Berlin 2008 – wissenschaftliches Programm
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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 6: Electronic+Optical Properties
CPP 6.3: Vortrag
Montag, 25. Februar 2008, 14:30–14:45, C 230
Magnetic field effects on charge carriers in OLEDs — •Ulrich Niedermeier1,2, Wiebke Sarfert1, and Heinz von Seggern2 — 1Siemens AG, CT MM 1, Günther-Scharowsky-Str. 1, 91058 Erlangen, Germany — 2TU Darmstadt, Department of Materials Science, Petersenstr. 23, 64287 Darmstadt, Germany
Magnetic field effects in organic light emitting diodes (OLEDs) have attracted more and more attention in recent research activities. In an external magnetic field both the current flow through an OLED and the light emission from the device are increased. We present results from magnetoresistance measurements suggesting that the presence of triplet excitons within the device is linked to the appearance of the magnetoresistive effect. In fluorescent emitters the effect occurs at voltages above turn-on where both electrons and holes are injected and form excitons. Introducing phosphorescent emitters in a fluorescent matrix results in a decrease of the magnetoresistance effect since triplet excitons are effectively removed from the system by radiative decay. Using different cathode and emitter materials we furthermore show a dependence of the magnetoresistance on the charge carrier balance within the device. In photoluminescence measurements of fluorescent emitters a magnetic field has no influence since optical excitation creates only singlet excitons. Finally, we discuss our results in the framework of two recently proposed theories on the origin of the organic magnetoresistance effect.