A microscopic view of charge transport in polymer transistors

Charge transport in polymer transistors has been investigated experimentally on a sub-micrometer length-sale by means of scanning-probe techniques. First, we present measurements of the microscopic potential landscape inside operating polythiophene and polyfluorene transistors, which reveal details about the current flow. Transistors based on poly [(9,9-dioctylfluorene-2,7-diyl)-co-(bithiophene)] (F8T2) in a nematic glassy state show a distinct microstructure in the potential. This microstructure has been analyzed quantitatively, and can be explained by a mobility anisotropy of 10 for current flow parallel and perpendicular to the alignment direction of the F8T2 chains. Furthermore, we find that domain boundaries are not limiting the charge transport in F8T2 transistors. Secondly, the dynamics of the charge accumulation process is discussed on the basis of time-resolved measurements of the charge carrier density. Finally, we introduce light-assisted scanning potentiometry as a tool to investigate trap states in polymer transistors. The technique allows a direct mapping of the density of trapped charges. Photon-assisted trap-release in transistors based on poly [(9,9-dioctylfluorene-2,7-diyl)-alt-phenylene-(N-(p-2-butylphenyl)imino-phenylene)] (TFB) and a SiO2 gate dielectric has been studied. We find that the efficiency of this release process is governed by the absorption properties of TFB. This indicates that trapping occurs in the polymer. (C) 2004 Elsevier B.V. All rights reserved.