Influence of Mechanical Rubbing of Polyhexylthiophene Layers on the Field-Effect Mobility using Differently Treated Isol
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Influence of Mechanical Rubbing of Polyhexylthiophene Layers on the Field-Effect Mobility using Differently Treated Isolator Surfaces H. Heil, T. Finnberg, R. Schmechel and H. von Seggern Institute of Materials and Geo Science, Darmstadt University of Technology, D-64287 Darmstadt, Germany. ABSTRACT This paper reports on the influence of the field-effect mobility of transistors based on regioregular head-to-tail coupled poly (3-hexylthiophene) by mechanically induced alignment on differently treated insulator surfaces. It is demonstrated that on hydrophilic insulator surfaces mechanical rubbing of the polyhexylthiophene layers perpendicular to the source drain contacts can increase the field-effect mobility whereas rubbing parallel to the source drain contacts results in a reduced mobility. In contrast it is shown that in transistors with a hydrophobic insulator surfaces, which show much higher mobilities no further improvement can be achieved. The rubbing induced polymer alignment is deduced from optically polarized transmission spectroscopy on polymer-coated quartz glass substrates. The different behavior of the field-effect mobility will be explained in terms of different degrees of crystallinity. INTRODUCTION Low cost electronic devices fabricated from polymeric functional materials have a great potential to become a competitor to amorphous silicon thin film transistors (TFT). Advantages of organic field-effect transistors (OFETs) are the inexpensive fabrication combined with a good performance in terms of charge carrier mobility. High values of charge carrier mobilities comparable to those of amorphous silicon devices have been reported [1-4]. The reason for such high mobilities is the high molecular order and hence the global alignment of molecules or polymer chains in the charge transport channel. Highly regioregular head-to-tail coupled poly-(3-hexylthiophene) polymers (P3HT) show very high charge carrier mobilities up to 0.1cm2/(Vs) due to their super-molecular self-ordering properties [1,5]. With respect to silicon substrates it has been reported that a treatment of the SiO2 gate-oxide with a silylating agent such as hexamethyldisilazane (HMDS) is the key to high mobility P3HT transistors. In this case the π−π orbital systems arrange in a way that the stacking occurs normal to the insulator plane and in addition promotes the self-organization properties of P3HT [3]. Besides the approach of structural self ordering of molecules different techniques to align polymers, e.g., mechanical alignment in the form of rubbing, have been applied [6,7]. The aim of this paper is to show the influence of aligned P3HT films on the transport properties with differently treated insulator surfaces of a bottom gate organic field-effect transistor. Therefore the alignment and ordering properties of differently prepared transistor substrates and P3HT films are investigated utilizing polarized optical absorbance measurements. These results will be correlated to the field-effect mobilities of equally treated P3HT-OFETs. E
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