Molecular Orientation Depth Profiles in Organic Glasses Using Polarized Resonant Soft X-ray Reflectivity

Molecular orientation anisotropy can be critical in functional organic thin films. For instance, it is known that molecular orientation can affect the performance of organic electronic devices such as light emitting diodes (OLEDs), thin film transistors (OTFTs), and photovoltaics (OPVs); however, the impact of molecular orientation on device performance tends to be obscured by complexities of the multilayer device structure, and control over molecular orientation in the organic layer(s) is often limited. Thus, techniques that can depth profile molecular orientation in thin films are sorely needed. We demonstrate that polarized resonant soft X-ray reflectivity (p-RSoXR) can extract molecular orientation depth profiles with the needed nanometer-level resolution. Leveraging developments in stable molecular glass formation using physical vapor deposition (PVD), we create molecular glass films of posaconazole, a rod-like molecule, with controlled orientation. We show that p-RSoXR is highly sensitive to thin oriented layers at the posaconazole surface, while providing quantitative molecular tilt values for the bulk film. Finally, we demonstrate that p-RSoXR can detect and characterize buried interfaces based solely on molecular orientation, providing insight into the structure of complex films that was previously unmeasurable.