Browsing by Subject "OLEDs"

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    Kinetics of degradation and exciton quenching in organic light-emitting devices
    (2020-06) Myers-Bangsund, John
    Organic light-emitting devices (OLEDs) are next-generation, thin film light sources which have significant advantages over conventional display and lighting technologies, including: high contrast, high power efficiency, tunable color, and compatibility with low-cost fabrication techniques on flexible substrates. These attributes have driven the rapid commercialization of OLEDs in mobile phone displays over the last two decades, but OLEDs have yet to gain traction in high brightness applications such as lighting, automotive head lights, imaging light sources, and lasers. In part, this is because OLED performance at high brightness tends to be limited by two processes: reversible efficiency roll-off (where efficiency decreases under high current) and irreversible degradation. Both of these processes are tied to excited state (or exciton) quenching reactions which dissipate energy non-radiatively and can drive chemical reactions within the active layers of an OLED. In this work, we seek to better understand these limiting phenomena so that they can ultimately be overcome. Our overarching strategy toward this end is to develop and apply combined electrical and optical analysis techniques to decouple efficiency loss pathways. Comparing measurements of electro- and photoluminescence (EL and PL), we unraveled how OLED lifetime depends on the spatial distribution of excited states and charge carriers. We found that multiple kinetic pathways determine the degradation rate, where the emitter radiative efficiency is deteriorated by exciton reactions in the emissive layer, while charge trapping and leakage are aggravated by reactions with charges at interfaces or outside the emissive layer. These methods have allowed us to identify design principles for mixed host emissive layers and molecular screening criteria to accelerate materials development. These techniques also revealed a surprising result which contradicted conventional models: luminescence quenching can occur even at low electrical bias levels, reducing peak efficiency by more than 20% in some cases. We connect this effect to preferred molecular orientation and net polarization of organic films, and we identify strategies to eliminate this loss pathway. Together, the findings in this work highlight the advantages and limitations of combined electrical and optical characterization of OLEDs. Wider application of these approaches may help researchers more quickly develop new materials and design strategies for efficient and durable OLEDs.