| The search for low-cost, large area, flexible devices has led to a remarkable increase in the research and development of organic semiconductors. Single-crystal organic field-effect transistors (OFETs) are ideal device structures for studying fundamental science associated with charge transport in organic materials and have demonstrated high mobility and outstanding electrical characteristics. For example, an exceptionally high carrier mobility of 20 cm2/Vs has been demonstrated for rubrene single crystal field effect transistors. However, it remains a technical challenge to integrate single-crystal devices into practical electronic applications. A key difficulty is that organic single-crystal devices are usually fabricated one device at a time by handpicking a single crystal and placing it onto the device substrate. This makes it impossible to mass-produce at high density with reasonable throughput. Therefore, there is a great need for a high-throughput method for depositing large arrays of organic semiconductor single crystals directly onto device structures.;In this dissertation, I develop several approaches towards realizing this goal. The first approach is a solution-processing technique, which relies on solvent wetting and de-wetting on substrates with patterned wettability to selectively direct the deposition or removal of organic crystals. The assembly of different organic crystals over centimeter-squared areas on Au, SiO 2 and flexible plastic substrates is demonstrated. By designing line features on the substrate, alignment of needle-like crystals is also achieved. As a demonstration of the potential application of this approach, arrays of organic single crystal FETs are fabricated by patterning organic single crystals directly onto and between transistor source and drain electrodes. Besides organic single crystals, this self-assembly strategy is also applicable for patterning other objects such as metallic nanowires.;In the second technique, organic single crystals are selectively nucleated on patterned templates of carbon nanotube (CNT) bundles. Several organic semiconductor materials are successfully patterned, including p-type pentacene, tetracene, sexiphenylene, and sexithiophene, as well as n-type tetracyanoquinodimethane. This study suggests that the selective growth of crystals onto patterned carbon nanotubes is most likely due to the coarse topography of the CNT bundles. Moreover, I observe that the crystals nucleate from CNT bundles and grow onto CNT bundles in a conformal fashion. The crystal growth can be directly applied onto transistor source-drain electrodes and arrays of organic single-crystal field effect transistors are demonstrated. To investigate the impact of CNTs on device performance, CNT bundles are incorporated into thin-film FETs and a mobility enhancement of organic semiconductors is observed.;In the third approach, organic single crystals with well controlled sizes and shapes are successfully grown using patterned Au films as templates. It is observed that sexithiophene crystals nucleate from the edge or the top surface of Au films and then grow two dimensionally on SiO2 surface. The sizes and shapes of sexithiophene crystals are precisely determined by that of the Au patterns. After removing Au templates, large arrays of sexithiophene crystals with controlled sizes and various shapes such as stripes, squares, hexagons, etc. are achieved. Top-contact FETs made of sexithiophene ribbons are demonstrated. Besides organic single crystals, Au templates can also act as templates to pattern vapor- and solution-deposited organic semiconductor thin films. Patterned organic thin-film FETs exhibit superior performance compared to unpatterned devices.;Finally, oriented growth of organic semiconductor single crystals on templates with various features is studied. On substrates with aligned features, such as friction-transferred poly(tetrafluoroethylene) thin films, organic semiconductor thin films rubbed by a cheese cloth, and polymer films prepared through photo-irradiation, arrays of aligned crystals such as sexithiophene and sexithiophene are achieved. Using substrates with organic semiconductor wires as templates, several organic materials grow into interesting "centipede-like" structures. |
