Field effect transistors based on single grains and single grain boundaries of an organic semiconductor

Devices based on organic semiconductors have the potential to transform the display industry as we know it today. Organic thin film transistors, for example, have received much interest due to their potential utility in large area, low-cost, and flexible displays. This thesis addresses two issues that affect the overall performance of organic thin film devices, namely, contacts to the organic materials and grain boundaries in the films, by exploring transport over length scales of the individual grains. Specifically, charge transport studies are carried out using field effect transistors based on individual grains and individual grain boundaries of the organic semiconductor sexithiophene, an extensively characterized oligomer of thiophene. The devices are fabricated via vacuum deposition of the organic onto SiO2/Si substrates pre-patterned with pairs of metal electrodes spaced 0.5--2.0 mum apart. The electrodes serve as source and drain contacts to the organic semiconductor and the heavily doped Si substrate serves as the gate. The gate is an important element in these experiments, as it allows convenient control over the charge carrier density in the sexithiophene. Current-voltage characteristics are taken as a function of gate voltage, temperature, and time in order to determine threshold voltages, activation energies, and charge trapping effects. Contact resistance and the resistance of a single grain boundary are measured as well. Together, the single grain and single grain boundary studies strongly suggest that contacts and grain boundaries are the chief bottlenecks to charge transport in organic thin film devices.