Differences of interface and bulk transport properties in polymer field-effect devices

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 octadecyltrichlorosilane (OTS) can increase the field-effect mobility by up to a factor of 50, reaching values in saturation of more than 4 × 10−2 cm2/V s 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. Thus, our investigations reveal significant differences between interface and bulk transport properties in polymer field-effect devices.