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44251-AC10
Field Effect Transistors Based on Discrete Organic Semiconductor Grains

C. Daniel Frisbie, University of Minnesota

This project aims to understand the role of microstructure on charge transport in crystalline organic semiconductor films that are under investigation for applications in plastic electronics. A specific target, for example, is to measure the resistance of discrete grain boundaries in organic semiconductor films as a function of gate induced charge density and grain-to-grain misorientation. We are also interested in the role of defects, such as line dislocations on transport. To date, work has focussed on imaging microstructure and defects in ultrathin films of the benchmark semiconductor pentacene by atomic force microscopy (AFM). We have observed that we can image the crystallographic orientation of pentacene grains by a variant of AFM that we call transverse shear microscopy (TSM). TSM grain orientation maps allow us to determine the degree of crystallographic misorientation across grain boundaries in pentacene films only a few monolayers in thickness. The next step is to couple this imaging methodology with microscopic field effect transport measurements in which we use a conducting AFM tip as a source contact in a working field effect transistor. We can position the tip on either side of the grain boundary to determine its impact on electrical resistance.

In conjunction with these measurements we have used Kelvin probe force microscopy to image electrostatic potentials in ultrathin pentacene films and have observed negative grain boundary potentials. This information will be correlated with the grain boundary transport measurements described above. The expectation is that negative grain boundary potentials will impede the transport of holes by serving as traps.

We have also combined chemical etching of pentacene films with AFM to reveal line dislocations. This classical etching and microscopy approach opens up new opportunities for quantifying defect densities in crystalline organic semiconductor films and understanding stresses that develop in these films when they are grown. Ultimately, this information will be key to a complete understanding of microstructure-transport relationships in organic semiconductor films.

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