Structural Rearrangement of Organic Semiconductor Molecules with an Asymmetric Shape in Thin Films

2-Decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10) exhibits excellent performances as an active layer in organic thin-film transistors, and its performances are greatly influenced by the molecular packing, i.e., the crystalline polymorphs. This compound has the so-called thin-film phase in a vapor-deposited film, which is different from the single-crystal structure (the bulk phase). In this work, thin films of Ph-BTBT-C10 are prepared by spin coating, and the effect of aging on the film structure is investigated by high-resolution infrared (IR) spectroscopy in combination with X-ray diffraction. The results show that the as-spun film has the same thin-film phase as the vapor-deposited film and that the thin-film phase is rapidly converted to the bulk phase by aging at room temperature. In addition, the spectral analysis also reveals conformational changes in the decyl chain during the crystalline transition process. This work highlights the importance of using IR spectroscopy with a high-wavenumber resolution for structural analysis of molecular thin films.