Storage of charge carriers on emitter molecules in organic light-emitting diodes

Caroline Weichsel*, Lorenzo Burtone, Sebastian Reineke, Susanne I. Hintschich, Malte C. Gather, Karl Leo, Bjoern Luessem

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

103 Citations (Scopus)

Abstract

Organic light-emitting diodes (OLEDs) using the red phosphorescent emitter iridium(III)bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate) [Ir(MDQ)(2)(acac)] are studied by time-resolved electroluminescence measurements. A transient overshoot after voltage turn-off is found, which is attributed to electron accumulation on Ir(MDQ)(2)(acac) molecules. The mechanism is verified via impedance spectroscopy and by application of positive and negative off-voltages. We calculate the density of accumulated electrons and find that it scales linearly with the doping concentration of the emitter. Using thin quenching layers, we locate the position of the emission zone during normal OLED operation and after voltage turn-off. In addition, the transient overshoot is also observed in three-color white-emitting OLEDs. By time-and spectrally resolved measurements using a streak camera, we directly attribute the overshoot to electron accumulation on Ir(MDQ)(2)(acac). We propose that similar processes are present in many state-of-the-art OLEDs and believe that the quantification of charge carrier storage will help to improve the efficiency of OLEDs.

Original languageEnglish
Article number075204
Number of pages9
JournalPhysical Review. B, Condensed matter and materials physics
Volume86
Issue number7
DOIs
Publication statusPublished - 13 Aug 2012

Keywords

  • TRIPLET-TRIPLET ANNIHILATION
  • TRANSIENT ELECTROLUMINESCENCE
  • EFFICIENCY
  • DEVICES
  • TRANSPORT
  • GENERATION
  • MOBILITY
  • LEDS

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