Kinetic Versus Thermodynamic Orientational Preferences for a Series of Isomorphic Molecular Semiconductors

Due to the anisotropic nature of charge transport through most organic semiconductors, the orientation of the conjugated backbone is of great relevance because it may affect final device properties. Herein, we present a set of four nearly isostructural molecular organic semiconducting materials whose orientation changes drastically with a two-atom change in the conjugated framework. We investigate the X-ray diffraction patterns of these materials in the thin film, both as-deposited from solution and following melt-annealing. Following melt-annealing of the films, crystallites of all four materials orient edge-on with respect to the substrate, which indicates that this orientation is thermodynamically preferred. We can infer that the initial face-on orientation of some of the materials is due to kinetic trapping during the spin-coating process. Previous observations from the literature suggest that the edge-on orientation is the thermodynamically preferable state for many organic semiconducting materials. However, a cohesive explanation for this phenomenon remains elusive.