High-mobility poly-Si TFTs directly printed on paper

December 15, 2017 // By Julien Happich
Printed electronics, hand-in-hand with flexible electronics, is a hot topic where both active materials and substrates are being investigated for their processability as cheap and scalable roll-to-roll alternatives to traditional silicon transistors.

Popular ink materials include organic and metal-oxide semiconductors, though they lag behind silicon in terms of electrons and holes mobility (orders of magnitude lower than poly-silicon) reliability, and energy efficiency. Commonly used substrates found in flexible electronics include thin films of polyethylene naphthalate (PEN), polyethylene terephthalate (PET) and even polydimethylsiloxane (PDMS) used for stretchable or conformable electronics.

Sample of electronics on a paper substrate.

But driving costs further down, a team of researchers from Delft University of Technology (The Netherlands) wants fast electronic circuits to operate on paper (an order of magnitude cheaper than these polymers), without the uneconomical complications of thin-film transfers from a native silicon substrate to paper. 

A couple of years ago, Dr. R. Ishihara, an associate professor in TUDelft's Department of Microelectronics, lead his team to demonstrate how cyclopentasilane (CPS) could be used as a liquid silicon precursor to synthesized poly-Si at low temperature (under 100ºC). Now Ishihara and his team have successfully fabricated both p-channel and n-channel poly-Si thin-film transistors (TFTs) directly on top of paper, with field-effect mobilities of 6.2 and 2.0 cm2/V s, respectively.

The low temperature and roll-to-roll compatible fabrication method they describe in the npj's Flexible Electronics journal is an eight-step process, reviewed in detail in a paper titled "Solution-based polycrystalline silicon transistors produced on a paper substrate".