Molecular design principles of deformable polymer semiconductors for soft electronics
Keywords: Chemical Engineering.

Since the discovery of semiconducting conjugated polymers, their electrical properties have evolved tremendously. Their mobility can now exceeds that of amorphous silicon (1 cm2 V-1 s-1), making them attractive candidates for commercial use. In addition, with the recent interest in soft electronics, their intrinsic softness has attracted enormous attention. However, it remains highly challenging to achieve a polymer system possessing both high mobility and mechanical deformability. While ordered microstructures are desirable for efficient charge transport, amorphous counterparts are also essential to confer high mechanical deformability. Thus, this limitation necessitates precise molecular-level engineering of semiconducting polymers for soft electronics applications. This work contributes to the understanding of deformable semiconductors at the molecular level by exploring various design principles and examining their effects on microstructures and film properties. First, conjugation breakers are introduced to improve the ductility of polymer semiconductors without significant compromise of mobility. We discover that polymer crystallinity is greatly reduced by dynamic behaviors of the non-conjugated spacers, thereby enhancing the film ductility. Also, tie-chains of high molecular weight polymer chains enable efficient charge transport and relatively high mobility. The effects of conjugation breaker, i.e. flexibility, on the mechanical properties of semiconducting thin films are also examined. We show that more flexible spacers result in more ductile semiconducting thin films with lower elastic modulus. This study hence establishes the fundamental structure-property relationships of conjugation breakers. Second, we discuss the use of conjugated carbon nanorings as molecular additives for stretchable semiconductors. The conjugation of the additives is observed to enable efficient charge transport between host semiconductor chains and the additives, while the cyclic structures interrupt intermolecular interactions and reduced crystallinity of the polymer chains. As a result, the nanoring additives effectively improve stretchability of polymeric semiconductors without compromising the mobility. In addition, the nanoring additives improve the ductility of several different semiconducting polymers, hence confirming the general applicability of this additive-based approach. Fully stretchable transistors fabricated using our developed semiconductor-additive blend films exhibit high initial mobility as well as mobility retention capabilities under strain and after repeated strains. Third, a molecular p-type dopant is introduced as an additive to impart polymer semiconductors with high mobility, stability, and ductility. We find that charge transfer between the dopant and host semiconductor molecules results in improved mobility and stability, while reduced crystallinity from the additive leads to enhanced stretchability. In a fully stretchable transistor, the doped semiconductor films exhibit significantly improved stability, initial mobility, and mobility retention under strain. This is the first report that enables the simultaneous improvement of the above three desirable properties. Next, a fully conjugated donor-acceptor terpolymer design is presented. The fully conjugated backbone and strong aggregation allow for efficient intra- and inter-chain charge transport, respectively, while reduced crystallization arises from backbone irregularity of the terpolymers. The semiconducting terpolymers exhibit superior ductility as compared to semiconductor-blend films that suffer from phase separations. The terpolymer design is applicable to different donor and acceptor combinations, and we observe record high mobility retention in a fully stretchable transistor. We conclude this work by summarizing the current understanding on stretchable semiconductors and offering future outlooks. Collectively, this work provides molecular design principles of deformable semiconducting polymers, especially from their microstructure perspectives. With increasing needs for mechanically compliant electronic materials and devices, this work offers guidelines for future research on stretchable polymer semiconductors