Anisotropic Dynamic Disorder of π-Stacking and Static Disorder in Organic Electron Transporting Materials with Isomorphic Crystal Structures

Abstract

In organic semiconductors, charge transport is strongly correlated with intermolecular π–π interactions and the dynamic disorder caused by solid-state phonon vibrations. However, for pure π–π stacking of semiconducting cores, solid-state phonon vibrations and their directional characteristics are relatively unexplored. In this work, solid-state phonon vibrations using experimental and theoretical phonon-resolved analyses of single crystals are studied, for which a series of organic electron-transporting naphthalene diimide (NDI) analogs as a model system is designed. The three NDI analogs in this series exhibit isomorphic crystal structures with a brickwork-type assembly characterized by pure face-to-face π–π stacking, but different π–π stacking distances. The as-grown NDI single crystals exhibit strong anisotropic phonon vibrations, with substantial differences observed between the directions parallel and perpendicular to the π–π stacking, appearing in two distinct THz frequency ranges (below and above 7 THz). These vibrations originate from the out-of-plane and in-plane movements of the NDI cores, respectively. For vacuum-deposited polycrystalline films, despite having isomorphic crystal structures with equivalent dominant (001) facets, the static disorder, which is influenced by crystalline perfection and trap density, varied markedly among the three NDI analogs.