Five-membered 1,3,4-oxadiazole (OZ) and 1,3,4-thiadiazole (TZ) heterocycles have high electron affinity, good coplanarity, and multiple heteroatom characteristics, which is the promising building block for n-channel polymeric semiconductors. On the basis of OZ and TZ moieties, two A1–D–A2–D type polymeric semiconductors (PNOZ and PNTZ) were developed. Moreover, a comparative study had been investigated to reveal the effect of different heterocyclic acceptors on thermal properties, electronic properties, ordering structures, and carrier transport performance of the polymers. The results indicated that, compared with PNOZ, the TZ-containing PNTZ exhibited stronger self-assemble ability and obtained a fairly homogeneous surface structure with a good film interconnectivity, which is helpful for high-mobility carrier transport.

 Fig.1 Polymer structure, OFET device structure, and film morphology

To investigate the effect of different heterocycles on carrier transport performance of the two polymers, polymer field-effect transistors (PFETs) with a top-gate top-contact (TGBC) configuration were fabricated by solution spin-coating techniques. For OZ-containing PNOZ, the maximum electron mobility was demonstrated to be 0.026 cm² V^-1 s^-1. Compared with PNOZ, the TZ-containing PNTZ exhibited excellent electron transporting characteristics, affording 0.36 cm² V^-1 s^-1 maximum electron mobility. More importantly, it was found that both polymers-based PFETs exhibited an excellent air-stability with a negligible decay in more than 60 days. The remarkable air-stability for the TGBC PFETs mainly arises from the encapsulation effect of PMMA dielectric layer.

To investigate crystallinity and stacking structure of the polymer thin films, GIXRD measurements were also carried out based on the polymer thin films. According to the diffraction data, we can conclude that both polymers take mainly face-on stacking respective to substrates, which is similar with many high-performance polymers. As shown in out-of-plane patterns, both polymer thin films show sharp, multiple (h00) diffraction peaks corresponding to the lamellar structure, where (100) peaks appear at 2q = 4.0° for PNOZ and PNTZ, with the same interlayer distance of 22.06 ？. On the basis of the out-of-plane pattern data, both polymers thin films exhibited the compact (010) features at 2θ = 23.0^o for PNOZ and 23.4 ^o for PNTZ, and the corresponding π–π stacking distances are 3.86 ？ for PNOZ and 3.80 ？ for PNTZ. The results reveal that the TZ-containing PNTZ thin-films afford smaller π–π stacking distance, thereby facilitating more efficient intermolecular carrier hopping for PNTZ.

This work was published in ACS Applied Materials & Interfaces (2016, 8, 33051–33059). The GIXRD data was obtained at 1W1A, Beijing Synchrotron Radiation Facility. The authors gratefully acknowledge the assistance of scientists of Diffuse X-ray Scattering Station during the experiments.