Synthesis and Characterization of Thiophene-Based Copolymers: Effects of Comonomer Design and Doping Method on Solubility, Morphology, and Conductivity

Conjugated polymers are promising materials for organic electronic devices due to their low cost, light weight and flexible nature, and use of naturally abundant elements. However, achieving metal-like conductivities requires chemical doping. This dissertation investigates the complex relationship between repeat unit design and polymer properties, such as solubility, morphology, and conductivity, with a particular focus on doping methods. A series of expanded core thiophene-based copolymers were synthesized to examine how reduced steric demand along the polymer backbone, relative to the benchmark poly(3-hexylthiophene), influences solubility, polymer-dopant interactions, and conductivity. The dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) was used to evaluate three doping approaches: solution doping, sequential doping, and serial doping, which is a two-step method that successively utilizes both solution and sequential doping. This work demonstrates that increasing the core size of these copolymers enhances doping efficiency and conductivity. Consistent with prior reports, sequential doping outperforms solution doping by achieving higher conductivity at lower dopant concentrations; however, this efficiency strongly depends on the choice of dopant solvent. A key contribution of this research is the development of a predictive framework for solvent selection to optimize conductivity based on the polymer–solvent interaction energies that is universally shared by polymers of different structure and chemical type. In addition, serial doping is shown to further enhance thin-film conductivity by inducing crystallite formation, offering a strategy to improve the performance of otherwise amorphous conjugated polymers. Altogether, the systematic studies presented herein offer new insights into design-structure-property relationships of conjugated polymers and support the development of broadly applicable doping strategies that maximize conductivity across a wide range of polymer designs.