Chen Xian-Kai, Kim Dongwook, Brédas Jean-Luc
Center for Organic Photonics and Electronics and School of Chemistry and Biochemistry , Georgia Institute of Technology , Atlanta , Georgia 30332-0400 , United States.
Department of Chemistry , Kyonggi University , 154-42 Gwanggyosan-Ro , Yeongtong-Gu, Suwon 16227 , Korea.
Acc Chem Res. 2018 Sep 18;51(9):2215-2224. doi: 10.1021/acs.accounts.8b00174. Epub 2018 Aug 24.
Since the seminal work of Tang and Vanslyke in 1987 on small-molecule emitters and that of Friend and co-workers in 1990 on conjugated-polymer emitters, organic light-emitting diodes (OLEDs) have attracted much attention from academia as well as industry, as the OLED market is estimated to reach the $30 billion mark by the end of 2018. In these first-generation organic emitters, on the basis of simple spin statistics, electrical excitation resulted in the formation of ∼25% singlet excitons and ∼75% triplet excitons. Radiative decay of the singlet excitons to the singlet ground state leads to a prompt fluorescence emission, while the triplet excitons only lead to weak phosphorescence due to the very small spin-orbit couplings present in purely organic molecules. The consequence is a ca. 75% energy loss, which triggered wide-ranging efforts to try and harvest as many of the triplet excitons as possible. In 1998, Thompson, Forrest, and their co-workers reported second-generation OLED emitters based on coordination complexes with heavy transition metals (e.g., iridium or platinum). Here, the triplet excitons stimulate efficient and fast phosphorescence due to the strong spin-orbit couplings enabled by the heavy-metal atoms. Internal quantum efficiencies (IQE) up to 100% have been reported, which means that for every electron injected into the device, a photon is emitted. While these second-generation emitters are those mainly exploited in current OLED applications, there is strong impetus from both cost and environmental standpoints to find new ways of exploiting purely organic emitters, which in addition can offer greater flexibility to fine-tune the electronic and optical properties by exploiting the synthetic organic chemistry toolbox. In 2012, Adachi and co-workers introduced a promising strategy, based on thermally activated delayed fluorescence (TADF), to harvest the triplet excitons in purely organic molecular materials. These materials now represent the third generation of OLED emitters. Impressive photophysical properties and device performances have been reported, with internal quantum efficiencies also reaching nearly 100%. Our objectives in this Account are threefold: (i) to lay out a comprehensive description, at the molecular level, of the fundamental photophysical processes behind TADF emitters; (ii) to discuss some of the challenges facing the design of TADF emitters, such as the need to balance the efficiency of thermal activation of triplet excitons into the singlet manifold with the efficiency of radiative transition to the ground state; and (iii) to highlight briefly some of the recent molecular-design strategies that pave the way to new classes of TADF materials.
自1987年Tang和Vanslyke关于小分子发光体的开创性工作以及1990年Friend及其同事关于共轭聚合物发光体的工作以来，有机发光二极管（OLED）引起了学术界和工业界的广泛关注，因为据估计到2018年底OLED市场将达到300亿美元大关。在这些第一代有机发光体中，基于简单的自旋统计，电激发导致形成约25%的单重态激子和约75%的三重态激子。单重态激子向单重态基态的辐射衰变导致即时荧光发射，而由于纯有机分子中存在的非常小的自旋-轨道耦合，三重态激子仅导致微弱的磷光。结果是约75%的能量损失，这引发了广泛的努力，试图尽可能多地捕获三重态激子。1998年，Thompson、Forrest及其同事报道了基于与重过渡金属（如铱或铂）的配位络合物的第二代OLED发光体。在这里，由于重金属原子实现的强自旋-轨道耦合，三重态激子激发高效且快速的磷光。已报道内部量子效率（IQE）高达100%，这意味着注入器件的每个电子都会发射一个光子。虽然这些第二代发光体是当前OLED应用中主要使用的发光体，但从成本和环境角度来看，都有强大的动力去寻找利用纯有机发光体的新方法，此外，通过利用合成有机化学工具包，纯有机发光体还可以提供更大的灵活性来微调电子和光学性质。2012年，Adachi及其同事引入了一种基于热激活延迟荧光（TADF）的有前景的策略，以在纯有机分子材料中捕获三重态激子。这些材料现在代表了第三代OLED发光体。已报道了令人印象深刻的光物理性质和器件性能，内部量子效率也接近100%。我们在本综述中的目标有三个：（i）在分子水平上全面描述TADF发光体背后的基本光物理过程；（ii）讨论TADF发光体设计面临的一些挑战，例如需要平衡三重态激子热激活到单重态的效率与辐射跃迁到基态的效率；（iii）简要强调一些最近的分子设计策略，这些策略为新型TADF材料铺平了道路。
