Polarized X-ray scattering reveals non-crystalline orientational ordering in organic films

Molecular orientation critically influences the mechanical, chemical, optical and electronic properties of organic materials. So far, molecular-scale ordering in soft matter could be characterized with X-ray or electron microscopy techniques only if the sample exhibited sufficient crystallinity. Here, we show that the resonant scattering of polarized soft X-rays (P-SoXS) by molecular orbitals is not limited by crystallinity and that it can be used to probe molecular orientation down to size scales of 10 nm. We first apply the technique on highly crystalline small-molecule thin films and subsequently use its high sensitivity to probe the impact of liquid-crystalline ordering on charge mobility in polymeric transistors. P-SoXS also reveals scattering anisotropy in amorphous domains of all-polymer organic solar cells where interfacial interactions pattern orientational alignment in the matrix phase, which probably plays an important role in the photophysics. The energy and q -dependence of the scattering anisotropy allows the identification of the composition and the degree of orientational order in the domains. Molecular orientation, which critically influences the properties of organic materials, could until now only be characterized if the sample exhibited sufficient crystallinity. Resonant scattering of polarized soft X-rays by aromatic carbon bonds has now been used to probe non-crystalline ordering and molecular orientation in thin films with a resolution down to 20 nm.