Self-junctioned copper nanofiber transparent flexible films are produced using electrospinning and electroplating processes that provide high performances by eliminating junction resistance at wire intersections.Metal-coated nanofiber mats that are both conductive and transparent could make flexible electronic devices easier to produce, according to researchers from the US and Korea [An et al., Adv. Mater. (2016), DOI: 10.1002/adma.201506364].
Electronic devices like LEDs, displays, touch screens, solar cells, and smart windows rely on transparent conducting electrodes. Indium-tin-oxide (or ITO) dominates the market because it offers a reasonable trade-off between the mutually incompatible requirements of high transparency and low resistance.
The problem is that low resistance requires highly mobile charge carriers, which inevitably interact with light to reduce transparency. Nanomaterials like carbon nanotubes, graphene, metal nanofibers, and conductive polymers are all being investigated as possible alternatives to ITO for flexible electronics. Mats of metal nanofibers or wires are particularly promising because of their high conductivity and transparency, as well as offering flexibility. Their only major limitation is the increase in resistance at the junctions where nanowires intersect.
To get around this shortcoming, researchers from the University of Illinois at Chicago, the University at Buffalo, and the Universities of Korea, King Saud, and Yonsei electroplated electrospun polymer nanofibers with Cu to create a highly conductive but transparent thin films. The electroplating effectively smoothes out the intersections between fibers to reduce resistance while preserving the ‘holey’ nature of the nanofiber mat.
The result is a highly conductive thin film that is still 92% transparent. In fact, say the researchers, the material achieves a “world-record combination of high transparency and low electrical resistance” ten-fold better than previously reported.
“We can take the metal-plated fibers and transfer to any surface – the skin of the hand, a leaf, or glass,” says researcher Alexander L. Yarin of the University of Illinois.
When mounted on a flexible plastic substrate, like commercially available Eco-flex, the material can withstand stretching of up to 580% with little increase in resistance, as well as repeated flexing.
The combination of very low sheet resistance, very high transmittance, and very good mechanical flexibility and stretchability makes these transparent electrodes among the best reported, says Zijian Zheng of the Institute of Textiles and Clothing at Hong Kong Polytechnic University.
“The enabling technology of electrodepositing Cu on a nanofiber membrane, which eliminates the contact resistance often occurring in nanowire junctions, should make an impact in the field of flexible optoelectronics,” Zheng believes.
Moreover, both electrospinning and electroplating are high-throughput techniques that could be readily incorporated into continuous, roll-to-roll manufacturing processes to produce conductive electrode materials cheaply, in large volumes, and without the need for low-temperature, high-vacuum conditions.
This article was originally published in Nano Today (2016), doi:10.1016/j.nantod.2016.06.005