Within the last decade, organic light-emitting diodes (OLEDs) have evolved to be one of the major players in the display panel market. For instance, in 2023, it is expected that about half of the produced smartphones incorporate an OLED display. This rapid development is based on the high image quality and fast response time of OLEDs compared to the previously dominating technology of liquid crystal displays. Additionally, OLEDs feature interesting properties like mechanical flexibility, areal light emission, and semi-transparency that allow for futuristic device designs in display or lighting applications.
One of the major drawbacks of OLEDs is their limited efficiency compared to conventional light-emitting diodes (LEDs). Due to the high refractive index of their active, light-emitting layers, a large portion of the internally generated light is lost because of optical effects like total internal reflection. One promising approach, which increases the amount of outcoupled light, is to align the transition dipole moment (TDM) of the emitter molecules parallel to the interface planes of the device. In the best case, this method can yield an efficiency improvement of more than 50%.
This thesis focuses on both the accurate measurement and the exploration of control strategies of the emitter orientation. Furthermore, a software tool is developed, which supports the device design and data evaluation.
First, the state-of-the-art emitter orientation measurement technique is analyzed, which is angle-resolved photoluminescence spectroscopy (ARPS). A ray-optics model is developed in order to quantify the impact of experimental deviations from the ideal measurement configuration. In particular, a displacement of the light-emission spot from the rotation center of the measurement setup is investigated. The resulting alteration of the observable angle-resolved emission spectrum is calculated and the impact on the consequent orientation factor is estimated. Based on the optical model, a refined setup structure is proposed, which not only circumvents a part of the identified problems but also yields a ten-times increased signal-to-noise ratio (SNR).
Subsequently, it is explored how the emitter orientation can be controlled by external physical parameters during and after processing. Selected phosphorescent organic model systems are exposed to elevated temperatures and electric fields. In the first measurement series, the impact of the substrate temperature during deposition (𝑇sub) is investigated. It is found that the emitter orientation can be tuned from a more horizontal configuration at room temperature (RT) to an isotropic distribution if 𝑇sub approaches the glass transition temperature (𝑇g) of the material. This observation fits well to previous results of glass physics obtained with similar materials. In a second, alternative test series, OLEDs are treated after processing. Here, an emitter system comprising the host material NPB and the emitter Ir(piq)3 is investigated. For a treatment temperature of 125℃ and a simultaneously applied reverse bias of -20V the external quantum efficiency (EQE) of the OLEDs is increased by more than 50%. The effect is observed for two different emitter concentrations of 1 wt% and 10 wt% and OLEDs in the optical minimum and maximum. Finally, the long-term stability of the emitter orientation is experimentally
demonstrated over a time frame of 1.5 years and for storage temperatures up to 95% of the host material’s 𝑇g.
Preceding the presentation of the experimental results, the software-tool simojio is introduced, which is developed in this thesis. It enables an efficient and convenient workflow throughout the emitter orientation investigations and supports various tasks such as the device and setup design, the processing and visualization of xperimental data, and the extraction of orientation factors from angle-resolved emission spectra.
Simojio provides a graphical user interface (GUI), which enables a flexible configuration of input parameters and one-dimensional layer structures. The corresponding numerical and graphical results are neatly arranged in a separate, tab-structured window. The actual calculation and processing algorithms are implemented in custom-made python modules, which can be modified and extended by the users according to their specific needs. Simojio is applied to most of the emitter orientation related simulation and data-processing tasks, which are presented in this thesis. However, due to its flexible, modular architecture, it is not restricted to this use case but may be utilized for highly diverse numerical problems, which are based on the evaluation of generic parameters or one-dimensional structures.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:93144 |
| Date | 12 August 2024 |
| Creators | Hänisch, Christian |
| Contributors | Reineke, Sebastian, Neyts, Kristiaan, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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