Overshoot effect in transient electroluminescence from single layer organic light-emitting diodes

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Abstract
We investigate the transient electroluminescence (EL) from organic light-emitting diodes based on tristilbeneamine dispersed in polysulfone. An EL overshoot is observed a few microsecond after reversing the driving voltage. The amplitude of the EL spike can considerably exceed the previous EL level. The maximal EL intensity is strongly influenced by temperature, time duration of voltage pulse, and magnitude of reverse voltage. The time integrated overshoot intensity of EL scales as a power law with the pulse length. We attribute the overshoot effect to (i) the accumulation of holes on the interface formed by a polymer and a thin insulating layer of aluminum oxide next to the aluminum cathode under application of positive bias and (ii) the drift of holes back to the anode when the external field is being reversed and recombination with electrons in the bulk of the material. It turns out that the surface roughness of the Al2O3 layer is important for the recombination kinetics under reverse bias.