Inkjet-Printed Conductive Electrodes: Design, Fabrication, and Characterization

Abstract

Conductive patterns for printed electronics can be printed by adding metal nanoparticles to a solvent with specific agents to improve printability and prevent particle agglomeration. Printed materials require drying to evaporate the solvent and solidify the material. Electrical conductivity is one of the most important properties of printed metal nanoparticle conductors. Conductivity should be the same irrespective of pattern design, size, substrate, location, or density of adjacent patterns. However, we demonstrate here that inconsistencies in the drying process for printed patterns with proximity cause resistivity variations. We studied these resistivity variations experimentally in arrays of printed square electrodes. This variation depends not only on the location of each electrode in an array but also on the number of electrodes. This means that for the same drying temperature and duration, the array with a larger number of electrodes shows higher resistivity variation. After drying, nanoparticles are sintered in a second post-treatment process to improve the electrical conductivity of the printed metal nanoparticle film. In the sintering process, metal nanoparticles melt and merge to form larger grains, so the modified morphology of the printed structure can improve the electrical conductivity. In order to achieve a uniform drying pattern, optimized intense pulsed light (IPL) sintering can be considered as a solution. However, our findings demonstrate that selective CO2 laser sintering provides better control over patterns according to the pattern density. Additionally, when different materials or patterns are printed on the same substrate, IPL sintering is not an ideal technique due to the varying parameters required for each material or pattern. Laser sintering, on the other hand, can be programmed to sinter different areas with specific parameters, thereby improving the resolution of the sintering process. Furthermore, frequency domain thermoreflectance (FDTR) measurements allow for local monitoring of the resistivity of complex patterns that cannot be analyzed with a four-point probe. Our study shows that resistivity variation in complex patterns, such as spiral patterns, can be reduced the most with laser sintering. Laser sintering is able to reduce the resistivity variation from 17% observed with thermal sintering and 5.4% with IPL sintering to around 3.2%.