Influence of cavity thickness and emitter orientation on the efficiency roll-off of phosphorescent organic light-emitting diodes
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This article describes the first systematic investigation of how the efficiency roll-off in organic light-emitting diodes (OLEDs) is influenced by the position and orientation of the emitter molecules within the OLED cavity. The efficiency roll-off is investigated for two OLED stacks containing either the phosphorescent emitter Ir(MDQ)2(acac) or Ir(ppy)3 by varying the distance between emitter and metal cathode; a strong influence of emitter position and orientation on roll-off is observed. The measurements are modeled by triplet-triplet-annihilation (TTA) theory yielding the critical current density and the TTA rate constant. It is found that Ir(MDQ)2(acac) shows the lowest roll-off when the emitter is located in the first optical maximum of the electromagnetic field, whereas the roll-off of the Ir(ppy)3 stack is lowest when the emitter is positioned closer to the metal cathode. Measurement and modeling of time-resolved electroluminescence show that the different roll-off behavior is due to the different orientation and the corresponding change of the decay rate of the emissive dipoles of Ir(MDQ)2(acac) and Ir(ppy)3. Finally, design principles are developed for optimal high-brightness performance by modeling the roll-off as a function of emitter-cathode distance, emissive dipole orientation, and radiative efficiency.
Murawski , C , Liehm , P , Leo , K & Gather , M C 2014 , ' Influence of cavity thickness and emitter orientation on the efficiency roll-off of phosphorescent organic light-emitting diodes ' Advanced Functional Materials , vol 24 , no. 8 , pp. 1117-1124 . DOI: 10.1002/adfm.201302173
Advanced Functional Materials
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. This is the peer reviewed version of the following article: Murawski, C., Liehm, P., Leo, K. and Gather, M. C. (2014), Influence of Cavity Thickness and Emitter Orientation on the Efficiency Roll-Off of Phosphorescent Organic Light-Emitting Diodes. Adv. Funct. Mater., 24: 1117–1124., which has been published in final form at https://dx.doi.org/10.1002/adfm.201302173. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.