This thesis is concerned with the development and study of ambipolar
organic field-effect transistors (OFETs), with a specific focus on
devices and structures that are relevant to opto-electronic processes.
After giving an outline of the relevant theory and experimental methods,
the thesis is divided into three experimental chapters.
In the chapter on organic phototransistors (OPTs), the fabrication of
low-voltage bilayer OPTs is carried out using self-assembled monolayer
gate dielectrics. By combining two low-voltage OPTs, the output
voltage of a so-called photo-inverter was observed to be modulated
over a range of 1.5 V with an incident optical power density of 0 to
1.2 mWcm-2. The characteristics of a high-voltage polymer:fullerene
blend photo-inverter device are modelled using variable-range hopping
and simple circuit models, suggesting that the device operation
is dominated by changes in the threshold voltage.
In the second experimental chapter the relationship between the morphology
of polymer:fullerene blends and the characteristics of OFETs
is studied. It is shown that the thermally-induced clustering of fullerenes
is manifest as a reduction in electron mobility, due to a reduction in
percolating pathways. It is additionally found that higher molecularweight
fullerenes require greater annealing temperature and/or times
for this process to occur.
The final experimental chapter is concerned with polymer:fullerene
diffusion processes and the study of such phenomena using bilayer
OFETs. A model based upon the diffusion equation and percolation
theory is employed to quantify this process. Again, higher molecularweight
fullerenes are observed to require greater annealing temperature
for similar phenomena to be observed.