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Development of Narrow Band Blue and Red Emitting Macromolecules for Solution-Processed Solid State Lighting Devices

Periodic Reporting for period 1 - NarrowbandSSL (Development of Narrow Band Blue and Red Emitting Macromolecules for Solution-Processed Solid State Lighting Devices)

Reporting period: 2019-04-01 to 2021-03-31

Organic light-emitting diodes (OLEDs) have emerged as a preferred choice of technology for applications such displays and lighting. OLEDs are bright, thin, efficient, and have high contrast ratios and can be viewed at wide angles. Adoption of OLED displays in the smartphone market has revolutionized these devices. Commercialized OLEDs are manufactured by thermal evaporation, which makes larger displays expensive. Switching the device fabrication technique to solution processing is one way to reduce the cost of device manufacturing. Another attractive alternative to vacuum deposited OLEDs is Light-emitting electrochemical cells (LEECs). LEECs are typically engineered to contain only a single or bilayer of organic materials sandwiched between two air stable electrodes. Through this project we are investigating MR-TADF systems in solution processed devices.
Even with more economical and sustainable solution processed devices, there is still room for improvement in the emitting material used in the devices. Commercial displays rely on fluorescent emitters for blue emission and iridium based phosphorescent compounds for green and red pixels. Iridium based phosphorescent compounds are by far the best electroluminescent (EL) materials due to their ability to harvest 100% of the excitons in the device to produce light. However, this type of emitter is not suitable for pure blue emission due to its instability. In addition, availability of iridium on the Earth’s crust is limited. Organic fluorescent compounds are stable blue emitters; however, this class of materials can only harvest 25% of the excitons formed in the EL device due to spin statistics. Lack of proper pure blue emitters has meant that display manufacturers have kept inefficient fluorescent compounds in use. Since the global energy demand is increasing at a swift rate, there exists an urgent demand for developing materials that are low cost, easily accessible, environmentally friendly, stable, and efficient. A sustainable solution to this issue is to develop purely organic emitters that can harvest 100% exciton in an EL device. Thermally activated delayed fluorescent (TADF) compounds can harvest dark triplets via thermal back population of singlets from triplets. Conventional TADF compounds are designed to have an electron donor unit (D) and acceptor (A) unit connected through a spacer. It is possible to generate efficient blue, green, and red emitters with this design, however, the emission from these systems are very broad. Efficiency of broad emitters will be significantly reduced when applied in commercial displays as colour filters must be used. We have developed MR-TADF emitters for solution processed devices such as OLEDs and LEECs. The efficiency of MR-TADF OLEDs can compete with commercial devices that use colour filters. Thus, energy loss will be negligible when MR-TADF compounds are used in commercial displays. We developed our emitters combining our knowledge of chemistry, physics and material science. The outcome of this project will strengthen EU’s leading role as material supplier for OLED displays.
Each WP consists of designing materials, computational screening, designing synthetic routes, successful synthesis with high purity, optoelectronic characterization and finally investigating their EL performance.

1.2.1 Work Package 1
Modified triangulene type MR-emitter: We designed and synthesized a new MR-TADF emitter and investigated its photophysical and optoelectronic properties compare to its parent emitter. This work was published in Adv. Opt. Mater. 2020, 8, 1901627 and Chem. Sci. 2021, 12, 1121-1125.
1.2.2 Work package 2
Dimeric triangulene type MR-emitter for red shifted emission: We designed and synthesized a new dimeric MR-TADF narrow band green emitter and investigated its photophysical and optoelectronic properties. This work was published in Mater. Chem. Front. 2020, 4, 2018-2022.
1.2.3 Work package 3
Solution processable MR-TADF emitters for LEECs: We designed and synthesised three derivatives of solution processible ionic MR-TADF compounds for LEEC applications. We achieved colour tuning in ionic MR-TADF compounds by synthesizing blue, green, and red emitters by modifying a MR fragment with side groups. This project is at the device fabrication stage.
1.2.4 Work package 4
B,N-doped heptacene: We designed and synthesized a deep blue TADF B,N-doped heptacene that shows narrow emission band and short delayed lifetime in dilute solutions and in solid state. Solution processed deep blue device was fabricated with the same material. This work was published in (J. Am. Chem. Soc. 2020, 142, 14, 6588–6599 and SID Symposium Digest of Technical Papers, 2021, 52, 228-231). We further investigated modifying the parent molecule to tune the emission colour to the pure blue region and studied its photophysical and optoelectronic properties. (A manuscript is under preparation based on this project).
1.2.5 Work package 5
Effect of hetroatom doping position on the efficiency of TADF: In this work we are systematically investigating the effect of doping position on the efficiency of MR-TADF systems. Materials are successfully obtained and photophysical characterization is on-going.
1.2.6 Work package 6
Hybrid deep blue MR systems: We designed and synthesized new narrowband deep blue MR-TADF emitters that can improve contribution from higher triplet states towards TADF. Materials are successfully synthesized and photophysical characterization is on-going.
We demonstrated reduced efficiency roll-off for MR-TADF emitters in OLEDs with simple structural modification without sacrificing their narrow emission and TADF property. The results from this project presented new design directions for efficient MR emitters. Using coupled cluster calculations, we further investigated colour tunability and factors controlling the TADF efficiency in MR-TADF systems. The outcomes of this project will provide vital insight into complex energy relaxation pathways in MR-TADF systems and strategies to improve the efficiency in these systems.
This project saw the development of a series of pure blue MR-TADF emitters that are potential candidates to replace inefficient fluorescent blue emitters. These materials can be made in gram scale with high purity from inexpensive commercially available starting materials. The outcome of this project will open new molecular designs for stable and efficient blue emitters and high triplet energy host materials.
All the work carried out during this project is aimed at developing cost-effective, easy to access and sustainable solution processable emitters for display applications. The work carried out in this project is of potential impact to the display industry who seek solutions for next-generation blue OLEDs.