Depth profiling molecular orientation with polarized resonant soft X-ray reflectivity*

Organic molecules often exhibit complex molecular packing that results in anisotropic behaviors. Advances in thin-film processing make it feasible to harness this asymmetry and fabricate devices with superior properties. For instance, vapor deposition of glassy molecules can produce OLEDs with increased charge transport and light-outcoupling when components are aligned ‘face-on’ to the substrate. While many measurements are capable of investigating bulk orientation, few can simultaneously interrogate buried or free interfaces. Here we will discuss progress in developing polarized resonant soft X-ray reflectivity (P-RSoXR) for molecular orientation depth profiling in soft matter thin-films. P-RSoXR derives orientation contrast by probing near-edge X-ray absorption fine structure dipole moments allowing for continuous depth profiling of molecular orientation. We will demonstrate such capabilities by investigating orientationally inhomogeneous bilayers comprised of a model glassy molecule, TCTA, where P-RSoXR reveals a buried layer with a distinct orientation compared to the bulk. Modeling methodologies will be discussed along with future opportunities in systems with composition and orientation heterogeneity.