The aim of the present work is to investigate the nature of spin dependent processes in an organic light emitting diode based on a ITO/ α-NPD/Alq3 structure. The electroluminescence time response of the sample is monitored while the OLED is exposed to a high power resonant microwave pulse. Measurements are carried out at room temperature. The time scale of the induced transition is found to be independent of the bias voltage. It is shown, by way of a simulation, that this behavior appears inconsistent with models which attribute a change in the electroluminescence to a variation in charge mobility. Spin dependent processes directly related to a change in the rate of charge recombination play therefore a relevant role in Alq 3 light emitting diodes.

Using pulsed electroluminescence detected magnetic resonance (PELDMR), we study the dynamics of spin processes in Alq 3 based light-emitting diodes. The transitions induced by magnetic resonance are found to be much faster than the space charge reaction time that is measured by looking at the electroluminescence frequency response to an ac bias voltage. This observation excludes a change in the equilibrium space charge distributions as the cause of PELDMR, in favor of a change of the electron-hole recombination rate. At low temperatures the effect of electron spin resonance on the electroluminescence changes sign and lasts longer. The postpulse electroluminescence recovery is well fitted by a biexponential function characterized by two very different time scales, which are consistent with a detailed balance for the singlet and triplet states, in conformity with the electron-hole pair model.

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