(588bn) Polarized Soft X-Ray Scattering (PRSoXS) Measures Tunable Interfacial Molecular Orientation in Phase-Separated Organic Thin Films

Control over phase separation and molecular orientation in organic thin films is critical to design effective and stable organic electronic materials. Different preferred structures and mobilities at the free and buried interfaces impact both the molecular organization during the non-equilibrium deposition process and the ultimate function and stability over the lifetime of a device. It can be difficult to simultaneously engineer both the compositional phase separation and the molecular orientation; moreover, it can be difficult to measure the two structural parameters in all-organic blends. Here, we use physical vapor deposition (PVD) to co-deposit blends of organic semiconductors into films with thicknesses ranging from 20 to 300 nanometers. Atomic force microscopy (AFM) is used to quantify phase separation and height fluctuations at the free surface. Transmission Polarized Resonant Soft X-ray Scattering (PRSoXS) measures hierarchical phase-separated structure through the bulk of the thin films and reveals a surprisingly high degree of in-plane molecular orientation at the interface between components. The in-plane interfacial orientation depends strongly on the film thickness, deposition temperature, and deposition rate. The PRSoXS is interpreted using a computational scattering simulation framework which quantifies the magnitude and length of the molecular orientation and the purity of domains. The two techniques show increasing length scales of phase separation with decreasing film thickness at both the free surface and through the bulk, suggesting that the films are uniformly phase-separated throughout their thickness. The thin films show enhanced kinetic and thermal stability similar to that of homogeneous single- and multi-component vapor-deposited glasses [1].

Our results suggest that cooperative molecular orientation and phase separation are highly correlated, and that strong ordering can be induced over long length scales by physical vapor deposition. We show that vapor depositing into highly confined films can prepare new morphologies that are inaccessible when depositing thicker films, as has been recently shown for single-component vapor-deposited glasses [2] and is shown for the first time here for multiple components. Our work may enable design rules to make thin organic films with finely controlled phase separation and molecular orientation, which can be used to optimize properties for organic electronic applications. Furthermore, the measurement and analysis framework used throughout this work strengthens the information available from the PRSoXS technique, which can be used and expanded to measure nm-scale interfacial features in a wide range of soft matter systems.

[1] S. Cheng, Y. Lee, J. Yu, L. Yu, M.D. Ediger. Chem. Mater. 36, (2024), 3205-3214.

[2] Y. Jin, A. Zhang, S.E. Wolf, S. Govind, A.R. Moore, M. Zhernenkov, G. Freychet, A.A. Shamsabadi, Z. Fakhraai. Proc. Nat. Acad. Sci. 118, (2021), e2100738118.