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Development of highly efficient and stable organic light-emitting diodes based on delayed fluorescent materials for solid-state lighting applications

Periodic Reporting for period 1 - THF-OLED (Development of highly efficient and stable organic light-emitting diodes based on delayed fluorescent materials for solid-state lighting applications)

Reporting period: 2017-06-01 to 2019-05-31

At present, about 20% of the world’s electricity consumption is devoted to display and lighting in one form or another. Existing display technology (particularly mobile display) rely on inefficient liquid crystal display (LCD) technology for outdoor usage while most luminaires rely on very energy inefficient technologies such as incandescent and fluorescent light bulbs. Organic light-emitting diode (OLED) based display technology and solid-state lighting (SSL) devices hold great promise as an alternative to conventional LCD and luminaires with impressive economic and environmental savings . OLEDs are an emerging display and SSL technology that offers several advantages over commonly used LCD and luminaires. These advantages include: self-illumination, low power consumption, vibration resistance, and most importantly, environmental friendliness. OLEDs also provide enticing manufacturing design possibilities such as a wide range substrate choice, including flexible devices. The development of cheap and energy-efficient OLED lighting devices would be a transformative World-wide advance. Through this project we proposed to develop such OLED device based on thermally activated delayed fluorescent (TADF) materials that are efficient, stable and inexpensive. The overall object of this research proposal was to devise the solutions for these two pertinent challenges in TADF-OLED technology: minimizing efficiency roll-off and improving device operational stability.
As mentioned in the research proposal, in order to achieve the objectives of this research project, entire project was divided into work packages. The project initiated with computational screening of the potential TADF emitters and host materials and evolved with organic synthesis, chemical characterization, photophysical and electrochemical studies, and finally OLED device fabrication and testing. The project further categorized in terms of TADF emitters and host materials. An up-to-date overview of entire project and main results is as follows:

(1) 2nd generation TADF-OLED emitters: In this work, we have designed and synthesised four derivatives of a well-known green TADF emitter, 2CzPN and the effect of phosphine chalcogenide on their photophysical and optoelectronic properties was studied. Phosphine oxide-substituted carbazole derivatives were used as donor moieties to strategically blue shift the emission of 2CzPN. This work has been completed and a manuscript is under preparation.

(2) High triplet energy ambipolar TADF-OLED host materials: In this work, we have designed and synthesized four derivatives of triphenyl phosphine oxide substituted with carbazole and 9,9-dimethyl acridan. These materials found to be possess high triplet energy and ambipolar charge transport character; and eventually have turned out to be excellent host materials in terms of suppression of efficiency roll-off of TADF-OLED devices. With these results, we have been able to successfully achieved one of the objective of this research proposal. This work has been published in Chem. Lett., 2019, 48, 1225-1228.

(3) TADF-OLED emitters based on through space charge transfer (TSCT): In this work, a number of TADF emitters based on charge transfer through space were designed and screened through computation simulation. Acenapthene has been chosen as bridging unit to link donor and acceptor in co-facial arrangement. The synthesis of some these derivatives has been completed and photophysics in under investigation, while others are still under preparation.

(4) Effect of heavy-atom on TADF-OLED host materials: 1,3-Bis(cabazolyl)benzene (mCP)_is well-known host material for TADF-OLED. In this work, we designed and synthesized halogen (X = Br & I) substituted mCP derivatives to study the effect of heavy atom on performance of TADF emitters based OLED. The synthesis of Br-substituted mCP derivatives has been completed while synthesis of I-substituted mCP derivatives is under preparation.

(5) Electrochemiluminescence (ECL) in TADF emitters: As part of an established collaboration between EZ-C and Prof. Zhifeng Ding at Western University, Canada, we have observed highly efficient ECL properties in some of the newly synthesised TADF emitters. Further investigation into this observation is in progress.
An overview of progress beyond the state of the art, expected results until the end of the project and potential impacts is as follows:

(1) 2nd generation TADF-OLED emitters: The outcome of this work will be able to provide a road map to achieve high performance and operational stability for existing TADF emitters by simply tweaking their molecular structure.

(2) High triplet energy ambipolar host materials: This work will provide the direction to the OLED community to pursue the design for high triplet energy ambipolar host materials to successfully suppress the efficiency roll-off in TADF-OLED device.

(3) TADF emitters based on through space charge transfer (TSCT): The outcome of this work will provide an alternative TADF emitter design to improve the color purity and stability of TADF emitters compared to the conventional emitter design where donor-acceptor are linked directly. Further, this work will provide the insight of TADF mechanism in through space charge transfer based emitters.

(4) Effect of heavy-atom on TADF-OLED host materials: It would be interesting to look at the outcome of this work. If successful, it will prompt the OLED community to incorporated heavy atom in molecular design of host materials.

(5) Electrochemiluminescence (ECL) in TADF emitters: The outcome of this work will prompt the OLED community to explore the application of TADF emitters in ECL devices.
Oral presentation at ECECR 2019, St Andrews, UK
Poster presentation at MCAA General Assembly and Annual Conference 2018, Belgium