Synthesis of thieno[3,2-b]thiophene derived conjugated oligomers for field-effect transistors applications

Abstract

A novel series of soluble thieno[3,2-b]thiophene (TT) oligomers with alternating TT and bithiophene or fluorene triad architectures have been synthesized for field-effect transistor (FET) applications. Their optical, thermal and electronic properties were investigated using UV-Vis and photoluminescence spectroscopy, thermal gravimetric analysis, and cyclic voltammetry. Compared with α,α′-dihexylsexithiophene (D6HT), these oligomers exhibit blue-shifted absorption spectra, 0.1 eV to 0.3 eV lower than the highest occupied molecular orbital (HOMO) energy levels and accordingly higher ambient stability. Their crystallinity and morphology features of these oligomers were further investigated with X-ray diffraction and atomic force microscopy using vacuum-deposited thin film on Si/SiO₂ substrate. Symmetrically structured TT-oligomers including HT2TT, HTTT2, HTTTT and DDFTT exhibit ordered film morphology and promising FET performance with devices fabricated by either vacuum deposition or solution processing techniques. Interestingly, HTTT2 shows interconnected terrace island morphology, which is often observed for pentacene. All p-type transistors show promising performance, with HT2TT demonstrating a hole mobility up to 0.025 cm²V⁻¹s⁻¹ and an on/off current ration ∼1.2 × 10³ with vacuum sublimated film deposited on 70 °C substrate.