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Tobias Schmidt
Photophysics of organic light-emitting diodes - Device efficiency and degradation processes
Supervisor: Prof. Wolfgang Br%C3%BCtting [Experimental physics IV]
Date of oral examination: 06/25/2013
161 pages, english , OPUS Uni Augsburg (7.8.13)
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 a better understanding of degradation effects, especially their influence on the emitting guest/host systems, can lead to an enormeous 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 electrical characteristics of a complete OLED should not be changed by the method for the RQE determination. Hence, this approach was first not applicable, due to changes of the charge carrier balance for heavily differing layer thicknesses. Therefore, another technique was investigated to change the cavity strength at the emission zone inside complete OLED stacks, using a second reflective layer on top of the typically transparent anode. Thus, a thin silver layer with variable thickness was introduced to a conventional stack. Analyzing changes of the excited states lifetime of the emitting system inside the microcavity with a variable strength at the emitter position and comparing these results with numerical simulations allows for a determination of both, the intrinsic excited states lifetime and the RQE of the emitting system under investigation. Unfortunately, the electrical characteristics of these microcavity OLEDs have been unstable and hence, another approach was needed. The solution of this problem of changing the optical properties but not the electrical characteristics was the introduction of well established conductivity doped transport layers, for which the resistance of the layer is independent of its actual thickness. Hence, the first approach changing the distance of the emitting system to a highly reflective metal layer was implemented by a subsequent variation of the electron transporting layer thickness that is typically sandwiched between the metallic cathode and the emission layer of an OLED. Therewith, it was possible to determine the RQE of several emitting systems with two different experiments, namely external quantum efficiency measurements with and without outcoupling enhancements based on ray optics and time-resolved photoluminescence spectroscopy. Comparing these results with numerical simulations allows for a determination of the RQE 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. Therefore, 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.