(404f) A Molecular Design Approach Towards Elastic and Multifunctional Polymer Electronics

Skin-like electronics, which is soft, conformable and stretchable, have great potentials in health monitoring, robotics, prosthetics and medical implants. To realize these applications, it is necessary that the electronic materials are able to maintain their structural integrity and electrical functionality when placed on or inside human body, while encountering various types and levels of strain during usage. To this end, polymer semiconductors (π-conjugated) are promising as the active charge transporting layer, due to their mechanical flexibility, chemical versatility and solution processability. In ideal cases, it combines the semiconductivity of silicon and deformability of soft plastics. However, a polymer semiconductor (PSC) with high electrical performance typically fractures under low strain, resulting from its semi-crystalline thin film morphology. Virtually all previously reported molecular design rules focused on enhancing the ultimate fracture strains of polymer semiconductors without loss of electronic functionalities, in which ‘reducing long-range crystalline order’ has been a general principle. However, the semiconductor films in these cases suffered from permanent plastic deformation prior to crack formation. Upon removal of strain, the semiconductor film form wrinkles on a supported elastic dielectric, resulting in interfacial delamination and degradation in electrical performance of the devices. For practical applications, the semiconductor needs to reliably function more than just a single stretching event, but rather through multiple stretching-releasing cycles. Therefore, elasticity/reversibility beyond simple stretchability is highly demanded.

Here, we report a rationally designed single precursor BA for covalently-embedded in-situ rubber matrix formation (iRUM), which results in an elastic semiconductor (iRUM-s) with high cyclic reversibility while maintaining high charge transporting ability. Furthermore, iRUM-s film has superior solvent-resistance and photo-patternability, which is especially advantageous for solution-based multilayer device fabrication and complex circuit manufacturing. The iRUM precursor, BA, consists of perfluorophenyl azide (PFPA) end-capped polybutadiene and has the following key features: (i) the flexible backbone structure and compatible surface energy enable its good mixing with PSC in high BA-to-PSC ratio, allowing for high crosslinking density; (ii) BA undergoes self-crosslinking to generate a rubber matrix through azide/C=C cycloaddition, ensuring high stretchability and elasticity of the composite semiconductor film; (iii) BA also undergoes crosslinking with PSC alkyl side chains through azide/C–H insertion, ensuring solvent-resistance and photo-patternability; (iv) the azide groups of BA have seven times higher reactivity with C=C bonds than C–H, leading to finely controlled competition between forming a rubber matrix and linking with conjugated polymer side chain, which contributes to the maintenance of PSC aggregation and charge transport pathway.

When applied in stretchable transistors, the iRUM-s film retained its mobility after stretching to 100% strain, and exhibited record-high mobility retention of 1 cm² V^-1 s^-1 after 1000 stretching-releasing cycles at 50% strain. The cycling life was stably extended to 5000 cycles, five times longer than all reported semiconductors. Furthermore, we fabricated elastic transistors via consecutively photo-patterning of the dielectric and semiconducting layers, demonstrating the compatibility of the developed materials with solution-processed multilayer device manufacturing. The iRUM strategy is also promising for mass production when considering the cost-effectiveness and scalability of precursors as well as the reduced cost of expensive active materials resulting from the high content of iRUM precursors (~50%-75%). The highly accessible and reactive double bonds in iRUM films further provide unique opportunities for pre- and/or post-modification and interfacial engineering through chemical functionalization. This iRUM represents a molecular-level design approach for the transition from soft/stretchable to elastic and multifunctional skin-inspired electronics.