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Researchers of the LGP2
(May 2011)
 
Grenoble INP-Pagora, International school of paper, print media and biomaterials
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Laboratory of Pulp and Paper Science and Graphic Arts

IV - Printing Processes

IV - 3 - Development of inkjet inks for electronic printed field

In the electronic industry, fabrication of conductive patterns is crucial. Traditional methods to make electronic devices such as electroplating, etching processes and lithography technology are not only time consuming but they also produce large quantities of waste. For these reasons, the development of cost effective and fast processing techniques to fabricate conductive elements (for display technologies, solar cells, flexible electronic, etc.) has attracted more and more attention in recent years. One plausible alternative is the development of conductive inkjet inks which allow to draw a conductive pattern in one step.

Towards this meaning, water based conductive inks that combine the processability of conductive polymer to the high electrical properties of carbon nanotubes (CNTs) were synthesized. In this configuration, the conductive polymer acts as a binder to promote the adhesion of CNTs on the substrate and enhances the electric charge circulation within the CNTs network. Concerning the ink formulation, inks have to meet strict physicochemical properties to match with inkjet printing process requirements (low viscosity, surface tension ~ 35 mN/m). High homogeneous and stable aqueous CNTs suspensions were developed by using efficient techniques of dispersion (sonication, centrifugation) and were mixed with the conductive polymer suspension. Trials have been performed to determine which composite ink formulation provides the highest electrical performances. Up to now, the lowest sheet resistance obtained is 225 Ω/sq on a polymer film substrate. Figure 1 shows the morphology of a printed conductive film by optical and scanning electron microscopy. The printing quality was high and the presence of CNTs conducting pathways was emphasized..

Morphology of a printed conductive film by optical and scanning electron microscopy
Figure 1 - Morphology of a printed conductive film by optical and scanning electron microscopy

The conductive coating capability of different paper substrates was also explored to determine which type of paper is suitable for printed electronics. In this field, surface properties like roughness, permeability and surface energy are crucial properties. In order to minimize the impact of surface irregularities on electrical properties, specialty papers with very smooth and closed surface were selected. A polymer film (Polyethylene Terephtalate) was also tested as reference.

Figure 2 shows the variations of voltage with current intensity, corresponding to the conductive film on the different substrates. Each curve can be assimilated to a straight line whose slope corresponds to the sheet resistance (U = R.I). The lower the sheet resistance (i.e. the slope), the higher the conductivity. The polymer film gave the best results in terms of sheet resistance.

This was expected because its surface allows the formation of homogeneous films. For the paper substrates, surface irregularities disturb significantly the electrical conductance. However, the results obtained with the superimposition of several conductive layers lead to a significant decrease in sheet resistance, whatever the substrate.

Variations of voltage with current intensity, 
   for the conductive film on the different studied substrates
Figure 2 - Variations of voltage with current intensity, for the conductive film
on the different studied substrates.
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