Large magnetic field effects in organic light emitting diodes based on tris(8-hydroxyquinoline aluminum) (Alq3)/N,N′-Di(naphthalen-1-yl)-N,N′diphenyl-benzidine (NPB) bilayers

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Abstract
Magnetic field effects (MFEs) in the luminescence and photoconductivity of organic crystals such as anthracene have been known since the 1960s. While engaged in a project to use spin polarized current to manipulate electroluminescence (EL) in organic light emitting diodes based on a bilayer of tris(8-hydroxyquinoline) aluminum (Alq3) and N,N′-Di(naphthalen-1-yl)-N,N′diphenyl-benzidine (NPB), we observed MFE similar to those reported in anthracene. The MFE consist of an increase in EL with increasing magnetic field of a few percent for small magnetic fields, a decrease in EL of greater than 20% at high fields and an increase in conductivity, i.e., negative magnetoresistance (MR), for all magnetic fields. The high field effect (HFE) is enhanced at lower temperatures and higher current densities and is similar to that reported for delayed luminescence in anthracene single crystals suggesting that triplet–triplet annihilation (TTA) influences light emission in Alq3. Transient EL studies were performed in an attempt to identify a delayed component for the MFE with a time scale appropriate for triplets in Alq3, but the MFE develops at least ten times faster than that expected using a recently published triplet lifetime for Alq3. However, the temperature and drive dependence of the HFE suggests that TTA involving triplet entities with shorter lifetimes might be responsible for the HFE. In contrast, the low field effect (LFE) is insensitive to temperature and drive conditions and exists even when the HFE does not, indicating that the LFE is caused by a separate mechanism.