Universität zu Köln

Dillmann, Claudia (2024). Optimisation and Analysis of a Circularly Polarised Organic Light Emitting Diode (CP-OLED). PhD thesis, Universität zu Köln.

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Abstract

Because of their excellent properties and low energy requirement, organic light emitting diodes (OLEDs) are commonly used in display applications. Improvements and new concepts are widely studied, scientifically as well as commercially. This advancement has brought forward novel, innovative technologies like bendable or transparent displays, as well as increased efficiency and stability of established devices. Nevertheless, due to their amorphous nature, regular OLEDs only emit unpolarised light. In this thesis, the development of a highly functional circularly polarised OLED (CP-OLED) is pursued, characterised by both, excellent device performance and large circular polarisation (CP) effects. To quantify these effects, a measurement set-up for circularly polarised electroluminescence (CP-EL) is assembled, and different ways to evaluate and present this data are developed. For best expressiveness and comparability, the g value at the wavelength of the EL spectrum maximum (g@maxEL) is reported here. As chosen CP-OLED systems, devices from three categories are analysed: 1. I-CPL emitters 2. one component systems 3. mixed systems With the exception of intrinsic CPL approaches, all systems require thermal annealing to produce a chiral liquid crystalline phase, which is responsible for later CPL emission. For this, ideal curing conditions are developed for each system. All materials were evaluated in thin film as well as in OLED devices. Firstly, two helicene based molecules, H1-Me2 and ThiahelalkBZT, are utilized as potential intrinsic CPL emitters. As expected from calculations and similar materials in literature, no measurable CP-effect is found. Additionally, both molecules are tested as chiral inducers within an F8BT matrix. As such, they achieve g@maxEL values of up to 0.42 and 0.41, respectively. Secondly, multiple chirally functionalised polyfluorenes (PFs) are analysed as one component CP-OLED systems. Both, homo- and co-polymers are compared. To minimize complexity, CP-OLEDs are also built in a cavity system, thereby reducing the number of emission peaks. The disruptive influence of keto-defects within the chirally functionalised polymer is explained. Based on the position of the chiral centre, an odd-even effect resulting in opposite handedness for circular dichroism (CD) and gEL curves is found. Over all, g@maxEL values of up to -0.55 are measured with chiral side chain functionalised PFs. Thirdly, an F8BT based mixed CPL system is analysed in depth. The opposite CP-effect of both chiral inducer enantiomers, R5011 and S5011, is demonstrated. As was done for all polymer based systems, ideal curing conditions to achieve maximum CP-effects are developed. Supported by DSC measurements, required curing times and temperatures increase with increasing molecular weight of the polymer. Best CD and gEL values are found for a Mw of 7 - 21 kg/mol. The result of thickness variations of different layers within this stack are explained and strategies to reduce cathode reflections leading to optical interference are proposed. The influence of the chiral inducer concentration is analysed. It is demonstrated that using different enantiomeric excess (e.e.) ratios of both chiral inducer enantiomers with a constant combined chiral inducer concentration produces more comparable results than changing the chiral inducer concentration directly. CP-effects of a specific effective chiral inducer enantiomer concentration are equivalent to the same chiral inducer concentration with only one enantiomer used. Over all, g@maxEL values of up to -1.12 are found with F8BT + R5011 mixed systems.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Dillmann, Claudia c.dillmann@mailbox.org UNSPECIFIED UNSPECIFIED
Contributors:	Contribution Name Email Censor Meerholz, Klaus UNSPECIFIED Censor Lützen, Arne UNSPECIFIED
URN:	urn:nbn:de:hbz:38-721548
Date:	2024
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
Uncontrolled Keywords:	Keywords Language OLED English circularly polarised English chiral phase English
Date of oral exam:	7 June 2023
Referee:	Name Academic Title Meerholz, Klaus Prof. Dr. Lützen, Arne Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/72154