PhD thesis - Evans Research Group - University of Wisconsin-Madison

40between the metal electrodes and the pentacene monolayers. The large difference betweenthe mobilities observed with two and four terminals shows that the vast majority of thetotal resistance was assigned to the contact resistance between metal and pentacene.Even though the second layer islands did not come into contact, the sheet conductanceand mobility increased with increased coverage. The second layer islands can contribute toelectrical charge transport by adding a parallel conduction path and locally reducing theresistance of areas covered by a second layer island. The improvement in the two-terminalmobility μ fet observed in two-terminal devices could be caused by either the largeimprovement in the channel mobility or, more likely, by improvements in the physicallyincomplete contact between the metal and the pentacene.2.6 ConclusionsWe have found that pentacene monolayer transistors are sensitive probes of the chargetransport in pentacene. Molecular layers of organic semiconductor can be incorporated intonumerous applications such as OLEDs and organic solar cells. The transport of carriers inthe layers can affect the performance of the device. Our system can be applied to study theelectrical properties of other organic semiconductor materials that could potentially beused as a charge transport layer in the devices. The effects of crystal structure andmorphology of organic semiconductors at low coverages grown under different depositionconditions on charge transport in organic semiconductors can be explored more precisely.