Advancements in Inkjet Printing Techniques for Improving Field-Effect Transistors in Printed Electronics

This thesis centers on the enhancement of printed field-effect transistors (FETs) with a primary focus on addressing challenges through innovative inkjet printing methodologies. The study concentrates on the utilization of hydrophobic fluoropolymers, particularly Teflon amorphous fluoropolymer (Teflon-AF), as gate dielectrics in organic thin-film transistors (OTFTs). Teflon-AF, while promising for its low charge trap density, thermal stability, and low dielectric constant, presents impediments due to its low surface energy, resulting in issues like dewetting and bulging of printed patterns. This obstructs the creation of uniform lines using low-viscosity ink. To resolve these challenges, this thesis presents novel strategies for inkjet printing micro-patterns on hydrophobic surfaces.

Two approaches are outlined to successfully inkjet print micro-patterns on hydrophobic surfaces. The first involves a sequential inkjet printing and drying process, which maintains ink adherence to the surface. Meanwhile, an energy minimization technique predicts the equilibrium shape and volume of patterns, which is influenced by surface tension forces. The simulation accurately predicts the required ink volume for achieving dry patterns with smooth edges, advancing inkjet printing techniques for electronics. The second approach demonstrates stacked-coin methodology to form smooth lines on hydrophobic surfaces. Variations in drop spacing, stage speed, and stage temperature map out various regimes: isolated droplets, isolated groupings, broken lines, true stacked-coin, and delamination.

The study further investigates the fabrication of OTFTs employing Teflon-AF gate dielectric. Plasma treatment is applied to render the hydrophobic layer amenable to inkjet printing of silver electrodes. Morphological and surface chemical properties of Teflon-AF films change after plasma treatment and gradually reverse after annealing of the electrodes. This affects the organic semiconductor/dielectric interface and, consequently, the OTFT performance.

Moreover, the research explores inkjet printing capabilities by incorporating carbon nanotubes (CNTs) into the TFT channel. Strategic utilization and engineering of the coffee ring effect in inkjet-printed CNT channel obviate the need for surface treatment. Inkjet printing leads to the aligning and bundling of CNTs on the line edges, which improves device performance.