Thursday, February 21, 2013
106 Spalding Lab, Hartley Memorial Seminar Room
Chemical Engineering Seminar
Guiding crystallization around bends and corners: probing inter- and intraspherulite transport in solution-processed organic semiconductors
Yueh-Lin Loo, Professor, Chemical and Biological Engineering, Princeton University
Large-area electronics based on organic materials promise low-cost fabrication, lightweight construction, mechanical flexibility and durability. To truly realize the low-cost aspects of organic electronics, however, conventional high-vacuum deposition technologies – costly both in terms of instrumentation as well as operation – will have to be replaced by solution processing methodologies, like inkjet printing or spin casting. This need has in turn driven the development of solution-processable organic semiconductors, and even solution-processable organic conductors.
We have fabricated bottom-contact thin-film transistors with spun-cast triethylsilylethynyl-anthradithiophene (TES-ADT) comprising the electrically-active channel. As-spun, TES-ADT exhibits limited ordering and the device characteristics are unremarkable. Subjecting the same transistors to brief solvent-vapor annealing, however, induces large-scale crystallization and spherulite formation in TES ADT and, accordingly, improves the device characteristics dramatically. Specifically, the carrier mobility and on-off current ratio increase by more than two orders of magnitude, to > 0.1 cm2/V-sec and > 104, making the performance competitive for display backplane applications.
With this same materials system, we have demonstrated control over the spherulite size of TES ADT within the channels of thin-film transistors by seeding the crystallization process with fractional amounts of additives. Tuning the spherulite size from 30 microns to 3 millimeters has afforded us the opportunity to examine its effects on the electrical characteristics of thin-film transistors. Not surprisingly, the device mobility increases with increasing spherulite size as we proportionally decrease the spherulite boundary density within the active channels; this trend quantitatively agrees with the simple composite mobility model predicted by Horowitz.
More excitingly, we have recently developed a simple and versatile method for controlling the molecular orientation of TES ADT with precise in-plane spatial resolution over large areas as it crystallizes. By exploiting the differential crystallization rates of TES ADT on substrates having different surface energies, we have disrupted the radial symmetry of spherulitic growth by preferentially selecting for the molecular orientations that promote growth along the paths of the underlying patterns. We have thus been able to direct crystallization of organic compounds around sharp corners and along paths of arbitrary shapes over millimeter length scales. The ability to design free-form crystalline structures within organic thin films through a priori patterning of underlying substrates will enable emerging applications in materials engineering, such as organic electronics and optics.