Naphthodithiophene Diimide (NDTI)-Based Semiconducting Copolymers: From Ambipolar to Unipolar n-Type Polymers

A series of naphthodithiophene diimid (NDTI) based semiconducting polymers with various comonomer units, i.e., thienylenevinylene (TV), naphtho[1,2-b:5,6-b']dithiophene (NDT), vinylene (V), benzo[c][1,2,5]thiadiazole (BTz), and naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole (NTz), were synthesized by Stille coupling or copper iodide-assisted Stille coupling using 2,7-dibromo-NDTI or 2,7-bis(trimethylstannyl)-NDTI, respectively. Their HOMO and LUMO energy levels were estimated by cyclic volttammetry and photoelectron spectroscopy. The HOMO energy levels of the NDTI-based polymers were strongly affected by the electronic nature of the comonomer units. In contrast, their LUMO energy levels were almost identical, likely owing to their localized LUMOs on the NDTI moiety. All the polymers showed air-stable electron transport in the field-effect transistors (FETs), thanks to their low-lying LUMO (similar to-4.0 eV), as ambipolar (PNDTI-TV, -NDT, similar to 0.082 cm(-2) V1 s1, mu(e) = similar to 0.029 cm(-2) V-1 s(-1)) or unipolar n-channel materials (PNDTI-V, -BTz, PNDTI-NTz, mu(e) = similar to 0.21 cm(-2) V-1 s(-1)), depending on their HOMO energy levels. These results indicate that the type of active carrier in the NDTI-based polymers can be controlled by the nature of comonomer units. Moreover, the two-dimensional grazing incidence X-ray diffraction (2D GIXD) indicated that the polymers with linear backbone structures and large space between the branched alkyl chains tend to afford well-organized crystalline thin film with the edge-on orientation, consistent with good transport characteristics in their FET devices.