Structure of Organic Crystals

Abstract

Organic semiconductors comprise small-molecule crystals and polymers, both featuring strong intramolecular bonding of carbon atoms by a system of conjugated π electrons. By contrast only weak intermolecular Van der Waals forces act between neutral nonpolar molecules, yielding usually defective crystals with a low stability. In two-component crystals additional longer-ranging dipole and Coulomb interactions occur. The weak intermolecular bonding favors a close molecule packing with large coordination numbers, often leading to a herringbone stacking in small-molecule crystals. Structures of thin films often deviate from the bulk structure and depend on the interaction to the substrate and on film thickness; organic crystal layers usually show epitaxial coincidence modes not observed in inorganic heterostructures. Grain boundaries between single-crystal domains and vacancies are prevailing defects in organic crystals. Polymers consist of long chain-like molecules packed largely uniformly in lamella of crystalline domains, which are separated by amorphous regions with tangled polymer chains. Crystallinity usually increases with the molecular length, get maximum at a critical molecular weight, and decreases beyond.