Impact of ink formulation on carbon nanotube network organization within inkjet printed conductive films

The inkjet printing of an aqueous suspension of carboxylic acid-functionalized single walled carbon nanotubes (SWCNT-COOH) and of a conductive ink combining SWCNT-COOH with the conductive polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT-PSS) was studied. A dimensionless study predicted the behavior of these two fluids in a given printing system. Observations at different scales were performed on the printed samples to visualize the arrangement of the carbon nanotube (CNT) network within the printed layer. An innovative way to localize CNTs within the printed patterns was developed by using a mapping technique of surface sample, based on a scanning electron microscope coupled with an energy dispersive X-ray spectroscope. The SWCNT-COOH aqueous suspension is subject to the halo (or “coffee ring”) effect, which is a well-known phenomenon in inkjet printing, whereas the SWCNT-COOH/PEDOT-PSS ink offers a more homogeneous CNT network. The CNT orientation has also been under investigation. For the SWCNT-COOH suspension, specific orientations of the CNTs were recorded, whereas for the SWCNT-COOH/PEDOT-PSS ink, a more homogeneous CNT distribution with a random orientation was obtained. This study proved also that the droplet ejection velocity can have an impact on the CNT distribution and consequently on the electrical performances of the ink.