Charge transport and thermoelectric conversion in solution-processed semicrystalline polymer films under electrochemical doping

Charge transport and thermoelectric conversion mechanisms in doped semicrystalline polymer films are key issues in the field of wearable electronics, whereas the complex film structure consisting of crystalline domains and non-crystalline boundaries prevents sufficient understanding of them. In this study, we fully clarify the roles of the domains and the boundaries in a typical semicrystalline polymer on macroscopic charge transport under continuous electrochemical doping. In the crystalline domains, a multi-step transformation of the transport properties from effectively metallic behavior to weak localization (WL) to variable-range hopping (VRH) is found with decreasing temperature and doping level. On the other hand, at the domain boundaries, the effectively metallic conduction changes directly to VRH. Based on these results, the extremely complicated phase diagram, including the coexistence of the WL and VRH processes, is well explained. The proposed transport mechanism further explains the thermoelectric properties of the film. Polymer films are flexible, conductive materials with an expected application to a range of electronic devices, but the complexity of the underlying transport mechanisms inhibit improvements in performance. Here, the authors investigate the transport properties of doped semicrystalline polymer films and determine the role of crystalline domains and boundaries, finding evidence of weak localisation and variable range hopping, which vary with doping level.