Device and metasurface designs for next-generation blue-emitting organic LEDs
Creators: name: Carter, Catrice Monet
Publisher: name: No Publisher Supplied
Year: 2018

description: Polymer-based organic light-emitting diodes (P-OLEDs), have potential to be a fully-solution-processable alternative to current display and lighting options. Currently, organic light-emitting diodes (OLEDs) can have internal quantum efficiencies of 100%. However, for white OLEDs the light-extraction efficiency is, at best, between 20-31%. The external quantum efficiency is lower for red and green (5.5% and 9%, respectively) phosphorescent OLEDs (Ph-OLEDs) and even lower for blue phosphorescent Ph-OLEDs (3.5%) as a result of poor light extraction. External quantum efficiency results are even lower for fluorescent P-OLEDs; therefore, further improvements must be made for white light and single color P-OLEDs. The use of alternative device architectures (such as top-emitting devices), integrated light management structures (such as using noble metal nanostructures), and improved charge transport layers, have shown to improve light-extraction from OLEDs. Device operational lifetime (i.e., stability) improvements are also needed; currently OLED luminaires on the market have lifetimes of approximately 10,000 hours to 100,000 hours, depending upon the operating luminance. Short OLED lifetimes are a result of unstable charge injection layers, non-radiative excited state interactions, and corrosion of the electrodes under ambient conditions. The implementation of encapsulants or getters and inverted device configurations can help to circumvent some of these stability issues.    In this thesis, various P-OLED device architectures are theoretically and experimentally studied to determine efficiency and stability enhancement approaches, with consideration for economic and environmental impacts. First, an economic, efficiency, and environmental assessment of four different P-OLED device configurations: bottom-emitting conventional, bottom-emitting inverted, top-emitting conventional and top-emitting inverted devices is carried out with regards to the following metrics: device cost, yearly operating cost, optical power cost and CO2 emissions. For context, the metrics for the P-OLED devices are compared to those for a ubiquitous blue inorganic LED device architecture. The results show that the top-emitting inverted device architecture performs competitively at the laboratory scale with commercial-scale inorganic LEDs for all metrics and significantly reduces the device cost, yearly operating cost, optical power cost and CO2 emissions for the P-OLED devices, due to elimination of indium tin oxide and its comparatively high luminous efficacy and longer lifetime. A scenario analysis is also carried out which projects economic and environmental impacts for P-OLEDs fabricated at a large scale.    Next, an experimental investigation of the photoluminescence (PL) stability, PL lifetime, and PL quantum yield of conjugated polymer:organometallic (PVK:FIrpic), blue, phosphorescent thin-films blends on silver metasurfaces is carried out in comparison to corresponding data for the phosphorescent thin-film blend on planar silver films. Certain silver metasurfaces are found to have the ability to increase the radiative decay rate of triplet emission from the blue organic phosphorescent thin-films and this results in an improvement in the stability of the emission. In particular, this work shows that nanoparticle (NPT) Ag metasurfaces cause the greatest improvements in stability and brightness from the PVK:FIrpic thin films, with an average PL stability enhancement factor of 2, a reduction in the average PL lifetime by a factor of 1.29, and a PL intensity enhancement factor of 6.6 relative to PVK:FIrpic on planar Ag. Overall, the results have shown a correlation between enhanced PL stability and shorter PL lifetimes of PVK:FIrpic on silver plasmonic metasurfaces relative to a planar silver surfaces.    Finally, theoretical electromagnetic simulations are used to assess the light-extraction efficiency four different P-OLED configurations: conventional bottom- and top-emitting P-OLEDs and inverted bottom- and top-emitting P-OLEDs. The electromagnetic simulation results show that the total light extraction efficiency is the highest (28 %) for the bottom-emitting device configurations and the top-emitting conventional device has the lowest light extraction efficiency of 1 %. Further, it is shown that in-plane oriented dipoles contribute the most to the light extraction efficiency. The power absorbed in each device layer is also quantified and shows that a large portion of the power loss occurs when the dipole is oriented in the out-of-plane direction, particularly for the metallic layers, due to surface plasmon polariton modes at the metal/semiconductor interface.    In summary, this thesis identifies device designs and metasurface electrode types that can lead to efficiency and stability gains in polymer-based OLED devices using experimental and theoretical methods. The work is original in that it consists of the first quantitative assessment of economic, energy and sustainability impacts of different OLED device architectures. Additionally, the demonstration that the local electromagnetic fields of metasurfaces can be used to improve the stability of phosphorescent OLED materials is unique and is relevant to the implementation of blue phosphorescence emitters in commercial OLEDs. The approaches to improve OLED device performance reported in this thesis have the potential to save on capital costs and on energy consumption, and to minimize the carbon footprint associated with OLED devices.