Low haze structures for transparent flexible electrodes by electrospinning processes

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Flexible, transparent, and conductive electrodes (FTCEs) are a key enabling technology for a generation of flexible, printable and wearable electronics.

Tapping and wiping can only work on flexible devices, when flexible materials are used for touchscreens and electric circuits, but not brittle materials like indium tin oxide or silicon. For this purpose, INM - Leibniz Institute for New Materials is working with the process of electrospinning, a technique that produces ultra-fine fibres that are up to 100 times thinner than a human hair. These fibres are collected on glass or on foils in an unstructured, wide mesh net. When conductive materials are spun, flexible conductive transparent electrodes could be produced. These FTCEs have transparencies comparable to indium tin oxide with low haze less than 2%.

Electrospinning relies on the electro-hydrodynamics of a polymer droplet in a strong electromagnetic field. The polymer droplet deforms into a cone under the electromagnetic field and ejects a jet of liquid polymer to reduce the charge on the droplet. Once in the air, the polymer jet experiences a bending instability causing the fibre to whip through the air, effectively drawing the fibre to diameters below 500nm.

"Our innovation lies in the choice of starting materials,” explains Peter William de Oliveira, Head of the InnovationCenter INM. “We can use sols, which have to be calcined, or polymers and composites with no further heat treatment. Depending on the starting material, it is possible to produce both intrinsically conductive fibres and those which are electrically conductive in a further step via silvering,"

In contrast to patterning processes via stamps or printing methods, electrospinning easily produces unstructured fibre networks with sufficient space between the fibres. At the same time, the length of the fibres reduces the number needed for conductivity with sufficient coverage, along with the connections between the fibres which in turn, descreases the contact resistance.

With fibre thicknesses well under 500nm, they’re not visible to the human eye and appear transparent. The net-like, random nature of the fibre deposition also eliminates typical diffraction phenomena, such as distracting rainbow effects.

"This process is machine-compatible and therefore, allows a very efficient path for the manufacture of such electrodes. At the InnovationCenter we have a spinning station with which we can meet the different needs of the interested parties," says de Oliveira.

Oliveria believes electrodes could be developed for flexible displays, photo-voltaics or passive sensors. The fibres produced by electrospinning could not only be used as electrodes, Oliveria continues, but could also be woven into electronics, or used for active water treatment due to their high surface area and material properties.