A study on the luminescence properties of high-performance benzothieno[3,2-b][1]benzothiophene based organic semiconductors

In terms of molecular structures, benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives qualify as the best-performing organic semiconductors for hole transport, but it exhibits poor luminescence properties. For instance, a classical molecule of 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) only shows photoluminescence quantum yields (PLQYs) of 5.0% in chlorobenzene. Herein, we present three kinds of organic semiconductors based on phenyl-BTBT molecule core, namely, 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT), 2,7-bis(4-ethylphenyl) benzothieno[3,2-b][1] benzothiophene (DBEP-BTBT), and 2,7-bis(4-methoxyphenyl)benzothieno[3,2-b][1]benzothiophene (DBOP-BTBT). Notably, blue fluorescence with relatively high PLQYs of 27%, 36%, and 48% in chlorobenzene was shown. Due to the surface defects and grain boundaries in the thin films, the PLQYs decrease to 3.4% for DPh-BTBT and 7.4% for DBOP-BTBT, respectively. Nevertheless, the DBEP-BTBT film still exhibits an absolute PLQY as high as 12%, which is a rare and precious characteristic in BTBT derivatives. Meanwhile, the differences in photophysical characteristics, crystal structures, and charge transport properties of the three BTBT derivatives were systematically studied. Similar to DPh-BTBT and DBOP-BTBT, the DBEP-BTBT crystals display typical layered herringbone packing structure with multi- and strong intermolecular interactions, which are conductive to form a two-dimensional charge transport network in layered thin films for high mobility. The mobility of DBEP-BTBT in polycrystalline thin film transistors is up to 3.22 cm2V−1s−1 in ambient air, which is close to DPh-BTBT (3.64 cm2V−1s−1) and much higher than that of DBOP-BTBT (0.5 cm2V−1s−1). Moreover, the calculations show that the intermolecular transfer integrals of HOMOs are almost isotropic for six nearest neighbour contacts in DBEP-BTBT and DPh-BTBT crystals, implying their possibility of isotropic hole transport property in single crystals.