Organic semiconducting materials are more cost effective and easier to manufacture than their inorganic counterparts, which makes them highly attractive for use in electronic devices such as solar cells. Devices from these materials are often fabricated using single crystals, as they are highly ordered and often free of defects. For many materials, the electronic properties have been shown to rely on the specific packing of molecules within the crystal lattice. Rubrene (5,6,11,12-tetraphenyltetracene) is a model organic semiconductor with extremely high charge mobility. Three different polymorphs, or crystalline arrangements, of rubrene exist, with one polymorph displaying a charge mobility an order of magnitude larger than the others. This polymorph is unique, as the molecules arrange in a herringbone formation of slipped pi-stacks within the crystal lattice. To effectively utilize rubrene crystals in devices, it is important to both understand how this herringbone formation influences charge transport and be able to efficiently detect this formation within a crystal. Lattice phonon Raman spectroscopy can help accomplish these goals by identifying intermolecular vibrations, or phonons, between molecules of a crystal lattice. To determine what phonon modes may be used to distinguish the herringbone formation, we synthesized a library of rubrene derivatives and grew crystals of the same herringbone structure. These derivatives, while different in their molecular structure, showed a similar pattern of phonon modes through Raman spectroscopy. Computational modeling of these crystals further identified a pi-stacking intermolecular vibration consistent throughout the derivative library. When comparing computational and experimental results of the herringbone structure with a different rubrene polymorph, this same pi-stacking motion was no longer present, suggesting the pi-stacking motion may be playing a role in the unique charge mobility of the herringbone structure. Detecting this pi-stacking mode with Raman spectroscopy could be used as an efficient method to identify future derivatives that are good candidates for use in electronic devices. Additionally, we have used the unique phonon modes of the herringbone structure in a recently developed low-frequency Raman imaging technique to create polymorphic images across entire crystalline batches and identify the percentage of herringbone rubrene crystals in a mixture of polymorphs.

Lattice phonon Raman spectroscopy as a tool to understand polymorphism in organic semiconductors

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.