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Aggregation of guest molecules and photoluminescence quenching processes in phosphorescent organic light-emitting diodes: an experimental and simulation study
Niyonkuru, Paul
Niyonkuru, Paul
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2024
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In the field of organic electronics, film morphology is critically important in influencing device performance. Specifically, in phosphorescent organic light-emitting diode (PhOLED), the aggregation of phosphorescent dopants within the emissive layer (EML) enhances photoluminescence (PL) quenching processes that are detrimental to device efficiency. A comprehensive understanding of how guest molecule aggregation affects these quenching phenomena, alongside the identification of factors and dynamics promoting such aggregation, is indispensable for the creation of more efficient devices. This dissertation explores PL quenching processes, their connection to morphological factors, and also investigates the role of material polarity in controlling guest aggregation.
The initial phase of this research focuses on the dynamics of triplet-triplet annihilation (TTA) and its relationship with the spatial distribution of guest molecules. An intermediate model is introduced, offering a detailed representation of the TTA process that incorporates the influence of morphological complexities on exciton dynamics. Validated through a combination of simulation and varied experimental conditions, this model consistently provides more accurate fits than conventional approaches, thus establishing a reliable framework for examining TTA in the context of realistically complex morphologies.
Subsequently, the dissertation investigates how the polarity of host and guest materials impacts guest aggregation and its effect on PL efficiency. Through comparative analyses of various EML configurations, utilizing two well-studied phosphorescent guests in differing host environments, optical characterization is performed using techniques such as photoluminescence quantum yield (PLQY), temperature-dependent PL, and time-resolved photoluminescence (TRPL). These analyses are corroborated by direct morphological observations based on high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and spatial point-pattern analysis. Simulation studies further elucidate guest aggregation dynamics, demonstrating that an increase in both guest and host permanent dipole moment (PDM) can lead to reduced aggregation and enhanced PhOLED performance. This research highlights the efficacy of a polar environment in significantly reducing aggregation, consequently promoting a more even distribution of dopants and, thereby, boosting device efficiency.
This work advances the understanding of material interactions within PhOLEDs. It identifies key factors impacting device performance and suggests targeted strategies for material selection and device architecture, contributing to addressing the challenges posed by dopant aggregation. The insights from this study not only benefit PhOLED technology but also offer avenues for the advancement of more efficient and reliable organic semiconductor devices that are subjected to morphological effects.
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