Interfacial exciton-polaron quenching (EPQ) in organic light-emitting diodes (OLED) refers to the remote and direct energy transfer from the excitons in the emission layer (EML) to the charges accumulated at the interface of the adjacent charge transport layers (CTLs). Here, interfacial EPQ is investigated by using a bilayer hole-only device (HOD), where an ultra-thin dopant layer is selectively introduced near the interface. At the heterointerface, positive-charge accumulation is induced due to the energy difference between distinct organic layers, while excitons are optically pumped in the dopants exclusively. The interfacial EPQ is characterized by measuring a shift in the photoluminescent intensity of the dopants. Such interfacial EPQ indeed depends on the interfacial energy offset and the distance between charges and excitons, and universally occurs regardless of the emission mechanism of OLEDs. We propose the device architecture to potentially reduce interfacial EPQ for achieving high-performance OLEDs.
We present a comprehensive model to analyze, quantitatively, and predict the process of degradation of OLEDs considering polaron, exciton, exciton–polaron interactions, exciton–exciton interactions and a newly proposed impurity effect. The loss of efficiency during degradation is presented as a function of quencher density. The density and generation mechanisms of quenchers are extracted using a voltage rise model. The comprehensive model is applied to stable blue phosphorescent OLEDs, and the results show that the model describes the voltage rise and external quantum efficiency loss very well, and that the quenchers in emitting layer are mainly generated by polaron-induced degradation of dopants. Quencher formation was confirmed from a mass spectrometry. The polaron density per dopant molecule is reduced by controlling the emitter doping ratio, resulting in the highest reported LT50 of 431 hours at an initial brightness of 500 cd/m2 with CIEy<0.25 and high EQE>18%.
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