  The influence of substrate treatment with self-assembled monolayers and
thermal annealing was analysed by electrical and structural measurements on
field-effect transistors (FETs) and metal-insulator-semiconductor (MIS) diodes
using poly(3-hexylthiophene) (P3HT) as a semiconducting polymer and Si/SiO2
wafers as a substrate. It is found that surface treatment using silanising
agents like hexamethyldisilazane (HMDS) and octadecyltrichlorosi-lane (OTS) can
increase the field-effect mobility by up to a factor of 50, reaching values in
saturation of more than 4E-2 cm^2/Vs at room temperature. While there is a
clear correlation between the obtained field-effect mobility and the contact
angle of water on the treated substrates, X-ray diffraction and capacitance
measurements on MIS diodes show that structural and electrical properties in
the bulk of the P3HT films are not influenced by the surface treatment. On the
other hand, thermal annealing is found to cause an increase of grain size, bulk
relaxation frequency and thereby of the mobility perpendicular to the SiO2/P3HT
interface, but has very little influence on the field-effect mobility.
Temperature dependent investigations on MIS diodes and FETs show that the
transport perpendicular to the substrate plane is thermally activated and can
be described by hopping in a Gaussian density of states, whereas the
field-effect mobility in the substrate plane is almost temperature independent
over a wide range. This investigations reveal significant differ-ences between
interface and bulk transport properties in polymer field-effect devices.
