Phase separation mechanisms in vapor-deposited multi-component glasses

Physical vapor deposition can prepare uniform, non-equilibrium thin films, kinetically trapping structures that can’t be formed by cooling from the bulk liquid. During deposition, molecules at the free surface partially equilibrate towards an anisotropic surface structure before becoming buried and kinetically frozen, building a glass with free surface-like structure throughout its depth. While the deposition-induced development of molecular orientation and enhanced packing density is well-understood for single-component glasses, co-depositing with a second component results in additional structural features due to phase separation. In this work, we use a combination of thermal and structural characterizations, primarily differential scanning calorimetry and resonant soft X-ray scattering, to measure phase separation in co-deposited blends of TPD and TCTA, two organic semiconductors. We synthesize the two measurements with physically informed computational X-ray scattering simulations to model the phase separation mechanisms. We show results from rate- and substrate temperature-dependent experiments to systematically explore the mechanisms underlying the multi-component assembly. We find that the degree of phase separation can be finely tuned by changing the deposition parameters, providing an opportunity to design new materials for organic electronics with precise control over their morphology.