In situ electrical characterization of DH4T field-effect transistors

Abstract We present an in situ electrical analysis of organic thin film transistors (OTFTs) during the deposition of the active layer of dihexylquaterthiophene (DH4T). At elevated temperatures (90 °C) DH4T films exhibit a smectic mesophase with extremely large 2D island formation. Upon cooling to room temperature they convert to extraordinary large crystalline domains ideal for transistor operation. The whole process is reflected in the in situ recorded transport properties. The data allow the determination of the transport layer thickness, which is given mainly by the first two monolayers (MLs). Furthermore, we find a threshold thickness of 1.5 nm for the onset of transistor operation. Thickness dependent mobility oscillations are reported for the first time, which correspond to the first two MLs. Upon cooling the transition from the smectic mesophase to crystalline phase increases the mobility by 45%. The morphology of the DH4T films shows 2D layer growth and close to the contacts an increased layer thickness.