Proceedings of September Meeting 2016 (NFM16)
Publication date: 14th June 2016
Advanced molecular materials that combine two or more physical properties are typically constructed by combining different molecules, each being responsible for one of the properties required. Ideally, single molecules could take care of this combined functionality, provided they are self-assembled correctly and endowed with different functional subunits whose strong electronic coupling may lead to the emergence new and exciting properties. Although solution processable and highly organized molecules, having either semiconducting or ferroelectric properties have been reported before, the combination of these properties in a single compound so far remained elusive. Here, we present a class of disc-like semiconducting organic molecules that are functionalized with strong dipolar side groups.
Supramolecular organization of these materials provides long-range polar order that supports collective ferroelectric behavior of the side groups as well as charge transport through the stacked semiconducting cores. The existence of a truly ferroelectric phase in the various compounds is demonstrated through a combination of techniques, including IR absorption and dielectric relaxation spectroscopy. Importantly, the ferroelectric polarization in these supramolecular polymers is found to couple to the charge transport and leads to a bulk conductivity that is both switchable and rectifying. On basis of the unique sense of the conductivity hysteresis loop, and control experiments, we can explicitly rule out alternative explanations like Schottky barrier modulation or filament formation. Instead, a detailed analysis of the device’s current-voltage characteristics shows that it is the bulk current that is modulated by the ferroelectric polarization. It can be described as a combination of Ohmic and space-charge-limited currents. The evident correlation between current-voltage and polarization-voltage hysteresis loops, and the parallel transients of polarization retention and current on/off-ratio demonstrate the direct connection between polarization and conductivity.
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