Emission from outside of the emission layer in state-of-the-art phosphorescent organic light-emitting diodes

Abstract

The emission zone profile in an organic light-emitting diode was extracted by fitting the experimentally measured far-field angular electroluminescence spectrum of a purposely designed device. It is based on a thin 10 nm emission layer doped with the red emitting phosphor Ir(MDQ)2acac. We find strong indications for light emission originating from outside of the emission layer, even though the device has electron and hole blocking layers. These are commonly assumed to completely confine the charge carrier recombination and hence the light emission to the emission layer. Since the calculated internal spectrum of the emission matches the emitter photoluminescence spectrum well, diffusion of the emitter molecules outside of the emission layer is hypothesized

Analysis of the Bias-Dependent Split Emission Zone in Phosphorescent OLEDs

“…Different techniques have been used to provide information about the emission zone, namely, the use of sensing layers, ,− electro-optical simulations, ,− and measuring and modeling the angle-dependent electroluminescence (EL). − In this work, we use the latter two techniques to investigate the changes in the EMZ with increasing current densities. Our findings reveal that the emission zone has two peaks at either side of the emission layer, and the maximum of this split emission zone shifts with the applied bias.…”

Single emission spectrum measurement enables control of organic LED emission zone giving ultralong device lifetime

“…This factor and weights for the different EML positions were varied with the Solver tool in Excel to give a minimum RMS error for the fitting. The fitting was stopped when the RMS fitting error was equal to the experimental error [13]. This means that the fitting error will not be lower than the experimental error which would result in EZ solutions having spatial frequencies below the system resolution limit [20].…”

Bottom-up honeycomb top layer for light outcoupling enhancement in blue organic light emitting diodes

“…This is done to avoid light coupling to guided modes and favor extraction. However, it is known that the dipole emission depends on its electromagnetic environment [11,12], so that one cannot be sure a priori that using a parallel oriented emitter would be enough to solve the problem of light trapping, as initially thought see Fig. 15 of [10].…”