Functionalized benzothieno[3,2 b]thiophenes (BTTs) for high performance organic thin-film transistors (OTFTs)†
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 Ion/Ioff = (3.3 ± 1.6) × 108 for Bp-BTT, and µ = 0.12 cm2 V−1 s−1 (max) and Ion/Ioff = (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 Ion/Ioff = (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.