Emission properties of small molecule phosphor emitters in organic light emitting diodes
Knowledge of emitter molecule optical properties such as emission zone profile, emitter orientation and intrinsic quantum efficiency are essential in developing and understanding state-of-the-art organic light emitting diodes. In this work, established approaches for emitter property characterization from electroluminescence patterns have been adapted and a novel approach established for use with specialised devices with the following key results: (i) For a single emitter device, for the first time, evidence of an emission zone profile extension into charge blocking layers is given. This might reduce the intrinsic quantum efficiency of electroluminescence as observed when comparing electroluminescence and photoluminescence efficiencies. Such an extended emission zone also has a significant effect on the extracted orientation ratio, which is crucial in determining outcoupling efficiency. (ii) Standard emitter orientation analysis yields the ratio of the second moments, but no further details of the orientation distribution. Therefore, temporally resolved experiments of OLED stacks that feature a plasmon induced orientation dependent lifetime have been conducted. The results, barring an emitter orientation fluctuating much quicker than the emitter excited lifetime, reveal an unexpectedly narrow angular distribution of co-evaporated phosphors. This measurement technique should be of utmost interest towards emitter orientation distribution tailoring to reach maximum external quantum efficiencies. (iii) Dual emitter devices were analysed to access the individual optical properties of each emitter with sufficient accuracy by separating an overlapping emission spectrum. This approach should be very useful in understanding and optimising multi-emitter RGB OLEDs.
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