Enhanced Mobility of Organic Field-Effect Transistors with Epitaxially Grown C 60 Film by in-situ Heat Treatment of the
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Enhanced Mobility of Organic Field-Effect Transistors with Epitaxially Grown C60 Film by in-situ Heat Treatment of the Organic Dielectric Th. B. Singh,1,§ N. Marjanović,1 G. J. Matt,1 S. Günes,1 N. S. Sariciftci,1 A. Montaigne Ramil,2 A. Andreev,2,† H. Sitter,2 R. Schwödiauer3 and S. Bauer3 1
Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University Linz, Austria 2 Institute of Semiconductor- and Solid State Physics, Johannes Kepler University Linz, Austria 3 Soft Matter Physics, Johannes Kepler University Linz, Austria § Corresponding author. e-mail: [email protected] ABSTRACT Electron mobilities were studied as a function of thin-film growth conditions in hot wall epitaxially grown C60 based field-effect transistors. Mobilities in the range of ~ 0.5 to 6 cm2/Vs are obtained depending on the thin-film morphology arising from the initial growth conditions. Moreover, the field-effect transistor current is determined by the morphology of the film at the interface with the dielectric, while the upper layers are less relevant to the transport. At high electric fields, a non-linear transport has been observed. This effect is assigned to be either because of the dominance of the contact resistance over the channel resistance or because of the gradual move of the Fermi level towards the band edge as more and more empty traps are filled due to charge injection. INTRODUCTION Research in Organic Field-Effect Transistors(OFETs) resulted in very interesting devices such as displays [1-3], all organic integrated circuits [4], gas sensors [5], identification tags [6-8], light sensors [9] and memory elements [10]. The measured charge carrier mobilities are 15 cm2/Vs for rubrene single crystal [11] and in excess of 1-2 cm2/Vs in the case of vacuum evaporated pentacene thin-films [12]. On the other hand, effort to increase charge carrier mobility in OFETs with n-type organic materials have been difficult due to several physical reasons: rapid degradation under ambient condition and electron transport properties are sensitive to purity of the crystal [13]. Among the n-type organic semiconductors, to mention a few, C60 [14], methanofullerenes [15], fluorinated phthalocyanines [16], naphthalenes and Nsubstituted naphthalene 1,4,5,8-tetracarboxylic diimide [17] or N,N´dialkil-3,4,9,10-perylene tetracarboxylic diimide (PTCDI, PTCDI-C5 [18] and PTCDI-C8H, correspondingly) show highest obtained mobilities of up to 0.6 cm2V-1s-1[19]. Most of these devices are grown on untreated inorganic SiO2 or Al2O3 dielectrics. Because of the very different physical nature of the two media, the deposition may result in highly disordered films, leading to a poor performance [20]. Although the van der Waals type interactions between organic molecules and inorganic substrates are rather weak, the crystallographic phases, the orientation, and the morphology of the resulting organic semiconductor films critically depends on the interface and growth kinetics. In field- effect devices, the first few monolayers cl
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