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.