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I1.8.1

Large Magnetoresistance at Room Temperature in Organic Semiconductor Sandwich Devices Govindarajan Veeraraghavan1, Omer Mermer2, Thomas Lee Francis1, Yugang Sheng2, Tho Duc Nguyen2 and Markus Wohlgenannt2 1 Department of Electrical & Computer Engineering, University of Iowa, Iowa City, Iowa-52242. 2 Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa-52242. ABSTRACT We describe a recently discovered magnetoresistance (MR) effect in semiconducting polymer and small molecule sandwich devices. The MR effect reaches up to 10% in a magnetic field of 10mT at room temperature. This MR effect is therefore amongst the largest of any bulk material. We characterize this effect and discuss its dependence on voltage, film thickness, temperature, electrode materials and (unintentional) impurity concentration in three different organic semiconductors. We found that the MR effect is only weakly temperature dependent and does not depend on sign and direction of the applied magnetic field. To the best of our knowledge, the discovered MR effect is not adequately described by any of the mechanisms known to date. INTRODUCTION Organic π-conjugated semiconductors (OSEC), which are usually divided into the classes of small molecular weight compounds and macromolecular polymers, have been used to manufacture promising devices such as organic light-emitting diodes (OLEDs) [1], [2], photovoltaic cells [3] and field-effect transistors [4]. Increased research interest in the electronic and optical properties of conjugated polymers began after the successful synthesis of doped semiconducting polymers and the discovery of electroluminescence (EL) in phenyl-based polymers (e.g poly (p-phenylene vinylene) (PPV)). The intrinsic processing advantages of conjugated polymers, such as solution processing, make them attractive for potential large-scale ink jet printing of display screens and other electronic circuits. Π-conjugated polymers exhibit electronic properties that are quite different from those observed in inorganic metals or semiconductors. These unusual electronic properties may essentially be attributed to their quasione dimensionality owing to their strong intra-molecular but relatively weak inter-molecular interactions. This results in weakly screened electron-electron interactions. In addition, the charge carriers in these materials are positive and negative polarons, rather than holes and electrons. OSEC used in OLEDs are undoped and the charge carriers are injected into the device from the metal electrodes. In addition to π-conjugated polymers, small molecular weight, organic compounds have also been extensively investigated. EL from OLEDs made from small molecules was first observed and extensively studied in the 1960s [5]. Intense research in both academia and industry has yielded OLEDs with remarkable color fidelity, device efficiency and operational stability. Davis and Bussmann [6] recently showed that the EL intensity could be modulated in OLEDs based on Alq3 sandwich devices by application of a magnet