Contact resistance in organic thin film transistors

Organic thin film transistors were fabricated by vacuum evaporation of pentacene using different growth conditions in order to tune the pentacene layer morphology. We have performed in situ transport measurements on completed samples, and ex situ morphological characterization by atomic force microscopy. Electric properties of the microscopic interface environment at the drain/pentacene and source/pentacene interfaces were probed by Kelvin force microscopy. From the observed voltage drops at the metallic contacts we have calculated contact resistances that may be decomposed into contributions due to structural defects in the channel, due to structural defects at the metal/pentacene interface, and due to charge carrier injection at the source/pentacene interface. We observe that samples fabricated by different growth conditions exhibit different contact resistances. The most important variable contribution to the contact resistance is due to the charge carrier injection at the source/pentacene interface.