Overview
This study investigates the kinetic arrest of phase separation in vapor-deposited organic thin film blends, specifically focusing on the relationship between deposition rate and the resulting domain structures. The characteristic length scale of phase-separated organic thin film blends is identified as a critical structural parameter influencing the performance and functionality of organic electronic devices.
Research Context
The morphology of vapor-deposited organic thin films is understood to arise from an interaction between thermodynamic driving forces and kinetic constraints during the deposition process. Previous observations in homogeneous vapor-deposited films have described a surface equilibration mechanism, where enhanced surface mobility allows molecules in the growing film to partially equilibrate into distinct surface-templated states during deposition.
Approach
The researchers aimed to isolate the contribution of kinetic effects to phase separation. This was achieved by systematically varying the deposition rate while maintaining a constant substrate temperature. The chosen material system was a co-deposited molecular glass blend comprising N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) and Disperse Orange 37 (DO37).
Morphology dependence on deposition rate was quantified through power spectral density (PSD) analysis, applied to data acquired from atomic force microscopy (AFM) images. To assess the extent of phase separation throughout the film, complementary Resonant Soft X-ray Scattering (RSoXS) measurements were performed.
Findings
- AFM analysis revealed two distinct deposition rate-dependent length scales at the film surface, indicating a direct influence of deposition kinetics on domain size and film topography.
- RSoXS measurements indicated that the observed phase separation extends through the entire thickness of the film.
- These observations are consistent with the surface equilibration mechanism, suggesting that enhanced surface mobility permits the multi-component blend to phase separate and coarsen into a structure with multiple length scales during deposition.
- The extent of this phase separation and coarsening before kinetic arrest was found to depend on the deposition rate.
Why This Matters
The demonstration of finely tunable domain size through the manipulation of deposition rate provides strategies for designing organic electronic devices with desired morphologies, which are critical for their performance and functionality.