Conjugated polymers are an important class of organic semiconductors (OSCs) which are core components in organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic field-effect transistors (OFETs). Compared to small-molecule semiconductors, these materials are semi-crystalline or amorphous with intricate molecular packing and a mixed variety of structural orders and disorders [1]. The susceptibility of polymer semiconductors to ‘burn-in degradation’ [2] can induce blend-demixing and photo-induced ordering/disordering [3] during device operation, thereby intensifying structural heterogeneities and impeding charge transport in planar heterojunction devices [4, 5], ultimately resulting in performance losses. 

Although structural disorders are recognized as the root cause of trapping sites, they also contribute to charge carrier transport for polymer semiconductors [4]. Controlling the transport properties in polymer semiconductors necessitates a deeper understanding of the type and variation of disorders at a length scale commensurate with charge carrier transport. Yet direct structural characterization of disorders and their spatial distribution at the nanoscale are limited due to the susceptibility of these materials to damage under exposure to the high-energy electron beams typically used for nanoscale structural examination [6]. Consequently, the role of particular molecular packing disorders and interface structures between molecular phases in hindering charge transport remains ambiguous. 

We have developed low-dose four-dimensional scanning transmission electron microscopy (4D-STEM) techniques, which allow complete access to reciprocal space over the size of a spatially localized probe (~3 nm). We apply this technique to characterize the microstructures and nanoscale atomic variation in typical polymer blends (F8:F8BT) in both plane-view films and cross-sectioned device models. With this technique, we can map the nanocrystalline and amorphous components to evaluate domain orientation and size and conduct electron pair distribution function analysis to probe the atomic structure of the amorphous matrix. These capabilities allow us to quantify the structural differences between polymer blends, reshape the understanding of temperature-induced phase segregation, and assess beam-induced damages caused by site-selective extraction techniques commonly employed in cross-section device characterizations. 

References

[1] L. Ding et al. Chemical Reviews. 2023, 123, 12, 7421–7497. 

[2] L. Duan et al., ACS Appl. Mater. Interfaces 2020, 12, 24, 27433–27442.

[3] N. Li et al., Nat. Commun. 2017, 8, 14541.

[4] L. G. Kaake et al. J. Phys. Chem. Lett. 2010, 1, 3, 628–635.

[5] K. Zhou et al. J. Mater. Chem. C, 2021,9, 13761–13769.

[6] J. Donohue et al. iScience, 2022, 25, 103882.