane directly on a paper surface using a doctor-blade in an oxygen- and water-free environment. Next, they polymerised the film using UV light and then annealed the layer by exposing it to an XeCl excimer laser. The paper is not damaged because the duration of the laser pulse is so short: in the range of tens of nanoseconds. 

 “The process is unexpectedly easy!” explains Ishihara. “Several measurements confirmed that the layer consists of silicon crystals and nothing else and a thin-film transistor made using the layer exhibited mobilities as high as those of the conventional poly-Si.”

Ishihara expects the process to scale-up easily for large-scale manufacturing. Doctor-blade coating is already used in the roll-to-roll printing process used for printing newspapers and the lasers are used for annealing materials in the manufacture of smartphone displays, he explains. More optimization is however required before electronics made this way are ready to hit the shelves.

Once this is done, “the most immediate application will be wearable electronics having ultra-high frequency radiofrequency identification (RF-ID) tag with sensors,” says Ishihara. These could be used for medical, fitness, identification or security purposes. “The process can also be expanded to solar cells, and will also realize stretchable and even edible electronics.”