Monitoring the Crystallisation Process of NDI-T2 Based Oligomers Using Spectroscopic Ellipsometry
Alexander Ehm a, Rukiya Matsidik b, Michael Sommer b, Dietrich R. T. Zahn a
a Institut für Physik, Technische Universität Chemnitz, 09126 Chemnitz, Germany
b Institut für Chemie, Technische Universität Chemnitz, 09111 Chemnitz, Germany
Poster, Alexander Ehm, 204
Publication date: 6th February 2024

Finding ways to tune the (opto-)electronic properties of organic semiconductors and their thin films can be a viable tool in the search for best-performance photovoltaic structures. It is well established that the crystallisation of organic molecules may lead to significant changes in their optoelectronic properties. In part, this is due to molecular alignment and corresponding changes in the molecular order of the organic thin films, as was demonstrated for the semiconducting polymer P(NDI2OD-T2) [1]. In its thin films the alignment could even be tuned to be parallel or perpendicular to the substrate surface. The favourable orientation of intramolecular transport may thus be gauged depending on the specific needs of the resulting device.

The average molecular orientation in an organic thin film can be determined using variable-angle spectroscopic ellipsometry (VASE), by tracking the orientation of the p-p* transition dipole moment using anisotropic modelling techniques [2]. This works because VASE measures the changes in the p- and s- polarised components of reflected polarised light. If there is a preferential orientation of a particular optical transition, then it will result also in a difference in the absorption coefficients oriented parallel or perpendicular to the plain of incidence. Moreover, VASE modelling yields information about film thickness, surface and interface roughness, and the general behaviour of the optical constants.

We study the crystallisation upon thermal annealing of novel oligomers with discrete chain length based on naphthalene diimide (NDI) and bithiophene (T2) monomers [3]. As these molecules consist of the same molecular units as the polymer P(NDI2OD-T2) and have a very similar structure, it is likely that they adapt preferential average orientations upon crystallisation as well. We used spectroscopic ellipsometry to monitor the general changes of the thin film properties in dependence on the annealing temperature. We aim to elucidate especially the dependence of the molecular orientation on thermal treatment and the changes occurring in the optical transitions upon thermally induced crystallisation.

We thank the Deutsche Forschungsgemeinschaft (DFG) for funding this work (Research Unit FOR 5387 POPULAR, project no. 461909888).

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