Spatial Orientation and Order of Molecular Subunits in Films of Organic Semiconductors

A. M. Anton and F. Kremer

Peter Debye Institute for Soft Matter Physics, Leipzig University, Linnéstr. 5, 04013 Leipzig

Conjugated donor/acceptor copolymers have obtained significant attention due to their soft matter properties combined with semiconducting characteristics, which allows for their operation in organic field effect transistors or solar cells, for instance [1]. Because the devices’ macroscopic properties arise from the materials’ molecular organization, a detailed understanding of the microscopic structure is essential for targeted developments. In order to shed light on the spatial orientation and order in thin films of P(NDI2OD-T2) the technique of Infrared Transition Moment Orientational Analysis (IR-TMOA) is employed. Therefore, the absorbance of structure-specific bands depending on the inclination of the sample and on the polarization of the IR light is evaluated [2,3]. This enables to determine the tensor of absorption separately for the respective molecular moieties as well as to deduce the orientation of atomistic planes defined through the polymer subunits, relative to the substrate and hence relative to each other. We found that the solvent used for spin coating (chlorobenzene or a chloronaphthalene:xylene mixture) determines the alignment of the T2 part (either face on or edge on), whereas the NDI unit is not affected. On the other hand, the inclination of the NDI plane is well retained for diverse sample thicknesses in between nano- and micrometers.

[1] A. C. Arias, J. D. MacKenzie, I. McCulloch, J. Rivnay, A. Salleo, Chem. Rev. 110, 3 (2010).
[2] A. M. Anton, R. Steyrleuthner, W. Kossack, D. Neher, F. Kremer, JACS 137, 6024(2008).
[3] A. M. Anton, R. Steyrleuthner, W. Kossack, D. Neher, F. Kremer, Macromolecules 49, 1798 (2016).