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Keywords:

  • organic field-effect transistors;
  • polycrystallinity;
  • semiconducting polymers;
  • x-ray microscopy;
  • NEXAFS

Abstract

We utilize scanning transmission X-ray microscopy (STXM) to study the domain structure of polycrystalline films of the semiconducting polymer poly(9,9’-dioctylfluorene-co-benzothiadiazole) (F8BT). By taking several images at different orientations of the film with respect to the polarization of the X-ray beam, we are able to compute quantitative maps of molecular alignment/order and molecular orientation, including both the backbone direction and phenyl ring plane orientation, as well as the in-plane and out-of-plane components. We show that polycrystalline F8BT films consist of well-ordered micron-sized domains with the transition from one domain orientation to another characterized either by a smooth transition of orientation or by ∼ 200 nm wide disordered domain boundaries. The morphology of the disordered domain boundaries resemble the electroluminescence patterns observed previously in F8BT light-emitting field-effect transistors suggesting that charge trapping at these disordered domain boundaries facilitates charge recombination in ambipolar operation. A relatively narrow distribution of local average tilt angles is observed that correlates with film structure, with the ordered domains in general showing a higher tilt angle than the disordered domain boundaries. We also use secondary electron detection to image the surface domain structure of polycrystalline F8BT films and demonstrate that the polycrystalline structure extends to the film/air interface. Finally, we calculate ideal NEXAFS spectra corresponding to a perfect F8BT crystal oriented with the 1s – π* transition dipole moment parallel and perpendicular to the electric field vector of a perfectly linearly polarized X-ray beam.