THE IMPACT OF INTERMOLECULAR INTERACTIONS ON THE THIN-FILM MORPHOLOGY OF NAPHTHALENE TETRACARBOXYLIC DIIMIDES

Abstract

Molecular semiconductor thin films have garnered significant interest for their use as the active components in lightweight, large-area, and flexible electronic devices. The kinetic constraints associated with rapid film deposition often leave films in non-optimal polycrystalline or amorphous states. Because organic solids are held together by weak non-covalent interactions, proper manipulation of molecular interactions can provide control over the film structure through subsequent post-deposition processing.

In this thesis, we develop robust processing-structure-function relationships governing the structural development of a series of naphthalene tetracarboxylic diimide (NTCDI) derivatives through exploring the relative contributions of molecule-molecule, molecule-solvent, and molecule-substrate interactions. Like many other molecular systems, the NTCDI derivatives exhibit polymorphism, or the ability to adopt multiple crystal structures. While subtle changes in chemistry between the NTCDI derivatives do not greatly impact their attainable polymorphs, they do induce changes in the molecule-molecule interactions present between molecular layers within the crystal structures of each derivative; we found the presence of short intermolecular contacts to directly correlate with polymorphic stability of these NTCDI derivatives. This short interlayer contact framework for assessing polymorphic stability is broadly applicable to other molecular semiconductors with disparate chemistries and to a variety of organic materials, including biological building blocks and pharmaceutics.

While molecule-molecule interactions can be used as a proxy for polymorphic stability, manipulating molecule-solvent and molecule-substrate interactions can further unravel the rich structural phase-space of the NTCDI derivatives. We demonstrate that controlling the molecule-solvent interactions during solvent-vapor annealing alters the relative growth rates along different crystallographic directions. This results in crystals that can adopt a continuum of habits, ranging from plate-like to needle-like domains, depending on the solvent choice and vapor concentration. Introducing a templating layer prior to deposition effectively modulates molecule-substrate interactions; properly lattice-matched templates can enable heteroepitaxial growth of specific polymorphs that are otherwise inaccessible.

By understanding the relative contributions of different types of intermolecular interactions in molecular systems, we demonstrate the ability to unlock the rich structural phase space of the NTCDI derivatives. The processing-structure-function relationships developed within this thesis can assist future materials development, not only for molecular semiconductors, but for organic materials in general.