Disentangling Polymorphism, Texture, and Molecular Orientation in TIPS-Pentacene Thin Films by Polarized Raman Spectroscopy

Abstract

Thin-film processing can alter the crystal structure of organic semiconductors, making it difficult to disentangle the respective roles of polymorphism, texture, and molecular orientation in the charge transport. Here, highly oriented thin films of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) prepared by bar-assisted meniscus shearing are investigated by combining grazing-incidence X-ray diffraction, polarized Raman spectroscopy, and periodic first-principles calculations. We show that bar-assisted meniscus shearing does not generate a distinct thin film polymorph under the investigated conditions. Instead, the films adopt the bulk Form I structure while developing pronounced out-of-plane and in-plane textures. Grazing-incidence X-ray diffraction establishes the film texture, while polarized Raman spectroscopy and periodic density functional theory calculations support the Form I assignment through the angular dependence and low-frequency vibrational fingerprint. In particular, the calculations reproduce the phonon frequencies and polarization dependence, support the vibrational assignment, and show that low-frequency modes in TIPS-pentacene have mixed inter- and intramolecular character, which makes them especially sensitive to subtle packing differences. Comparison among the reported polymorphs further shows that their inherent structure energetics, Raman fingerprints, and transport-related properties remain clearly distinguishable. Overall, this work establishes polarized Raman spectroscopy, combined with grazing-incidence X-ray diffraction and theory, as an effective route to resolve polymorph, texture, and molecular orientation in highly ordered organic semiconductor films.