Heteroacenes that contain chalcogenophene have been actively investigated as semiconducting materials because they are more stable in air than acenes and various chemical modifications of the compounds are possible. While there have been many attempts to combine benzene with thiophene rings to produce higher-performance materials, only a few derivatives are known to show high mobilities above 1 cm² /Vs. Also OFETs of high performance have been often achieved by using expanded heteroaromatic of π-conjugated including thiophene oligomers as the semiconducting materials.

In this work, we synthesize new type of fused chalcogenophene based transistor materials, 54DBTTT and 8-67DBTTT (Fig. 1), and investigate the characterization of organic single-crystal transistors based on the newly developed materials.

We made organic single crystals of 54DBTTT and 8-67DBTTT by physical vapor transport (PVT) and drop-cast method. We used an n⁺-doped Si wafer with 220 nm thickness thermally grown SiO₂, which was covered with a thin film (ca. 27.8 nm) of CYTOP insulator or a thin film (ca. 30 nm) of poly(methyl methacrylate) (PMMA) insulator. Source and drain electrodes were painted with a colloidal graphite paste. The field-effect mobility (μ) of the OFET devices were measured under vacuum from the saturation regime according to the equation: I_d=μC_i(W/2L)(V_g-V_th)², where C_i is capacitance of the insulator, V_d and V_th are the gate and threshold voltages, respectively.

To investigate the molecular conformation and stacking of the molecules in the crystal states, single-crystal X-ray analysis was performed. In contrast to the herringbone structure 54DBTTT, 8-67DBTTT molecules exhibit a two-dimensional brick-wall packing arrangement (Fig. 2).

Organic single-crystal transistors based on DBTTT derivatives showed typical p-channel FET characteristic. The best field-effect mobility of 0.3 cm² /Vs was obtained for 54DBTTT based single-crystal device with a CYTOP covered Si/SiO₂ substrate. On the other hand, the mobility for the 8-67DBTTT based transistor was 8.1x10^-3 cm² /Vs. As shown above, molecular packing structure of the 8-67DBTTT is brick-wall packing. Acoording to the Fig. 2(d) alkyl chain placed between the DBTTT cores, which is not preferable to the high performance OFET device.