Observation of Large Magnetic Field Effects in Organic Light Emitting Diodes

摘要

Large magnetic field effects (MFE) have been observed in organic light emitting diodes (OLED) based on a bilayer of tris (8-hydroxyquinoline) aluminum (Alq_3) and N,N' -Di(naphthalen-1-yl)-N,N'diphenyl-benzidine (NPB). They consist of an increase in electroluminescence (EL) of a few percent at low magnetic fields followed by a decrease in EL of 20+% at high fields. Associated with these two effects is a decrease in resistance of typically 1-3% as the magnetic field is increased. The magnitude of the high field effect (HFE) varies with temperature and current density, while the low field effect (LFE) survives even when the HFE is not present. The HFE is enhanced at low temperature and/or high current density. These effects are similar to those reported for anthracene single crystals suggesting a large triplet-triplet annihilation (TTA) component for the EL in Alq_3. However, transient EL studies fail to definitively identify a delayed luminescence component with a time scale appropriate for TTA in Alq_3. We discuss this and other questions concerning the origin of MFE in this system.