Experimental Tests of Possible Mechanisms for the Organic Magnetoresistive Effect
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1032-I10-20
Experimental Tests of Possible Mechanisms for the Organic Magnetoresistive Effect Tho Duc Nguyen, James Rybicki, Yugang Sheng, and Markus Wohlgenannt Physics and Astronomy, The University of Iowa, 188 IATL, Iowa city, IA, 52242 ABSTRACT We experimentally test three existing models of organic magnetoresistance (OMAR) which are all based on carrier spin dynamics. We first prove that hyperfine field originating from the hydrogen nuclei in organic materials is necessary for observing OMAR by studying C60 sandwich devices using several different electrode materials. We show that C60, unlike many other organic semiconductors, does not exhibit any intrinsic OMAR effect. However, we find that as soon as the carriers in C60 are brought in proximity with hydrogen-containing compounds, either in the form of a polymeric electrode, or side-chain substituents, a weak OMAR effect is observed. Next, we perform charge-induced absorption and electroluminescence spectroscopy in a polyfluorene organic magnetoresistive device. Our experiments allow us to measure the singlet exciton, triplet exciton and polaron densities in a live device under an applied magnetic field, and to distinguish between three models of OMAR. These models are based on different spindependent interactions, namely exciton formation, triplet exciton-polaron quenching and bipolaron formation. We show that the singlet exciton, triplet exciton and polaron densities and conductivity all increase with increasing magnetic field. Our data are inconsistent with the exciton formation and triplet-exciton polaron quenching models. INTRODUCTION Recent years have seen a surge in interest in spin transport in organic semiconductor devices [1], including the study of the organic magnetoresistive effect (OMAR). OMAR is a recently discovered large, low field, room temperature magnetoresistive effect in non-magnetic organic light-emitting diodes (OLEDs) [2-5]. The effect can be as large as 10% relative change in resistance for a magnetic field B = 10 mT. To the best of our knowledge, the mechanism causing OMAR is not yet established with certainty. Three kinds of models have been put forward to explain OMAR. (i) Electron-hole pair mechanism (EHP) models based on MFE in radical pairs [4, 6-12]. In this model the spin dependent reaction P+ + Pexciton between oppositely charged polarons to form an exciton ("recombination") is of central importance. (ii) The triplet-exciton polaron quenching model [13] (TPQ) that is based on the spin-dependent reaction TE + P P + GS* between a triplet exciton (TE) and a polaron to give an excited singlet ground state (GS*). (iii) The bipolaron mechanism (BP) [14, 15] which treats the spin-dependent formation of doubly occupied sites (bipolarons) P+ + P+ BP2+ (and an analogous reaction for negative carriers) during the hopping transport through the organic film. We note that the BP model does not assume the formation of stable bipolarons, but is merely based on the occurrence of doubly occupied hopping sites whose energy may be higher than t
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