Simulating PVD Glass: Over What Length Scale Does an Inorganic Substrate Perturb the Structure of a Glassy Organic Semiconductor?

Understanding the structure of vapor deposited (PVD) glasses at buried interfaces is important for organic electronic applications. Here, we combine coarse-grained molecular dynamics (MD) simulation and experiments to study the molecular structures at the buried interfaces of PVD glass of an organic semiconductor, DSA-Ph (1,4-di-[4-(N,N-diphenyl)amino]styrylbenzene), on different substrates. For simulations, custom interaction potentials are used to control anchoring and orientation on different substrates. Simulation results show that the length scale over which the substrate perturbs the structure of a vapor-deposited glass is approximately 5σ (3 nm). In experiments, the structure of PVD glasses of DSA-Ph are examined as a function of film thickness with grazing incidence X-ray scattering. Experiments show that interfacial molecular packing is more disordered compared to the bulk, and they suggest that the film thickness at the buried interface that exhibits a modified molecular packing is ∼8 nm, consistent with the simulation results. In both simulations and experiments, the deposition temperature controls glass structure beyond this interfacial layer of a few nanometers.