Berlin 2012 – wissenschaftliches Programm
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DS: Fachverband Dünne Schichten
DS 16: Organic electronics and photovoltaics: simulations and optics II (jointly with CPP, HL, O)
DS 16.2: Vortrag
Dienstag, 27. März 2012, 11:45–12:00, H 2032
Charge Carrier Storage on Emitter Molecules in Organic Light-Emitting Diodes — •Caroline Weichsel1, Sebastian Reineke1,2, Björn Lüssem1, and Karl Leo1 — 1Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str. 1, D-01069 Dresden, Germany — 2Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
In this contribution, we study trapping of charge carriers in phosphorescent organic light-emitting diodes (OLED) using the red emitter iridium(III)bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate) [Ir(MDQ)2(acac)] by transient electroluminescence. We observe a transient overshoot exceeding regular light emission after voltage turn-off, which can be explained by delayed charge carrier recombination of charges stored in the emission layer. We study this mechanism by variation of the off-voltage and by adding thin quenching layers, which allow to determine the position of the emission zone during regular light emission and after voltage turn-off. Additionally, we show that the signal intensity is linearly proportional to the doping concentration. Investigations on the applied current and the pulse length show a saturation of the overshoot intensity, which we ascribe to the limited ability of emitter molecules to store electrons. We propose that this storage process negatively affects the external quantum efficiency of the OLED. We assume that the effect can also take place in other OLED structures and suggest that the methods presented here can help identifying charge carrier storage on emitter molecules.