Molecular engineering of benzothiadiazole-based polymers: balancing charge transport and stretchability in organic field-effect transistors†

Fluorination is a common strategy for improving the electronic properties of π-conjugated materials. This has shown to be important for applications in organic electronics, especially for the fabrication of organic field-effect transistors or organic photovoltaics. Despite being an efficient tool to generate high-performing materials, fluorination of conjugated materials and its effect on thermal transitions and mechanical properties are not fully understood. Evaluated through multiple techniques, this work investigates the glass transition temperatures and mechanical properties of a series of differentially fluorinated and/or alkylated benzothiadiazole-based polymers. In addition to the evaluation of these properties, their electronic properties under strain were measured through the fabrication of organic field-effect transistors. The crack onset strains of the prepared polymers ranged from as low as 10% for the polymer containing only a fluorinated backbone to as high as 30% elongation for the polymer containing an alkoxy appended backbone. Ultimately, fluorination embrittles conjugated polymer thin films, whereas alkylation softens them. Furthermore, the crack onset strains, and electronic properties of the synthesized polymers have an inversely proportional relationship; similarly, the electronic properties under strain increase with decreasing elastic moduli. These results show the importance of molecular design on the mechanical properties of conjugated polymers and in the development of stretchable conjugated polymer semiconductors.