Magnetic field effect in organic light emitting diodes.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2010)
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|Item Type:||Ph.D. Thesis|
|Title:||Magnetic field effect in organic light emitting diodes|
The discovery of a magnetic field dependent resistance change of organic light emitting diodes (OLEDs) in the year 2003 has attracted considerable scientific and industrial research interest. However, despite previous progress in the field of organic spin-electronics, the phenomenon of the "organic magnetoresistance (OMR) effect" is not yet completely understood. In order to improve the understanding of the microscopic mechanisms which ultimately cause the OMR effect, experimental investigations as well as theoretical considerations concerning the OMR are addressed in this thesis. In polymer-based OLED devices the functional dependencies of the OMR effect on relevant parameters like magnetic field, operating voltage, operating current and temperature are investigated. Based on these results, previously published models for potential OMR mechanisms are critically analyzed and evaluated. Finally, a concept for the OMR effect is favored which suggests magnetic field dependent changes of the spin state of electron-hole pairs as being responsible for changes in current flow and light emission in OLEDs. In the framework of this concept it is possible to explain all results from own measurements as well as results from literature. Another important finding made in this thesis is the fact that the value of the OMR signal in the investigated OLED devices can be enhanced by appropriate electrical and optical conditioning processes. In particular, electrical conditioning causes a significant enhancement of the OMR values, while at the same time it has a negative effect on charge carrier transport and optical device characteristics. These results can be explained by additional results from charge carrier extraction measurements which suggest that electrical conditioning leads to an increase in the number of electronic trap states inside the emission layer of the investigated OLED devices. The positive influence of trap states on the OMR effect is furthermore emphasized by measurements of the OMR effect under illumination with infrared light as well as by theoretical considerations in the scope of the electron-hole pair concept. Finally, it is demonstrated that the enhancement of the OMR values achieved in OLED devices after electrical or optical conditioning can largely be reversed by appropriate annealing. This suggests that the conditioning procedures do not cause a permanent degradation of the OLEDs. Hence, it is speculated that morphological modifications inside the emission layer of the devices might be a possible explanation for the observed consequences of device conditioning.
|Place of Publication:||Darmstadt|
|Uncontrolled Keywords:||Organic magnetoresistance, OMR, Magnetic field effect, OLED, PPV, Conditioning, Charge carrier traps|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 500 Naturwissenschaften
500 Naturwissenschaften und Mathematik > 530 Physik
600 Technik, Medizin, angewandte Wissenschaften > 620 Ingenieurwissenschaften
|Divisions:||11 Department of Materials and Earth Sciences > Material Science > Electronic Materials|
|Date Deposited:||19 Jan 2010 11:53|
|Last Modified:||07 Dec 2012 11:56|
|Referees:||von Seggern, Prof. Dr. Heinz and Alff, Prof. Dr. Lambert|
|Refereed:||14 December 2009|