Enhancing Thin Film Morphology and Conductivity Through Serial Doping of Conjugated Thiophene-Based Copolymers

Chemical doping has been widely used to enhance the electrical performance of conjugated polymer thin films for organic electronic devices with specific attention given to dopant molecule selection, modifications of copolymer design to enhance dopant interactions, and the doping process itself. Here, a set of expanded core thiophene-based copolymers are doped via solution, sequential, or “serial” doping processes. Serial doping, which utilizes solution and sequential doping methods in succession, was utilized to investigate how the use of a two-step doping method impacts the doping efficiency, thin film conductivity, and morphology. Results show that serial doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) increases the conductivity of all of the thiophene-based copolymers relative to either one-step solution or sequential doping processes. This increase in conductivity of serial-doped films is not attributed to greater levels of incorporation of F4TCNQ; rather, the serial-doped thin films display more ordered crystalline microstructures, despite the pristine films having amorphous character. This indicates that serial doping enhances the conductivity by inducing crystallization. This study also shows that as the spacing length between alkyl side chains of the expanded core copolymers is increased, the conductivity and the ordering of crystallites increases. Overall, this work provides insight into how copolymer design and the doping method impact conductivity through complex and intertwined relationships between dopant integration, effective electron transfer, and thin film microstructure.