Microstructural Characterization of Conjugated Organic Semiconductors by X-Ray Scattering

In this chapter, we highlight the contribution of X-ray scattering techniques to investigate the microstructure of conjugated organic semiconductors. In thin films, conjugated semiconductors exhibit a complex multi-phase microstructure that strongly affects their optoelectronic properties and functionality in optoelectronic devices. Thus, it is of crucial importance to attain in-depth characterization and quantification of important microstructural features such as crystal structure, molecular packing, the degree of crystallinity, and the size, orientation and packing defects of crystals. In multi-component systems, such as in bulk heterojunctions photovoltaics, there is more complexity and characterization includes the phase fractions and domain size of different phases. X-ray scattering techniques provide unique insights into these microstructural features over a wide range of length scales. This chapter begins with a brief review of the fundamentals of X-ray scattering. We then cover grazing-incidence X-ray scattering for characterizing the organic semiconductor microstructure highlighting the applications of X-ray scattering to probe the molecular order and orientation at the small-length-scale using wide-angle x-ray scattering (WAXS) and phase separation at the large-scale using small-angle x-ray scattering (SAXS). We further discuss probing the microstructure evolution in situ during thin film processing to elucidate the mechanisms of film formation.