Organic field-effect transistors via inkjet printing

• Main Author: Cheng, X.
• Published: University of Cambridge 2009
• Subjects: 621.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.597572

This thesis is concerned with the fabrication process, charge injection, dielectric preparation and downscaling of organic field-effect transistors via inkjet printing. To provide a general guideline of fabricating inkjet-printed short channel transistors, this thesis starts with an improved self-aligned printing (SAP) technique implemented with gold nanoparticle inks which defines a nanometer scale channel between source and drain electrodes. Electron and hole injection and transport in the channel is studied using ambipolar semiconductor F8BT. Contact resistance effects are found to be significantly more pronounced in these short channel devices and the current is limited by insufficient charge injection. Self-assembled monolayers (SAMs) are effectively employed to reduce the contact resistance in the SAP. Next, the systematic control of electron and hole charge injection in top-gate F8BT ambipolar transistor via SAM modification is investigated. Gold electrodes are modified with PFDT and 1DT to reduce contact resistance and improve charge injection. Furthermore, a crosslinked fluoropolymer Cytop (C-Cytop) is adopted as an ultrathin gate dielectric, which is necessary for proper gate control of the accumulation layer in short channel devices. The C-Cytop can be spin-coated in air with uniform thin films, low gate leakage and high dielectric breakdown strength. The yield and stability of C-Cytop devices are remarkable compared to other non-fluorinated polymer dielectrics and less dependent on the underlying semiconductor roughness. In particular high performance in-type transistors are demonstrated using C-Cytop and small-molecule ActivInk N1400 as the semiconductor. Finally, downscaled inkjet-printed nano-channel n-type transistors are demonstrated by printing silver nanoparticle inks with N1400 and C-Cytop in the SAP structure. The printed silver nanoparticles serves as source contact combining with thin C-Cytop layer as gate dielectric.