The Impact of Capping on the Mobility and Thermal Stability of Organic Thin Film Transistors
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The Impact of Capping on the Mobility and Thermal Stability of Organic Thin Film Transistors Stephan Meyer1, Stefan Sellner2, Frank Schreiber3, Helmut Dosch4, Gerhard Ulbricht4, Matthias Fischer5, Bruno Gompf5, and Jens Pflaum1 1 3rd Physics Institute, University of Stuttgart, Stuttgart, 70550, Germany 2 Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA, 02138 3 Institute of Applied Physics, University of Tübingen, Tübingen, 72076, Germany 4 Max-Planck-Institute, Stuttgart, 70569, Germany 5 1st Physics Institute, University of Stuttgart, Stuttgart, 70550, Germany
ABSTRACT We performed temperature-dependent studies on pentacene thin film transistors (TFTs) with and without encapsulation. The capping layer is realized either by a sputtering layer of aluminum oxide (AlOx.) or, alternatively, by a polymeric layer of poly-para-xylylene (PPX). A field-effect can be demonstrated for both capping materials up to temperatures of about 140 170 °C, which is about 50 °C above the desorption point of uncapped pentacene thin films on SiO2 substrates. Complementary studies by thermal desorption spectroscopy and temperaturedependent x-ray diffraction show that the organic layer remains crystalline on the substrate far above the electrical breakdown temperature of the encapsulated device.
INTRODUCTION In the last years organic electronic devices have found their way into commercial applications. Thereby, devices based on organic materials have to meet certain stability criteria to find their way into commercial products. As many organic materials are sensitive to photooxidation, to the exposure to ambient gases and humidity and also to elevated temperatures, new challenges arise concerning the design and stability of organic devices [1, 2]. The exposure of organic semiconducting films to air might lead to doping or oxidation of the organic film, elevated temperatures to a degradation or even to the desorption of the active transport layer. Especially the stability at elevated temperatures under operating conditions is important to guarantee a good long term performance of the device [3, 4]. A possible concept to increase the device stability is the encapsulation of the electronic device, in particular the organic semiconducting layer [5-7]. We have shown recently for the organic semiconductor diindenoperylene (DIP) that by providing the molecular film grown on SiO2 with an aluminum oxide capping layer, the thermal stability can be strongly enhanced [6, 7]. The DIP remains crystalline on the surface even at temperatures about 200 °C above the desorption point of the uncapped organic layer. In addition, cross-sectional transmission electron microscopy (TEM) and x-ray diffraction have shown that the aluminum oxide layer forms a well defined heterostructure with a sharp interface on the molecular scale on top of the organic film. Hence aluminum oxide shows great promise for a significant enhancement of the stability of organic devices at elevated temperatures.
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