Fabrication of an inkjet printed ion gel gated organic thin film field effect transistor for biomedical implants

This thesis work reports the experimental fabrication of an inkjet printed ion gel-gated organic thin-film field-effect transistor. The motivation for this project is the need for flexible, biocompatible and low-consumption transistor arrays for applications in the biomedical field, with particular regard to the on-going researches on retinal prostheses and biomimetic tactile sensors at the University of New South Wales, Sydney, Australia. The properties, materials, modes of operation and fabrication methods of organic thin-film transistors are reviewed. In this project, PEDOT:PSS was selected as the gate electrode material, P3HT as the semiconductor, ion gel as the dielectric, polyimide as the substrate and gold as source and drain contacts. Inkjet printing was preferred as the deposition process because of its non-contact, direct-write and low-cost nature. A piezoelectric-based FUJIFILM Dimatix Materials Printer DMP-2800 was employed. Before printing, suitable inks for each of the organic materials were formulated in order to comply with the requirements in terms of viscosity, surface tension and volatility set by the printer. Afterwards, the printing process was performed in terms of pattern design, printing parameters optimization, droplets deposition and solvents evaporation. Finally, both single materials and the full transistor were electrically characterized. The objective of effectively printing the three polymer inks was achieved, but the whole transistor could not be eventually fabricated because of the unsatisfactory interaction between the conductive polymer and the ion gel. However, the issue will be addressed by revising the ink compositions. The electrical properties of PEDOT:PSS and ion gel resulted to be satisfactory. The relationship between the drain current and both the gate and the source-drain voltage showed a typical transistor behavior. However, issues of excessive gate current and P3HT photo-oxidation must be addressed in the near future.