Organic electronic materials have potential applications ranging from portable organic photovoltaics (OPVs) and organic field-effect transistors (OFETs) to wearable sensors and actuators. The most widely used organic electronic materials are conjugated polymers that have both semiconducting and conducting variants. In these materials, the electronic performance is closely related to, and underpinned by, their mechanical performance. This is especially true for portable and wearable devices that may be subjected to large stresses from everyday use and the environment. Another complication is that most thin-film electronics require multiple layers to create a fully functioning device. A recent study by Finn et al. showed that the dominant failure mechanism in roll-to-roll printed thin-film OPVs was delamination of the device stack. The mechanical failure of these devices arise from poor cohesion in individual layers and weak adhesion between layers and the substrate. Additionally, failure can arise as a result of elastic mismatches between the layers that causes stress to concentrate at the interface leading to cracking and shorting.
Scratch testing is a method of rapidly characterizing the cohesion and adhesion of thin films and coatings. This technique is usually employed to compare the adhesion of a range of thin films to a particular substrate or to characterize the adhesion of one thin film to a range of substrates. In a progressive load scratch test, a stylus is moved across the surface of a sample with a linearly increasing load until failure occurs at critical loads, Lci. The Lci’s associated with failure of the thin films are a function of film-substrate adhesion, film thickness, loading rate, and the mechanical properties of both the substrate and the thin film. In this study, we utilize progressive load scratch testing to compare the cohesive strength and adhesion of (1) P3ATs as a function of alkyl side-chain length and (2) P3HT in a range of molecular weights. The goal of this work is to provide insight into improving the cohesion and adhesion of thin-film conjugated polymers and to report the results from a mechanical testing technique not typically applied to organic electronic materials.