Tuning the Isomeric Fused Heteroaromatic Core of Small Donor–Acceptor Molecules to Alter Their Crystalline Nature and Enhance Photovoltaic Performance

Novel donor–acceptor-type molecules, endo-DBST(BT-Cz)2 and exo-DBST(BT-Cz)2, were designed with benzothiadiazole-carbazole (BT-Cz) based arms and isomeric cores of dibenzosexithiophene (DBST), in which sulfur atoms at two lateral thienos in DBST are face-to-face for the endo-derivative and back-to-back for exo-derivative. In contrast to the endo-type, the exo-form exhibited highly crystalline properties, leading to an increased hole mobility up to 1.1 × 10–4 cm V–1s–1. A significant effect of the isomeric DBST cores on the photovoltaic properties was also observed in the organic photovoltaic (OPV) cells. The maximum power conversion efficiency of 2.13 % with a fill factor up to 61.41 % was obtained for the exo-DBST(BT-Cz)2-based devices, which are 1.2 times higher than that in the endo-DBST(BT-Cz)2-based cells. Our results demonstrate that tuning the isomeric core would increase the carrier mobility significantly and consequently enhance the photovoltaic performance of the resulting molecules in OPV cells.