Magnetoresistance and Magnetic Field Effect on Electroluminescence in Polyfluorene OLEDs
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0937-M10-04
Magnetoresistance and Magnetic Field Effect on Electroluminescence in Polyfluorene OLEDs Govindarajan Veeraraghavan1, Tho Duc Nguyen2, Yugang Sheng2, Omer Mermer2, and Markus Wohlgenannt2 1 Department of Electrical and Computer Engineering, Optical Science and Technology Center, University of Iowa, 188 IATL, University of Iowa, Iowa City, IA, 52242 2 Department of Physics and Astronomy, Optical Science and Technology Center, University of Iowa, 188 IATL, University of Iowa, Iowa City, IA, 52242 ABSTRACT We report on the experimental observation of large magnetoresistance and magnet field effect (MFE) on electroluminescence in polyfluorene organic light emitting diodes (OLEDs). To the best of our knowledge, the mechanism causing these effects is currently not known. Moreover, we show that these experiments do not allow determination whether the magnetic field acts on the carrier density or carrier mobility making any attempt of explaining it ambiguous. As a remedy, we performed magnetoresistance measurements in hole-only devices and show that the MFE acts on the carrier mobility rather than carrier density. INTRODUCTION Organic π-conjugated semiconductors, 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. There has been growing interest in spin and magnetic field effects (MFE) [4-8] in these materials. MFE 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-(8hydroxyquinoline) (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 = 10 mT. OMAR [7-10] is therefore amongst the largest MR effects of any bulk material. The devices we describe can be manufactured cheaply on flexible substrates and may also be transparent. Our devices therefore hold promise for applications where large numbers of MR devices are needed, such as magnetic random access memory (MRAM); and applications related to OLED display screens such as touch screens where the position of a magnetic stylus is detected (patent pending, see demonstration video at http://ostc.physics.uiow
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