A Study on the Mechanical Deformation dependent Electrical Property Change in PEDOT:PSS Conductive Polymer for Stretchable Electronic Conductor

Abstract

Recently, conducting polymers have been widely used to produce flexible and stretchable electronics, which are exposed to large amounts of mechanical strains. In contrast to elastic strain regions, extrinsic contributions such as defect generation, microstructural and dimensional changes, and neighboring materials can influence the change of electrical properties in concerted ways under large and plastic deformation. Therefore, systematically extinguishing each extrinsic factor and understanding the mechanism of change in the resistivity of a material subject to mechanical deformation is becoming increasingly important for the realization of stretchable electronic conductors. Here, we investigated the use of a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) conductive polymer as an electrode material and developed two classes of stretchable electronic conductors that could endure large amounts of mechanical stretch. First, we suggested the use of a film-type PEDOT:PSS conductor by applying a polyimide (PI) substrate that had matching mechanical properties (Poissons ratio and elastic modulus). The PEDOT:PSS films successfully stretched above 60% strain without any generation of defects (buckles or cracks). Interestingly, upon stretching the films, the resistivity of the pristine PEDOT:PSS decreased by 80%, however, it exhibited almost invariant change for dimethyl sulfoxide (DMSO)-doped PEDOT:PSS, which were directly caused by a morphological change in PEDOT:PSS. Due to the dissolution of PEDOT chains after doping with DMSO, the electrical path was not changed under mechanical strain. In case of the pristine films, the equivalent growth of conductive PEDOT-rich cores led to significant enhancements in charge conductivity. For the first time, we revealed a mechanism for the strain-induced growth of PEDOT-rich cores and discovered the mechanically tunable conductivity of PEDOT:PSS films. Secondly, we developed a new class of soft material-based electronic conductors based on PEDOT:PSS organogel. Due to its natural softness and compatibility with human-skin, this material enabled the stable transport of electrical signals upon severe mechanical deformation. Even under large amounts of stretch, the electrical resistance increased insensitively when the strain up to 350% and became lower than the theoretical increase in resistance resulting from geometrical shape changes. This was because of the formation of a polymeric conducting path that had closely packed PEDOT:PSS in the gel. The resistance change in response to strain was invariant even when the material was stretched to 50% strain in a cyclic deformation. Furthermore, by using ethylene glycol as a liquid constituent for the organogel, purely electrical conduction was enabled without the use of any electrochemical reactions, successfully resulting in long-term environmental stability. The PEDOT:PSS stretchable electronic conductors can be widely applied to electrical interconnection applications such as human wearable and attachable devices that cover any arbitrary curved surface or three-dimensional structures that require large amounts of stretchability. This work is a multidisciplinary project that includes studies in materials science, polymer science, and electronics and will enable researchers to design and tailor entirely new classes of conductive organic hybrid materials

such materials will also lead into a new area of biocompatible electronic devices.