Magnetic field effects on the conductivity of organic bipolar and unipolar devices at room temperature

Magnetic field effects on the conductivity of different types of organic devices: undoped and dye doped aluminium (III) 8-hydroxyquinoline (Alq3)-based organic light emitting diodes (OLEDs), electrononly Alq3-based diodes, and a hole-only N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (a-NPD)-based diode were studied at room temperature. Only negative magnetoresistance (MR) was observed for the Alq3-based devices. The addition of a rubrene dye in Alq3-based OLEDs quenches the MR by a factor of 5. The a-NPD hole-only device showed only positive MR. Our results are discussed with respect to the actual models for MR in organic semiconductors. Our results are in good agreement with the bipolaron model.