Sublimation Mechanism for Small Molecule Organic Semiconductors

Sublimation Mechanism for Small Molecule Organic Semiconductors

Yi Zhang, Nathan T. Morgan, Russell J. Holmes, and Edward L. Cussler

Department of Chemical Engineering and Materials Science

University of Minnesota, Minneapolis MN 55455

Organic semiconductors continue to receive substantial application for next-generation display, photovoltaic and solid-state lighting technologies. Processes capable of producing high purity materials at large scales are desirable. Presently, the majority of these materials are purified using thermal gradient sublimation.  We find that the sublimation rate of the archetypical, small molecule hole transport material N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPD), is controlled by a combination of viscous flow and physical vapor deposition.  Other steps like diffusion within the solid feed, reaction at the feed particle surface, and mass transfer within the bed of feed particles do not significantly affect the sublimation rate.  Such a mechanism is in contrast with other commercially practiced sublimations.  The results obtained here for NPD suggest guidelines for the large-scale purification of active materials for organic electronic devices.