Steric effect of halogen substitution in an unsymmetrical benzothienobenzothiophene organic semiconductor

We report a crystal structure analysis of an unsymmetrical organic semiconductor 2-bromo-7-butyl[1]benzothieno [3,2-b][1]benzothiophene (Br-BTBT-C-4) and its transistor characteristics. Unlike the parent compound 2-butyl-[1]benzothieno [3,2-b][1]benzothiophene (BTBT-C-4) forming a bilayer-type structure composed of headto-head herringbone packings, Br-BTBT-C-4 forms a dense head-to-tail herringbone packing arranged so as to avoid the steric hindrance between bromine atoms. The intermolecular transfer integrals in Br-BTBT-C-4 suggest its advantage for carrier transport; however, the transistor characteristics of Br-BTBT-C-4 and BTBT-C-4 were found to be almost the same. This is because BTBT-C-4 has a higher crystallinity than Br-BTBT-C-4 in vacuum deposited thin films.