Microstructural effects on charge transport in the molecular semiconductor sexithiophene

This thesis describes experiments probing microstructural effects on charge transport in molecular semiconductor thin films. It reports two techniques of isolating crystalline grains, grain boundaries, and single molecular monolayers, and the experimental results of these techniques on a specific system-sexithiophene (chosen as a model material because of its high charge mobility), but the experiments will be expanded to other molecular materials in future work.; Effects of microstructure on transport were probed in two ways: (1) using field-effect devices, and (2) using conducting probe atomic force microscopy (CPAFM). The first technique isolates individual sexithiophene crystals ($\sim$l μm) between closely-spaced electrodes in transistor geometries. Gold electrodes are fabricated on a SiO₂ wafer, and trace amounts of sexithiophene are then vacuum-deposited generating thin crystals, 1–6 molecules tall. Electrically-isolated crystals grown between electrodes allowed electrical characterization. Charge transport is observed with: (1) no discernible dependence on film thickness, (2) thermally-activated transport above 100K and temperature insensitive transport below 100K, and (3) time-dependent transport attributable to charge trapping. In cases where a pair of crystals was isolated, the resultant grain boundary severely limited conduction.; The second technique uses a modified AFM in which a tip is metal-coated and used as a positionable electrical contact. Individual sexithiophene crystals with one electrical contact made to the substrate can be electrically assessed using the AFM tip as the second. CPAFM allows imaging with minimal applied tip force while allowing current to flow from tip to sample. Monitoring the current/voltage behavior as the tip is repositioned on the sexithiophene crystals allows connection of nanoscale electrical properties to structural features for individual sexithiophene crystals. Experiments isolate conduction through individual layers of a molecular material and discrete grain boundaries. Measured contact and grain boundaries resistances demonstrate that transport in polycrystalline sexithiophene films is likely limited by these features.; The combination of electrical and physical characterization of sexithiophene crystals demonstrates the importance of microstructure on transport, and can be generalized to other molecular semiconductors. Furthermore, the ability to electrically contact molecular “sheets” provides a technique for probing the effects of reduced dimensionality on transport in organic systems.