Nanopaper transistors for the coming age of flexible and transparent electronics

(Nanowerk Spotlight) The coming age of wearable, highly flexible and transparent electronic devices will rely on essentially invisible electronic and optoelectronic circuits. In order to have close to invisible circuitry, one must have optically transparent thin-film transistors. In order to have flexibility, one needs bendable substrates.

Transparent plastic substrates are widely explored for flexible electronics (read more: "The rise of flexible electronics") – but they have intrinsic problems: Most of them are not thermally stable; are not based on green (i.e. renewable and biodegradable) materials; and are not as easily printable as paper.

For these applications, transparent nanopaper has many advantages over regular paper as well as plastic substrates. Nanopaper, made from cellulose like traditional paper, shows much lower surface roughness and much higher transparency than traditional paper. This is due to the nanoscale dimensions of the cellulose fibers used for its production.

With regard to fabricating electronics, and compared to plastic substrates, the crucial advantages of nanopapers are their better thermal stability and the fact that they tolerate a much higher processing temperature than plastic (see: "Cellulose nanomaterials review: structure, properties and nanocomposites").

A research group, led by Liangbing Hu, an Assistant Professor in the Department of Materials Science and Engineering at the University of Maryland, has now fabricated transistors on specially designed nanopaper.

Reporting their findings in a recent online edition of ACS Nano ("Highly Transparent and Flexible Nanopaper Transistors"), co-first authored by Hongli Zhu, a post doctoral researcher in Hu's group, the researchers show that flexible organic field-effect transistors (OFETs) with high transparency and excellent mechanical properties can be fabricated on tailored nanopapers.

A picture of a fabricated transparent and flexible nanopaper transistor. (Image: Bing Research Group, University of Maryland)

Nanopaper is transparent and, in oder to keep the high transparency of the device, the semiconductor materials also need to be transparent. To that end, the researchers used a highly conductive single-walled carbon nanotube (SWCNT) film to serve as the transparent gate electrode of the transistor.

"Instead of a transparent conductive oxide (TCO) film we used a carbon nanotube film because TCO is brittle and can crack during the fabrication process," Hu explains to Nanowerk. "Furthermore, carbon nanotube film can be deposited by various low-cost methods such as rod coating and simple drawing methods, while the deposition of high quality TCO film usually requires expensive methods such as vacuum deposition or high temperature annealing."

"The nanopaper OFETs exhibit good transistor electrical characteristics," says Hu. "To demonstrate the flexibility of nanopaper OFETs, we measured devices before and during bending. We observed only a 10.2% and a 9.8% decrease in mobility when we bent the device in the direction parallel to the conduction channel direction and vertical to the conduction channel direction, respectively."