Anomalous room temperature magnetoresistance in organic semiconductors

We report the substantial change in the large room temperature (similar to 8% at 100 Oe, up to 15% at 1000 Oe) magnetoresistance of thin organic semiconductor films of tris-(8-hydroxyquinoline) aluminum (Alq(3)) upon doping with PtOEP and Ir(ppy)(3) complexes. The origin of magnetic field effects on charge transport properties of organic semiconductors until now has remained obscure. We propose a model for the anomalous magnetoresistance and its change with doping based on the charge transport in these semiconductors being electron-hole (e-h) recombination limited. The process of e-h recombination includes formation of correlated e-h pairs and the subsequent annihilation of e-h pairs with different rates for the singlet and triplet spin states. The e-h pairs may also dissociate back into free charge carriers. We suggest that a magnetic field controls spin interconversion of e-h pairs. In the absence of field the singlet mixes with the entire triplet manifold by hyperfine interaction. The magnetic field lifts the triplet degeneracy, and for strong field, the mixing remains only between the singlet and the T-0 component of the triplet, thus changing the e-h recombination rate and hence the current. The experimental results are consistent with the model. (c) 2006 Elsevier B.V. All rights reserved.