Influence of a magnetic field on the device performance of OLEDs

Magnetic field effects in organic light emitting diodes have emerged as subject of intense research activities. We investigated the recently discovered organic magnetoresistance effect, i. e. the phenomenon that the presence of an external magnetic field can influence both the current flow through an organic light emitting diode and the light emission from the device. Magnetoresistance measurements were performed in different device structures as a function of magnetic field and driving voltage. We demonstrate that electrical conditioning can be used as an efficient method to enhance the organic magnetoresistance effect in devices based on polymers and small molecules. Depending on duration and intensity of the conditioning process the magnetoresistance effect can be increased from ~1% to values exceeding 15% at 40mT in devices with poly(paraphenylene-vinylene) as light emitting polymer. Qualitatively the increase in magnetoresistance can be correlated with a decrease in luminance during the conditioning process. From this we conclude that degradation of the bulk emitter material is responsible for the enhancement of organic magnetoresistance. In addition, we show a dependence of the magnetoresistance effect on the charge carrier balance within the device. In bipolar devices the magnetoresistance effect is significantly larger than in hole-dominated devices which suggests that electron-hole pairs play an important role in the fundamental mechanism causing the organic magnetoresistance effect.

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