Ulrich Niedermeier

Electron-hole pair mechanism for the magnetic field effect in organic light emitting diodes based on poly(paraphenylene vinylene)

We investigated the magnetic field effect (MFE) on current and electroluminescence in organic light emitting diodes based on poly(paraphenylene vinylene). The MFE was strictly positive in the full range of device operation and showed nonmonotonic dependencies on applied voltage and temperature. Furthermore, the MFE on current obtained in bipolar devices was significantly larger than in hole-dominated devices. We discuss our results in the framework of an electron-hole pair model and show that the model can explain all functional dependencies observed in our devices.

Origin of magnetic field effect enhancement by electrical stress in organic light emitting diodes

Recently, it has been discovered that the magnetic field effect (MFE) in organic light emitting diodes (OLEDs) based on poly(para-phenylene vinylene) can be enhanced by exposing the diode to moderate electrical stress. Here, we disclose the mechanism behind this way of improving the MFE. We first show that electronic traps in general play an important role for the MFE. Optical depletion of available trap states by infrared illumination leads to a decrease in the MFE. Furthermore, we demonstrate that annealing of the OLED at high temperatures eliminates the MFE improvement of the previously performed electrical conditioning. However, the improvement can be restored by subsequent conditioning at higher current or voltage. Thus it is likely that electrical stress is accompanied by a transformation of the polymer morphology or conformation resulting in a formation of energetic traps for charge carriers.

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.