Molecular designs of polymer semiconductors for skin-inspired electronics
Keywords: Chemistry.

Polymer semiconductors (PSCs) are a promising class of materials to produce skin-inspired electronics for advanced wearable and implantable applications. These electronics are designed to have the properties of human skin, which is soft, conformable and stretchable. Compared to silicon-based inorganic semiconductors, polymers offer the distinct advantages of intrinsic mechanical flexibility, low-cost solution processability and chemical tunability. However, a PSC with high electrical performance typically exhibits low stretchability/elasticity, poor compatibility with solution-processed multilayer device manufacturing and environmental instability. Therefore, the research work in this dissertation aims to tackle these critical challenges for PSCs by developing molecular engineering approaches, thus expediting the commercialization of polymer electronics. Chapter 1 provides a background introduction on stretchable PSCs. Chapter 2 addressed the challenge of PSC brittleness. In order to improve PSC stretchability, I designed hydrogen-bonding conjugation breakers with optimized self-association constant and linker flexibilities, and introduced them into diketopyrrolopyrrole-based PSCs. The breakage of these dynamic interactions dissipated strain energy, resulting in an improved fracture strain of PSC thin film. Chapter 3 addressed the challenge of simultaneously achieving high stretchability and electrical performance in PSCs. Specifically, I found that an indacenodithiophene-co-benzothiadiazole (IDTBT) polymer is a promising candidate, which possessing high backbone coplanarity but low packing tendency. The polymer chains in amorphous region effectively dissipate strain energy through conformation change, while the rigid backbone configuration results in efficient intrachain charge transport. As a summary of Chapter 2 and 3, through rational conjugated polymer backbone designs, I achieved highly stretchable PSCs with high electrical functionalities maintenance under mechanical deformation. For realistic consumer electronics, the PSC needs to function through repeated stretching-releasing cycles beyond a single stretching event. Chapter 4 addressed the challenge of elasticity and device manufacturing compatibility issue of PSCs. In order to enable high cyclic reversibility in PSC films, I designed a single precursor BA for covalently-embedded in-situ rubber matrix (iRUM) formation, obtaining a composite semiconductor film with PSC network interpenetrating with rubber matrix through covalent linkages. This approach results in an elastic PSC film due to the high covalent crosslinking density. Furthermore, iRUM-semiconductor is solvent-resistant and photo-patternable, which are especially beneficial for solution-processed complex circuits manufacturing. Chapter 5 addressed the challenge of the environmental instability issue of stretchable PSCs during long-term use in organic field-effect transistors. In order to protect PSC films against water-induced electrical performance degradation, I developed a nanostructured fluorinated molecular protection layer (FMPL) with significantly low water permeability. More importantly, the nanostructured FMPL maintained its protecting function under mechanical deformation. Chapter 6 provides a summary of the thesis and outlook for stretchable PSCs. Collectively, the research works in this dissertation demonstrate the reliable operation of polymer electronic devices, which is enabled by high mechanical and environmental stability of polymer semiconductors. At the same time, the developed semiconductor materials are compatible with solution-processed manufacturing and exhibited high charge transport properties