High magnetic field effect in organic light emitting diodes

Fluorescent and phosphorescent organic light emitting diodes (OLEDs) were measured in high magnetic field up to 9 T. The current efficiency steeply increased below 100 mT as reported but gradually decreased when the field was larger. In the range of 0.1–6.5 T, the decrease was proportional to the square of the magnetic field, whereas in the range between 6.5 and 9 T, it was proportional to the square root of the magnetic field. In contrast, phosphorescent OLED did not show magnetic field dependence. Unipolar devices of n-type Alq3 (tris-(8-hydroxyquinolino) aluminum) and p-type α-NPD (N′,N′-di(naphthalene-1-yl)-N,N′-dipheyl-benzidine) showed positive linear magnetoresistance only for minority carriers. The mechanism of the quadratic dependence of the electroluminescent intensity is discussed based on the experimental results.