Tailoring self-assembly and optoelectronic properties of organic semiconductors via macromolecular engineering

Conjugated polymers and small molecules are a promising class of semiconducting materials for application in macroelectronic and energy conversion devices. The development of high performance devices employing this class of semicrystalline materials ultimately depends on the precise control of crystalline domain size, orientation and connectivity due to the complex nature of their molecular interactions and chemical structure. It is there-fore of paramount importance to control self-assembly processes of π-conjugated molecules and polymers into functional microstructures over a wide set of length scales suitable for device optimal operation. In the present thesis, we investigated the effect that novel materials design criteria have on the control of the active layer morphology and long-term stability in organic field effect transistors and bulk heterojunction solar cells. A first strategy to reduce microstructure complexity in semiconducting polymer thin-films is to separate the contributions to morphology and charge transport resulting from different polymer molecular weights. Combined morphological and electronical characterization revealed a high degree of isotropy in charge transport mechanisms in semicrystalline and poorly aggregating conjugated systems as a result of the broad variety of self-assembled microstructures observed at different molecular weights. Next, we designed and synthesized a new class of semi-conducting materials based on a flexible linker concept. Initially, we showed the successful preparation and purification of a prototype in a class of semiconducting polymer that allows independent control over the conjugated segment length and overall chain length by covalently linking low-MW conjugated segments with flexible aliphatic linkers. Our flexibly linked polymeric material exhibited improved thin-film formation compared to the low-MW starting polymer and unique thermal properties. Importantly, our linking strategy had a clear effect on the chain self-assembly and allowed structural control between distinct thin film morphologies without altering the chain length. Next we extended the flexible linker design motif to small molecule derivatives that are employed in high performance bulk heterojunction solar cells. In this study we showed, that active layer degradation under continuous thermal stress can be inhibited by the formation of a more robust thin film microstructure with the additive present. Finally, the efficient functionalization of a series of polymeric blocks was successfully used to synthetize a new class of alternated multi-block copolymers. The facile nature of the synthetic procedure ena-bles high degree of polymerization and offers the possibility to include a large number of semiconducting polymers in block copolymer architectures. Ultimately, when our alternating multi-block copolymers are solution processed in thin films, a relatively high degree of self-assembly and micro-domains phase separation is ob-served at length scale ideal for well-ordered heterojunction needed to improve solar cells efficiency.

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