Electrical Characterization of Vacuum Deposited and Solution Processed DH4T Thin Film Transistors
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L10.3.1
Electrical Characterization of Vacuum Deposited and Solution Processed DH4T Thin Film Transistors Tobias Muck1, Michael Leufgen1, Amira Lebib1, Tanja Borzenko1, Jean Geurts1, Georg Schmidt1, Laurens W. Molenkamp1, Veit Wagner2, Henrique L. Gomes3 1 Experimentelle Physik III, Physikalisches Institut, Universität Würzburg 97074 Würzburg, Germany 2 School of Engineering and Science, International University Bremen 28759 Bremen, Germany 3 Universidade de Algarve, Campus de Gambelas, FCT, 8000 Faro, Portugal
ABSTRACT We present organic field-effect transistors with dihexylquaterthiophene (DH4T) as active material, a derivative of the oligothiophene α-4T with two hexyl chains as end groups. This substitution makes this molecule suitable not only for vacuum sublimation but also for solution processing which enables cheaper production. Additionally, the layer ordering is improved. We compare vacuum deposited and solution processed OFETs based on DH4T. The former ones show nearly ideal I-V characteristics. The latter ones show deviations from ideal behavior and lower currents. Furthermore, temperature dependent measurements of drain-source current in vacuum deposited DH4T-OFETs shows an unusual variation of the OFET apparent mobility. These apparent changes in mobility are caused by a threshold voltage shift, which becomes pronounced at around 290 K
INTRODUCTION Oligothiophenes are used for the production of organic field effect transistors due to their high charge mobility µ. In particular, sexithiophene (α-6T) [1] and quaterthiophene (α-4T) have found a large interest. Because of the decreasing solubility with increasing length the oligothiophenes α-4T and α-6T are most preferentially deposited by evaporation (OMBD: organic molecular beam deposition). The derivative dihexylquaterthiophene consists of α-4T molecules whose end groups are substituted by hexyl chains (figure 1). On the one hand, DH4T can be evaporated [2] and on the other hand this material is soluble in different solvents, e.g., toluene, hexane and chloroform. Furthermore, it shows a two orders of magnitude higher mobility than α-4T, essentially due to improved layer ordering [3]. A steplike mobility decrease was observed beyond 86°C [4]. DH4T offers the potential to produce casting-deposited OFETs, as were reported previously, e.g., for DE4T (diethylquaterthiophene) [5].
Figure 1: The molecule dihexylquaterthiophene
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In this paper we present a comparison of vacuum-deposited and solution-processed DH4T-based OFETs. Also, we investigated the temperature-dependent behavior of the source-drain current, mobility and threshold voltage of OMBD-deposited DH4T-OFETs. The results are discussed in terms of a bias induced stress of the organic material.
EXPERIMENTAL DETAILS The OFETs were laterally structured by optical lithography. The drain and source (20nm thick) gold electrodes with channel lengths in the range of 50 µm down to 1 µm were deposited with a titanium adhesion layer (10 nm) on top of a 200 nm SiO2 layer (gate insulator), thermally
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