Probing Molecular Orientation using Polarized Resonant Soft X-ray Reflectivity

Charge transport in organic semiconducting films is inherently anisotropic: it depends on the orientation and packing of the molecules, which in turn are influenced by molecular structure and film processing conditions. A technique capable of non-destructively depth-profiling molecular orientation with (sub)nanometer-level depth resolution would enable a significant improvement in the understanding of structure-property-processing relationships in organic semiconductor devices. We explore the feasibility of using polarized resonant soft X-ray reflectivity (p-RSoXR) as a tool to profile molecular orientation by studying a model set of glassy small molecule (posaconazole) films with different net orientations. By comparison with variable angle spectroscopic ellipsometry (VASE) and near edge X-ray absorption fine structure (NEXAFS) data, we show that p-RSoXR is sensitive to both surface and bulk molecular orientation in the films. We conclude by demonstrating our approach to extracting orientation profiles from p-RSoXR data, as well as discussing the current strengths and limitations of the technique.