Controlling domain size of phase-separated organic semiconductor blends using physical vapor deposition

Physical vapor deposition can produce phase-separated glassy thin films of organic semiconductor blends with tunable domain size, the control of which is crucial for optimizing organic semiconductor devices. Domain size can be tuned by adjusting the substrate temperature during deposition, which is due to a combination of thermodynamic and kinetic effects.[1] Here, to isolate the role of kinetic effects on phase separation during deposition, we vary the deposition rate at constant substrate temperature for a 50/50 mixture of N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) and Disperse Orange 37 (DO37). Power spectral density (PSD) analysis of AFM images reveals deposition rate dependency of the domain size, while soft X-ray scattering confirms uniform phase separation through the film depth. Our results show that length scales of both phase separation and surface roughness vary smoothly with deposition rate.

Our results align with the surface equilibration mechanism where enhanced surface mobility enables molecules to partially equilibrate into distinct surface-templated states during deposition.[2] Here, this mechanism allows the blend to phase separate and coarsen before kinetically arresting, resulting in tunable domain size and morphology.

1. C. Bishop et. al. Chem. Mater.36(18) (2024)

2. Fiori, M. E. et. al. PNAS,118(42) (2021)