The mechanism of enhancing the conductivity of PEDOT: PSS films through molecular weight optimization of PSS

Poly(3,4-ethylenedioxythiophene) (PEDOT): poly (styrene sulfonic acid) (PSS) is a promising intrinsically conductive polymer. Researchers have been modifying PSS to enhance the conductivity of PEDOT: PSS films, however, the relationship between PSS molecular weight and PEDOT: PSS conductivity remains unclear. Herein, the conductivity mechanism is discussed in detail, and it is observed that, as the molecular weight of PSS increases, the conductivity initially increases but then decreases after reaching a peak. This behavior can be attributed to the different aggregation state structures, the degree of oxidation, and the molecular conformation of PEDOT. Specifically, the optimal PSS molecular weight leads to an increase in PEDOT crystallinity and the appearance of a continuous nanoproto-fiber morphology. Additionally, the highest degree of PEDOT oxidation and the largest molecular weight of PEDOT polymerized after reduction treatment, along with the transition from coiled-coil conformation to linear or swollen coils, synergistically increase the carrier mobility and carrier concentration, resulting in a significant improvement in electrical conductivity. However, further increase in PSS molecular weight leads to decreased crystallinity, reduced continuous crystal domains, lower PEDOT oxidation levels, and molecular weights, preventing the generation of effective charge pathways. The electrical conductivity mechanism reported here provides theoretical models and specific strategies for improving the electrical conductivity of thermoelectric, electromagnetic shielding, and bioelectronic materials.