Naphthobisthiazole diimide-based n-type polymer semiconductors: synthesis, π-stacking, field-effect charge transport, and all-polymer solar cells

New n-type conjugated polymer semiconductors bearing an electron-deficient naphthobisthiazole diimide (NBTDI) moiety have been synthesized and their electronic energy levels, solid-state morphology, field-effect charge transport and photovoltaic properties were investigated. Stille coupling polymerization of the new monomer 5,11-bis(4-bromophenyl)-2,8-bis(2-decyltetradecyl)-benzothiazolo[4,5,6,7-lmn]thiazolo[5,4-f][3,8]phenanthroline-1,3,7,9(2H, 8H)-tetrone with distannyl derivatives of the arylene moiety (1,4-phenylene/2,5-thienylene/vinylene) yielded poly(naphthobisthiazole diimide)s (PNBTDIs) with number-average molecular weights of 73–89 kDa and polydispersity indexes of 2.46–3.08. Thin films of the PNBTDIs have strong absorption bands in the visible region, resulting in an absorption edge optical band gap of 1.73–2.02 eV. X-ray diffraction analysis of neat films of PNBTDIs revealed lamellar crystalline materials with a rather short intermolecular π–π stacking distance of 0.34–0.35 nm. In thin film transistors, the PNBTDIs showed unipolar n-channel transport with electron mobility as high as 1.5 × 10−2 cm2 V−1 s−1. All-polymer solar cells incorporating the PNBTDIs as an electron acceptor and thiazolothiazole copolymer (PSEHTT) as a donor had power conversion efficiencies of 1.1–1.5%. The results demonstrate that poly(naphthobisthiazole diimide)s are promising n-type materials for field-effect transistors and all-polymer solar cells.