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.
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Discussions with and help from A. L. D. Kilcoyne, T. Tyliszczak, T. Young, A. Hexemer, G. Cody, S. Kevan and J. Kortright are gratefully acknowledged. P-SoXS development and blend film characterization by B.A.C., H.Y., E.G. and H.A. are supported by DOE (DE-FG02-98ER45737), GAANN Fellowship (E.G.) and ALS Fellowship (H.Y.). Characterization of PBTTT films by B.A.C., H.Y. and H.A. are supported by NSF (DMR-0906457, ARRA). M.L.C. and J.E.C. are supported by NSF (DMR-0906224, ARRA). C.H. and R.F. are supported by BMBF (contract 05 K10WEA). C.R.M. acknowledges support from the EPSRC (EP/E051804/1) and ARC (FT100100275). We graciously thank I. McCulloch and M. Heeney (Imperial College, UK) for samples of PBTTT. Part of this research was undertaken on the soft X-ray beamline at the Australian Synchrotron, Victoria, Australia. Data were acquired at beamlines 126.96.36.199., 7.3.3. and 188.8.131.52. at the ALS, which is supported by DOE (DE-AC02-05CH1123) and the LDRD programme under DOE (DE-AC02-05CH11231).
The authors declare no competing financial interests.
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Collins, B., Cochran, J., Yan, H. et al. Polarized X-ray scattering reveals non-crystalline orientational ordering in organic films. Nature Mater 11, 536–543 (2012). https://doi.org/10.1038/nmat3310
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