An isotropic three-dimensional organic semiconductor 2-(thiopyran-4-ylidene)-1,3-benzodithiole (TP-BT): asymmetric molecular design to suppress access resistance

In this study, we have synthesised two novel p-type organic semiconductors in which chalcogen atoms are introduced along the long axis of molecules: 2-(pyran-4-ylidene)-1,3-benzodithiole (P-BT) and 2-(thiopyran-4-ylidene)-1,3-benzodithiole (TP-BT). P-BT forms a conventional two-dimensional (2D) herringbone arrangement, while TP-BT forms a similar herringbone arrangement, but also includes overlap integrals between the herringbone layers, realising isotropic three-dimensional (3D) intermolecular interactions. We have fabricated bottom-gate/top-contact organic thin-film transistors (OTFTs) using these materials as the active layers and have investigated the film thickness dependence of their mobility. The results demonstrate that the mobility of 2D P-BT decreases as the thickness of the active layer increases. In contrast, the mobility of the 3D TP-BT does not change as the film thickness increases, and the corresponding OTFTs have little film thickness dependence. Further results regarding the thickness dependence of the contact resistance estimated by the transfer line method indicate that TP-BT can significantly suppress access resistance due to its 3D electronic structure.