Engineered Grain Boundaries and their Properties in Crystalline Organic Semiconductors

Stanford University, Stanford CA

Investigators

Abstract

Technical Description: The goal of this research project is to unravel structure-property relationships at the grain boundaries of crystalline organic semiconductors. Such fundamental understanding is expected to lead to materials or processing techniques that mitigate the effect of grain boundaries. In order to study grain boundaries, engineered microstructures are fabricated using shear-coating or capillary force lithography. These techniques allow to form grain boundaries across which the molecular misorientation can be controlled and characterized accurately using synchrotron-based X-ray diffraction. Charge transport across these well-characterized grain boundaries is studied using field-effect transistors. Furthermore, the presence of trap states in the bandgap of the semiconductor is assessed using ultra-sensitive sub-bandgap spectroscopies. The correlation of transport data and the sub-gap absorption data helps to elucidate the nature of transport bottlenecks across grain boundaries. In addition to transport, the effect of grain-boundary structure on the electronic stability of the semiconductor and the role of polar molecules adsorbed from the atmosphere in tuning the electronic properties of grain boundaries is studied. By conducting these studies as a function of grain-boundary structure, a complete picture of the role of structure in governing the electronic properties of grain boundaries is made to emerge. Non-technical Description: Organic semiconductors are already finding applications as light emitters in the display industry. The versatility of this family of electronic materials, which can be synthesized and functionalized to exhibit a broad range of properties, promises the realization of entire electronic circuits with organic semiconductors as well as solar cells and chemical sensors. Because these materials are held together by weak intermolecular forces however, they suffer from the presence of imperfections or defects. This research project aims at elucidating how defects affect the electronic behavior of organic semiconductors. In particular this project focuses on the boundaries between crystals that form in the film, which often limit the performance of the electronic material. By fabricating model systems, where these boundaries are well controlled, the effect of the structure of these boundaries on electronic properties is revealed. The societal impact of the research is amplified by outreach activities. For instance, remote optical and electronic microscopy, where high-school students are engaged to study materials at the micro and nanoscale, provide a direct link between structure (as observed in the microscope) and properties.