Copper nanowires enable cheaper foldable electronics, solar cells

Ashley N. Paddock, ashley.paddock@photonics.com

Copper nanowires could bring down production costs for electronic displays, foldable electronics and solar cells — helping engineers build more affordable e-readers, iPads, cell phones, photovoltaic panels and more.

A new technique arranges copper atoms in water to form long, thin, nonclumped nanowires, which are then transformed into transparent conductive films and coated onto glass or plastic.

Images (a) and (b) represent copper nanowire (CuNW) ink before and after coating on polyethylene terephthalate with a Meyer rod. (c) A bent CuNW film (25 Ω sq^—1 and 83% transparent) completing an electrical circuit with a battery pack and a LED. Images courtesy of Aaron Rathmell, Duke University.

"I was contacted by a solar cell company frustrated with the low production rate of transparent conducting films made by depositing indium tin oxide from a vapor," said Benjamin Wiley, a Duke University chemist. "They were interested in coating silver nanowires from solution to increase the production rate by at least 100 times ... I thought, if I can obtain the same properties with copper nanowires as have previously been obtained with silver nanowires, we could reduce the cost of producing thin-film solar cells to an even greater extent."

The copper nanowire films have the same characteristics as those currently used in solar cells and electronic devices, but they are less expensive to manufacture. In electronic screens, films that currently connect pixels are made of indium tin oxide (ITO). This highly transparent material transmits information well but is an expensive rare-earth element and must be deposited from a vapor in a process that is a thousand times slower than newspaper printing. Also, ITO-containing devices can crack easily.

Researchers have sought alternatives to ITO, such as using inks containing silver nanowires. This approach has been used to develop a cell phone screen of silver nanowires that will be on the market this year. But like indium, silver is expensive.

Images (a) and (b) depict dark-field optical microscope images showing uniformly dispersed networks of copper nanowires that are 90% and 85% transparent, with sheet resistances of 186 and 30 Ω sq ^—1, respectively. Images (c) and (d) are corresponding scanning electron microscope images of the copper nanowire films from images (a) and (b) showing the average length (20 ± µm) and diameter (52 ±17 nm) of the copper nanowires.

Copper, however, is an abundant element and is about 100 times less expensive than silver and ITO. Wiley and graduate student Aaron Rathmell showed that it is possible to form a layer of copper nanowires on glass to make a transparent conducting film. The film did not perform well at first because the wires clumped together, but the team developed a new method of growing the copper nanowires and coating them on a glass surface to combat the clumping problem.

The scientists hope that the flexibility and high performance of the nanowires will make them an ideal choice for next-generation displays and solar cells.

"As copper nanowires are flexible (ITO is brittle and cracks), they can also enable new applications such as solar curtains, or electronics that can conform to the body," Wiley explained. Other applications that will benefit from the new technology include organic LED panel lighting, flat panel displays, e-ink systems, touch screens, electro-active glass and electromagnetic-interference shielding systems, he noted.

The work appeared online Sept. 23 in Advanced Materials (doi: 10.1002/adma.201102284).

To manufacture the copper nanowires for commercial applications, Wiley co-founded NanoForge Corp. in 2010. The company received a $45,000 grant earlier this year from North Carolina IDEA for refinement and scale-up of the manufacturing of the nanostructures. It is now filling orders.