Trap effect of triplet excitons on magnetoresistance in organic devices

In an organic bipolar device, injected electrons and holes can form spin singlet and triplet excitons, which are manipulated by an applied magnetic field. We suppose that the localized intra-molecule triplet exciton has a blocking effect on charge carrier transport by assuming that the intra-molecule triplet exciton can increase the on-site binding and make the electron states more localized. By considering the magnetic field-dependent transition between singlet and triplet excitons, from the master equation based on the hopping mechanism, we calculate the magnetoresistance (MR) in organic devices and compare the results with some experimental data. Our research reveals the importance of hyperfine interaction in organic magnetoresistance (OMAR). Especially, our investigation indicates that a bipolar organic device should have a larger MR value than a unipolar one due to the trap effect of triplet excitons on hopping electrons or holes, which is confirmed by some experimental observations. (C) 2015 Elsevier B.V. All rights reserved.