Charge transport in organic semiconductors is highly sensitive to film heterogeneity and intermolecular interactions, but probing these properties on the length scales of disorder is often difficult. Here we use micro-Raman spectroscopy to assign vibrational modes of isomerically pure syn and anti 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES ADT) by comparing to density functional theory calculations. With polarization-dependent measurements, we determine the orientation of crystallites in pure isomers. In mixed-isomer samples, we observe narrow linewidths and superposition spectra, indicating coexistence of isomerically pure sub-domains on length scales smaller than the probe area. Using the ring breathing modes close to 1300 cm−1 as indicators of the pure isomer crystalline sub-domains, we image their spatial distribution with 200-nm resolution. These results demonstrate the power of micro-Raman spectroscopy for investigating spatial heterogeneities and clarifying the origin of the reduced charge carrier mobility displayed in mixed-isomer diF-TES ADT.
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A portion of this work was performed using the Raman microspectrometer in the UNC EFRC Instrumentation Facility established by the UNC EFRC Center for Solar Fuels, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DE-SC0001011. P.J.D. and O.D.J. acknowledge support from the NSF ECCS 1254757. A.J.P. and J.E.A. thank NSF DMREF-162748 for support.
The authors declare that the data within the article and Supplementary Information, as well as additional data supporting the findings of this study, are available upon reasonable request.
Supplementary information accompanies this paper at https://doi.org/10.1038/s42004-019-0122-7.
Huang, Chiung-Wei; You, Xiao; Diemer, Peter J.; Petty, Anthony Joseph II; Anthony, John E.; Jurchescu, Oana D.; and Atkin, Joanna M., "Micro-Raman Imaging of Isomeric Segregation in Small-Molecule Organic Semiconductors" (2019). Chemistry Faculty Publications. 160.