The Excited State Properties of Thermally Activated Delayed Fluorescence Emitters: A Computational Study Towards Molecular Design

Lighting and displays form essential parts of our daily lives and consume approximately 20% of the electricity used worldwide. Consequently, significant energy and cost savings can be achieved by improving the efficiency of these devices. Due to their lightweight, flexibility and high-performance optical and electrical properties, Organic Light-Emitting Diodes (OLEDs) are a central focus of this research and have huge potential for application in technologies such as smart phones, televisions and lighting. OLEDs are, like classic LEDs, able to transform electrical energy into visible, ultra-violet (UV) or near Infra-red (NIR) light. However, unlike LEDs, OLEDs consist of several very thin, stacked layers organic materials and do not rely on small, point-shaped single crystals. In addition, organic systems are highly attractive for mass production stemming from their ability to be deposited on a variety of low-cost substrates such as glass, plastic or metal foils, and due to their relative ease of processing. Indeed, because production costs of these devices are typically dominated by fabrication and packaging, the relatively weak van der Waals bonded organic films also create the opportunity for a new suite of innovative fabrication methods, including direct printing through the use of contact with stamps, or alternatively via ink-jets and other solution-based methods.

Even though OLEDs have huge potential to achieve a higher energy efficiency than LEDs and may also be processed under more sustainable conditions, today's state of the art white OLEDs still have higher power consumption than white LEDs. In terms of efficiency, initial attempts to implement OLEDs based upon purely organic materials were restricted by the type of excited state which emits the light. Indeed, upon electrical excitation 25% of the emitting molecules are in a so called singlet excited state, while 75% are in triplet excited states. However, conventional organic materials cannot emit from the triplet excited states, meaning that only a maximum efficiency of 25% could be achieved. An extensive research effort successfully led to 2nd generation (so called phosphorescence) OLEDs that use heavy metals to promote light emission from the triplet states and, in principal, achieve 100% efficiency. However, until now the only phosphorescent materials found practically useful are iridium and platinum complexes that are unappealing for commercial applications due to their high cost and low abundance.

This research proposal seeks to investigate, using multi-scale modelling, the fundamental properties crucial to molecules and materials for a new class of OLEDs that exploits thermally activated delayed fluorescence. This exploits a small energy gap between the two emitting states (singlet and triplet) so that thermal energy can transfer population from the triplet state to the singlet state. Importantly this mechanism opens the possibility to achieve, in principal, 100% efficiency and crucially precipitates the potential to return to materials containing only lighter more abundant elements, such as organic molecules. By combing quantum chemistry, molecular and quantum dynamics, this multidisciplinary approach will produce a detailed physical and chemical understanding of the material properties on a wide variety of time and length scales. Critically, these simulations will underpin our understanding of the properties that lead to their efficiency. This bottom up approach will consequently provide important insight into achieving systematic material design with the potential for vastly improved and cheaper devices.

This research proposal seeks to address a number of important questions aimed at achieving a deeper understanding of complexes and materials that exploit thermally activated delayed fluorescence (TADF) for applications within 3rd generation organic light emitting diodes (OLEDs). The core research objectives contain elements of knowledge generation that are both closely related to fundamental academic research and, importantly, which are of direct relevance to applied practical questions for realising improved devices. Consequently, it is anticipated that the successful execution of this proposal will have implications over a broad field of research with, on the longer term, the potential for improved and cheaper devices.

This relevance to a wide field of research domains leads to a broad scope of potential beneficiaries. In the short term (<5 years) the most likely beneficiaries will be academic researchers, both theoretical and experimental. It is anticipated that the theoretical/computational approaches adopted and exploited within this proposal will provide impetus for other theoretical researchers in related areas, such as organic photovoltaics. From an experimental perspective, it is expected that the foreseen results will help the interpretation and understanding of experiments, especially those specifically probing the dynamics in the excited states. In addition, this work will potentially motivate additional experiments aimed at proving and/or developing the concepts described by our computations. Importantly, the direct links with experimental research (Prof. Monkman, University of Durham) fostered through this research proposal will enhance the initial knowledge transfer to experimental academic beneficiaries. In the mid-term (5-10 years) it is expected that the insight and understanding obtained from this proposal, perhaps once fully verified with experimental observations, will be exploited by the commercial private sector with a direct interest in manufacturing these devices. Here the collaboration with Cynora GmBH (a technological leader in the field of iridium-free emitter systems for OLEDs) will accelerate this aspect of knowledge transfer. In the long-term (>10 years), successful application of TADF emitters for efficient OLEDs opens the possibility for significantly improved and cheaper devices used within a variety of everyday display and lighting appliances. Consequently it has the potential for a broad impact to the general public, leading to a significant economical and societal impact.

The impact on people will arise from the training and career development of the researchers involved in this multidisciplinary proposal. This funding will provide the PI research independence and opportunities for career development, specifically to foster exciting collaborations with academic and industrial partners. Most importantly it will enable the PI to implement the research aims for his group, in which fundamental academic research is performed in close corroboration with research directly focused towards technological applications enabling a detailed understanding of the important processes over a wide variety of time and length scales. The postdoctoral researcher funded through this proposal will work in close collaboration with a doctoral researcher (funded by Newcastle University). This postdoctoral researcher will have the opportunity to supervise projects run by Master's students and also contribute to the supervision of the postdoctoral researcher, providing opportunities for develop academic management skills. The postdoctoral researcher will be encouraged to attend training courses to enhance his/her multidisciplinary skills (programming, theory and computational software). Specific training and career development requirements will be identified at the start of employment in a meeting the PI.