We examine the influence of film thickness on the optical and ambipolar field-effect transistor properties of solution-processed dicyanomethylene- substituted quinoidal oligothiophene [QQT(CN)4] thin films. Threshold voltages for both p- and n-channels show a linear thickness dependence due to an increase in the bulk conductance in thicker films. Electron mobility is found to increase gradually when decreasing film thickness. In contrast, hole mobility remains nearly unchanged except in films thinner than 50 nm. Film morphology is characterized by atomic force microscopy and X-ray diffraction techniques. Direct correlation between crystalline grain size and thickness dependence of the electron field-effect mobility is observed. This result can be attributed to a strong effect of the grain boundaries on the electron trapping properties and suggests the possibility to improve the charge transport properties of QQT(CN)4 thin films by controlling their morphology. The influence of contact resistance effects on the electron transport properties of the devices is also discussed. Devices with optimized structure show hole and electron mobilities in the saturation regime as high as 0.08 and 0.015 cm2/(V s), respectively. Overall, this study provides new important insight into the ambipolar charge transport properties of quinoidal oligothiophene derivatives for organic field-effect transistors.
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