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Organic Light Emitting Diodes: Photophysics of Thermally Activated Delayed Fluorescence Emitters and Device Characterization
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In this thesis, the influence of bromine atoms on the mechanism responsible for the observation of thermally activated delayed fluorescence (TADF) is investigated. Heavy atoms, such as bromine, are well known to promote spin-orbit coupling (SOC), consequently facilitating the transition between singlet and triplet states by intersystem crossing (ISC). For that reason, organic heavy metal complexes are considered organic phosphors because their presence facilitates the radiative decay directly from the triplet state to the ground singlet state, giving origin to the observation of phosphorescence at room temperature. In TADF emitters triplet states are up-converted to high energy singlet excited state through thermally activated reverse intersystem crossing (RISC). Therefore, the presence of heavy atoms in the structure of TADF molecules poses an interesting question of knowing if whether it will assist RISC or if it will promote triplet decay directly to the ground state either through non-radiative processes or instead through phosphorescence.
This work gives clear evidence that bromine causes direct triplet state radiative decay to the ground state, causing an otherwise strong TADF emitter to show significant phosphorescence, even at room temperature.
A second objective of this thesis was to develop an experimental technique to directly probe the excited state dynamics and electroluminescence (EL) mechanism in working organic light emitting diodes (OLEDs), under device operation. This was accomplished by measuring the transient electroluminescence emission of TADF-based OLEDs on a newly assembled transient EL spectroscopy system developed within this project.
These studies show that charge trapping plays an important role in the operation of OLEDs, and largely dominates the EL generation. This is a direct consequence of the intrinsically disordered nature of organic emitters.
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Photophysics TADF SOC OLED Transient EL