Organic Semiconductor Devices
Organic semiconducting materials are widely recognized as having tremendous potential for certain electronics applications. The charge transport properties of such materials are determined by both the microstructural arrangement of molecules (as in inorganic semiconductors) and by the chemical structure of the molecules themselves.
In previous years, the Natelson group pursued a research effort aimed at understanding transport mechanisms and contact phenomena in organic semiconductors, based largely on bottom-contacted organic field-effect transistors (OFETs). Material systems that we examined included polythiophene and solution-processable derivatives of pentacene. Our accomplishments included:
- We examined the relationship between semiconductor mobility and contact resistance in devices based on P3HT and Au electrodes, showing that, for the diffusion-limited injection regime, contact resistance is inversely proportional to the mobility over four decades in mobility.
- We developed a method for extracting the current-voltage characteristics of just the metal-organic contact.
- We showed that surface chemistry may be used to manipulate those metal/organic interfacial energetics. With self-assembly of the appropriate work function-raising molecule, Au electrodes may be modified so that their injection properties act like those of Pt. This is one route to optimizing contacts in organic FETs.
- We pointed out that transport in these materials crosses over from thermal activation to a field emission regime at low temperatures and very large electric fields.
- Finally, we found good evidence that interfacial charge transfer at the Pt/P3HT interface effectively dopes the P3HT with mobile holes very locally, and that similar physics is at work in graphene devices.
We continue to have an interest in the physics of organic semiconductor systems, and are thinking about radio-frequency approaches to infer greater understanding about contact effects and transport in these materials.