Performance enhancement of air-stable thieno[2,3-b]thiophene organic field-effect transistors via alkyl chain engineering†

In this study, four novel thieno[2,3-b]thiophene (TT) small molecules, 2,5-bis((5-octylthiophen-2-yl)ethynyl)thieno[2,3-b]thiophene (1), 2,5-bis((5-(2-ethylhexyl)thiophen-2-yl)ethynyl)thieno[2,3-b]thiophene (2), 3,4-dimethyl-2,5-bis((5-octylthiophen-2-yl)ethynyl)thieno[2,3-b]thiophene (3), and 2,5-bis((5-(2-ethylhexyl)thiophen-2-yl)ethynyl)-3,4-dimethylthieno[2,3-b]thiophene (4), were synthesized and explored as channel layers for organic field-effect transistors (OFETs). Conjugated triple bonds and flexible alkyl side chains were strategically integrated into the TT core to promote efficient carrier transport. The compounds were characterized using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV-visible spectroscopy (UV-vis), and cyclic voltammetry (CV) to evaluate their thermal stability, optical properties, and electrochemical behavior. Organic thin films were prepared through solution shearing, and their surface morphology and microstructure were analyzed using atomic force microscopy (AFM) and X-ray diffraction (XRD). Among the four, compounds 1–3 showed p-channel activity. Notably, compound 1, which possesses linear alkyl side chains, demonstrated decent electrical performance under ambient conditions, achieving a hole mobility of 0.42 cm² V⁻¹ s⁻¹ and a current on/off ratio exceeding 10⁸. These results reveal that appropriate alkyl chain engineering enhances molecular packing and crystallinity, thereby improving device performance. Furthermore, devices based on compound 1 maintained stable operation upon 90-day storage, demonstrating excellent air stability.