Thin-film and single-crystal transistors based on a trifluoromethyl-substituted alternating (thiophene/phenylene)-co-oligomer

We demonstrated the performance of thin-film transistors (TFTs) and single-crystal field-effect transistors (FETs) based on a trifluoromethyl-substituted alternating (thiophene/phenylene)-co-oligomer (AC5-CF3), 1,4-bis(5′-(4″-trifluoromethylphenyl)thiophene-2′-yl)benzene. An FET with a fine AC5-CF3 single-crystal demonstrated field-effect mobility as high as 3.1 cm2 V−1 s−1. This value implies that AC5-CF3 must be a useful n-type organic semiconducting material. The performance of AC5-CF3 TFTs depended on the substrate temperatures at which AC5-CF3 thin films were deposited. From the viewpoint of mobility, threshold voltage and sub-threshold slope, we obtained the highest performance at the substrate temperature of 100 °C. This was because a higher substrate temperature for deposition enlarged the size of grains in AC-CF3 thin films and improved the characteristics of grain boundaries. However, 120 °C depositions of AC5-CF3 induced deep valley-like cracks in the thin films, probably because of the difference between the coefficients of thermal expansion for AC5-CF3 thin films and silicon wafer substrates, resulting in effects such as worsening mobility. AC5-CF3 TFTs prepared at 100 °C deposition showed no channel length dependence of the field-effect mobility, and their average field-effect mobility was 0.55 ± 0.05 cm2 V−1 s−1.