Regensburg 2013 – scientific programme
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HL: Fachverband Halbleiterphysik
HL 50: Organic semiconductors
HL 50.9: Talk
Wednesday, March 13, 2013, 11:45–12:00, H13
Comparing the transition dipole orientation of different phosphorescent emitters in organic light-emitting diodes under electrical excitation — •Philipp Liehm, Caroline Murawski, Mauro Furno, Björn Lüssem, Karl Leo, and Malte C. Gather — Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str. 1, 01062 Dresden, Deutschland
We investigate the average transition dipole orientation of different phosphorescent matrix/emitter systems that are used in state-of-the-art highly efficient organic light-emitting diodes (OLED). The orientation is determined using two different methods: in-situ angle resolved electroluminescence spectroscopy and analysis of the exciton decay time under electrical excitation. Both methods are supported by optical simulations. We find that the dipoles of the green emitter Ir(ppy)3 align nearly isotropically in the matrices of TCTA/TPBi and CBP. By contrast, we show evidence that the Ir(MDQ)2(acac) and the Ir(ppy)2(acac) dipoles are preferentially horizontal when embedded in a NPB and a CBP matrix, respectively. Within the range of experimental error, we obtain similar results for both investigated methods. Using optical simulations, we provide a quantitative estimate of how the difference in average orientation influences the external quantum efficiency of OLEDs based on Ir(ppy)3 and Ir(ppy)2(acac) embedded in a CBP matrix and compare against experimental results.