AVS 51st International Symposium
    Organic Films and Devices Thursday Sessions
       Session OF+EM-ThA

Paper OF+EM-ThA4
Magnetic Field Effects in Transient Electroluminescence (EL) from Alq@sub 3@/NPB Bi-layer Organic Light Emitting Diodes

Thursday, November 18, 2004, 3:00 pm, Room 304C

Session: Molecular and Organic Films and Devices - Optoelectronic
Presenter: J. Wilkinson, Naval Research Laboratory
Authors: J. Wilkinson, Naval Research Laboratory
A.H. Davis, Naval Research Laboratory
K. Bussmann, Naval Research Laboratory
J.P. Long, Naval Research Laboratory
Correspondent: Click to Email
The long electron spin lifetimes commonly found in organic materials make organic light emitting diodes (OLEDs) potential candidates for spin-injection controlled light sources. However, direct-current measurements of electroluminescence (EL) in OLEDs with non-magnetic electrodes (i.e. with no spin injection) show a variation in EL with applied magnetic field that must be understood. For example, EL can increase by as much as 6% around 0.1 Tesla (T) before decreasing by up to 20%, at 2 T.@footnote 1@ This shows that magnetic field effects are not due to spin injection, but are an intrinsic property of the light emission process. To probe the causes of these processes, we have performed sensitive transient EL experiments in the low-field regime (H = 80 mT) where the magnetic field enhances EL. The OLED is driven with rectangular voltage pulses producing a temporal response in EL with features down to the system resolution of 70 ns. The time-dependent magnetic field effect, defined as @DELTA@EL/EL = [EL(H) - EL(0)]/EL(0) increases EL by 6% for a 3.6 V drive at room temperature, as in direct-current measurements. But in addition @DELTA@EL/EL has interesting transient behavior when the device is first turned on, and again after the drive pulse is turned off during a long-lived delayed EL signal. As the device turns on, @DELTA@EL/EL doubles relative to its steady state value, which it attains on the microsecond timescale. Immediately after device turn-off, a long-lived increase in @DELTA@EL/EL is detected as well. The measured tendency for the magnetic field enhancement of EL to decrease as the transient EL increases is consistent with drive-dependent measurements. Together, these experiments indicate that high concentrations of non-equilibrium carriers or excitons interfere with the magnetic enhancement process. @FootnoteText@ @footnote 1@ A.H. Davis and K. Bussmann, Organic Light-Emitting Materials and Devices VII, eds. Z.H. Kafafi and P.A. Lane, 5214, 57-63 (2004).