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Title: Organic transistors and complementary circuits
Author: Higgins, Stuart Gregory
ISNI:       0000 0004 6348 0275
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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This thesis describes the development of a process flow to allow the fabrication of organic field effect transistors with self-aligned source and drain electrodes, and sub-micron channel lengths on flexible plastic substrates. This was achieved using a combination of bilayer nanoimprint lithography, gravure printed or photolithographically patterned dielectric, and bilayer self-aligned lithography. Effective channel lengths over an order of magnitude could be defined from 375-3800 nm. Electrode overlaps between gate-drain and gate-source of 100-200 nm are demonstrated, yielding very low channel-length normalised overlap capacitances of 0.1-0.6 fF per μm. The viability and behaviour of these architectures was investigated using zone-cast, gravure and inkjet printed semiconductors. State-of-the-art transition frequencies in the range 1-6 MHz were achieved at operating biases < 30 V. The molecular packing of a zone-cast small molecule p-type semiconductor (TIPS-pentacene) was observed to be influenced by the underlying architecture. Simple techniques were developed to quantify gravure printed film quality. Periodic modulation of printed films was observed to be a function of gravure cliché cell geometry and ink formulation. Both gravure and inkjet printed p- and n-type semiconductors (DPPT-TT and P(NDI2OD-T2)) were studied. Thin gravure printed dielectrics were observed to systematically increase effective mobilities, at the expense of an increase in leakage current. Complementary self-aligned inverters are presented, with a peak gain of 28 achieved for devices operating below 20 V, along with NAND and NOR logic gates operating up to 20 kHz. The viability of gravure printing an indacenodithiophene-benzothiadiazole (C16IDT-BT) on plastic is demonstrated yielding effective mobilities in the range 0.04-0.4 cm squared per V per s at V(DS) < 20 V, exceeding previous reports for the behaviour of this material in a bottom-gate, bottom-contact configuration. Keywords: plastic electronics; organic electronics; flexible; organic field effect transistor (OFET); complementary circuit; inverter; logic gate; printing; gravure; inkjet; zone-casting; photolithography; nano-imprint lithography (NIL); self-aligned; cliché; shim; image analysis; transition frequency; megahertz.
Supervisor: Campbell, Alasdair Sponsor: Engineering and Physical Sciences Research Council ; European Commission
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral