High Electron Mobility in [1]Benzothieno[3,2‑b][1]benzothiophene-Based Field-Effect Transistors: Toward n‑Type BTBTs

The first example of an n-type [1]­benzothieno­[3,2-b]­[1]­benzothiophene (BTBT)-based semiconductor, D­(PhFCO)-BTBT, has been realized via a two-step transition-metal-free process without using chromatographic purification. Physicochemical and optoelectronic characterizations of the new semiconductor were performed in detail, and the crystal structure was accessed. The new molecule exhibits a large optical band gap (∼2.9 eV) and highly stabilized (ΔELUMO = 1.54 eV)/π-delocalized lowest unoccupied molecular orbital (LUMO) mainly comprising the BTBT π-core and in-plane carbonyl units. The effect of out-of-plane twisted (64°) pentafluorophenyl groups on LUMO stabilization is found to be minimal. Polycrystalline D­(PhFCO)-BTBT thin films prepared by physical vapor deposition exhibited large grains (∼2–5 μm sizes) and “layer-by-layer” stacked edge-on oriented molecules with an in-plane herringbone packing (intermolecular distances ∼3.25–3.46 Å) to favor two-dimensional (2D) source-to-drain (S → D) charge transport. The corresponding TC/BG-OFET devices demonstrated high electron mobilities of up to ∼0.6 cm2/V·s and Ion/Ioff ratios over 107−108. These results demonstrate that the large band gap BTBT π-core is a promising candidate for high-mobility n-type organic semiconductors and, combination of very large intrinsic charge transport capabilities and optical transparency, may open a new perspective for next-generation unconventional (opto)­electronics.