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Non-isotropic emitter orientation and its implications for efficiency analysis of organic light-emitting diodes

: Schmidt, T.D.; Flämmich, M.; Scholz, B.J.; Michaelis, D.; Mayr, C.; Danz, N.; Brütting, W.


Rand, B.P.; Adachi, C.; Elsbergen, V. van ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Organic Photonics V : 16 - 18 April 2012, Brussels, Belgium
Bellingham, WA: SPIE, 2012 (Proceedings of SPIE 8435)
ISBN: 978-0-8194-9127-5
Paper 843513
Conference "Organic Photonics" <5, 2012, Brussels>
Conference Paper
Fraunhofer IOF ()
organic light-emitting diode; external quantum efficiency; oriented emitting systems; radiative quantum efficiency; outcoupling efficiency

The efficiency of organic light-emitting diodes (OLEDs) is limited since only a small fraction of the consumed electrical power is converted into visible light that is finally extracted to air. Most of the efficiency loss is caused by suboptimal radiative quantum efficiency (RQE) of the emitting guest-host system and by dissipating a huge part of the radiated energy to optical modes such as surface plasmons or waveguided modes, which cannot easily be extracted by common outcoupling structures. In order to increase the external quantum efficiency (EQE) of OLEDs new approaches are needed. Recent studies show that the EQE can be enhanced considerably by horizontally oriented emitters, a feature that is well known for fluorescent emitters and has lately been demonstrated in phosphorescent state-of-the-art OLEDs. By means of optical simulations we investigated the influence of non-isotropic emitter orientation on the effective RQE and the outcoupling factor. We show that in order to achieve a consistent efficiency analysis it is indispensable to account for possible deviations from isotropy. Ignoring these orientation effects leads to significant misinterpretation of the RQE and other factors, which determine the external quantum efficiency of a device. Furthermore, we demonstrate the huge potential for efficiency enhancement of mainly parallel dipole emitter orientation in both fluorescent and phosphorescent OLEDs.