Magnetic Field Sensors Based on Magnetoresistance Effect in Organic Semiconductor Sandwich Devices
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Magnetic Field Sensors Based on Magnetoresistance Effect in Organic Semiconductor Sandwich Devices Govindarajan Veeraraghavan1, Ömer Mermer2, Yugang Sheng2, Tho Duc Nguyen2, Thomas Lee Francis1,3 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. 3 OMR Sensors Inc., Dubuque, Iowa-52002. ABSTRACT We describe magnetic field sensors based on a recently discovered magnetoresistance (MR) effect in nonmagnetic organic semiconductor sandwich devices. The MR effect reaches up to 10% in a magnetic field of 10 mT at room temperature. We perform an extensive experimental characterization of this effect. We found that the MR effect is only weakly temperature dependent and does not depend on sign and direction of the applied magnetic field. We also measured the device response to alternating magnetic fields up to 100 kHz. To the best of our knowledge, the discovered MR effect is not adequately described by any of the MR mechanisms known to date. INTRODUCTION Organic π-conjugated semiconductors (OSEC), which are divided into the classes of small molecular weight compounds and macromolecular polymers, are used to manufacture promising devices such as organic light-emitting diodes (OLEDs) [1], photovoltaic cells [2] and field-effect transistors [3]. 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 quasi-one 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. Magnetic field effects in organic semiconductors are currently an active research area. Kalinowski and coworkers [4], Davis and Bussmann [5], and Frankevich and coworkers [6] recently showed that the electroluminescence intensity can be modulated in OLEDs by application of magnetic field, B. In these works, the magnetic field effect was assumed to be due to spin dependent processes acting on excitons. While studying semiconducting polymer sandwich devices we recently discovered [7] a large and intriguing magnetoresistance (MR) effect, which we dubbed organic magnetoresistance (OMAR). Later we extended our results to small molecules [8] such as the prototypical Tris-(8-hydroxyquinoline) (Alq3) [9]. OMAR reaches up to 10% at room temperature (defined as ∆R / R ≡ (R (B)-R (0)) / R (0); R is the device resistance) for B =
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