Pankert, Elisabeth
(2025).
Excited State Dynamics of Emissive
Organic Materials: Singlet-Triplet
Transitions and OLED Device Integration.
PhD thesis, Universität zu Köln.
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2025-Elisabeth Pankert-Excited State Dynamics of Emissive Organic Materials - Singlet-Triplet Transitions and OLED Device Integration.pdf - Accepted Version Download (10MB) |
Abstract
The development of organic light-emitting diodes (OLEDs) has revolutionized display technology, driven by their high efficiency, flexibility, and superior color quality. A critical challenge in OLED technology is the efficient utilization of triplet excitons, which make up 75% of the excitons generated during charge recombination. Traditional fluorescent OLEDs harness only singlet excitons, limiting their internal quantum efficiency (IQE) to 25%. The introduction of phosphorescent OLEDs, utilizing heavy-metal complexes, e.g. containing iridium, overcame this limitation by enabling triplet exciton emission through strong spin-orbit coupling, achieving near 100% IQE. However, the high cost and limited availability of heavy metals have driven research into alternative mechanisms, including thermally activated delayed fluorescence (TADF) and hybridized locally and charge-transfer (HLCT) excitons that can undergo reverse intersystem crossing (rISC) from higher lying triplet states. This thesis investigates a range of organic photoluminescent molecules, focusing on their underlying photophysical mechanisms and integration into OLEDs. A combination of steady-state and time-resolved spectroscopy across varying temperatures is employed to analyze these materials. In the first part a donor-acceptor (D-A) molecules with HLCT character is studied, achieving high photoluminescence quantum yield (PLQY) and the emitter is successfully implemented in an OLED. By studying the excited states in varying environment, the relaxation behavior of the compound is unraveled. The second part covers the systematic investigation of D-A molecules incorporating an N-phenyl-phthalimide acceptor with varying carbazole-based donors. The effects of donor modifications on the photophysics, particularly TADF behavior, are explored, emphasizing the interplay between molecular structure and TADF efficiency in OLEDs. Part three offers the exploration of a fluorinated acridone derivative demonstrating rISC from the T2 to the S1 state. A concept was developed to apply the mechanism in an OLED, showing promising potential for successful implementation. In the fourth part investigations into a D-A TADF emitter with chiral substituent are presented for potential application in a circularly polarised OLED (CP-OLED). By systematically correlating molecular photophysics with OLED device performance, this work highlights the potential of TADF and related mechanisms as sustainable, cost-effective alternatives to phosphorescent emitters. These
Item Type: | Thesis (PhD thesis) | ||||||||
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URN: | urn:nbn:de:hbz:38-786911 | ||||||||
Date: | 2025 | ||||||||
Language: | English | ||||||||
Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||
Divisions: | Faculty of Mathematics and Natural Sciences > Department of Chemistry > Institute of Physical Chemistry | ||||||||
Subjects: | Chemistry and allied sciences | ||||||||
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Date of oral exam: | 21 March 2025 | ||||||||
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Refereed: | Yes | ||||||||
URI: | http://kups.ub.uni-koeln.de/id/eprint/78691 |
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