Organic field-effect transistors (OFETs) is the basic unit of complementary logic circuit, however, the development of n-type OFETs lags behind of p-type ones due to the barrier of electron injection and the interference from oxygen and water, which hinders the development of complementary logic circuit. Therefore, the design and synthesis of high-performance n-type organic semiconductors and the improvement of device performance and stability have important scientific significance. In this work, a novel naphthalene diimide (NDI)-vinylogous tetrathiafulvalene derivative (BDTNDI-DTYA)₂ was designed and synthesized via a π-expanded strategy by fusing the benzene-1,2-dithiol (BDT) and 2-(1,3-dithiol-2-ylidene) acetonitrile (DTYA) moieties onto the NDI core. The chemical structure of the compound was characterized by ¹H NMR, ¹³C NMR, Fourier transform infrared spectroscopy (FT-IR) and high-resolution mass spectrometry (HRMS). The thermal, optical and electrochemical properties of (BDTNDI-DTYA)₂ were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), ultraviolet-visible (UV-Vis) absorption spectra and cyclic voltammetry (CV). The energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of (BDTNDI-DTYA)₂ calculated from CV were -5.66 and -4.01 eV, respectively. The edge absorption of (BDTNDI-DTYA)₂ in thin film showed obvious red-shift (68 nm) relative to that in CHCl₃ solution, indicating strong intermolecular interactions in solid state. The bottom-gate and top-contact (BGTC) OFETs based on (BDTNDI-DTYA)₂ fabricated by spin-coating method, showed n-type electron transporting characteristics. The average electron mobility of the untreated devices was 0.04 cm² V^-1 s^-1 when measured in nitrogen atmosphere and was increased of up to 1.00 cm² V^-1 s^-1 when the thin films of (BDTNDI-DTYA)₂ were thermal annealed at 160 ℃. On the other hand, azulene was used as an additive to treat the thin films of (BDTNDI-DTYA)₂ via sublimation, the average electron mobility of OFETs was increased to 0.98 cm² V^-1 s^-1. The effect of thermal annealing treatment and azulene-treatment on the performance of (BDTNDI-DTYA)₂-based OFETs were investigated by UV-Vis absorption spectra, atomic force microscopy (AFM) and X-ray diffraction (XRD). For UV-Vis absorption spectra of thin films of (BDTNDI-DTYA)₂, after thermal annealing at 160 ℃ and azulene-treatment, the absorption peak in long-wavelength was enhanced and widened relative to that of the untreated thin films with obvious shoulder peaks and red-shifts (35 and 39 nm, respectively). The AFM and XRD results indicated that the improvement of device performance originated from the improved (BDTNDI-DTYA)₂ thin film crystallinity and morphology. In this work, a π-expanded NDI-VTTF derivative as n-type organic semiconductor was designed and synthesized, and azulene was used for the first time to effectively regulated the structure and morphology of the active layer of OFETs, which both provide new insights for development of novel organic semiconductors and their high performance OFET devices.

Key words: naphthalene diimide, n-type organic field-effect transistors, electron mobility, structure-performance relationship, additive