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

照明和显示器是我们日常生活的重要组成部分，消耗了全球约20%的电力。因此，通过提高这些设备的效率，可以实现显著的能量和成本节约。由于其重量轻，灵活性和高性能的光学和电学特性，有机发光二极管（OLED）是这项研究的中心焦点，并在智能手机，电视和照明等技术中具有巨大的应用潜力。与传统的LED一样，OLED能够将电能转化为可见光、紫外线（UV）或近红外线（NIR）光。然而，与LED不同的是，OLED由几种非常薄的堆叠层有机材料组成，并且不依赖于小的点状单晶。此外，有机体系对于大规模生产是非常有吸引力的，这是由于它们能够沉积在各种低成本基底如玻璃、塑料或金属箔上，并且由于它们相对容易加工。实际上，因为这些装置的生产成本通常由制造和包装主导，所以相对弱的货车德瓦尔斯键合的有机膜也为一套新的创新制造方法创造了机会，包括通过使用与印模接触的直接印刷，或者经由喷墨和其他基于溶液的方法。尽管OLED具有实现比LED更高的能量效率的巨大潜力并且还可以在更可持续的条件下加工，但是当今的现有技术水平的白色OLED仍然具有比白色LED更高的功耗。在效率方面，最初尝试实现基于纯有机材料的OLED受到发射光的激发态类型的限制。实际上，在电激发时，25%的发射分子处于所谓的单重激发态，而75%处于三重激发态。然而，传统的有机材料不能从三重激发态发射，这意味着只能实现25%的最大效率。广泛的研究工作成功地导致了第二代（所谓的磷光）OLED，其使用重金属来促进来自三重态的光发射，并且原则上实现100%的效率。然而，到目前为止，发现的唯一实用的磷光材料是铱和铂的配合物，由于其高成本和低丰度而不适合商业应用。本研究计划旨在使用多尺度建模来研究对利用热激活延迟荧光的新型OLED的分子和材料至关重要的基本性质。这利用了两个发射态（单重态和三重态）之间的小能隙，使得热能可以将布居从三重态转移到单重态。重要的是，这种机制原则上打开了实现100%效率的可能性，并且关键地沉淀了返回到仅含有更轻更丰富元素（例如有机分子）的材料的可能性。通过结合量子化学，分子和量子动力学，这种多学科的方法将在各种时间和长度尺度上产生对材料性质的详细物理和化学理解。至关重要的是，这些模拟将巩固我们对导致其效率的属性的理解。因此，这种自下而上的方法将为实现系统的材料设计提供重要的见解，并有可能大大改进和廉价的设备。

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.