AVS 50th International Symposium
    Organic Films and Devices Tuesday Sessions
       Session OF+EM-TuA

Paper OF+EM-TuA4
Large Magnetic Field Effects in Organic Light Emitting Diodes (OLED) based on Alq3

Tuesday, November 4, 2003, 3:00 pm, Room 318/319

Session: Molecular and Organic Films and Devices-Optoelectronics
Presenter: A.H. Davis, Naval Research Laboratory
Authors: A.H. Davis, Naval Research Laboratory
K. Bussmann, Naval Research Laboratory
Correspondent: Click to Email
A study of the spin statistics of exciton formation in organic semiconductors predicts that the amount of light produced by an organic light emitting diode (OLED) can be modulated by controlling the relative spin-polarization of the holes and electrons that combine to form excitons. Organic materials with intrinsically low spin-orbit coupling show long electron and hole spin lifetimes (tS > 1 ms), allowing for the possibility of spin-coherent transport from ferromagnetic electrodes to the recombination zone of a bilayer organic LED. It is important to distinguish this spin-dependent exciton formation and luminescence from other various magnetic field effects (MFE's) observed in the photoconductivity, photoluminescence, delayed luminescence and electroluminescence (EL) of certain organic crystals. We have grown a number of OLED's with magnetic and non-magnetic electrode materials based on a conventional Alq3/NPB organic bilayer and report two magnetic field effects that are present in all devices. The first is a low field effect (LFE) consisting of a one to seven percent increase in EL with an applied field that typically saturates below 1000 Oersteds (Oe). The second is a high field effect (HFE) consisting of an EL decrease by as much as 20+% at 1.8 Tesla (T). Unlike the LFE, the HFE is dependent on temperature, current density and electrode material. This suggests that the LFE and HFE are caused by separate mechanisms and that conditions near the electrode/organic interfaces are important to the HFE. We discuss these results in terms of several possible mechanisms, some of which should be sensitive to the spin-polarization of the injected holes and electrons.