Mechanisms for the enhancement of the thermal stability of organic thin films by aluminum oxide capping layers
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. Gerlacha) and F. Schreiberb) Physical and Theoretical Chemistry Laboratory, Oxford University, Oxford OX1 3QZ, United Kingdom
M. Kelsch Max-Planck-Institut für Metallforschung, 70569 Stuttgart, Germany
N. Kasper Max-Planck-Institut für Metallforschung, 70569 Stuttgart, Germany; and ANKA, FZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany
H. Dosch Max-Planck-Institut für Metallforschung, 70569 Stuttgart, Germany; and Institut für Theoretische und Angewandte Physik, Universität Stuttgart, 70550 Stuttgart, Germany
S. Meyer and J. Pflaum III. Physikalisches Institut, Universität Stuttgart, 70550 Stuttgart, Germany
M. Fischer and B. Gompf I. Physikalisches Institut, Universität Stuttgart, 70550 Stuttgart, Germany
G. Ulbricht Max-Planck-Institut für Festkörperforschung, 70569 Stuttgart, Germany (Received 4 July 2005; accepted 31 October 2005)
We present a detailed study of the thermal stability of organic thin films of diindenoperylene encapsulated by sputtered aluminum oxide layers. We studied the influence of capping layer thickness, stoichiometry, and heating rate on the thermal stability of capped films and their eventual breakdown. Under optimized encapsulation conditions (thick and stoichiometric capping layer), the organic films desorb only at temperatures 200 °C above the desorption of the uncapped film. Moreover, the capped organic films retain their crystalline order at these elevated temperatures, whereas they would normally (i.e., uncapped) be in the gas phase. This study therefore also shows a way of studying organic materials under temperature conditions normally inaccessible. Considering results from complementary techniques, we discuss possible scenarios for the eventual breakdown. The results have implications for the performance and long-term stability of organic devices for which stability against elevated temperatures as well as against exposure to ambient gases is crucial.
I. INTRODUCTION
In recent years, great progress has been made in the area of organic semiconductor devices.1–5 Such devices eventually have to meet certain stability requirements to
a)
Present address: Institut für Angewandte Physik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany. b) Address all correspondence to this author. Present address: Institut für Angewandte Physik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany. e-mail: [email protected] DOI: 10.1557/JMR.2006.0052 J. Mater. Res., Vol. 21, No. 2, Feb 2006
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find their way into commercial products. Exposure of the active organic films to ambient gases may cause gas penetration into the organic film and lead to oxidation or unintentional doping of the active transport layer.6–8 Stability against elevated temperatures is important to guarantee a good long-term performance of such devices.9–16 An obvious way of protecting the active layer against these degradation and failure mechanisms is encapsulation.17 Aluminum oxide is frequently used in devic
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