Samsung Advanced Institute of Technology

Semiconducting polymer thin films are essential elements of soft electronics for both wearable and biomedical applications1？11. However, high-mobility semiconducting polymers are usually brittle and can be easily fractured under small strains (<10％)12？14. Recently, improved intrinsic mechanical properties of semiconducting polymer films have been reported through molecular design15？18 and nanoconfinement19. Here we show that engineering the interfacial properties between a semiconducting thin film and a substrate can significantly delay micro-crack formation in the film. We present a universal design strategy that involves covalently bonding a dissipative interfacial polymer layer, consisting of dynamic non-covalent crosslinks, between a semiconducting thin film and a substrate. This enables high interfacial toughness between the layers, suppression of delamination, and delocalization of strain. As a result, crack initiation and propagation are significantly delayed to much higher strains. Specifically, the crack onset strain of a high-mobility
semiconducting polymer thin film improved from 30％ strain to 110 ％ strain without any noticeable microcracks, i.e. becoming extrinsically stretchable. Despite of the presence of a large mismatch in strain between the plastic semiconducting thin film and an elastic
substrate after unloading, the tough interface layer helped to maintain bonding and exceptional cyclic durability and robustness. Furthermore, we found that our interfacial layer reduces the mismatch of thermal expansion coefficients between different layers. This
38 interface engineering approach was found to improve crack onset strain of various semiconducting polymers, conducting polymers and even metal thin films. This significantly broadens the material choices for stretchable electronics.