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Title: Microstructure, morphology and device physics of gravure printed and solution processed organic field-effect transistors
Author: Guite, Alexander David John
ISNI:       0000 0004 2713 5060
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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This thesis explores the relationship between microstructure, morphology and device physics in gravure printed and solution processed organic field-effect transistors (OFETs). Chapter 1 introduces the key concepts encountered in this work: the properties of organic semiconductors and OFETs; the use of printing techniques in organic electronics; and the relationship between microstructure and OFET performance in poly(3-hexylthiophene) (P3HT). Chapter 2 details the materials and experimental techniques used in this thesis. In Chapter 3, gravure printing is demonstrated for high throughput fabrication of OFETs. Printed devices are achieved with typical saturated mobility of 0.03cm2/Vs and on/off ratio in the range 103:9-4:6, which exceeds that achieved with spin coated devices with the same material system and geometry. Chapter 4 presents a systematic comparison of the microstructure and OFET characteristics of gravure printed and spin coated P3HT thin films. First light scattering is used to understand the conformation of P3HT chains in various solvents, then grazing incidence wide angle X-ray scattering (GIWAXS), absorption characteristics and atom force microscopy (AFM) are used to characterise the microstructure of the P3HT lms. In turn, this is compared to OFET performance. In Chapter 5 two solvent based techniques are investigated as alternatives to thermal annealing as methods to enhance microstructure. A blend of a high and low boiling point solvent is first examined as the casting solvent for P3HT and is found to moderately improve P3HT field-effect mobility. Secondly, solvent vapour treatment (SVT) - exposing a P3HT film to a solvent vapour after spin coating - is studied by in-situ GIWAXS. The time resolved measurement of interchain and interlamella distances allowed the dynamics of SVT to be investigated. SVT was found to decrease P3HT crystallinity, although AFM showed it lead to smoother films. In Chapter 6 two emerging materials are investigated for use in OFETs. Preliminary work on fabricating OFETs with single crystal copper phthalocyanine is presented. Finally, work towards a metal-free OFET is described in which the source and drain electrodes are formed of high conductivity PEDOT deposited by vapour phase polymerisation.
Supervisor: Campbell, Alasdair Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral