Study Of The Magnetic Field Effects In Organic Light Emitting Diodes

Organic magnetic field effects （MFEs） refers to the changes of theelectroluminescence （EL） and electrical current in organic semiconducting devicescaused by an applied external magnetic field. Although the investigation of MFEs hasbeen made great progress, its underlying mechanisms still deserve furtherdeliberation.In this thesis, the magnetic field effects on the carriers’ mobility have beeninvestigated for poly（3,4-ethylenedioxythiophene）-poly（styrenesulfonat）（PFO） basedlight emitting diodes by the transient electroluminescence （EL） method. The ELpulses can be divided into the following six regions: a) a delay region; b) a fast initialrising edge; c) a second slower rising edge; d) a quasisteady state; e) a fast fallingedge; f) a slow decay region. The Definitions are as: td:the delay time; t₁: the mainbody of the hole packet arrives at the cathode;t₂: electrons propagate into the device,resulting in a slower rise in the EL.t_d+t₁and t₂are thought to be the transit time ofhole and electron in PFO layer, respectively. The effect of the external magnetic fieldon the different regions can be obtained by measuring the EL pulses both with andwithout a magnetic field. The values of t_d+t₁and t₂have no change. It is found thatthe external magnetic field has no effect on both the electron and hole mobility inPFO based light-emitting diodes when the driving voltage is above the onset of theelectroluminescence. The EL decay curves for the device, are separated from eachother in the slow component with and without the external magnetic field. It indicatesthe external magnetic field actually affects the bimolecular recombination process. Soour result may support the electron-hole pair model （EHP）, which explain the organicMFEs without changing the carrier mobility.The other work in this paper is that study the MFEs of the fluorescent and thephosphorescence in a white organic light emitting devices through the method ofinserting a filter. The structure of the OLED includes a fluorescent layer of DPVBiand a phosphorescence layer of Ir（ppq）2（acac）. The electric luminance spectrum hastwo obvious peaks which are belonged to the red and the blue light. We ha ve measured the MEL response for different bias with and without the magnetic throughinserting a filter. We explain the MFEs in fluorescent through the E-H pair model, andthe MFEs in phosphorescence through the E-H pair model（EHP） and the triplet-tripletannihilation （TTA） model. Furthermore, we have proved the MEL and OMR offluorescence exist in the OLED simultaneously, and the MEL increases with thedegradation of the OLED. We get the conclusion that the HEP model is suitable toexplain the MFEs in fluorescent, and TTA is crucial to MFEs in phosphorescence.