Organic field effect transistors based on biphenyl, fluorene end-capped fused bithiophene oligomers

Organic field effect transistors based on a structural combination of fused bithiophene with fluorene (BFTT) or biphenyl units (BPTT) are described. The two molecules have similar molecular structures, except that the fluorene unit of BFTT is substituted for a biphenyl in BPTT. A 1.5-2 times higher field effect mobility was achieved with BPTT compared to that of BFTT. To determine the reason for the superior mobility of BPTT compared to that of BFTT, single-crystal X-ray structures of BPTT and BFTT were determined and a clear correlation between the crystal structure and the electrical characteristics was found. The BFTT based on a fluorene unit adopts a slightly more distorted conformation that deviates from the quasi-planar structure in the single crystal with larger dihedral and bending angles than those of BPTT. On the other hand, BPTT shows a quite planar structure with relatively small dihedral and bending angles. BFTT also shows longer shortest intermolecular distances than BPTT. Those are the results of the carbon at the 9 position of fluorene units and attached hydrogen atoms on that position in BFTT, which is regarded as the only difference of the two molecules. These differences in molecular conformation and packing characteristics in the single-crystal consequently affect the transport properties of the electric carrier. In addition, device stability was checked after storage in air for more than 1 month or after exposure to strong UV irradiation for several hours. Both molecules showed a high device stability, but BPTT was slightly better. This better air and UV stability of biphenyl-based BPTT, Compared to fluorene-based BFTT, can be attributed to the lower highest occupied molecular orbital energy level of the BPTT and keto-defect formation in the fluorene unit of BFTT, after UV irradiation.