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Photophysics of small molecule organo-metallic complexes for OLED applications



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Over the last 3 decades, advances in chemistry have produced new semiconducting materials with very different properties, transitioning from inorganic to organic semiconductors, allowing previous material constraints to be worked around. Organic semiconductors can be transparent, flexible, and soluble and are able to be tuned to absorb or emit across a wide range of wavelengths. This has revolutionised device technology, already competing in global lighting and display industries. Whilst organic semiconductors avoid some of the problems inherent in inorganic technologies, they have their own limitations. The electronic transitions in organic materials are highly spin sensitive, as excitons form due to lower dipole moments. Triplet excitons cannot directly couple radiatively to the ground state, so unless additional processes to alter spin angular momentum exist 75% of excitations formed cannot emit light. I have studied a new class of small molecules, Carbene Metal Amides (CMAs) which display efficient emission, and have been used in OLEDs, breaking the efficiency records for both solution processed devices (27.5%), and evaporated host free emissive layers (23%). Herein I present my work using a variety of time resolved spectroscopic techniques to study the photophysical properties this new class of emitters, explaining the rapid and efficient excited state spin conversion, and the effects of molecular modification. I show that this class of emitter sits between two existing technologies for emission from triplet states, namely heavy metal phosphorescence and organic Thermally Activated Delayed Fluorescence (TADF). The incorporation of both sets of physical processes which have previously allowed emission from triplet states has resulted in a class of materials with near 100% photoluminescence and internal device efficiencies, and I show the nature of the states involved in the emission process, how they respond to a number of modifications to the molecular structure, and discuss how this class of materials may be further developed for device applications.





Friend, richard
Credgington, dan


OLED, TADF, CMA, photophysics


Awarding Institution

University of Cambridge