Thermal Properties, Molecular Structure, and Thin-Film Organic Semiconductor Crystallization

The crystallinity of a group of organic small molecules is investigated by vapor depositing the materials into thin films followed by a thermal annealing step. The materials are categorized into three groups: platelet-forming, spherulite-forming, and those that resist crystallization. Differential scanning calorimetry is utilized to determine the bulk thermal properties of these materials, which provide a reliable indicator of a material's crystallization motif. Platelet-forming materials tend to be characterized by high melting points (Tm) and high magnitude crystallization driving force at the material's crystallization temperature (ΔGc). The materials that resist crystallization as a thin film have small ΔGc. These results provide guidelines that can help determine which organic molecules have a greater likelihood of growing into large-scale crystalline frameworks, a key step for improving the charge carrier mobility and exciton diffusion length in organic semiconductors.