From Monolayer to Bulk: Thin-Film-Specific Polymorphic Transitions of a Molecular Semiconductor

Organic compounds have the potential to form distinct crystal structures on a substrate surface. These are typically referred to as thin-film and monolayer phases. The properties of these phases are often key for developing high-performance devices. Nevertheless, many thin-film-specific phases remain unidentified, and the known bulk phase is instead used as a structural model to discuss structure-property relationships also in thin films. For example, the polymorphism of dinaphtho-[2,3-b:2',3'-f]-thieno-[3,2-b]-thiophene (DNTT) has long been overlooked, even though this compound is widely used as a benchmark material for organic thin-film transistors. For a comprehensive understanding of polymorphic transitions in organic semiconductors, the present study investigates the thickness-dependent structural changes in DNTT vapor-deposited films using high-resolution infrared Brewster-angle transmission spectroscopy, grazing incidence X-ray diffraction, and density functional theory calculations. This multimodal approach identifies three different crystal structures depending on the film thickness: the monolayer phase, the thin-film phase, and the bulk phase. Furthermore, the structure solutions of the monolayer phase and candidate structures of the thin-film phase are obtained. This study not only provides an overall model for the thin-film growth of organic semiconductors but also discusses a powerful combination of analytical and modeling techniques for identifying unknown thin-film phases of organic materials.