(271h) Using Resonant Soft X-Ray Scattering (RSoXS) to Study Phase Separation in Smooth and Rough Vapor-Deposited Glasses

Well-controlled phase separation in organic thin films is essential to their application in devices such as organic photovoltaics, thermoelectrics, and flexible electronics. Quantifying phase separation in these materials can be difficult, however, as the small amount of contrast between two organic components can lead to low signal-to-noise for many common measurement methods.

In this work, we show that we can measure phase separation in vapor-deposited glasses using experimental Resonant Soft X-ray Scattering, or RSoXS, in conjunction with GPU-accelerated scattering simulations. For a co-vapor-deposited 50:50 blend of the small glass-formers TPD and Disperse Orange 37, we find that the extent of phase separation can be controlled by the choice of substrate temperature during physical vapor deposition. These vapor-deposited films show significant height fluctuations (up to 40% of the film thickness in the most extreme cases), and therefore would generally be considered “too rough” for RSoXS. We quantify the phase separation by a combination of experimental RSoXS, conventional AFM, and chemically sensitive photo-induced force microscopy (PiFM). We carefully analyze the dependence of the scattering profiles on the variable X-ray photon energy to separate the effects of compositional and height contrast. Finally, we construct a model of the film and simulate the RSoXS measurement to find good agreement with the experimental data. We find that RSoXS is sensitive to compositional length scales that are not apparent from surface-sensitive microscopies alone. We then will present preliminary work on the application of this method to other vapor-deposited systems.

Through this work, we show that RSoXS is a robust method of measuring phase separation, even for features that are buried below the free surface of the film. We extend the traditional analysis framework to make it more robust for very rough films. Finally, from a materials perspective, we show a route to finely control phase separation in vapor-deposited glasses.