Open Access

Tobias D. Schmidt

• Organic light-emitting diodes are promising new light sources for both general lighting and display technologies. Although first commercial products are already available, the efficiency and the long-term stability during electrical operation are not really satisfying, yet. Thus, there is still much room for improvement of both factors influencing future applications based on OLEDs. The motivation of this thesis was the better understanding of the photo-physical processes inside complex OLED structures. Especially the influence of cavity effects on the efficiency of the devices and a detailed analysis of energy dissipation to the optical modes of an OLED was in the focus of the first part, resulting in a comprehensive efficiency analysis of state-of-the-art devices. Additionally, the degradation induced changes of the photo-physical properties of the emitting molecules have been less investigated in the past. Hence, the second part of this thesis deals with this topic because aOrganic light-emitting diodes are promising new light sources for both general lighting and display technologies. Although first commercial products are already available, the efficiency and the long-term stability during electrical operation are not really satisfying, yet. Thus, there is still much room for improvement of both factors influencing future applications based on OLEDs. The motivation of this thesis was the better understanding of the photo-physical processes inside complex OLED structures. Especially the influence of cavity effects on the efficiency of the devices and a detailed analysis of energy dissipation to the optical modes of an OLED was in the focus of the first part, resulting in a comprehensive efficiency analysis of state-of-the-art devices. Additionally, the degradation induced changes of the photo-physical properties of the emitting molecules have been less investigated in the past. Hence, the second part of this thesis deals with this topic because a better understanding of degradation effects, especially their influence on the emitting guest/host systems, can lead to an enormous increase of lifetimes in terms of long-term stability and a deeper physical understanding. However, the external quantum efficiency of organic light-emitting diodes is determined by four different factors, namely the charge carrier balance, the radiative exciton fraction, the effective radiative quantum efficiency of the emitting system and the outcoupling factor of the device. The first factor is mainly influenced by electrical properties of the used organic layers such as charge carrier mobility and injection barriers between them. The radiative exciton fraction is caused by quantum mechanical selection rules and is unity for phosphorescent emitting systems, while it can be significantly less for fluorescent emitters. However, only the determination of all four factors of the EQE would lead to a consistent comprehensive efficiency analysis of state-of-the-art OLEDs, which was one aim of this thesis. Therefore, the main focus was on developing and evaluating an approach to determine the radiative quantum efficiency of an emitting guest/host system inside a complex OLED structure, because measurements of isolated thin films in integrating spheres can lead to a wrong estimation of this important factor. Thus, a method based on a subsequent variation of the cavity strength, mainly formed by the typically used metallic cathode of an OLED, at the position of the emission zone was investigated by using simplified structures. Thereby, changes of the cavity strength were achieved by a variation of the distance of the emission layer to a highly reflecting silver layer which was obtained by a variable optical spacer thickness between both layers. Therewith, the interference effects and the power dissipation between the different optical modes of this system are changed, which is known as Purcell effect. Thus, the radiative rate of the emitting molecules is modified while the non-radiative rate remains unchanged. The corresponding excited states lifetime of the emitting molecules inside the cavity is hence changed with the optical spacer thickness. Using time-resolved photoluminescence spectroscopy and comparing the extracted excited states lifetimes to numerical simulations leads to a determination of the radiative quantum efficiency of the emitting system under investigation. The second part of this thesis focused on the analysis of degradation processes by means of electrical aging of the devices. First, a new sample structure exhibiting low latency has been developed. With this new structure it was possible to investigate the changes of the excited states lifetime during electrical aging via time-resolved electroluminescence spectroscopy. This method is much closer to the real processes arising during electrical operation than probing the whole emission layer with an optical excitation. The decrease of the excited states lifetime of the emitting system can be correlated with the drop in luminance during electrical aging. Thus, the common assumption of an unchanged radiative rate while the non-radiative rate is increased due to electrical operation was proved for the emitting system Ir(ppy)3:CBP. Additionally, the decrease of the excited states lifetime and the corresponding radiative quantum efficiency of the emitting system was identified to be the main reason for the drop in luminance although strong changes in the electrical characteristics have been detectable via IVL measurements and impedance spectroscopy. Finally, the presented approach for efficiency analysis of OLEDs was then used for analyzing degradation effects in state-of-the-art devices. Therefore, the efficiency analysis was performed via EQE measurements and time-resolved photoluminescence spectroscopy before and after an accelerated degradation process. Therewith, it was possible to explain the electrical aging induced drop in luminance exclusively by a decrease of the radiative quantum efficiency of the emitting system Ir(MDQ)2(acac):alpha-NPD, while the other three factors determining the external quantum efficiency of an OLED remained constant. It should be noted, that a possible deviation of the emitter orientation due to the electrical degradation has been included in the analysis. Additionally, the calculation of the radiative and the non-radiative rates of the emitting species for the pristine state and after degradation disproved the assumption of an unchanged radiative rate due to electrical aging. It was found, that both rates are modified by the degradation process, the non-radiative rate is increased while the radiative rate is reduced. In order to explain theses unexpected changes, two different degradation mechanisms, namely aging of the matrix and of the emitting molecules, have been assumed and partial evidence has been achieved.…
• Organische Leuchtdioden (OLEDs) sind verheißungsvolle neue Lichtquellen sowohl für den allgemeinen Beleuchtungssektor, als auch für Displayanwendungen. Obwohl einige kommerzielle Produkte bereits auf dem Markt angeboten werden, sind Langzeitstabilität und Effizienz dieser Produkte noch nicht zufriedenstellend und müssen weiterhin verbessert werden. Essentiell für eine Steigerung der beiden genannten Kenngrößen, ist die genaue Analyse der Energiedissipation der angeregten organischen Moleküle in die verschiedenen optischen Moden einer OLED. Dazu präsentiert diese Dissertation eine Methodik, die es erlaubt, alle wichtigen Kenngrößen der Effizienz einer OLED mittels Messungen der externen Quanteneffizienz der Bauteile, in Kombination mit optischer Kurzzeitspektroskopie, zu bestimmen. Diese Methodik basiert auf einer systematischen Variation des Abstandes des emittierenden Systems zu einer hochreflektierenden Schicht. Durch den sogenannten Purcell Effekt wird dadurch sowohl die effektiveOrganische Leuchtdioden (OLEDs) sind verheißungsvolle neue Lichtquellen sowohl für den allgemeinen Beleuchtungssektor, als auch für Displayanwendungen. Obwohl einige kommerzielle Produkte bereits auf dem Markt angeboten werden, sind Langzeitstabilität und Effizienz dieser Produkte noch nicht zufriedenstellend und müssen weiterhin verbessert werden. Essentiell für eine Steigerung der beiden genannten Kenngrößen, ist die genaue Analyse der Energiedissipation der angeregten organischen Moleküle in die verschiedenen optischen Moden einer OLED. Dazu präsentiert diese Dissertation eine Methodik, die es erlaubt, alle wichtigen Kenngrößen der Effizienz einer OLED mittels Messungen der externen Quanteneffizienz der Bauteile, in Kombination mit optischer Kurzzeitspektroskopie, zu bestimmen. Diese Methodik basiert auf einer systematischen Variation des Abstandes des emittierenden Systems zu einer hochreflektierenden Schicht. Durch den sogenannten Purcell Effekt wird dadurch sowohl die effektive Effizienz des emittierenden Systems (und damit dessen Emissionslebensdauer), als auch die Verteilung der Energie innerhalb den optischen Moden der OLED variiert. Durch den Vergleich von Simulationen mit Messergebnissen lässt sich dann auf die einzelnen Größen, die die externe Quanteneffizienz von OLEDs bestimmen, schließen. Darüber hinaus wird diese Methodik im zweiten Teil der Arbeit benutzt, um elektrische Alterungsprozesse in hocheffizienten OLEDs zu untersuchen.…