Functionalized benzothieno[3,2 b]thiophenes (BTTs) for high performance organic thin-film transistors (OTFTs)

Abstract

New benzothieno[3,2-b]thiophene (BTT) derivatives, end-functionalized with biphenyl (Bp-BTT), naphthalenyl (Np-BTT), and benzothieno[3,2-b]thiophenyl (BBTT; dimer of BTT) moieties, were synthesized and characterized for bottom-gate/top-contact organic thin-film transistors (OTFTs). All three materials exhibit good environmental stability as assessed by thermogravimetric analysis, and no decomposition after extended light exposure, due to their wide band gaps and low-lying HOMOs. The single crystal structures of Bp-BTT and BBTT reveal flat molecular geometries, close π–π stacking, and short sulfur-to-sulfur distances, suggesting an ideal arrangement for charge transport. X-ray diffraction (XRD) measurements verify that the bulk crystal structures are preserved in the polycrystalline thin films. As a consequence, Bp-BTT and BBTT exhibit good OTFT performance, with µ = 0.34 cm² V⁻¹ s⁻¹ (max) and I_on/I_off = (3.3 ± 1.6) × 10⁸ for Bp-BTT, and µ = 0.12 cm² V⁻¹ s⁻¹ (max) and I_on/I_off = (2.4 ± 0.9) × 10⁷ for BBTT; whereas Np-BTT gives lower device performance with µ = 0.055 cm² V⁻¹ s⁻¹ (max) and I_on/I_off = (6.7 ± 3.4) × 10⁸. In addition, octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) treatment of the SiO₂ gate dielectric is found to be effective in enhancing the OTFT performance for all three BTT derivatives, by improving the interfacial semiconductor film morphology and in-plane crystallinity.