Charge transport in disordered organic field-effect transistors

Charge transport in disordered organic field-effect transistors

Meijer, E.J.

Klapwijk, T.M. (promotor)

Applied Sciences

2003-06-20

In this thesis we study charge transport in organic semiconductors. We do this by focusing on the physical characterization of disordered organic field-effect transistors. It will be made clear that the disorder in the polymer films is crucial for the interpretation of the data. The field-effect transistor geometry allows variation of the charge carrier density in the semiconductor, without the presence of counter ions. Therefore, the transistor allows a rather clean study of the charge transport in organic semiconductors as a function of the charge carrier density and temperature. In the experiments we find that the organic transistors are in several respects not comparable to silicon MOSFETs. Therefore, in this thesis we redefine and re-evaluate basic transistor parameters, such as the threshold voltage, the field-effect mobility, the contact resistance and the dopant density. Subsequently, we study the charge transport as a function of charge density, temperature and electric field, giving insight into the charge transport mechanism. Based on our observations we propose as the main charge transport mechanism: multiphonon hopping of polaronic charge carriers in a Gaussian density of states. We investigate the electrical stability of the polymer layer in metal-insulator-semiconductor diodes, where we determine and analyse the dopant density changes as a function of oxygen and light exposure. The presence of contact resistances in the transistors is addressed by analysing the scaling behavior of the electrical characteristics as a function of the transistor channel length, and an empirical relation between the contact resistance and the charge carrier mobility in the polymer layer is observed. Finally, we discuss why typically only unipolar transistor behavior is observed experimentally, and we demonstrate ambipolar transistor behavior in organic field-effect transistors based on blends of organic semiconductors and on low band gap organic semiconductors.

organic semiconductor
field-effect transistor
charge transport
modelling
polymers

http://resolver.tudelft.nl/uuid:6a2b93f3-4ef4-4363-8b2f-440e0a6d01e1

Optima Grafische Communicatie, Rotterdam

90-6734-306-4

Institutional Repository

doctoral thesis

(c) 2003 E.J. Meijer