Aram Amassian1,2
1Materials Science and Engineering, King Abdullah University of Science and Technology, Saudi Arabia
2CHESS
KAUST-Cornell Center for Energy and Sustainability, Cornell University
Abstract:
A key aspect of research in organic electronics and optoelectronics is the establishment of a structure-property relationship for molecular semiconductor materials. In the case of most high performance molecular semiconductors, charge transport is believed to be limited by grain boundary defects along the channel of the organic field-effect transistor (OFET). A key working assumption has been that additional improvements to the output characteristics of OFETs, including their field-effect mobility (m), on-off ratio, and threshold voltage, require increased crystallite or grain size or - as a corollary - decreased density of grain boundaries. We have used scanning probe microscopy and time-resolved synchrotron X-ray reflectivity and diffraction techniques extensively to investigate the mechanisms of growth and polymorphism of molecular semiconductor thin films (e.g., pentacene and diidenoperylene) deposited from molecular beam sources tunable in energy. We show that the quality of grain boundaries and the interconnectivity of grains near the semiconductor-dielectric interface influence the output characteristics and performance of OFETs much more profoundly than the size of the grains. Device-to-device variations of four orders of magnitude are reported for m and the on-off ratio simply by introducing small changes in the degree of filling of molecular layers near the semiconductor-dielectric interface. Such changes are achieved by tuning the state of incident molecules and/or by engineering the surface of the gate dielectric. These observations reveal a previously unknown relationship between the morphology and microstructure of molecular semiconductors and the performance of OFET devices.
2009 Run
Sept. 23rd to Nov. 10th
