Study of Polymorph Tuning at the Surfaces in an Organic Semiconductor

<b>Organic semiconductors (OSCs) are promising for thin-film transistor applications as they potentially offer distinctive advantages over their inorganic counterparts, particularly in terms of their properties, processing techniques and cost-effectiveness. Small molecules with extended aromatic core and solubilizing long chains are budding candidates for solution-processed organic semiconductors. However, as small molecular OSCs are held together by weak non-directional van der Waals force, they tend to exhibit many alternative packing arrangements with differences in structure and energy leading to polymorphism. The utilization of solid surfaces (substrates) as a nucleation or crystallization mediator was a strategy used in our investigation to identify new thin-film phases. We could successfully demonstrate that defined innovations in the experimental protocol (like the choice of solvent, temperature etc.) can promote tuning between polymorphs as different local energy minima become accessible.</b><br/><br/><b>Among the various OSCs, small molecules with[1]benzothieno [3,2-b]benzo thiophene (BTBT) cores are identified as the best p-type semiconductor. Here we investigated the crystal structure solution, film-forming and charge transport properties of FD44 (OEG BTBT), a BTBT derivative. The crystal structure of FD44 was resolved from the single crystal, and the film-forming properties of FD44 were investigated from the monolayer to the bulk for solution-processed and physical vapour deposited thin films. When we consider the structural aspect of the BTBT core, FD44 molecules arrange in a herringbone type packing which facilitates 2D carrier transport properties. Also, in addition to the three bulk phase polymorphs, extensive investigations carried out by altering the experimental protocols employed in the fabrication of thin films at the vicinity of a substrate revealed four new thin-film phases. Specular X-ray diffraction and grazing incidence X-ray diffractions were performed using in-house X-ray equipment and synchrotron to determine the crystallographic structure within the thin films. Furthermore, our detailed investigations helped us in understanding the origin and stability of these new polymorphic forms and thereby gain control over the conditions that led to their formations. Controlling the polymorphism of OSCs within thin films is crucial because they form the active channel layer in most organic electronic devices, and dramatic variations in charge transport can be induced even by small changes in the molecular packing.<br/><br/>Acknowledgement</b><br/><br/><b>This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska Curie grant agreement No 811284</b>