Resonant Soft\nX‑ray Scattering Reveals Hierarchical Structure in a Multicomponent\nVapor-Deposited Glass

Multiphase vapor-deposited glasses are an important class\nof materials\nfor organic electronics, particularly organic photovoltaics and thermoelectrics.\nThese blends are frequently regarded as molecular alloys and there\nhave been few studies of their structure at nanometer scales. Here\nwe show that a codeposited system of TPD (<i>N</i>,<i>N</i>′-bis(3-methylphenyl)-<i>N</i>,<i>N</i>′-diphenylbenzidine) and DO37 (disperse orange 37),\ntwo small molecule glass-formers, separates into amorphous, compositionally\ndistinct phases with a domain size and spacing ca. 10s of nanometers\nthat depends on substrate temperature during deposition. Domains rich\nin one of the two components become larger and more pure at higher\ndeposition temperatures. We use resonant soft X-ray scattering (RSoXS)\ncomplemented with atomic force microscopy (AFM) and photoinduced force\nmicroscopy to measure the phase separation, topography, and purity\nof the deposited films. A forward-simulation approach to RSoXS analysis,\nthe National Institute of Standards and Technology RSoXS Simulation\nSuite (NIST RSoXS simulation suite), is used with models developed\nfrom AFM images to evaluate the energy dependence of scattering across\nmultiple length scales and interpret the RSoXS with respect to structure\nwithin the films. We find that the RSoXS is sensitive to a length\nscale of phase separation buried within the film that is consistent\nwith the surface composition profile, and correlates to the topography\nto an extent that depends on substrate temperature. We demonstrate\nthat vacuum scattering, which is often ignored in RSoXS analysis,\ncontributes significantly to the features and energy dependence of\nthe RSoXS pattern, and then illustrate how to properly account for\nvacuum scattering to analyze films with significant roughness. We\nthen use this analysis framework to understand structure development\nmechanisms that occur during vapor deposition of a TPD-DO37 codeposited\nglass with results that outline paths to tune morphology in multicomponent\nmaterials.