We report on monochrome top emitting organic light emitting diodes (OLEDs) with inverted layer structure and discuss the optical and electrical optimization of OLED devices comprising the orange phosphorescent emitter Ir(MDQ)<sub>2</sub>(acac). We show that first, charge balance within the emitting layer is an important factor for efficient generation of light and second, optical outcoupling is a critical issue in top-emitting devices. We demonstrate the use of doped charge transport layers for efficient injection of charge carriers and optical modeling to improve outcoupling. The latter one is done via optimized cavity tuning and application of a dielectric capping layer. Finally, driving voltages of 4.2V at 1000 cd/m<sup>2</sup>, 19 lm/W and 17% external quantum efficiency (EQE) from devices made on metal substrates are reached.
The extraction of the luminous power internally generated by organic light emitting diodes (OLEDs) is still the
most severe limitation in the overall efficiency of these devices. We present a joint theoretical and experimental
study aimed to quantitatively evaluate the light outcoupling limitations of planar p-i-n type small-molecule
OLEDs, both in bottom and top emitting configuration. We discuss the physical origin of these limitations by
analyzing internal optical losses and overall light conversion efficiency in OLEDs.