Molecular Orientation Depth Profiles from Resonant Soft X-ray Reflectivity

Orientation within amorphous molecular glass films can impart unique optoelectronic properties that provide enhanced performance in devices such as organic light emitting diodes (OLEDs). Bulk molecular orientation in vapor-deposited glass films can be tuned through processing conditions; however, little is known about the molecular orientation at the film surface or at the buried interface(s), where anisotropic optoelectronic properties can have a large impact on device performance. We use vapor-deposited films of tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as a model system to develop soft X-ray techniques to extract molecular orientation from organic thin films. Variable incident-angle near edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to probe the average molecular orientation within the top few nanometers of the TCTA films, as well as to determine the energy-dependence of orientation sensitivity for TCTA. Informed by the NEXAFS spectra, polarized resonant soft X-ray reflectivity (pRSoXR) measurements are performed at selected X-ray energies to probe molecular orientation through the depth of the film. Using the combined NEXAFS and pRSoXR data, we demonstrate a methodology to extract molecular orientation depth profiles with (sub)nanometer-level resolution.