(544h) Influence of Copolymer Design and Doping Method on Electrical Conductivity of Conjugated Copolymer Thin Films

Conjugated polymers are promising materials for flexible and printed electronic devices due to their optoelectronic and physical properties. However, these materials are typically semi-conducting or insulating in nature and must be chemically doped to achieve metal-like electrical properties. Therefore, significant work has been done to optimize the doping process, specifically with the polymer:dopant pair of poly(3-hexylthiophene) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) being the archetypal system. In this work, novel thiophene-based copolymers are studied to determine how copolymer design as well as doping method impacts solubility, morphology, and electrical conductivity. Specifically, conjugated spacing units of thiophene, bithiophene, and terthiophene are incorporated into the backbone design to compare how increasing the distance between alkyl side chains impacts copolymer solubility, dopant integration, and film conductivity. A fused ring core of thienothiophene is also selected to enable investigations of how increasing rigidity and planarity of the comonomer affects solubility and conductivity. Thin films made from these copolymers are subjected to both solution and sequential doping to determine the strategy that optimizes conductivity. Results show that increasing the distance between alkyl spacing length increases the film conductivity, with conductivities eclipsing that of P3HT by several orders of magnitude. These studies also show that sequential doping offers higher conductivity at lower concentrations of anionic F4TCNQ with a strong dependence on the nature of the solvent used for doping. Overall, this work provides fundamental insight into how copolymer design, doping method, and strength of polymer-solvent interaction energy impacts morphology and electrical performance, paving the way for printed electronics.