Resonant soft X-ray scattering reveals hierarchical structure in a multi-component vapor-deposited glass

Abstract

Multi-phase vapor-deposited glasses are an important class of materials for organic electronics, particularly organic photovoltaics and thermoelectrics. These blends are frequently regarded as molecular alloys and there have been few studies of their structure at nanometer scales. Here we show that a co-deposited system of TPD and Disperse Orange 37, two small molecule glass-formers, separates into compositionally distinct phases with a domain size and spacing that depends on substrate temperature during deposition. Domains rich in one of the two components become larger and more pure at higher deposition temperatures. We use resonant soft X-ray scattering (RSoXS) complemented with Atomic Force Microscopy (AFM) and photo-induced force microscopy (PiFM) to measure the phase separation, topography, and purity of the deposited films. A forward-simulation approach to RSoXS analysis, the National Institute of Standards and Technology (NIST) RSoXS Simulation Suite (NRSS), is used with models developed from AFM images to evaluate the energy dependence of scattering across multiple length scales and interpret the RSoXS with respect to structure within the films. We find that the RSoXS is sensitive to a hidden length scale of phase separation that is not apparent from the AFM characterization alone. We demonstrate that vacuum scattering, which is often ignored in RSoXS analysis, contributes significantly to the features and energy dependence of the RSoXS pattern, and then illustrate how to properly account for vacuum scattering to analyze films with significant roughness. We then use this analysis framework to understand structure development mechanisms that occur during vapor deposition of a TPD-DO37 co-deposited glass with results that outline paths to tune morphology in multi-component materials.