Characterization of the interfacial orientation and molecular conformation in a glass-forming organic semiconductor*

Vapor-deposition enables precise control over the nanostructure in molecular glasses which are an important class of materials used in organic electronics. Extensive characterization of film properties has identified bulk molecular orientation as a key structural motif that determines optoelectronic performance. However, few studies have investigated how molecules orient near buried interfaces, despite its importance in electronics. Here, we use polarized resonant soft X-ray reflectivity to measure nanometer-resolved molecular orientation depth profiles of vapor-deposited thin films of an organic semiconductor, TCTA. Our approach characterizes the molecular orientation throughout the film and reveals how molecules near the substrate and free surface have a nearly isotropic orientation compared to an anisotropic bulk. Pairing our results with NEXAFS enables a determination of the molecular conformation. We support our results with molecular dynamic simulations that provide insight into the mechanisms of film formation that occur at interfaces between orientationally distinct layers. Applications to multicomponent glasses will be discussed and the potential for future studies to develop critical structure-function relationships.