11 A.M. MONDAY, NOV. 17 - SCIENCE LIBRARY, ROOM 212
Cathy Wong, University of California, Berkeley
"Inter-molecular order at a hidden interface in a small-molecule organic semiconductor thin film"
Solution processing is required for the large-scale manufacture of small-molecule organic semiconductors and can result in crystalline domains with high charge mobility. However, the interfaces between these domains impede charge transport, degrading device performance. Although understanding these interfaces is essential to improve device performance, their intermolecular and electronic structure is unknown: they are smaller than the diffraction limit, are hidden from surface probe techniques and cannot be directly resolved using X-ray methods. We have used transient absorption microscopy to inspect a drop-cast thin film of 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene, a material cited for high hole mobility and singlet fission. The crystal and electronic structure of the domains has been characterized, but analogous information for the domain interfaces is unknown. Using a judicious selection of light polarization, we isolate a signal at the interface that is not observed in either of the adjacent bulk domains, exposing the exciton dynamics and inter-molecular structure of this hidden interface. Surprisingly, instead of finding an abrupt grain boundary, we reveal that the interface can be composed of nanoscale crystalline aggregates interleaved by a web of interfaces that compound decreases in charge mobility.