Role of Dopant Counterion Distribution in Decoupling of the Strain-Dependent Electrical Performance from the Crack Onset Strain of Conjugated Polymers

Abstract Doping enhances the electrical performance of conjugated polymers but inevitably alters their mechanical properties by modifying their aggregation structure. However, the mechanism by which doping affects the mechanical properties of conjugated polymers remains poorly understood. In this work, we used three different dopants to phase-selectively dope poly(3-hexylthiophene) (P3HT) films to modulate carrier transport pathways and further investigated the mechanism by which the transport pathways affected the changes in electrical performance of P3HT under tensile strain. 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is a weak dopant and predominantly dopes the crystalline phase of P3HT, thereby resulting in a single-crystalline phase carrier transport pathway. Tris(4-bromophenyl)aminium hexachloroantimonate (Magic Blue, MB) can dope both crystalline and amorphous phases; however, its counterions are predominantly distributed in the amorphous phase of P3HT to form the accumulation layer as the primary carrier transport pathway. Gold chloride (AuCl3) can also dope both crystalline and amorphous phases with counterions distributed in both phases; thus, dual pathways are established as the crystalline phase and accumulation layer. For each dopant, the conductivity of P3HT first increases and then decreases, whereas the crack onset strain (COS) decreases monotonically with increasing dopant concentration. After stretching, the crystalline structures of the doped P3HT films deform and fracture, resulting in the formation of small crystalline regions and new crystalline–amorphous interfaces. In comparison with the corresponding lightly doped P3HT film, the highly doped film using F4TCNQ as the dopant, with its lower COS, results in a more rapid decrease in conductivity under tensile strain. In contrast, in P3HT films highly doped with MB or AuCl3, excess carriers in the amorphous phases migrate to adjacent accumulation regions at newly formed interfaces and participate in charge transport, allowing the film to maintain superior electrical performance under tensile strain and thereby decoupling strain-dependent electrical properties from the COS. The key to this decoupling lies in the distribution of dopant counterions within the amorphous regions of conjugated polymers to form the accumulation layer carrier transport pathway, regardless of whether the crystalline regions can be doped. The dual-pathway carrier transport endows the AuCl3-doped P3HT film with the highest initial conductivity and the best tensile stability of the conductivity among the three types of doped P3HT films. This work aids in the design of dopant-conjugated polymer systems that can overcome the bottlenecks in the development of flexible electronics.