Selective Deposition of Conductive Nanofiber Network with Minimized Contact Resistance for Large-Area Soft Electronics

Transparent conductors are applied in the soft electronics including displays, smart windows, and wearable devices in the form of transparent electrodes and heaters. Electrical conductivity, transmittance, and mechanical durability are important factors in the design of transparent conductors. Based on the trade-off between conductivity and transmittance, the performance of transparent conductors can be improved by optimizing both factors simultaneously. In this research, catalyst-embedded nanofibers were formed on various substrate such as glass, polymer film, and stretchable polymers via electrospinning, and then selectively plated. First, a catalytic seed layer was prepared by direct electrospinning of a polymer solution containing palladium. A conductive network was selectively deposited on the catalytic nanofiber seeds on the substrate by copper electroless reduction deposition based on autocatalytic reaction. In the process of heat treatment and electroless deposition, the contact resistance was minimized due to the removal of overlap between nanofibers, so high conductivity was ensured. The transparent conductors under various conditions can be produced with high reproducibility by actively adjusting the electrospinning time and electroless deposition time. Under optimal fabricating conditions, the transparent conductor with a transmittance (T_R) of more than 90 % and a sheet resistance (R_S) of several Ω/sq was prepared, which means that FoM value (the ratio of electrical conductivity to optical conductivity) is more than 1000. Conductive nanofiber networks can be uniformly fabricated on each rigid (glass), flexible (polymer film), and stretchable (stretchable polymer) substrate even with the same fabricating method and conditions. Since the electrospun catalytic nanofibers have a large aspect ratio of up to 10⁵ ~ 10⁶, they can form an effective percolation network even at a low density of the conductive path, thereby improving the transmittance and conductivity simultaneously. This fabrication technology includes direct patterning and solution-based process, so it can be applied to large-area production. The standard deviation of a network density of the fabricated large-area transparent conductor is about 1%, so it has a uniform distribution. An electrical circuit can be constructed by using the fabricated transparent conductor, and it can be applied as a transparent electrode. As a result of applying the transparent conductor as a Joule heating-based transparent heater, is has a uniform temperature distribution, over the entire area and operates stably even at a temperature of more than 100 °C. In addition, it was verified that the transparent heater stably maintains the heating characteristics even when an external force is applied. The fabrication method in this research has no lithography or transfer process, so it can be applied to large-area/mass production due to low process complexity and can be easily extended to the platform technology for soft electronics including displays, smart windows, and wearable devices.