  Contact resistances often contribute significantly to the overall device
resistance in organic field-effect transistors (OFETs). Understanding charge
injection at the metal-organic interface is critical to optimizing OFET device
performance. We have performed a series of experiments using bottom-contact
poly(3-hexylthiophene) (P3HT) OFETs in the shallow channel limit to examine the
injection process. When contacts are ohmic we find that contact resistivity is
inversely proportional to carrier mobility, consistent with diffusion-limited
injection. However, data from devices with other electrode materials indicate
that this simple picture is inadequate to describe contacts with significant
barriers. A generalized transmission line method allows the analysis of
nonohmic contacts, and we find reasonable agreement with a model for injection
that accounts for the hopping nature of conduction in the polymer. Variation of
the (unintentional) dopant concentration in the P3HT can significantly alter
the injection process via changes in metal-organic band alignment. At very low
doping levels, transport suggests the formation of a barrier at the Au/P3HT
interface, while Pt/P3HT contacts remain ohmic with comparatively low
resistance. We recently observed that self-assembled monolayers on the metal
source/drain electrodes can significantly decrease contact resistance and
maintain ohmic conduction under conditions that would result in nonohmic, high
resistance contacts to untreated electrodes. Finally, we discuss measurements
on extremely short channel devices, in the initial steps toward examining
transport through individual polymer chains.
