Crystal Structure of Fiber Structuredpentacene Thin Films: a Crystal Truncation Rod Study Using Synchrotron X-rays

It is widely recognized that the intrinsic charge transport properties in organic thin-film transistors (OTFTs) depend strongly on the crystal structure of the organic semiconductor layer. Pentacene, showing one of the highest charge carrier mobilities among organic semiconductors, is known to crystallize in at least four polymorphs, which can be distinguished by their layer periodicity d(001). Only two polymorphs grow as single crystals, and their detailed crystal structure has been solved. The substrate-induced 15.4 Å polymorph is the most relevant for OTFT applications; however, its crystal structure has remained incomplete as it only grows as a fiber structured thin film. In this work, the crystal truncation rod X-ray scattering technique is extended to fiber structured thin films. Through this novel technique, the complete crystal structure of this polymorph grown on technologically relevant substrates could be solved. It was also discovered, that the molecular arrangement within the unit cell is substrate dependent, which may lead to a controlled fine-tuning of intrinsic charge transport properties.