Berlin 2005 – scientific programme

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HL: Halbleiterphysik

HL 61: Organische Halbleiter

HL 61.4: Talk

Wednesday, March 9, 2005, 11:30–11:45, TU P270

Transport properties of ultra thin thiophene based thin film transistors — •T. Muck and V. Wagner — School of Engineering and Science, International University Bremen, Campus Ring 8, D-28759 Bremen, Germany

The aim of this study was to determine the properties of thin organic layers within a field-effect transistor geometry. Transport properties of standard OFETs are known to be crucially influenced by the first monolayer (ML). Deposition of the active material was performed by organic molecular beam deposition in ultra high vacuum. As organic semiconductor various thiophene derivatives, e.g. dihexylquaterthiophene (DH4T) were analyzed. These DH4T films were prepared at elevated temperatures (90 ^∘C) to achieve a growth in the smectic phase of DH4T. Electrical measurements were performed on these growing liquid crystal films. By characterizing the organic transistors at different film thicknesses in the monolayer range we observe a steplike behavior of the charge mobility. Charge transport starts at approx. 0.6 ML and the field-effect mobility shows a quadratic increase for increasing coverage due to percolation of DH4T monolayers. Saturation is observed while completing the first ML. We interpret this behavior with the growth mode of DH4T analyzed by AFM imaging and compare this with Monte-Carlo simulations. The deposition of the second monolayer influences the mobility first by worsening due to non-percolated islands and a subsequent improvement while closing the second monolayer. Additional monolayers do not increase the performance. These data allow the determination of the transport layer thickness, which is given mainly by the first two monolayers.