To develop flexible and transparent electrodes, INM is working with electrospinning, a technique that produces ultra-fine fibers that are up to 100 times thinner than a human hair. These fibers are collected on glass or on foils in an unstructured, wide mesh net. When conductive materials are spun, flexible conductive transparent electrodes could be procuced. These FTCEs have transparencies comparable to indium tin oxide with low haze less than two percent.
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 stability causing the fiber to whip through the air effectively drawing the fiber to diameters below 500 nanometers. The fibers are collected on glass or on a film in an unstructured, wide mesh net. The innovation lies in the choise of the starting materials: Sols, which have to be calcined, or polymers and composites with no further heat treatment are used. Depending on the starting material, it is possible to produce both intrinsically conductive fibers and those which are electrically conductive in a further step via photochemical metallization.
In contrast to patterning processes via stamps or printing methods, electrospinning easily produces unstructured fiber networks with sufficient space between the fibers that light scattering is reduced to less than two percent. At the same time, the length of the fibers reduces both the number of fibers needed for conductivity with sufficient coverage and connections between the fibers which reduce the contact resistance. With fiber thicknesses under 500 nanometers, the fibers are not visible to the human eye and appear transparent. The net-like, random nature of the fiber deposition also eliminates typical diffraction phenomena, such as distracting rainbow effects.