In this thesis we report the development of a range of high-performance thin-film
transistors utilising different solution processable organic dielectrics grown at temperatures
compatible with inexpensive substrate materials such as plastic. Firstly, we
study the dielectric properties and application of a novel low-k fluoropolymer dielectric,
named Hyflon AD (Solvay). The orthogonal nature of the Hyflon formulation, to
most conventional organic semiconductors, allows fabrication of top-gate transistors
with optimised semiconductor/dielectric interface. When used as the gate dielectric in
organic transistors, this transparent and highly water-repellent polymer yields high-performance
devices with excellent operating stability. In the case of top-gate organic
transistors, hole and electron mobility values close to or higher than 1 cm2/Vs, are
obtained. These results suggest that Hyflon AD is a promising candidate for use
as dielectric in organic and potentially hybrid electronics. By taking advantage of
the non-reactive nature of the Hyflon AD dielectric, the p-doping process of an organic
blend semiconductor using a molybdenum based organometallic complex as the
molecular dopant, has also been investigated for the first time.
Although the much promising properties of Hyflon AD were demonstrated, the
resulting transistors need, however, to be operated at high voltages typically in the
range of 50-100 V. The latter results to a high power consumption by the discrete
transistors as well as the resulting integrated circuits. Therefore, reduction in the
operating voltage of these devices is crucial for the implementation of the technology
in portable battery-operated devices. Our approach towards the development of low-voltage
organic transistors and circuits explored in this work focused on the use of
self-assembled monolayer (SAM) organics as ultra-thin gate dielectrics. Only few
nanometres thick (2-5 nm), these SAM dielectrics are highly insulating and yield high
geometrical capacitances in the range 0.5 - 1 μF/cm2. The latter has enabled the
design and development of organic transistors with operating voltages down to a few
volts. Using these SAM nanodielectrics high performance transistors with ambipolar
transport characteristics have also been realised and combined to form low-voltage
integrated circuits for the first time.
In the final part of this thesis the potential of Hyflon AD and SAM dielectrics
for application in the emerging area of graphene electronics, has been explored. To
this end we have employed chemical vapour deposited (CVD) graphene layers that can be processed from solution onto the surface of the organic dielectric (Hyflon AD,
SAM). By careful engineering of the graphene/dielectric interface we were able to
demonstrate transistors with improved operating characteristics that include; high
charge carrier mobility (~1400 cm2/Vs), hysteresis free operation, negligible unintentional
doping and improved reliability as compared to bare SiO2 based devices.
Importantly, the use of SAM nanodielectrics has enabled the demonstration of low
voltage (<|1.5| V) graphene transistors that have been processed from solution at low
temperature onto flexible plastic substrates. Graphene transistors with tuneable doping
characteristics were also demonstrated by taking advantage of the SAM’s flexible
chemistry and more specifically the type of the chemical SAM end-group employed.
Overall, the work described in this thesis represents a significant step towards
flexible carbon-based electronics where large-volume and low-temperature processing
are required
