Mechanical, Morphological, and Charge Transport Properties of NDI Polymers with Variable Built-in Π-Conjugation Lengths Probed by Simulation and Experiment

Abstract

Mechanically deformable polymeric semiconductors are a key material for fabricating flexible organic thin-film transistors (FOTFTs)—the building block of electronic circuits and wearable electronic devices. However, for many π-conjugated polymers achieving mechanical deformability and efficient charge transport remains challenging. Here the effects of polymer backbone bending stiffness and film microstructure on mechanical flexibility and charge transport are investigated via experimental and computational methods for a series of electron-transporting naphthalene diimide (NDI) polymers having differing extents of π-conjugation. The results show that replacing increasing amounts of the π-conjugated comonomer dithienylvinylene (TVT) with the π-nonconjugated comonomer dithienylethane (TET) in the backbone of the fully π-conjugated polymeric semiconductor, PNDI-TVT100 (yielding polymeric series PNDI-TVTx, 100 ≥ x ≥ 0), lowers backbone rigidity, degree of texturing, and π–π stacking interactions between NDI moieties. Importantly, this comonomer substitution increases the mechanical robustness of PNDI-TVTx while retaining efficient charge transport. Thus, reducing the TVT content of PNDI-TVTx suppresses film crack formation and dramatically stabilizes the field-effect electron mobility upon bending (e.g., 2 mm over 2000 bending cycles). This work provides a route to tune π–π stacking in π-conjugated polymers while simultaneously promoting mechanical flexibility and retaining good carrier mobility in FOTFTs.