Space-charge-limited magnetoresistance in organic semiconductors

We present a two-site model for large magnetic field effect on current of nonmagnetic organic semiconductor, based on bipolaron formation in the presence of both hyperfine field and an applied magnetic field. Under the framework of space-charge-limited current, we derived our current density and magnetoresistance expression. The magnetoresistance fit well with the Lorentzian empirical law. We discuss all parameters involved in, and elucidate the origin of positive and negative magnetoresistance under our model. -- Highlights: ► The MR model is based on formation of bipolaron in the presence of hyperfine field. ► Carrier hopping occurs via a limited number of percolation paths. ► A magnetic field lifts probability of parallel spin thus obstructs polaron hopping. ► The MR fit well with the Lorentzian empirical law. ► The sign of magnetoresistance depends on mobility of bipolarons to polarons.