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Recent Advances in the Computational Characterization of π-Conjugated Organic Semiconductors
Published in John R. Reynolds, Barry C. Thompson, Terje A. Skotheim, Conjugated Polymers, 2019
Jean-Luc Bredas, Xiankai Chen, Thomas Körzdörfer, Hong Li, Chad Risko, Sean M. Ryno, Tonghui Wang
In 2012, Adachi and co-workers proposed a promising approach to harvest triplet excitons in organic emitters by promoting reverse ISC (RISC) from T1 to S1 via simple thermal activation.285 This process, illustrated in Figure 2.8, gives rise to thermally activated delayed fluorescence (TADF) in metal-free organic conjugated compounds (see Figure 2.8). Since then, a large number of experimental and theoretical investigations have been devoted to these purely organic TADF third-generation OLED emitters.286–289 Impressive photo-physical properties and device performances have been reported with, in some instances, IQE reaching nearly 100% and EQE as high as 41.5%.290 However, there remain challenging issues, such as how to best balance the need for a small single (S1) – triplet (T1) energy gap with the need for a large S1 to S0 oscillator strength, the limited stability of the blue TADF emitters, and relatively low efficiencies of red and near-infrared TADF emitters. It is thus desirable to develop a better understanding of the basic TADF mechanisms. Our objective in this section is then to give a brief introduction to some recent advances in the theoretical understanding of TADF emitters.
Fluorescence Spectroscopy and Its Implementation
Published in Helmut H. Telle, Ángel González Ureña, Laser Spectroscopy and Laser Imaging, 2018
Helmut H. Telle, Ángel González Ureña
E-type delayed fluorescence was observed for the first time in eosine. Again, its fluorescence is spectrally near-identical to that of normal, prompt fluorescence, but its apparent lifetime is that of phosphorescence. The latter observation was interpreted that the mechanism-limiting step implies the formation of the triplet state. This type of delayed fluorescence is illustrated in Figure 9.9, representing the simplified Jablonski diagram for the two steps involved in the E-type delayed fluorescence (including the triplet state formation and the thermal activation of the relaxed T1 state to provide sufficient energy for the re-crossing into S1), and the fluorescence decay observed for the overall process. Because of the need for (thermal) activation energy of the triplet state, in order to overcome the S1 barrier, the process is also known as thermally activated delayed fluorescence (TADF). An experimental example for the TADF process is shown in Figure 9.10.
Effects of the phosphorescent sensitizer on charge dynamics in deep blue phosphor-sensitized-fluorescent organic light-emitting diodes
Published in Journal of Information Display, 2022
Hakjun Lee, Kyo Min Hwang, Ki Ju Kim, You Na Song, Young Kwan Kim, Taekyung Kim
After the success of the commercialization of organic light-emitting diodes (OLEDs) in consumer electronic devices, tremendous progress has been made in terms of their efficiency and life span [1–5]. For example, by adopting green and red phosphorescent emitting materials in the OLED panels for cellphones, current and power efficiency have been enormously increased, and the life span of the device has also been dramatically improved. Unlike green and red, the situation for blue color is completely different. Since the very first commercial OLED product, fluorescent materials with low efficiency have been used in blue pixels. Despite the high efficiency, the operational instability of deep blue phosphorescent OLEDs (PhOLEDs) hinders its commercialization. Since only a few metal elements such as iridium (Ir) or platinum (Pt) can be utilized practically, the molecular structures are also limited. About a decade ago, a significant breakthrough was reported [6]. Thermally activated delayed fluorescent (TADF) materials opened a new era of triplet-harvesting technology due to practically unlimited materials design possibilities. Thus, it is no wonder that TADF OLEDs are extensively explored as a candidate for the blue-emitting materials for the next generation OLEDs. However, deep blue TADF OLEDs are suffering the same operational instability as PhOLEDs. The extremely short life span can be attributed to many factors such as the long triplet excited state life span and the need for a high energy host for a broad spectrum of TADF dopant materials.
The role of the bulky blocking unit of the fluorescent emitter in efficient green hyper-fluorescent organic light-emitting diodes
Published in Journal of Information Display, 2021
Fluorescent organic light-emitting diodes (OLEDs) have been used for several decades because of their good stability [1–3]. However, they have limited internal quantum efficiency because only singlet excitons can be used for light emission [4]. To overcome such limitation by utilizing all generated excitons, thermally activated delayed fluorescence (TADF) emitters emerged to harvest the triplet excitons using the small singlet–triplet energy gap [5–8]. The small energy gap between the singlet state and the triplet state in the TADF materials can induce reverse intersystem crossing from the triplet state to the singlet state. Therefore, the TADF materials can achieve 100% internal quantum efficiency. Although the TADF materials have the merit of quantum efficiency, they do not have good color purity and good emission ability due to the torsion between their donor and their acceptor [9].
Theoretical insights on the luminescent mechanism of a highly efficient green-activated delayed fluorescence emitter using the QM/MM method
Published in Molecular Physics, 2023
Hai-Yang Sun, Zi-Yue Yu, Ai-Ping Zhou, Shu-Li Wei, Qiang Chang, Tian Zhang, Yu-Ping Sun
For the TADF system, the total fluorescence efficiency () is the sum of the immediate fluorescence efficiency () and the thermally activated delayed fluorescence efficiency (), . and can be calculated by the following Equations (1)–(4):