N-type charge carrier transport properties of bdopv-benzothiadiazole-based semiconducting polymers

Abstract

High-performance n-type organic semiconducting polymers are key components of next-generation organic electronics. Here, we designed and synthesized two electron deficient organic polymers composed of benzodifurandione-based oligo (p-phenylenevinylene) (BDOPV) and benzothiadiazole by Stille coupling polycondensation. BDOPV-benzothiadiazole-based copolymer (PBDOPV-BTT) possesses a D-A1-D-A2 type backbone with intramolecular charge–transfer interactions, while PBDOPV-BTTz is an all-acceptor polymer. The former has a higher molecular weight (Mn) of 109.7 kg·mol−1 than the latter (Mn = 20.2 kg·mol−1). The structural difference of these polymers was confirmed by the optical absorption spectra. PBDOPV-BTT showed a more bathochromically shifted absorption spectrum than PBDOPV-BTTz. The longer wavelength absorption of PBDOPV-BTT was due to the intramolecular charge transfer. Therefore, PBDOPV-BTT had a narrower band gap than PBDOPV-BTTz. However, this feature was not reflected by the lowest unoccupied molecular orbital (LUMO) levels. Both polymers displayed almost the same LUMO level of −3.8 eV. Accuracy of this observation was cross-verified by density functional theory (DFT) calculations. The electron-transporting properties were investigated by thin film transistors. PBDOPV-BTT showed an electron mobility (µe) of 1.02 × 10−2 cm2 V−1 s−1 under the optimized annealing conditions. PBDOPV-BTTz exhibited poorer transistor performances with the optimized µe of 9.54 × 10−6 cm2 V−1 s−1. Finally, the grazing-incidence wide angle X-ray scattering (GIWAXS) measurements of both polymer films revealed the higher crystallinity of PBDOPV-BTT with the edge-on orientation.