Low temperature magnetic field effects on the efficiency of aluminium tris(8-hydroxyquinoline) based organic light emitting diodes in the absence of magnetoresistance

This paper presents low temperature results of the magnetic field effect on current and efficiency in Alq3 based light emitting diodes (OLEDs). We report a magnetic field effect on light output at driving voltages where no magnetic effect is measured on the current. This is attributed to the presence of significant trapping, where trapped carriers are unable to de-trap at low temperature. At high current densities and corresponding bias, magnetoresistance is observed as a result of trap filling or field assisted tunnelling. A negative magnetic field effect (MFE) on efficiency at low temperatures and a deviation from the linear relationship between organic magnetoresistance (OMR) and singlet population are also observed. The latter are attributed to significant triplet–triplet annihilation (TTA) occurring within the device.