Jonghwa Jeong, Department of Materials Science and Engineering, University of Utah, 122 S. Central Campus Drive, Room 304, Salt Lake City, UT 84112 and Debra J. Mascaro, Department of Mechanical Engineering, University of Utah, 50 S. Central Campus Drive, Room 2110, Salt Lake City, UT 84112.
Organic field-effect transistors (OFETs) are being pursued as low-cost alternatives for numerous electronic applications, including full-color displays and RFID circuitry. It is well known that OFETs require a crystalline morphology for high charge carrier mobility and efficient device operation. As such, the organic semiconductors commonly studied for OFET applications tend to form either large polycrystalline grains or large single crystals, and significant effort has been made to improve the crystallinity through various deposition and processing techniques. In this work, we apply a novel processing technique to tris(8-hydroxyquinoline)aluminum (Alq3) and rubrene. Alq3 in its amorphous form is one of the most common organic materials used in organic light-emitting devices. Here we investigate crystalline Alq3 as a potential n-type semiconductor for OFETs. Rubrene is a p-type semiconductor, and has the highest reported room temperature hole mobility for a single-crystal OFET. Both Alq3 and rubrene are highly soluble in common organic solvents, and previous work has shown that amorphous films of these materials tend to crystallize when exposed to solvent vapor. Here, we use solvent-vapor-assisted recrystallization to achieve in-plane growth of Alq3 and rubrene single crystals directly on a device substrate, with the crystals positioned in pre-patterned transistor channels. In this technique, the device substrate is situated in solvent vapor ambient, and a solution of the organic semiconductor is drop-cast onto the source and drain electrodes. The crystal formation completes the fabrication of a transistor (gold gate, parylene gate insulator, gold source and drain electrodes, Alq3 or rubrene semiconductor). The crystal morphology is controlled by the solution concentration, the vapor concentration, and the vapor exposure time. In this presentation, we will discuss the materials properties of the crystals, the recrystallization technique, and the resulting transistor performance. In combination with ink-jet printing, the solvent-vapor-assisted recrystallization technique holds promise for facile fabrication of arrays of single crystal OFETs with high charge carrier mobilities.