In this paper, we present a method for a comprehensive analysis of the efficiency roll-off with current density in phosphorescent organic light-emitting diodes (OLEDs). By combining electrical and optical excitation in time-resolved spectroscopic experiments, we are able to measure the excited-state lifetime for different driving conditions. It is, thus, possible to correlate changes of the triplet lifetime with a decrease of the radiative quantum efficiency of the emitting system due to exciton quenching processes. As compared to the conventional analysis of the measured external quantum efficiency (EQE) in dependence of the applied current density, the lifetime analysis is not affected by changes of the charge-carrier balance with current, which can have a significant impact on the interpretation of the results. By performing timeresolved spectroscopy for a series of red phosphorescent OLEDs, triplet-polaron quenching (TPQ) is identified as the dominant mechanism behind the efficiency roll-off up to a current density of 100 mA=cm 2 , while the conventional EQE vs current plot rather suggests triplet-triplet annihilation as the main quenching mechanism. We show that this apparent discrepancy is caused by exciton quenching occurring already at very low current densities, where EQE measurements are not reliable due to significant changes of the charge-carrier balance in this region. In addition, we present evidence that the triplet-polaron quenching rate Γ TPQ is independent of the microcavity so that variations of the triplet lifetimes of a series of devices exhibiting different layer thicknesses can be described with a single parameter set.

Emitter orientation will play a major role in future applications of organic light-emitting diodes due to its strong impact on the efficiency of the devices. Up to now, determining the orientation of transition dipole moments required elaborate angular-dependent measurements of the light emission pattern. In this paper, we present a simplified and straightforward method to extract the emitter orientation from external quantum efficiency measurements. We demonstrate the validity of the method on three different dye-doped emitting systems

The non-isotropic alignment of molecules can increase the interaction efficiency with propagating light fields. This applies to both emissive and absorptive systems and can be exploited for achieving unprecedented efficiencies of organic opto-electronic devices such as organic light-emitting diodes. Optical analysis has revealed certain phosphorescent emitters to align spontaneously in an advantageous orientation. Unfortunately, established approaches only determine an average orientation because emission patterns solely depend on the second moments of the transition dipole vector distribution. In order to resolve further details of such a distribution, additional differences in the emission characteristics of parallel and perpendicularly oriented emitters need to be introduced. A thin metal layer near the emitters introduces plasmon mediated losses mostly for perpendicular emitters. Then, analyzing the emission at different polarizations allows one to measure emission lifetimes of mostly parallel or mostly perpendicular oriented emitters. This should alter the transient emission when observing the temporal phosphorescence decay under different directions and/or polarizations. The angular width of the orientation distribution can be derived from the degree of such lifetime splitting. Our results suggest a narrow but obliquely oriented molecular ensemble of Ir(MDQ)2(acac) doped into the α-NPD host inside an Organic LED stack.

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