Effect of a benzothiadiazole spacer on transport properties and N-doping of naphthalene-diimide-based copolymers

Abstract

We report the synthesis of two n-type semiconducting polymers containing a naphthalene diimide (NDI) monomer and thiophene spacers. The two polymers differ by the introduction of an additional 2,1,3-benzothiadiazole (BTD) moiety in the polymer backbone. Although there are examples of polymers combining these two moieties, the effects of introducing the BTD as a third co-monomer on the transport properties and, in particular, the doping mechanisms of these materials have not been elucidated. We describe the optoelectronic, structural organisation and transport properties of these two polymers and investigate the effect of BTD introduction on molecular doping with N-DMBI as a dopant using UV-Vis spectroscopy, DFT modelling, cyclic voltametry, and X-ray diffraction. We show that the incorporation of a BTD unit in the polymer backbone not only improves the charge transport in organic field effect transistors but also the doping efficiency with N-DMBI as well as the conductivity and stability in the conducting state. Our results highlight the need for post-deposition annealing to optimise the N-DMBI doping of the BTD-based polymer and ultimately its conductivity and thermoelectric properties.