**2. Fundamental concepts of TADF emitters**

LED, and perovskite LED) were also reported [6–10]. By dint of the strategies used in OLEDs, the performance of these LEDs can be greatly enhanced. Besides, with the increasing understanding of the insight of OLEDs, the concepts utilized in OLEDs can also be applied to other

To satisfy the requirements of energy-saving lighting and high-quality displays, white OLED (WOLED) has been considered to be one of the most promising candidates. Since the pioneer WOLEDs made by Kido and his coworkers, the WOLED technology is greatly improved [14, 15]. During the last 24 years, the power efficiency (PE) was enhanced from less than 1 lm/W to more than 100 lm/W [16–18], indicating that WOLEDs are promising for the lighting and displays filed. In terms of lighting application, WOLEDs require a standard fluorescent tube

Besides, the color rendering index (CRI) above 80 is required for the indoor lighting, and the Commission International de L'Eclairage (CIE) chromaticity coordinates of WOLEDs should be located near white light equal-energy point (0.33, 0.33) [22–25]. Moreover, for the high-quality lighting, other characterization parameters (e.g., correlated color temperature (CCT), color stability, and driving voltage) of WOLEDs are also required to be taken into

In 2012, Adachi et al. made a breakthrough on the thermally activated delayed fluorescence (TADF) material, which has been considered as the third-generation OLED emitter [31–33]. Different from the conventional fluorescent emitters in which only the singlet excitons (25%) can emit light since the radiative decay of triplet excitons (75%) is spin forbidden, TADF emitters could harness both singlet and triplet excitons since triplet excitons can be harvested as delayed fluorescence through their upconversion from a lowest triplet state to a lowest singlet state by inducing an efficient reverse intersystem crossing (RISC) [34–38]. Therefore, similar to phosphorescence emitters, a maximum internal quantum efficiency (IQE) of 100% can be realized [39–42]. Due to the outstanding properties (e.g., free noble metal, high efficiency, low driving voltage, bright luminance, lower power consumption, and potentially long lifetime), TADF emitters have been actively investigated to develop WOLEDs [43–45]. Although efficient TADF emitters were only demonstrated 6 years ago and WOLEDs with TADF emitters were just reported 4 years ago, the performance of WOLEDs with TADF emitters has been improved step by step [46]. For example, WOLEDs with TADF emitters can exhibit nearly 20% external quantum efficiency (EQE) [47], which is comparable to stateof-the-art phosphorescence WOLEDs and fluorescence/phosphorescence hybrid WOLEDs [29, 30, 48–54]. Thus, WOLEDs with all TADF emitters have great potential to the lighting

Herein, we first introduced the basic concepts of TADF emitters, which are beneficial to comprehend WOLEDs with TADF emitters. Then, we summarize the main approaches to realize WOLEDs with TADF emitters in recent years. More specifically, we highlight the recent development of WOLEDs based on all TADF emitters, WOLEDs based on TADF and conventional fluorescence emitters, and WOLEDs based on TADF and phosphorescence emitters. Particularly, the device structures, design strategies, working mechanisms, and electroluminescent processes of the representative high-performance WOLEDs with TADF emitters are

[19–21].

optoelectrical devices, which is beneficial to the development of related fields [11–13].

38 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

efficiency (40–70 lm/W) and ≥10,000 h of lifetime at the luminance of ≥1000 cd/m<sup>2</sup>

account [26–30].

and display field.

Due to the effect of spin statistics, when holes injected from the anode meet electrons injected from the cathode, singlet and triplet excitons will be formed with a ratio of 1:3 [55]. In the case of the first-generation OLED emitters (i.e., conventional fluorescence emitters), only the singlet excitons (25%) can emit light since the radiative decay of triplet excitons (75%) is spin forbidden, as shown in **Figure 1**. Therefore, the EQE of fluorescence WOLEDs is usually below 5%, considering that the outcoupling factor is ~20%. For the second-generation OLED emitters (i.e., phosphorescence emitters), they can not only harvest triplet excitons via the triplet-triplet energy transfer but also harvest singlet excitons via the singlet-triplet intersystem crossing process (ISC) due to the heavy-atom effect [56]. Thus, the EQE of phosphorescence WOLEDs can be as high as 20%.

In terms of the third-generation OLED emitters (i.e., TADF emitters), a small energy gap (△EST) between singlet (S1 ) and triplet (T1 ) excited states is required and can be attained by carefully designing organic molecules [31]. Generally, the S1 level was considerably higher in energy than the T1 level by 0.5–1.0 eV, because of the electron exchange energy between these

**Figure 1.** An energy diagram of a conventional organic molecule. H is hole and e is electron. Reproduced from Ref. [31].

levels. However, to enhance thermal upconversion (i.e., T1 → S<sup>1</sup> RISC), the molecular design of TADF materials requires small △EST, typically less than 0.2 eV, to overcome competitive non-radiative decay pathways, leading to highly luminescent TADF materials [57]. In addition, to enhance the photoluminescence efficiency of TADF materials, the geometrical change in molecular conformation between its ground state (S0 ) and S1 states should be restrained to suppress non-radiative decay. As a result, the maximum theoretical IQE of TADF emitters can be 100%.
